T11D-1282 0800h
Crustal seismicity of Kamchatka
The summary is given of tectonic seismicity of Kamchatka mainland, i.e., of areas to WNW from the main subduction zone of Kuril-Kamchatka arc. Volcano-tectonic earthquakes, abundant in this area, are not considered. Historical and pre-1960 instrumental data are almost absent for this territory. Paleoseismological data indicate quite considerable activity. During the last 40 years of detailed studies, hundreds of small and moderate earthquakes are recorded; they mostly occurred as clusters. Clusters and individual epicenters are often associated with known or assumed Holocene faults; however, they do not form well-expressed lineaments. The structures that have been revealed most distinctly are located in the ranges Sredinny, Kumroch, Tumrok, Valaginsky, Ganal?sky, in the Shchapinsky graben, and along the Eastern Volcanic Belt. A band of relatively weak seismicity stretches to the North, to Parapolsky-dol istmus and to Koryaksky uplands.
T11D-1283 0800h
The Channel Islands Thrust Fault, Southern California: Structure at the Juncture Between the Western Transverse Ranges and the Continental Borderland
Potential-field data over the northern Channel Islands and Santa Barbara basin and seismic reflection data collected near these islands show the crustal structure near the tip of the Channel Island thrust fault. This fault dips north to underlie the Santa Barbara basin and is part of the regional fault system that separates the western Transverse Ranges from the California Continental Borderland. Our investigation focuses on Santa Cruz Island, where a local exposure of mainly Jurassic ophiolitic basement rocks includes the Willows Plutonic Complex. These mafic and ultramafic igneous rocks produce strong magnetic and gravity anomalies, showing that fragments of the Willows Plutonic Complex have been carried northwestward into or below the basin by sinistral translation of hanging-wall blocks in the thrust system. The potential-field anomalies indicate a cumulative left-lateral offset of about 20 km along what is probably the Santa Cruz Island fault. This fault is known from onshore trenching to be primarily a left-lateral strike-slip fault that was active during late Quaternary time. Seismic-reflection data show that where the Santa Cruz Island fault projects into the offshore a fault-bend fold deforms stratified rock in the Santa Barbara basin. Slip along this fault is partitioned into strike-slip and southwest-vergent reverse components. The Santa Cruz Island fault formed where structures of the California Borderland terminate to the northwest against the rocks that make up the northern Channel Islands. Structures developed at this termination may be similar to ones that formed where the Newport-Inglewood and the San Pedro Basin faults end to the northwest against the Santa Monica Mountains. These terminating faults pose a considerable earthquake hazard, and findings from the area of Santa Cruz Island may help elucidate this hazard.
T11D-1284 0800h
Probabilistic Seismic Hazard Assessment in Mongolia. Amplification Frequency Microzoning at Ulaanbaatar
The last ten years, many studies about active deformation have been done in Mongolia. The description of the active fault have been improved and the recurrence time of large earthquakes where estimated by pale seismological investigations. A specific paleoseismological study on a thrust fault associated with cumulative surface ruptures has been done showing that large event with magnitude up to 7 take place in this central Asia region where most of large faults are strike slip faults. The seismic recordings have been completely reviewed and the installation of new seismic sensor allows us to better describe the present activity and the attenuation laws. From these recent knowledge, we propose a new seismotectonic model based on known active faults including thrust faults (associated to seismicity or paleoseismicity or to active morphology) and on a homogeneous seismic catalog. The probabilistic analysis, where we included an amplification factor on quaternary sediments, shows a new view of the level of hazard which is not only the mirror of the observed activity but the potential pick ground acceleration for various period of time. The association of geological and seismological characteristics of the region largely improves the results where we show the contribution, for various period of time, of the background seismic activity, the fault activity and the amplification due to soft sediments. We applied also the model for the capital, Ulaanbaatar where live half of the population and stay most of the economy. Using extensive site effect measurements, we estimated the amplified frequencies in the capital. These new results, despite some parameters can still be improved, gives now to the Mongols authorities an estimation of the hazard based on most recent knowledge in this area.
T11D-1285 0800h
Shallow Subsurface Geometry of Active Thrust Faults Along the Itoigawa-Shizuoka Tectonic Line, Central Japan, Determined From Closely Spaced Gravity Survey and Fault Dislocation Model.
The Itoigawa-Shizuoka Tectonic Line (ISTL) is one of the most active intraplate faults in central Japan with a slip rate as high as 6-9 mm/yr in the Late Quaternary. Although slip rate is rather uniform along the entire length of the ISTL, characteristics of Quaternary faulting change remarkably along strike. The northern portion of the ISTL is thrust with the east side upthrown (i.e., Okubo et al., 2000), whereas the southern portion is left-slip or thrust with the west side upthrown (Matsuta et al., 2000). First, we conducted closely-spaced gravity measurements along three lines across the southern portion of the ISTL: these are, from south to north, Ichinose upland, Fujimi, and Chino lines. We analyzed the gravity data using the two-dimensional Talwani's method, and determined the density structures along these three lines. In the Ichinose upland section, back tilting of 100 ka alluvial fans in the foothills suggests thrust component, whereas no evidence exists for strike slip. The density boundary between Middle Miocene rocks and Quaternary basin fill sediments lies in front of the foothills, not at the topographic range front. The boundary fault dips about 18 degrees west. Elongated pressure ridges develop along the range front in the Fujimi section. The pressure ridges are bounded on the west by left-slip faults; shallow excavations have revealed these faults nearly vertical. However, the density boundary between pre-Miocene rocks and Quaternary sediments dips about 15 degrees west, and is situated a few kilometers east of the pressure ridges, where middle Pleistocene alluvial fans have been back tilted. Thus the vertical faults bounding the pressure ridges are secondary faults formed as a result of oblique slip on the west-dipping, low-angle thrust. In the Chino section, 50 ka terrace risers have been displaced left-laterally by more than 400 meters, and hence strike slip seems dominant here. However, the density structure is essentially the same as the above two sections, although more detailed terrain corrections are needed for such a section with rugged topography. Next, we conducted try-and-error parameter calculations of the fault dislocation model in order to clarify whether the low-angle thrust fault could create observed land-surface tectonic features, such as backward tilting fan and asymmetric bulge in the Ichinose upland and the Fujimi regions. The result shows that the model of low-angle thrust fault at 200m beneath surface with short high-angle back thrust indicate good agreement with observed geomorphic features under the condition of fixed total slip amount derived from average slip rate and tephra chronology, compared to another candidate model of high angle strike-slip master fault with flower-structure type short faults. Our results indicate that the ISTL south of Suwa basin is basically a west-dipping fault, formed as one of west-dipping imbricated thrust faults due to collision of the Izu-Bonin arc against Honshu in Miocene-Pliocene time. Our results provide an example of how pre-existing structures rejuvenate under new stress fields.
T11D-1286 0800h
Timescale dependent deformation of orogenic belts?
The principle aim to link geodetic, paleoseismologic and geologic estimates of fault slip is to extrapolate the respective rates from one timescale to the other to finally predict the recurrence interval of large earthquakes, which threat human habitats. This approach however, is based on two often implicitly made assumptions: a uniform slip distribution through time and space and no changes of the boundary conditions during the time interval of interest. Both assumptions are often hard to verify. A recent study, which analysed an exceptionally complete record of seismic slip for the Wasatch and related faults (Basin and Range province), ranging from 10 yr to 10 Myr suggests that such a link between geodetic and geologic rates might not exist, i.e., that our records of fault displacement may depend on the timescale over which they were measured. This view derives support from results of scaled 2D sandbox experiments, as well as numerical simulations with distinct elements, both of which investigated the effect of boundary conditions such as flexure, mechanic stratigraphy and erosion on the spatio-temporal distribution of deformation within bivergent wedges. We identified three types of processes based on their distinct spatio-temporal distribution of deformation. First, incremental strain and local strain rates are very short-lived are broadly distributed within the bivergent wedge and no temporal pattern could be established. Second, footwall shortcuts and the re-activation of either internal thrusts or of the retro shear-zone are irregularly distributed in time and are thus not predictable either, but last for a longer time interval. Third, the stepwise initiation and propagation of the deformation front is very regular in time, since it depends on the thickness of the incoming layer and on its internal and basal material properties. We consider the propagation of the deformation front as an internal clock of a thrust belt, which is therefore predictable. A deformation front advance cycle requires the longest timescale. Thus, despite known and constant boundary conditions during the simulations, we found only one regular temporal pattern of deformation in a steady active bivergent-wedge. We therefore propose that the structural inventory of an orogenic belt is hierarchically ordered with respect to accumulated slip, in analogy to the discharge pattern in a drainage network. The deformation front would have the highest, a branching splay the lowest order. Since kinematic boundary conditions control deformation front advance, its timing and the related maximum magnitude of finite strain, i.e. throw on the frontal thrust are predictable. However, the number of controlling factors, such as the degree of strain softening, the orientation of faults or fluid flow and resulting cementation of faults, responsible for the reactivation of faults increases with increasing distance from the deformation front. Since it is rarely possible to determine the complete network of forces within a wedge, the reactivation of lower order structures is not predictable in time and space. Two implications for field studies may emerge: A change of the propagation of deformation can only be determined, if at least two accretion cycles are sampled. The link between geodetic, paleoseismologic and geologic fault slip estimates can only be successfully derived if the position of the investigated fault within the hierarchical order has not changed over the time interval of interest.
