U51A-0001
Resolving the Geometry of Global Subduction Zone Interfaces a Priori - Working Towards Improved Earthquake Source Modeling
A key step in many earthquake source inversions requires knowledge of the geometry of the fault on which the earthquake occurred. Our knowledge of this surface is often uncertain, however, and as a result fault geometry misinterpretation can map into significant error in the final temporal and spatial slip patterns of these inversions. Relying solely on an initial hypocenter and CMT mechanism can be risky when establishing rupture characteristics needed for rapid tsunami and ground shaking estimates. Here we attempt to improve the quality of fast finite-fault inversion results by combining, a priori, several independent and complementary datasets to more accurately constrain the geometry of the seismic rupture plane of subducting slabs. Unlike previous analyses aimed at defining the general form of the plate interface, we require mechanisms and locations of the seismicity considered in our inversions to be consistent with their occurrence on the plate interface, by limiting events to those with well-constrained depths and with CMT solutions indicative of shallow-dip thrust faulting. We construct probability density functions about each location based on formal assumptions of their depth uncertainty and use these constraints to solve for the 'most likely' fault plane, exploring fits with both planar and polynomial geometries. Where available, data from shallow active seismic experiments across trenches are also used as additional constraint. We show that by using the aggregated data sets this method produces a fault plane that is more consistent with all of the data available than is the plane implied by the initial hypocenter and CMT mechanism, allowing us to rapidly determine more accurate initial fault plane geometries for source inversions of future earthquakes. We use these geometries to explore the effect on finite fault model slip distributions, and show that the model changes can have a significant affect on assumed seismic hazard.
U51A-0002
Great earthquakes hazard in slow subduction zones
Research on the Sumatra-Andaman earthquake of 2004 has challenged two popular paradigms; that the strongest subduction earthquakes strike in regions of rapid plate convergence and that rupture occurs primarily along the contact between the basement of the overriding plate and the downgoing plate. Subduction zones presenting similar structural and geodynamic characteristics (slow convergence and thick wedges of accreted sediment) may be capable of generating great megathrust earthquakes (M>8.5) despite an absence of thrust type earthquakes over the past 40 years. Existing deep seismic sounding data and hypocenters are used to constrain the geometry of several key slow subduction zones (Antilles, Hellenic, Sumatra). This geometry forms the basis for numerical modelling of fore-arc thermal structure, which is applied to calculate the estimated width of the seismogenic portion of the subduction fault plane. The margins with the thickest accretionary wedges are commonly found to have the widest (predicted) seismogenic zone. Furthermore, for these margins there exists a substantial (20-60 km wide) region above the up-dip limit for which the contribution to tsunami generation is poorly understood. As the rigidity (mu) of these high-porosity sediments is low, co-seismic slip here can be expected to be slow. Accordingly, the contribution to seismic moment will be low, but the contribution to tsunami generation may be very high. Indeed, recent seismological data from Nankai indicate very low frequency shallow-thrust earthquakes beneath this portion of the accretionary wedge, long-considered to be "aseismic". We propose that thick accumulations of sediment on the downgoing plate and the presence of a thick accretionary wedge can increase the maximum size of the potential rupture fault plane in two ways; 1) by thermally insulating the downgoing plate and thereby increasing the total downdip length of the fault which can rupture seismically and 2) by "smoothing out" the interplate contact, which can increase the lateral length of the fault segment along the trench (between aperities). These factors can have a significant impact on the hazard assessment for the Hellenic, Calabrian and Gibraltar subduction zones, the Antilles arc, as well as Makran, Hikurangi and N. Colombia, which all share strong similarities with Sumatra, Nankai and Cascadia.
U51A-0003
Seismicity of the West Sumatran Subduction Zone as Revealed by Local Seismicity
The Sumatran margin played host to three great earthquakes in the preceding four years (Aceh-Andaman 26
December 2004 Mw = 9.2, Nias 28 March 2005 Mw = 8.7, Bengkulu 12 September 2007 Mw = 8.5), two of
which were associated with significant loss of life. Yet, a part of the margin near the northern Mentawai
islands remains unbroken, and the historical record suggests that only half of the accumulated tectonic strain
might have been released by the Bengkulu earthquake in 2007. Earthquakes along the strike-slip Sumatra
fault, though much smaller in magnitude, still present a significant seismic hazard, due to its proximity to
dense populations and shallow depth. We present first data from two local seismic networks, which were
installed in November 2007 and April 2008 along the active western Sumatran margin. The networks consist
of 80 continuously recording three component short-period and broadband stations, covering 750 km of the
active margin between 3°S and 1.5°N (provinces West Sumatra, Bengkulu and North
Sumatra). The stations were installed on the islands Nias, Batu and the Mentawai Islands and the adjacent
mainland. The rare occurrence of forearc islands along the Sumatran margin allows the deployment of
seismic landstations above the shallow part of the thrust fault, thus providing high hypocenter location quality
for the updip end of the seismogenic zone in an area where geodetic data are also available. The northern
network recorded a 6.0 Mw on the Sumatra Fault and its aftershocks. In the southern part the stations
captured the pronounced postseismic activity of the Mw 8.5 Bengkulu earthquake from 12 September 2007
including the Mw 7.0 event on 25 February 2008. Preliminary seismicity distributions suggest that aftershock
activity is particular intense between the Mentawai Islands Sipora and North Pagai; in this area the coseismic
slip distribution of the Bengkulu earthquake had a minimum. We present first results of this ongoing research
project including the seismicity distribution and first 1-D velocity models and relate them to the background
seismicity.