T11D-1287 0800h
Crustal shortening rate inferred from Late Quaternary fluvial terraces across the central part of the northeastern Japan arc
Crustal shortening and strain rates of the central part of the NE Japan arc are estimated using late Quaternary fluvial terraces. Fluvial terraces are correlated to marine isotope stages (MISs) by tephrochronology and 14C dating. The latest and older accumulation terraces are recognised and correlated to MIS 2-4 or 4, and MIS 6, respectively. Regional uplift rates are estimated as 0.17 +/- 0.01 m/kyr (outer arc) and 0.28 +/- 0.03 m/kyr (inner arc) using the relative heights of accumulation terrace surfaces. Assuming that the regional isostatic uplift is caused by crustal shortening, the shortening rates are 0.26 cm/yr (outer arc) and 0.40 cm/yr (inner arc). The strain rates are 0.03 ppm/yr (outer arc) and 0.05 ppm/yr (inner arc). These long-term rates are nearly one order of magnitude smaller than the short-term (several-year) strain rate obtained using global positioning system data. Keywords fluvial terrace; crustal shortening; northeastern Japan; late Quaternary
T11D-1288 INVITED 0800h
Measuring fold growth and crustal shortening: a case study from northern Tien Shan
Active folding can generally be documented from structural geology, geomorphology or subsurface imaging techniques. From the age and elevation of a folded terrace, one can primarily infer incremental uplift or incremental tilting since terrace abandonment, a period of time generally covering several seismic cycles. Structural geology and subsurface geometry are more appropriate to estimate deformation over the longer term, typically a few 100 kyr at least. The identification and dating of growth strata is then key to estimate the chronology of the deformation. The pattern of uplift, and its time evolution, is the first accessible information one can retrieve. For most applications, horizontal shortening needs to be estimated as well. This step requires some structural models of the fold development. For example, in the case of mature fault-bend fold with no growth strata, converting the uplift pattern derived from a folded terrace into horizontal shortening is straightforward: uplift at any point along the section is equal to the product of the sinus of local dip angle and of the shortening across the fold. In many cases this simple approach cannot be applied. Dip angles may not parallel the fault at depths either because the beds may postdate fold initiation (growth strata) or because the fold might not be mature enough, or because of non conservation of bed thickness during deformation . We will show how the methodology can be adapted and expanded in partitucal to identify and overcome these difficulties. We will focus on a case study of an active fold along the northern front of the Tien Shan. The time evolution of shortening across that particular fold can be estimated accurately from combining geomorphic measurements, structural measurements and a magnetostratigraphic section.
T11D-1289 0800h
Preliminary 3-Dimensional Geologic Map of the Santa Rosa Plain, Northern California
We have constructed a preliminary 3-dimensional geologic map of the Santa Rosa Plain as a tool to address earthquake hazard and groundwater issues. The map allows integration of diverse datasets to produce a stratigraphic and structural architecture for the region. This framework can then be used to predict pathways of ground water flow and potential areas of enhanced or focused seismic shaking beneath the Santa Rosa Plain. The 3D map also allows us to identify relations which will require further refinement to develop a coherent 3D image of the crust. The 3D map, built using EarthVision 3D geologic mapping software, consists of three bounding components: fault surfaces, stratigraphic surfaces, and a basement upper surface. Fault surfaces are derived from geologic mapping, subsurface projection of fault dips from the surface geology and earthquake hypocenters. Stratigraphic surfaces are derived from the mapped geology, a digital elevation model and stratigraphic information from wells. A basement surface, predominantly composed of Mesozoic rocks of the Franciscan Complex, the mafic Coast Range Ophiolite and strata of the Great Valley Sequence, is derived from inversion of regional gravity measurements and constrained by well data. The preliminary 3D map of the Santa Rosa Plain area highlights two large basins ($>$2 km deep): the Windsor and Cotati basins. These basins are divided by a structural high associated with the W-NW-trending, NE-dipping Trenton thrust fault. The Cotati basin is further subdivided by a deeper basement ridge subparallel to the Trenton fault, which separates the basin beneath Cotati from the basin of Petaluma Valley to the southeast. Neither of the basement ridges breaks the surface, yet faults associated with the ridges could displace or truncate aquifers, provide channelways for groundwater flow between aquifers, or create zones of impermeability that disrupt the vertical and lateral continuity of groundwater flow. The complex configuration of the buried basins and their crosscutting ridges could also enhance local ground shaking during earthquakes on the active Rodgers Creek, Healdsburg and Maacama faults, and might explain unexpectedly severe damage suffered at Santa Rosa in the 1906 San Francisco earthquake and in the 1969 earthquakes on the Rodgers Creek-Healdsburg fault system. A major challenge in refining this model will be to develop other stratigraphic surfaces within the blocks framed by the major fault surfaces, such as the base of the Holocene, the base of the Tertiary sedimentary section, and the bases of Tertiary volcanic units. Additional gravity, magnetic, seismic, material property and well data, coupled with inferences from the geology exposed around the margins of the Plain will be used in subsequent versions of this 3D map to improve constraints on the sedimentary architecture of the basins, the shape of the basement surface beneath the water-bearing units, the location, shape and offset of faults within and bounding the Santa Rosa Plain, and the distribution of mafic volcanics rocks.
T11D-1290 0800h
3-D Discrete Element Simulation of Strike Slip Faulting
Recent progress of the X-ray Computed Tomography(X-rayCT) enables us to observe not only the surface but also the inside of geomaterials with a certain level of resolution. X-rayCT was employed in a series of experiments on sand box to visualize the faulting mechanism. A three-dimensional discrete element simulation was performed in order to figure out not only the strain distribution but also the stress distribution during the process of strike slip faulting. In this simulation, a rectangular sand box made of a rigid basement and rigid lateral walls is modeled with 129300 particles. The Discrete Element Method(DEM), widely used in the field of soil mechanics, is a numerical simulation technique that solves an assembly of frictional granular bodies by means of pursuing a behavior of interacting particle governed by Newton's laws of motion. The strike slip faulting analysis successfully simulated the sequential Reidel shear development from basement and computed the stress distribution produced by shearing. A zone of intensive shear stress developed adjacent to the Reidel shear plane joint at the pre-peak stage of boundary shear stress time history. After the peak stage of boundary shear stress, the shapes of histogram of the second stress invariant kept almost unchanged, then the boundary shear stress reached the residual strength and a lumped shear band appeared near the center of the model.
http://unit.aist.go.jp/actfault/english/activef.html
T11D-1291 0800h
Coseismic fold scarp associated with historic earthquakes upon the Yoro active blind thrust, the Nobi-Ise fault zone, central Japan
We present structural models constrained by tectonic geomorphology, surface geologic mapping, shallow borehole transects and a high-resolution S-wave seismic reflection profile to define the kinematic evolution of a coseismic fold scarp along the Nobi-Ise fault zone (NIFZ). The NIFZ is an active intraplate fault system in central Japan, and consists of a 110-km-long array of active, east-verging reverse faults. Fold scarps along the Yoro fault are interpreted as produced during a large historic blind-thrust earthquake. The Yoro Mountains form the stripped core of the largest structure in the NIFZ and expose Triassic-Jurassic basement that are thrust eastward over a 2-km-thick sequence of Pliocene-Pleistocene strata deposited in the Nobi basin. This basement-cored fold is underlain by an active blind thrust that is expressed as late Holocene fold scarps along its eastern flank. Drilling investigations across the fold scarp at a site near Shizu identified at least three episodes of active folding associated with large earthquakes on the Yoro fault. Radiocarbon ages constrain the latest event as having occurred in a period that contains historical evidence for a large earthquake in A.D. 1586. A high resolution, S-wave seismic reflection profile at the same site shows that the topographic fold scarp coincides with the projected surface trace of the synclinal axis, across which the buried, early Holocene to historic sedimentary units are folded. This is interpreted to indicate that the structure accommodated coseismic fault-propagation folding during the A.D. 1586 blind thrust earthquake. Flexural-slip folding associated with secondary bedding-parallel thrusts may also deform late Holocene strata and act to consume slip on the primary blind thrust across the synclinal axial surfaces. The best-fitting trishear model for folded ca. 13 ka gravels deposited across the forelimb requires a 28\deg east-dipping thrust fault. This solution suggests that a 4.2 mm/yr of slip rate has been accommodated on the Yoro fault during the late Holocene, with an average vertical rate of 1.9 mm/yr. This is consistent with longer-term slip rates calculated by a structural relief across a ca. 7.3 ka volcanic ash horizon (1.6 mm/yr), and ca. 110 ka innerbay clays (1.3 mm/yr) deposited across the forelimb. Our trishear model is thus able to account for the bulk of the folding history accommodated at shorter millennial timescales, suggesting that this technique may be used to adequately define slip rates on blind thrust faults.
T11D-1292 0800h
Investigating Kinematics Of Folding From Sandbox Experiments
We analyze the kinematics of folding using sandbox experiments in order to provide guidelines to interpret uplift rates revealed by alluvial or fluvial terraces deformed by fault propagation folds. The experiment consists of sand layers intercalated with low friction glass beads layers over an horizontal rigid basement, pushed by a moving backstop. The total thickness, h, is 80 pixels (1m). The basal friction angle is estimated to 20°. Deformation along the cross section is monitored from a video system. The displacement field between two images is measured from the optical flow technique. This set-up allows describing quantitatively the development of a nascent fold formed at front of a propagating decollement. The experiment shows that in the early stage, when cumulative shortening is less than 2h/3, slip along the decollement tapers gradually to zero and the displacement gradient is absorbed by distributed deformation of the overlying medium. In this stage, horizontal displacements decrease linearly with distance. Vertical displacements reflect a nearly symmetrical mode of folding, with displacements varying linearly between relatively well defined hinges. When the cumulative slip on the decollement exceeds about 2h/3 deformation tends to become localized on a few discrete shear bands. When deformation exceeds h deformation is fully localized on a single frontal ramp reaching the surface. At this stage the fault geometry does not evolve any more and the hanging wall deforms by simple shear as it overthrusts the flat-ramp system. The ratio between maximum uplift rate and horizontal shortening rate increases until deformation gets fully localized, and then drops to a constant value imposed by the fault geometry. As long as strain localization is not fully established the sand layers experience a combination of thickening and horizontal shortening which induce gradual rotation of the backlimb and forelimb of the growing fold. During that stage the relationship between uplift rate and shortening rate depends on cumulative deformation. We propose an analytical expression that reproduces this behavior. The kinematics observed in these experiments might be used to relate finite deformation and incremental folding in the case of fault-propagation folds and hence derive shortening rate from deformed recent markers.
T11D-1293 0800h
Identifying Active Faulting in the Flinders Ranges, South Australia, Using a Temporary Seismometer Deployment
Compared to the rest of continental Australia, the Flinders Ranges region of South Australia stands out not only because of its high topographic relief, but also because of its high seismicity and high fault density. The high density of thrust faults combined with their relatively high Quaternary slip rates indicate that the Flinders and Mt. Lofty Ranges comprise a region of pronounced neotectonic activity. Applying the accumulating body of neotectonic evidence to an assessment of earthquake hazard in South Australia requires determining what relationship exists, if any, between the earthquakes and faults. If it can be shown that the seismicity tends to cluster along faults, then neotectonic and paleoseismological studies focused on these active faults can help constrain the size and frequency of earthquakes. If there is no evidence of clustering, then an assessment of earthquake hazard will rely more heavily on the very short history of recorded earthquakes We describe the use of a temporary seismometer deployment to monitor local earthquakes in the Flinders Ranges, South Australia. 16 seismograph stations were deployed over a 200 x 100 km area, which is one of the most seismically active regions in Australia. The instrumentation consisted of short-period and broadband Guralp seismometers combined with Reftek and Kelunji data loggers, which sample data continuously at 100-200 sps. Analysis of data from the period Sept.-Dec., 2003, resulted in the determination of hypocentres for over 175 earthquakes, most of which could not be located using PIRSA's permanent network. 54 of these earthquakes had depths resolved at 10 km or greater, and the proportion of deep events appears to increase from the southern to the northern part of the Flinders Ranges. The largest earthquake, ML-4, occurred near Hawker on 22 November, 2003, and has a depth of 17-A2 km, and a well-resolved normal focal mechanism. This result is surprising given that the regional stress field is widely assummed to be compressive, and we will discuss implications for lithospheric deformation in the Flinders Ranges.
T11D-1294 0800h
Seismic reflection profiling around the hypocentral area of the 2003 Miyagi-ken Hokubu earthquake (Mj6.4): Reactivated thrust faulting of a Miocene normal fault.
The 2003 Miyagi-ken Hokubu (northern Miyagi) earthquake occurred on July 26, which was preceded by the largest foreshock of Mj5.6 and was followed by the largest aftershock of Mj5.5. Although these earthquakes were not so large in magnitude, they caused large damages. The earthquakes occurred just beneath the Asahiyama hills, where exist the active Asahiyama flexure. Aftershock observations delineate a clear fault plane that extends toward the Sue hills in the east, not toward the Asahiyama hills. However neither surface ruptures nor active fault assocciated with the earthquakes were observed in this region. To clarify both the surface extension of the fault and geologic structure of this region, we conducted 17km-long seismic reflection profiling, using a 17.5-ton vibrator. Geologically, this region was subjected rapid EW extension in middle Miocene and thus produced rift basin was filled by the Matsushima-wan Group (syn-rift sediments) which was bounded by a normal fault, the Ishinomaki-wan fault, in the eastern side of the basin. The Matsushima-wan Group was unconformably overlain by the Shida Group (Miocene post-rift sediments). The Shida Group was unconformably overlain by the Pliocene and post-Pliocene sediments. Deformed Pliocene strata show thrust faulting, indicating EW compression after early Pliocene. Detailed data processing reveals that the seismic profile is essentially concordant with the structure inferred from surface geology. A west-dipping fault with about 50 degrees is found beneath the southeastern extension of the Sue hills where the Ishinomaki-wan fault was supposed to extend. The deeper part of the fault extends toward the earthquake fault plane determined by aftershocks and the shallower part shows a thrust-like structure, which indicate basin inversion using this fault. Thus the 2003 Miyagi-ken Hokubu earthquake occurred as reactivated thrust faulting of the Miocene normal fault bounding the eastern side of the rift basin.
T11D-1295 0800h
Validation of a Kinematic Semi Empirical Approach to Calculate Seismic Hazard Scenarios
After validating some source inversions of the regional patterns of intensities of earthquakes in the Greater Los Angeles Region (B.S.S.A., Vol. 93, pp. 47-60; B.S.S.A., 2004, Vol. 94, 5, in press), and in NE Italy (J.G.R., 2004, Vol. 109, in press), in the present work we use our kinematic "KF" approach to produce some regional seismic hazard scenarios in NE Italy. We do this by using the KF formula in the direct mode, and by applying a Montecarlo technique to its eleven source parametres. We remind that KF considers the nucleation coordinates, the seismic moment, the fault plane solution, the lengths of the line source and rupture velocities both along-strike and antistrike. In so doing, we obtain 62.325 sources to calculate the scenario for the M=5.8-6.2 Bosco Cansiglio, 1936 earthquake, and 20.625 sources for an hypothetical destructive earthquake caused by the so-called "Montello" tectonic structure (tentative origin of two earthquakes in 1695, and in 778 A.D.), WSW of the Cansiglio area. Both sources have an Alpine dip-slip mechanism. The Cansiglio scenario starts from i) the fault retrieved by intensity inversion, from ii) the Database of potential sources in Italy, of INGV, and iii) from neotectonic evidence. Both scenarios are obtained by mapping the mean values of intensities, and the corresponding standard deviations, all over the region. Note that in the former case a validation is feasible by comparing the theoretical scenario with the experimental intensities reported by the Italian DOM4.1 catalog. Thus, 44 out of 103 site intensities (within the 55x55 km study area) were forecasted exactly; in 39 sites the deviation between the mean scenario intensities and the reference intensities is $<=$0.5 in real numbers, and the difference is 1.5 in only 5 cases. The means plus 1 standard deviation are conservative when referred to the intensities observed in 1936. Taking the 1936 earthquake as reference event, we compared the fitness of our KF scenario and of a traditional scenario produced by an accredited empirical "attenuation law" (Grandori et al., 1987; however, similar results are obtained with other empirical isotropic relations). The fitness criterion was the sum S of the squared residuals r, with r equal to the intensity observed at each site minus the intensity calculated there. The KF scenario scored S=50.5, the empirical "attenuation" scored S=104.6. Regarding the "Montello" scenario, it starts from a causative fault based on the TRANSALP deep seismic traverse, and on neotectonic evidence. Interestingly, it seems that the only intensity datum available for the 778 A.D. earthquake (VII-VIII degree at Treviso) has a low compatibility with the aforementioned Montello tectonic structure; thus, this fault seems to be an unrealistic source for the VIII Century event.