http://www.noc.soton.ac.uk/gg/sumatra/
U51A-0004
Forearc Backthrusting as a Source of 26 December 2004 Sumatra-Andaman Tsunami
The massive tsunami generated during the Great Sumatra-Andaman earthquake of 26 December 2004 killed over 250000 around the coastlines of Southeast Asia, with the biggest loss in northern Sumatra of about 150000 lives where runup heights reached up to 30 metres. The source of the 26 December 2004 tsunami has been related to vertical coseismic uplift near the subduction toe and possible splay faults within the accretionary wedge, but these modelled sources do not completely reconcile with the observed near field characteristics of the tsunami in north Sumatra. The maximum coseismic slip estimated seismologically and even higher slip values postulated by tsunami modelling results do not explain the large runup heights reported, if the vertical uplifting of the water column was just localized near the subduction front. Using high- resolution seismic tomography results here we demarcate the presence of a backstop buttress structure beneath the outer arc high. On coincident deep seismic reflection profile we show the presence of an active backthrust that is imaged down to at least 15 km depth and reaches the surface near the western edge of the Aceh forearc basin. We suggest that coseismic slip along the forearc backthrust boundary might provide a reasonable explanation for the generation of this massive tsunami, and can explain the incongruities in the observed large runup heights and arrival times in the near field. Modelling results obtained by solving shallow water wave equations for tsunami propogation, from the proposed backthrust tsunami source further support our hypothesis and corroborate with near field tsunami arrival times recorded. The dip of backthrust being nearly orthogonal to that of the main thrust, it would be difficult to separate the rupture along the two thrusts seismologically. These results imply that slip along backthrust faults during a mega-thrust earthquake could prove destructive, and should be considered during tsunami modelling and mitigation.
U51A-0005
3-D Seismic Refraction Tomography in Sumatra Subduction Zone
A seismic refraction survey was conducted as part of the major UK and international project to image the 3-D
structures and the seismic velocity of the Sumatra subduction zone. The 3-D seismic refraction tomography
mainly focusses on the two segment boundaries identified by the earthquake ruptures in 2004 and 2005.
High quality seismic datasets (refraction, reflection, gravity and magnetics) were collected in the two survey
areas on the vessel R/V Sonne in 2008. The northern area, around the island of Simeulue, is about 196 km
long and 185 km wide. 50 Ocean Bottom Seismometers (OBS) were deployed in this area, and 10462 air-gun
shots were fired along 1550 km of profiles. 47 OBSs were then installed near the island of Nias, in an area of
246 km long and 180 km wide, and 9134 shots were fired on 1408 km of profiles. During the OBS
deployment, air-gun shooting, and OBS recovery, high resolution swathe bathymetry data were recorded,
and XBT data were collected in each OBS deployment location. Gravity data were also recorded during the
whole survey and magnetics data collected during the air-gun shooting. The 3-D refraction tomography
successfully sampled the two survey areas. Refractions from the oceanic and continental crust are clear and
easy to pick, and refractions from the mantle lithosphere are also visible at some locations at an offset up to
150 km, which enables us to image the deeper structures of the Sumatra subduction zone. A tomographic
inversion program JIVE-3D (Hobro et al. 2003) will be applied to the refraction/reflection travel times to invert
them into a minimum-structure velocity model. The high resolution bathymetry will be smoothed and put into
the model as a known interface. The XBT data will be used to calibrate the acoustic velocity in the water.
During the shooting period, several earthquakes of magnitude 5.0 and above occurred near the survey area,
which also provide extra information for the inversion. The well resolved 3-D structure models obtained will
give insight into the possible rupture barriers causing the observed segmentation of the subduction zone.
http://www.noc.soton.ac.uk/gg/sumatra/
U51A-0006
Ridge structure control of segmentation of the Sumatra subduction zone
During cruise SO198-2 on R/V Sonne in July 2008 we collected 3500km of multichannel seismic reflection data using a 5420cuin airgun source and a 2.4km 192 channel streamer. Here we show results from profiles between Nias, the Batu Islands, and Siberut, which span the southern end of rupture in the 28 March 2005, the 1935 and 1797 rupture zones. We are able to map reflections from the downgoing plate from the trench to the forearc high on a series of dip lines and an intersecting strike profile. The travel times to the plate generally mirror the seabed topography, except for a localised region in which the plate is anomalously shallow, which approximately corresponds to the location of the 1935 rupture zone. Removing the effect of the seabed from free air gravity data confirms that the apparently shallow plate reflection coincides with a local mass excess; moreover the residual gravity field is crenulated along the subduction zone parallel and perpendicular to the fracture zones that can be mapped in bathymetry and gravity offshore. The fabric of the downgoing plate inherited from its formation at the Wharton Ridge thus appears to control the segmentation of the subduction zone.