T11D-1296 0800h
Testing 3D fault configuration in the northern Los Angeles basin, California via patterns of rock uplift the since 2.9 Ma
Competing models of three-dimensional fault topology, starting from the Southern California Earthquake Center (SCEC) Community Fault Model (CFM), were tested for viability using numerical Boundary Element Method (BEM) models and patterns of rock uplift by folds in the northern Los Angeles basin Los Angeles basin. Thirteen structural cross-sections constrained by well and outcrop data were used to compile a structure contour map of the base of the Pico Formation (2.9 Ma) across about 50 km of the northern Los Angeles basin from the Coyote Hills on the east to Pacific Palisades on the west. A map of rock uplift rate was constructed from these data by measuring the structural relief relative to the central trough of the Los Angeles basin, a long-lived northwest-trending structural low that lies to the northeast of the Newport-Inglewood fault. BEM models of 3D fault topology were used to generate uplift rates over the same region using North-South contraction at 100 nanostrain/year. A suite of models investigate the sensitivity of uplift patterns to 1) dip of blind thrust faults (e.g. Las Cienegas and Elysian Park), 2) presence of low-angle (20 degree) thrust ramp below 10 km depths 3) regional extent of this low-angle ramp and 4) inclusion of near surface splays of the Santa Monica fault. Model-data compatibility was evaluated on the basis of structural trend, spatial variation in rates and location of major structures (i.e. key near surface folds). All models are consistent with the location and uplift pattern of the Coyote Hills and Santa Fe Springs structures, the location and orientation of the central trough, and a North-trending structure separating Santa Fe Springs on the east from Montebello to the northwest. Incorporation of the low-angle ramp below 10 km depth that is regionally extensive (i.e. many faults sole into this fault) improves model and geologic uplift compatibility. Furthermore, steepening the Las Cienegas and Elysian Park faults to 60 degrees reduces mismatch. If correct, our models are consistent with the emerging paradigms that 1) the north and northeastern boundaries of the Los Angeles basin's central trough are flanked by deep (> 10 km) and low-angle ($<$30 degree) fault ramps and 2) near surface thrust faults merge into this ramp with steep (60 degree) dips.
T11D-1297 0800h
Integrated Geophysical Methods to Examine the lithosphere beneath the of the Southern High Plains
The high plains of west Texas have a more complex basement structure than the flat featureless surface would imply. This region is subject to some of the largest gravity anomalies in the continental U.S. Analysis of gravity data suggests that mafic intrusions, uplifts, lower crustal upwarps and shallower layers in the upper mantle are responsible for most of the small wavelength (less than 150 km) gravity anomalies in this region. Anomalies of this wavelength have been modeled by several previous investigators. We focus here on modeling the long wavelength anomalies that have previously been filtered out as regional anomalies. Regional gravity anomalies still show a tremendous gradient from a gravity low in the west and high gravity in the east. To try to model deep lithospheric structure we apply long wavelength pass filters to focus on regional anomalies. The entire gravity anomaly for wavelengths greater then 400 km could be modeled as a layer then this to about 90 km beneath the Rift and deepens to 115 km at the Texas New Mexico boarder and then thins slightly to the east. We then filtered these data to analyze adapt with 250 km wavelengths and longer. We did not allow the depth to this previous 90 to 120 km layer to change and found the Moho to be 35 km deep beneath the Rio Grande Rift thickening to 56 km beneath Lubbock, Texas (these depths were derived from receiver function analysis and are consistent with LaRistra images) but this did not account for a gravity low across most of western New Mexico. To match this long wavelength gravity anomaly it was necessary to place a thick "rift pillow" beneath the Rio Grande rift that is thickest near the rift but gradually thinning and pinching out near the Texas boarder. The geometry of these layers are consistent with tomographic results form the LaRistra project.
T11D-1298 0800h
Seismogenic Structures Deduced from 3D Spatial Analysis for 1999 M$_{L}$ 6.4 Chaiyi Earthquake Sequences
Due to the convergent tectonics, Taiwan region undergoes frequent earthquakes including felted and feltless ones. The shallow recorded earthquakes ($<$ 20 km) are mainly distributed in western Taiwan. Among them the larger ones usually bring up damages and casualties due to urbanization over there. Since the earthquakes are generated from the activations of active faults, the distribution of the seismicities can be used as a good tool to image the subsurface faults that is essential to realize the framework of active faults. We evaluate an earthquake sequence that happened in Chaiyi area of southwestern Taiwan to clarify the seismogenic faults. The collected earthquake sequence took place at Chiayi in 1999 (namely Chaiyi earthquake; M$_{L}$ 6.4). In total there are 475 earthquakes including the main shock and aftershocks in ten days. We conducted the Double difference (hypoDD) method to relocate all the earthquakes and then used GOCAD (3-D modeling software) to visually image the seismogenic faults. Focal mechanisms are also solved for larger earthquakes (M$_{L}$ $>$ 4) to further check the faulting behavior. On the other hand, we independently reconstructed another structural model based on available subsurface and surface geological data. By comparison of two structural models, we found the seismogenic falults are closely related to the neotectonic system exposed on the ground surface. Three major fault planes (Fault A, B, and C) are identified in Chiayi area. The fault A strikes N-S and dips to the west in listric shape of 65>X to 40>X. Eight earthquakes including the main shock and 7 aftershocks (M$_{L}$ $>$ 4) located on this fault plane all show a focal mechanism of reverse faulting and a nodal plane parallel to the fault plane. The aftershocks distributed in the northern study area allow us further identify another two nearly vertical fault planes, striking N80 >XE (Fault B) and N80>XW (Fault C) respectively. Five focal solutions indicate they are pure strike-slip. Since the main shock is located at the bottom of the Fault A, we therefore interpret that the main shock caused by rupturing on the fault A and simultaneously triggered not only aftershocks on the same fault plane, but also the other two strike-slip faults in the north. Judging the spatial relationships of the subsurface structures derived from this study and surface geology, (1) Fault A may be related to the growth of the Hsiaomei Anticline; (2) Fault B and C to the Meishan fault.
T11D-1299 0800h
Terrace Development in Response to the Interference of two Active Structural Systems in Houli, central Taiwan
The study area, Houli, is located between the frontal two thrust faults, Changhua (CH) and Sanyi, in the western wedge-tip of fold-and-thrust belt of Taiwan. Neotectonically it plays as transition role of Miaoli and Taichung two domains in the north and south respectively. The former one experienced a large earthquake over magnitude 7 in 1935 and also the latter one in 1999. Houli has long been well known not only the abundant terraces but also their deformation related to neotectonics. We recently remapped the geomorphic features by using the 40 m DEM and aerial photos to carefully analyze the subtle features and to realize the degree of the inferences given by different active structural systems. Besides the Tuntzuchiao fault produced by 1935 earthquake, there are two regional scale active structure systems, Tiechanshan (TCS) anticline and CH fault, have been reported in Houli. The TCS anticline is actually dominant in Miaoli area and plunges southward. Its axis orientation turns from NS to NE-SW when reaches Houli. The CH fault is believed as generated from Taichung area and migrated northward into the study area. According to our results, a number of tectonic scarps and warped surfaces in association with CH fault and its coupled back thrust reflect the CH fault system has been active since Pleistocene. On the other hand, the development of staircase terraces distributed ENE-WSW does not follow the growth of TCS anticline. Instead, it is in response to the growth of another smaller anticline, tentatively named as Yuehmei (YM) anticline that is developed parallel to TCS anticline. Looking at the relevant seismic profiles, YM anticline is a local drag-fold attached on the southern limb of TCS anticline and distributed merely in Houli area. Since not much geomorphic deformation associated with TCS anticline, it can be concluded that the growth of TCS anticline is much slower than YM anticline. It indicates that the development of YM anticline is probably to accommodate the shortening of Taichung neotectonic system.
T11D-1300 0800h
Three Sequential Ruptures Along the Rueisuei Segment of the Longitudinal Valley Fault, Eastern Taiwan
The Mw 7.1 1951 October and November earthquakes in eastern Taiwan were the most destructive earthquakes in the past 100 years along the Longitudinal Valley fault. Although these earthquakes produced a substantial amount of damage, little is known about fault slip rates and recurrence intervals along the Longitudinal Valley fault. The November earthquakes produced about 65 km of surface rupture along three north-south trending fault segments along the Longitudinal Valley. During the earthquake, vertical offsets across the faults averaged 1.0 m, with a maximum of 2.1 m. Between June 2003 and February 2004, we initiated paleoseismic studies along the Rueisuei segment of the Longitudinal Valley fault at the Anding bridge site in order to determine the precise timing and magnitude of past earthquakes. Excavations along the 1951 surface rupture reveal evidence of two prehistoric earthquakes. Accelerator mass spectrometer (AMS) radiocarbon ages of detrital charcoal from alluvial deposits and fault scarp colluvium indicate that the penultimate earthquake occurred about 1640 - 1800 AD. The event horizon of the penultimate earthquake is bracketed between two alluvial deposits separated by buried colluvium. Trench wall exposures also revealed clear evidence of at least one pre-1500 AD rupture at the Anding bridge site. AMS dates on detrital charcoal fragments indicate some recycling of detrital charcoal. Vertical offset from the 1951 earthquake at the Anding bridge site was about 1.3 m. Because the scarps formed by the two prehistoric ruptures are similar in height to the scarp formed by the 1951 Mw 7.1 earthquake, the prehistoric ruptures was, at least locally, similar in size to the most recent event. The average dip slip inferred from vertical offset and the observed fault dip of ~27° for the Longitudinal Valley fault at the Anding bridge during the past three earthquakes was about 6.1 0.2 m, which implies a dip slip rate of ~10 mm/a. This rate is only 1/2 of the GPS geodetically determined shortening rate across the valley. Although the Longitudinal Valley fault currently creeps at about 10 mm/a, it is clear that the current creep rate does not fully release strain accumulation along the Rueisuei segment.
T11D-1301 0800h
Reconstruction of High-Resolution Horizontal Displacement Field Using Aerial Photogrammetry and Particle Image Velocimetry: An Example From the Taiwan Chi-Chi Earthquake Rupture Area
Surface displacement field may indicate 3-D fault geometry and provides basis for characterizing further fault behaviors. The GPS method reveals accurate but spatially discontinuous ground displacement because of limited number of geodetic monuments. Here we combine two digital techniques, aerial photogrammetry and particle image velocimetry (PIV), for determining the horizontal, spatially continuous ground displacement field of a portion of the Chi-Chi earthquake rupture area. We selected pairs of almost identical area of pre- and post-earthquake aerial photographs. We orthorectified the pre-earthquake aerial photograph using a previously constructed 40m DEM and several ground control points (GCPs). The post-earthquake aerial photograph was orthorectified using the same 40m DEM for avoiding systematic errors from two sets of DEM. We calibrated the coordinates of the GCPs on the hanging wall by adding GPS determined displacement vectors resulted from the earthquake, which gave best coordinate estimation of the post-earthquake GCPs. Finally, several sizes of the sliding window were used for image comparison applying the particle image velocimetry. Through these techniques, spatially continuous ground displacement field was reconstructed. The pattern indicates a clear trend of decreasing magnitude and flow-like distribution of displacement vectors towards the surface scarps on the hanging wall side. In addition, the results of the ground displacement show fault discontinuity, which generally coincides with the surface trace of Chi-Chi earthquake rupture. With simple assumptions on fault kinematics, the determined displacement pattern may provide better and more reliable constraints on 3-D fault geometry and earthquake slip distribution model.
T11D-1302 0800h
Seismogenic Structures in Hualien Region, eastern Taiwan
Due to extremely high seismicity and abundant tectonic-influenced geomorphic features, eastern Taiwan has long been known as a tectonically active region. The geological model of an on-going arc-continent collision was successfully proposed to explain the arrangement of the tectonic entities and their interaction. The convergent situation between Eurasia plate and Philippine Sea plate is believed still being existing because of no geomorphic evidence directly related to significant subsidence of the backbone mountain range. However, in the north of the Coastal Range, the Philippine Sea plate is moving northerly downward by the subduction mechanism. Accordingly the fault systems on-land and offshore should be significantly different. With an attempt to answer the puzzle mentioned above we therefore analyze the seismogenic structures in northern part of eastern Taiwan. We adopt the double difference (hypoDD) method to relocate earthquakes, apply the GOCAD (Geologic Computer Aided Design) to visually image the 3D subsurface structures, and determine the rupture plane by the Finite Dimension Source Model (FDSM) from first motion focal mechanisms. Selected earthquakes are (1) located within region between 121.2$\sim$$122\deg$E and 23.5$\sim$$24.5\deg$ N; (2) M$_{ L }$$\geq$3; (3) and showing clearly P or S arrived time at least 6 recorded stations. Additionally, we determine the M $_{ L }$ $\geq$4 focal mechanisms by using the first P wave polarities to examine the reliability of rupture planes determined above. A few of seismogenic structures are clearly identified in this study. Looking at the E-W profile, a major reverse fault dipping $60\deg$ to the east is found in depth of 20-40 km beneath the Coastal Range, which is probably the subsurface image of the plate boundary. On the other hand, within the Central Range several N-S oriented high-angle normal faults are found near the surface in the western part of the study area. The second one from the west reflects the subsurface extension of Lishan fault. In the eastern margin of the Central Range seismic clusters in depth of 10-20 km show high-angle reverse faults. Such a situation of compression down beneath but extension above may suggest a flower structure of the back-bone range. To the Hualien coast and offshore area no more mountains exist but we still found a west-dipping thrust in shallow depth, indicating the existence of the shortening. Further north, a complex seismic cluster is dominated by strike-slip and tensional earthquakes, but no fault plane can be recognized by spatial distribution.
T11D-1303 0800h
Paleoseismology of the 1999 Chichi earthquake rupture, central Taiwan
We integrate with the occurrence ages from the nine trench sites at the different locations, through the chronological dates are constrained the timing of individual seismic events, even the occurrence ages fall in very wide ranges. The chronological constrains for the paleoseismic events however can infer that the at least seven paleoearthquakes occurred during the past 3.1 ka, i.e. they occurred in 1999 AD, 430-300 Cal BP (365$\pm$65 year BP), 790-680 Cal BP (735$\pm$55 year BP), 920-890 Cal BP (905$\pm$15 year BP), 1380-1700 Cal BP (1540$\pm$160 year BP), 1710-1900 Cal BP (1805$\pm$95 year BP), and 2840-3160 Cal BP (3000$\pm$160 year BP), respectively. However, the paleoseismic events permit to be defined time intervals between the displacement events. Recurrence intervals can be estimated for 365$\pm$65 years, 380$\pm$65 years, 170$\pm$35 years, 635$\pm$88 years, 265$\pm$128 years, and 1195$\pm$128 years, respectively. Accordingly, the recurrence intervals are not very uniform except for the last two recurrence intervals. They are well constrained by chronological dates at least in three trenches which is approximately 370$\pm$70 years since 800 year BP. The recurrence intervals in the past 2 ka have occurred in less than 700 years, indicating the minimum interval of about 200 years. The oldest interval has a longer time span of about 1200 years, which are probably not consecutive. But we do not have direct evidence that the events were lacked or eroded in these investigated trenches. Estimate of the coseismic vertical slip rate per event can be obtained 1-5 mm/year. And the long-term vertical slip rate is about 2.4-4.7 mm/year combined with six trenching data.
T11D-1304 0800h
Paleoseismology of the 1951 earthquake rupture along the Longitudinal Valley Fault at the Guangfu, eastern Taiwan
The Longitudinal Valley fault, located at the eastern Longitudinal Valley, has been considered as the collision boundary between the Philippine Sea and Eurasian plates. A series of large earthquakes (M 6.2, M7.0) occurred along the Longitudinal Valley fault, on November 25, 1951. The historical records were documented that the surface rupture extended about 60 km from the Guangfu to Chihshang town, and the surface rupture showed an eastern dipping left-lateral reverse fault with 1.63 m horizontal offset and 1.3 m vertical offset nearby the trench site. We excavated three trenches across a fault scarp in the northern segment near the Guangfu. The excavations show that three bifurcated thrusts dip about 30$\deg$ toward the east. We distinguished three paleoseismic events as the evidence for the occurrence of fault intersected relation and truncation of fault within different stratigraphic units, the younger event break through the ground surface inferred the 1951 earthquake rupture. Based on the stratigraphic dislocation in both sides of the three faults, the vertical offset is 1.0 m, 1.5m and 1.5 m, respectively. By restoring the 1951 earthquake rupture, the major fault still deforms the youngest colluvial deposits (radiocarbon age, B.P. 300). We concluded that at least three paleoseismological events can be identified from those trenches and two events occurred in the past 300 years, and the recurrence interval is less than 250 years.
T11D-1305 0800h
Active fault (Liouchia fault) and related Holocene deformation along the frontal foothills, southwestern Taiwan
Fold-and-thrust belt of Taiwan is a neotectonic entity in response to on-going arc-continent collision occurring in western Taiwan. To figure out the fault geometry and kinematics is of great importance when we consider the hazard assessment in such a highly populated area. In southwestern Taiwan, the Liouchia fault bounds the foothills and the Chianan Coastal Plain. The frontal foothills display a clear lineation scarp facing to the west, which is trending NNE-SSW with a length of 17 km located on the forelimb of the Nioushan anticline. The anticline has been interpreted as hangingwall fold associated with the fault ramp of the frontal thrust. Holocene marine terrace (radiocarbon age: 9559-10147 yr BP) unconformably overlain the late Pleistocene strata on the hangingwall. The synchronous marine sediments are deposited underneath the Coastal Plain about 43-m-depth on the footwall. Based on the measurement of vertical displacement in both sides of the fault, it is obtained 4.7 mm/yr of uplifted rate on the up-throwing block.
T11D-1306 0800h
Coseismic displacement distribution of the strong motion records during the 10 December 2003 Chengkung, Taiwan earthquake
The Chengkung earthquake (Mw=6.8) took place on December 10, 2003 was located to 23.106N and121.324E along the eastern coast of Taiwan near the town of Chengkung with the focal depth of 15.7 km. The mainshock was felt throughout Taiwan Island. This earthquake triggered many strong motion stations of the Taiwan Strong Motion Instrumentation Program (TSMIP) that deployed by the Central Weather Bureau (CWB). For a large earthquake occurs at near source region, the ground motion can be on scale recording by strong motion instrument. Permanent displacement signal can be determined after twice integrations with necessary baseline correction. In this study, we calculated 120 coseismic displacements from forty TSMIP strong motion records to study the source dislocation model. Displacement results show, for the northern part (Chengkung) of the study region, magnitudes of horizontal displacements moved toward the north-east direction increase from 3-4 cm in the longitudinal valley (LV) to 7-10 cm in the coastal line. The most horizontal displacements in the southern part (Taitung) of the study area were measured 2-3 cm in the south-west directions. In the epicenter region the maximum uplift was about 18 cm. For understanding the coseismic displacements caused by the 2003 Chengkung earthquake, Okada (1992) method is used to inverse fault plane dislocation. Results show major fault was located from Taitung to Chengkung. Fault length is 40 km and width is 20 km initial at depth 10 km. Fault plane solution are strike 22, dip 50 and slip 64. The averaged slip is 60 cm and Mw is 6.7. This result is reasonable agree with the result of moment tensor inversion solution of Mw=6.8 from Harvard Centroid Moment Tensor solution (strike, 10; dip, 50; slip, 69).
T11D-1307 0800h
A Slow Slide in central Taiwan Induced by Chi-Chi Earthquake revealed by PIV Analysis
The Chi-Chi Earthquake occurred in the western foothills of Central Taiwan, which triggered two fast and catastrophic Chiufengershan landslide and Tsaoling rockslides. After two days of Chi-Chi event, the slow Hongtsaiping slide (Central Taiwan) is reported with several meters of slide in an area of about 1.2 km$^{2}$ (800 m x 1500 m). Therefore, we want to know if non-failure or unobvious slides are the omens of landslides or rockslides in this area. We also try to characterize in detail the slide directions, their magnitudes and region of slides. We use two gray aerial photos of the same selected area (Taiwan Grid 67, 1:5000, pixel size of 0.25 m) acquired from Central Geological Survey (CGS) and Chinese Society of Photogrammetry & Remote Sensing (CSPRS) which are taken in 1998 and 2002 through a software Particle Imaging Velocimetry (PIV) analysis. Trough PIV we can set reference points at will in order to let PIV correlate the pixels of two aerial photos so that it can tell us the displacement vector field due to the topographic change. Of course, PIV can only show displacement field instead of vectors, so we will differentiate regions of different amount of displacement. The results will be precise if we can remove the offsets between the two gray aerial photos and the amount of displacement is obviously large enough. Therefore, the results of PIV analysis will help us not only find the areas in which sliding has happened, but also evaluate the azimuth and magnitudes of sliding. We use PIV analysis in the Hongtsaiping area which covers a dimension of 3129 x 2538 pixels. Our results show that the maximum horizontal displacement is about 11 m towards NW in the slide area of about 1.2 km$^{2}$. This tremendous slow slide need to further investigation in combining with the geological data.
T11D-1308 0800h
The application of InSAR for crustal deformation and land subsidence in Taiwan
We apply InSAR technique between August 1995 and November 2001 for crustal deformation in central Taiwan. We utilize 2-pass and 3-pass methods to obtain interferograms from 12 ERS SAR images in central Taiwan. The result of D-InSAR (Differential-InSAR) reveals land subsidence and earthquake deformation in our study area. The critical land subsidence region was located in the mouth of Choshui River and Erhlin River. After comparing with the distribution of aquaculture in Changhua County, we find that in the aquacultural area of Tacheng Village has the greatest subsidence value of 14-17 cm/yr due to groundwater overdrafted. The subsidence region is not only located in aquacultural area but has also been detected in urban area near aquacultural area. Comparison with the result of our study is consistent with the report from Water Resources Agency during 1985-1995. In temporal analysis we find the interferograms would lose coherence on the mouth of Choshui River (approx. 408 km2) when the time interval between 2 images was longer than 3 months. The low coherence here is mainly affected by vegetation and lower backscattering buildings density. The Chi-Chi earthquake on 21 September, 1999, with magnitude M$_{w}$=7.6 and an epicenter near Chi-Chi in central Taiwan caused surface rupture along thrust Chenlungpu fault. Interferograms obtain during Chi-Chi earthquake reveal the crustal deformation on the footwall, and the result of interferograms shows that the earthquake caused more than 33.6 cm uplift in line of sight of ERS satellite relative to coastline at Chenlungpu fault east of Taichung City. There are 2 interferograms show the coseismic and 1 shows the post-seismic. The coseismic interferograms reveal an increasing gradient from the coastline and deep on Chelungpu fault. The post-seimic group shows a slightly increasing phase line relative to the global trend, which corresponds to the Changhua fault trace. We consider this maybe the reactive motion after the mainshock triggered the motion of this fault. Constructing the displacement map using the GPS data, we can simulate interferograms from GPS and reveal the same trend between the results. This technique doesn't need to do the field work, which is well suited for crustal deformation monitoring and analysis.
T11D-1309 0800h
Changes in Regional Stress Associated with the 1999 Chi-Chi, Taiwan, Earthquake Based on Focal Mechanisms
The 1999 Mw7.6 Chi-Chi earthquake was the most recent prominent earthquake in Taiwan. The mainshock was followed by substantial number of strong aftershocks. Pre- and post-mainshock focal mechanisms of the 1999 Chi-Chi earthquake are analyzed to characterize spatial and temporal variation around the Chelungpu fault, which ruptured during the mainshock. The Frohlich's triangle diagram used to define the classification of earthquake focal mechanisms. We categorized the types of earthquake focal mechanisms as either normal , strike-slip, or thrust according to the corresponding maximum plunges is the pressure axis (P), null axis(B), or tension axis (T). The strain and stress orientations was estimated based on moment tensor summation and focal mechanism stress inversion respectively. The stress is caused by tectonic sources, but the strain results from the application of stress to particular structure of the crust. We show the difference between stress and strain directions and on what we can learn from this observation about tectonic processes in the volume studied. A comparison of the directions of strain and stress tensors showed close agreement for subvolumes with predominantly thrust and strike-slip faulting. This article describe and characterize the spatial and temporal distribution of earthquake focal mechanisms in the region of the 1999 Chi-Chi earthquake before(107 event) and after(401 event) the mainshock within the period from 1 January 1991 to 31 December 2001. Whereas the earthquake focal mechanism data suggest some spatial and temporal complexity in Central Taiwan, the overall pattern can be characterized by cluster analysis of grouped earthquake according to the seismicity trend and type of focal mechanisms. The aftershock focal mechanisms are dominant by strike-slip type even though the mainshock is thrust fault type. In contrast, for temporal distribution of different kind of aftershock focal mechanisms, which occurrence sequence is strike-slip, thrust and then normal types. In general, The most P axes of earthquakes which occurred before and after the 1999 Chi-Chi earthquake are consistent with the regional strain and stress orientations. These results indicate that the structural impact caused by 1999 Chi-Chi earthquake show a consistent trend of west-northwest, parallel to the direction of plate convergence.
T11D-1310 0800h
Dynamic Deformational Characteristics of Co-seismic Shear Zone of the 1999 MW 7.6 Chi-Chi Earthquake,Taiwan
The structures of fault rocks formed pass entirely throughout faulting periods, due to a long history there is a broad suite of deformational mechanisms involved in their deformational structures. Fault rocks formed at deeper levels by ancient faulting have been exhumed by uplifting, and if fault movement continues throughout that process there will be exposed in the fault zone, a variety of fault rocks formed under different conditions. It is therefore possible to gain insight into the process operating throughout the faulting history by studying the structures of fault rocks exposed. The drill cores through the Chelungpu fault zone related to the Chi-Chi (Taiwan) earthquake provide fresh and continuous core samples of fault rocks for studying the faulting process and tectonic history. In this paper, we report the analytic results on the meso- and micro-scopic structures of the co-seismic slip zone on the basis of the observations of the fault rocks within the Chelungpu fault zone from both the shallow drill cores and fault outcrops, and discuss their tectonic implications. The analytic results of the meso- and micro-scopic structures show that the 100-km-long surface rupture zone of the 1999 Mw 7.6 Chi-Chi earthquake was controlled primarily by the pre-existing Chelungpu fault zone, which is distributed in a wide zone of up to 60 m, and that the main co-seismic slip is localized in a narrow shear zone of <0.3 cm. Our results support the idea that the seismic slip is concentrated in a mm-scale narrow zone along pre-existing active faults. The Chelungpu fault zone is composed of cataclasite, fault breccia, and gouge zones that are well observed in both the fault outcrops and the drill cores taken throughout the fault zone. The foliations developed in the cataclasite and fault breccia zones are oriented parallel to that of the fault gouge zone where the main co-seismic slip occurred. The structural analyses of the shear zone and fault rocks show that the Chelungpu fault zone has slipped as a thrust fault with a significant left-lateral slip component since it formed from the Pleistocene to present. This oblique thrust motion is caused by the oblique convergence between the Philippine Sea plate and the Eurasian plate from the southeast.
T11D-1311 0800h
Coseimic and postseismic deformation of Chengkung Earthquake in Eastern Taiwan Revealed by Strong Motion and GPS Data
The Mw 6.5 Chengkung Earthquake occurred on December 10th, 2003 in the Costal Range of eastern Taiwan is believed to rupture the NNE-striking Chihshang fault in the Longitudinal Valley. The Chihshang fault is the most active creeping segment of the Longitudinal Valley fault, which is considered as a plate suture zone between the Luzon arc of the Philippine Sea plate and the Chinese continental margin of the Eurasian plate. Based on the relocation of aftershock sequences, the main shock is a pure thrust event that ruptures the east dipping Chihshang fault plane. All the stations in the hanging wall show significant uplift. The maximum permanent vertical displacement shown by strong motion data in the hanging wall side is about 19 cm, meanwhile the horizontal displacement is about 9 cm trending NE. In general the coseismic horizontal displacement shows a fan-shape pattern. It is worthy to notice that the some stations on the footwall of the Chihshang fault were uplifted during this earthquake event. For characterizing the coseismic pattern, we use 40 strong motion and 6 continuous GPS data around the Chihshang fault to simulate the subsurface fault geometry and the distribution of coseismic slip along the fault surface by 3-D dislocation model. The results show that the model containing only single Chihshang fault can not well fit the vertical displacement on the footwall side. Thus we try to reinvestigate the coseismic displacement by adding an additional subfault plane appended under the Longitudinal Valley. Due to this subfault geometry, additional rupture surface will locate at a distance of 3.5 km away from Chihshang fault trace. The predicted coseismic displacemet by inversion of the two fault geometry model is much better than that of single fault model, especially around the Longitudinal Valley. The best-fitted model reveals that the maximal dislocation is about 1m dip-slip on the Chihshang fault plane near the hypocenter, and the dislocations near the surface are partly locked and decline to about 1~10 cm on both fault planes. The calculated scalar moment is 1.9E26 dyne-cm, which is quite compatible with the 2.0E26 dyne-cm based on the result of Harvard CMT.
T11D-1312 0800h
Surficial Characteristics of the Chelungpu Fault System Revealed by LiDAR-derived Digital Elevation Model in west Central Taiwan
We applied post-earthquake airborne LiDAR-derived digital elevation model to characterize a complex segment of the Chi-Chi earthquake rupture area in west central Taiwan. It is demonstrated that geomorphic and structural features can be more reliably delineated using the high-resolution LiDAR-derived DEM, especially the advantage applying its high vertical resolution of about 10-15 cm. First, the Chelungpu fault system includes two subparallel fault traces in the study area. The deformed footwall terraces show gentle folds trending about NNE in the north and about NE in the south. Second, creek profiles indicate that slope changes usually associated with the Chi-Chi earthquake rupture, fault scarps, and folded terraces. Third, the LiDAR DEM shows the Fengyuan city is situated on a gently folded terrace with a NE-trending fold axis. The LiDAR DEM also reveals well-defined paleo-channels and river-incision scarps within the populated Fengyuan city. The gradually shifted paleo-channels suggest that the Dachia river had migrated from southwest towards west to its present direction. To the east of the Fengyuan city, the Chi-Chi earthquake scarp was uplifted about 3 $\sim$ 7 meters during the coseismic event. However, the LiDAR DEM indicates that the scarp offset is more than 3 $\sim$ 7 meters and up to 10 meters. The height difference supports at least one rupture event had occurred prior to the Chi-Chi earthquake along his young fault scarp. We also compared 1:1000 scale pre-earthquake city planning map with the post-earthquake LiDAR DEM to better determine coseismic displacement on the hanging wall. In total, the presented surficial observations indicate the region to be tectonically active in the past and are consistent with the continual NW movement of the Chelungpu fault.
T11D-1313 0800h
New GPS Network on the Active Fault System in Taiwan
According to the historical records, disastrous earthquakes occurred in Taiwan were caused by reactivations of active faults. In last century, there were five with the surface rupture: 1906 Meishan Eq. (the Meishan F.), 1935 Hsihchu Eq. (the Shihtan and the Tuntzuchiao F.), 1946 Hsinhua Eq. (Hsinhua F.), 1951 Hualien-Taitung Eq. (Longitudinal Valley F.), and 1999 Chi-Chi Eq. (the Chelungpu F.). In order to identify earthquake associated surface rupture and further to mitigate potential hazards, the investigation and monitoring on the active fault system are of great urgency. Central Geological Survey (CGS) of Taiwan is currently executing a 5-year (2002-2006) project, integrating geological and geodetic data to better characterize short-term and long-term spatial and temporal variations in deformation across major already-known active faults of Taiwan. For the former, we use field survey, drilling, geophysical exploration, and trenching to recognize the long-term slip rate and recurrence interval of each fault. For the latter, we deploy near-fault campaign-style GPS and leveling monitoring networks. Here we further combine the result of other GPS networks including continuous-mode. This project is actually concentrated on fault-specific investigation.. Until Dec. 2004, we have set up 756 GPS stations and 27 precise leveling lines including 1024 leveling benchmarks. For the purpose of understanding temporal variability and receive continuous record, the CGS began to deploy 6~10 new GPS stations of continuous mode since 2004. Upon the completion of the geodetic project, we are supposed to provide information on short-term slip rates of major active faults. By integrating other geological datasets we will also evaluate the short-term and long-term behavior of the active faults, and further offer insight into spatial and temporal variability in deformation processes.
T11D-1314 0800h
A Study on Deformation of the Chelungpu Fault Zone in Shihgang Area, Central Taiwan
The 1999 Chi-Chi earthquake caused surface ruptures and severe deformation in Shihgang area, Central Taiwan. Many studies related to the Chelungpu fault were conducted in recent years, especially focused on the surface ruptures, paleoseismology and seismology. In this paper we use the precise and dense cadastral surveying to identify the deformation characteristics of the Chelungpu fault zone in Shihgang area. The resulted displacement is characterized by a clear E-W oriented break that coincident with the surface trace of the Shihgang backthrust. Besides, the result of strain analysis shows that most of the study areas are under compression in N-S direction, and extension in E-W. The contours computed by derived compressional stress also show a sharp strain contrast on both sides of the Shihgang backthrust and gradually decreases eastward. Therefore, the coseismic uplift and surface deformation in the Shihgang area during the Chi-Chi earthquake can be reasonably explained by adding compensation from Shihgang backthrust.
T11D-1315 0800h
Characterizing Brittle Deformation, Damage Parameters, and Clay Composition in Fault Zones: Insights From the Chelungpu and Mozumi Faults
Six outcrop sites along strike of the Chelungpu Fault (CF), Taiwan are compared with core samples through the CF zone and the Mozumi fault zone, Japan to evaluate how mesostructural and microstructural scale fault properties and clay mineralogy change as a function of strike, structural depth, and in different components of a fault zone. Characterization of core and outcrop samples will determine fluid flow properties and processes, and determine seismological processes at each study site. X-ray diffraction analysis shows an increase in smectite towards the CF at or very near the surface, and an increase in illite toward the fault at depth. Weathering and increased fluid flow in the exhumed fault zone plays an important role in the composition of the clay, but is enhanced in the clay gouge because of fabrics created by faulting. Weathering profiles based on samples and general weathering models for Taiwan-type climate will be used to determine the extent to which weathering plays a role in surficial fault gouge composition. The abundance of gouge, the dominance of smectite in outcrop samples, and the intensity of fault related damage at outcrop and microstructural scales increase in southern portions of the CF. Samples from the southern CF show almost no intact host rock as well as cross cutting relationships of veins, fractures, and foliated gouge suggesting multiple deformation events. Samples from the northern CF show mostly intact host rock with some unfoliated or weakly foliated clay. The southern portion of the CF is most likely older and has accumulated more damage relative to the north. Shale and clay-rich fault zones are not typically well preserved due to the ease with which they weather, and are therefore not well studied at the microstructural and outcrop scale. The Chelungpu and Mozumi faults provide a valuable opportunity to characterize shale and clay-rich fault zones. These fault zones may control fluid flow and distribution of fault-zone seismic waves differently than fault zones in crystalline rocks due to clay properties. Studies of fault zone properties provide important constraints for geologic modeling, and clay mineralogy and microstructures may control the role of fault core and damage zone as conduits or barriers to fluid flow.
T11D-1316 0800h
FAULT DETECTION USING RESISTIVITY IMAGE PROFILING METHOD AT HSINCHU SCIENCE INDUSTRIAL PARK, HSINCHU, TAIWAN
As Taiwan is located in the Neotectonic belt along the western Pacific Ocean, the detection of active faults is important for earthquake risk analysis. In addition, some active faults in Taiwan lie typically at the bases of urban or industrial area, their identification is often challenging because of environment changes and interferences, etc. The Hsinchu Science Industrial Park (HSIP), Hsinchu, N.W. Taiwan is one of the most important high technology sites in Taiwan. Based on previous geophysical and geologic surveys, the well-known active Hsinchen fault is crossed the site, although the exact location of active fault zone remains unclear. Different geophysical methods are used to detect an active fault. However, a successful recognition of a fault depends on the physical properties such as density contrast, acoustic impedance, and reisitivity contrast, etc. of the target and its surroundings. Due to a large resistivity contrast between the hanging wall (low resistivity Choulan shale) and the footwall (high resistivity Toukoshan gravel beds) of the Hsinchen fault, geoelectrical sounding may be one of the best ways to trace this fault. For the above reasons, the resistivity image profiling method was used to investigate the fault and also attempted to relate these DC resistivity measurements to the fault parameters. Correlated the electrical and fault parameters is one of the most important tasks for this study. All of the field measurements to be discussed were made during the period of 2000 to 2003. A portable resistivity meter used to make all resistivity measurements. Electrode spread was pole-pole arrays. Combined the RIP sounding results to the local topographic maps, geologic data and formal open pits information, fault zones are being recognized. Keywords: active fault; resistivity change; Hsinchen fault; Taiwan
T11D-1317 0800h
Earthquake-induced gravitational potential energy change at convergent plate boundary near Taiwan
The coseismic displacement induced by earthquakes will change the gravitational potential energy ({\bf {\it GPE}}). Okamoto and Tanimoto (2002) have shown that the gain of {\bf {\it {$\Delta$GPE}}} corresponds to the compressional stress regime while the loss of {\bf {\it {$\Delta$GPE}}} corresponds to the extensional stress regime. Here we show an example at a convergent plate boundary near Taiwan. The Philippine Sea Plate is converging against the Eurasian Plate with a velocity of 7-8 cm/yr near Taiwan, which has caused the active Taiwan orogeny and induced abundant earthquakes. We have examined the corresponding change of gravitational potential energy by using 757 earthquakes from the earthquake catalogue of the Broadband Array in Taiwan for Seismology ({\bf BATS}) from July 1995 to December 2003. The results show that the variation of the crustal {\bf {\it $\Delta$GPE}} strongly correlates with the different stage of the orogenesis. Except for the western Okinawa Trough and the southern Taiwan, most of the Taiwan convergent region exhibits a gain of crustal {\bf {\it $\Delta$GPE}}. In contrast, the lithospheric {\bf {\it $\Delta$GPE}} in the Taiwan region exhibits a reverse pattern. For the whole Taiwan region, the earthquake-induced crustal {\bf {\it $\Delta$GPE}} and the lithospheric {\bf {\it $\Delta$GPE}} during the observation period are 1.03$\times$10$^{17}$ joules and -1.15$\times$10$^{17}$ joules, respectively. The average rate of the whole {\bf {\it $\Delta$GPE}} in the Taiwan region is very intense and equal to -2.07$\times$10$^{10}$ watts, corresponding to about one percent of the global {\bf {\it$\Delta$GPE}} loss induced by earthquakes.
T11D-1318 0800h
Entrenched, Transverse Rivers as Geomorphic Strain Markers: Insights From South-Eastern Taiwan
In order to unravel continental deformation through time we need to know the strain rate history of a deforming region. Techniques such as GPS, earthquake moment tensor summation and palaeomagnetism generally lack the required temporal and/or spatial resolution to fully describe the deformation. In Taiwan, we explore the use of entrenched, transverse rivers to constrain the history of crustal deformation in geological time. Taiwan is built from the oblique collision of the Luzon Arc and Chinese continental margin. The collision propagates at 60 km/Myr from north to south. This space-time relationship provides a unique opportunity to study an evolving mountain belt through time. The area of interest is a series of seven river basins spanning nearly 60 km along the south-east coast of Taiwan, equivalent to nearly 1 Myr in time. The rivers drain roughly east-west perpendicular to the ridgepole of the mountain belt. The oldest, most northerly, basins show a pronounced asymmetry in their trunk stream position and appear to be rotated anticlockwise. Both the rotation rate and asymmetry factor increase steadily from south to north. The maximum finite rotation is $\sim$ $30\deg$ for a basin $\sim$ 1 million years old. The rotation of the drainage basins may result from the indentation of the Luzon Arc into the Chinese continental platform and the position of the relatively rigid Peikang High. We compare the estimates of rotation rate determined from the river basins to GPS rotation vectors. The deformation of passive markers, in this case river basins, have the potential to provide a continuous record of strain in tectonically active regions. We also note a linear increase in mean elevation and mean local slope of the drainage basins with distance from the southern tip of Taiwan. This is equivalent to a linear increase in elevation and mean slope through time. The mean slope reaches a threshold of $\sim$ $30\deg$ at $\sim$ 0.5 Myr despite a continuing increase in mean height. The space-time relationship allows us to constrain the rate at which a mountain belt attains steady state and has important implications for the development of mean elevation and threshold slopes.