U51A-0007
Deformation Structures on the Sumatra Subduction Margin from 2008 Multichannel Seismic Reflection Data
The earthquakes of December 26th 2004 and March 28th 2005 ruptured, respectively, 1200–1300 km and 300–400 km of the subduction boundary between the Indian-Australian plate and the Burman and Sumatra blocks. During the period May-July 2008, as a part of a multidisciplinary geophysical experiment, 3620 km of multichannel seismic data were acquired along a series of intersecting dip and strikes lines located on the Sumatra subduction zone between 2°S and 5°N, and includes data from both regions that ruptured. The downgoing plate is clearly imaged on all dip profiles and on strike lines located across the accretionary prism, up to 75 km landward of the trench. We present examples of deformation structures overlying the subducting plate and into the toe of the accretionary prism. From the MCS data landward vergent thrusts are clearly identified in parts of the prism toe that are consistent with the seafloor morphology from swath bathymetric data, where relatively gently seaward dipping slopes are observed on the front of the prism. Discrete fault structures are imaged that cut deeply through the prism sediments. The sediment thickness on the incoming plate is relatively thick, up to 3-seconds two-way time. Many profiles over the trench show a discrete reflection in the incoming sediment sequence, sometimes relatively high in amplitude and with negative polarity, that we interpret to be the seaward continuation of a décollement. The deformation in this region provides an insight into the stresses currently active close to the deformation front. We identify features that are related to: (1) the flexure of the subducting India-Australia oceanic plate; (2) compression of the sediments at the point of incipient accretion to the prism.
U51A-0008
New Images of Margin Structure within the 2004 Indonesian Great Earthquake Rupture Area: Preliminary Results from the Northern Extent of the CRUST Seismic Experiment
As part of U.K./Indonesian/U.S. project CRUST (Collaborative Research into Understanding Sumatran Tectonics), the National Science Foundation funded one week of multichannel seismic acquisition during July 2008. Using the German research vessel Sonne, we acquired 25 reflection profiles using an array of 12 G- guns totaling 5420 cu. in recorded on a 192 channel, 2.4 km hydrophone streamer. These profiles, totaling ~1250 km in length, image the southern portion of the rupture area of the December 26, 2004 Mw 9.3 earthquake, in the vicinity of the epicenter. Nineteen profiles image the subducting ~70 myr old Indian Plate crust as it enters the Sunda Trench and underthrusts the resultant accretionary prism. A clear negative-polarity décollement lies close to the top of the oceanic plate, and numerous faults cut the extremely thick (>5.5 km) incoming sediments. These sediments originated as part of the Bengal Fan and may arrive at the Trench having already undergone significant dewatering and consolidation. The accretionary prism consists largely of landward vergent thrusts for the outer ~40-45 km, followed by a marginal plateau up to 70 km wide within which vergence appears to switch from landward to seaward, and an inner prism ~50 km wide exhibiting primarily seaward vergence and allowing for lower acoustic penetration. There is no evidence for a megasplay fault similar to that observed in the Nankai Trough near the Kii Peninsula. Strata from the incoming plate are well preserved and minimally deformed in sections several kilometers thick as far inboard as the landward edge of the marginal plateau. Significant for tsunamigenesis, the suggested strength of the incoming section inherent in these observations supports models involving updip rupture propagation that appears to reach the Trench. Six profiles image the forearc basin and the structural boundary between the inner prism and the basin, which we interpret as the West Andaman Fault. This structure is near vertical and thought to be a strike-slip response of strain-partitioning in this obliquely convergent system. Several basin unconformities are present, as well as debris flows that together may yield insight into margin history and past seismic events.
U51A-0009
Bicentennial Earthquake Supercycles Inferred from Relative Sealevel Changes in Corals off West Sumatra
Records of strain accumulation and relief extracted from corals of the Mentawai islands, Sumatra, imply that this 700-km-long section of the Sunda megathrust has generated sequences of great earthquakes about every two centuries for at least the past 700 years. Episodes in the 14th and 16th/17th centuries resemble broadly the great rupture pair of 1797-1833. The MW8.4 earthquake of September 2007 represents the first in a series of large partial failures of the Mentawai section that will likely occur within the next several decades.
U51A-0010
Application of Microfossils to Reconstruct a Paleoeseismic Record of the Sunda Subduction Megathrust, Northern Sumatra
The 26th December 2004 tsunami affected a large part of the Indian Ocean with 280,000 killed, 14,000 missing, and 1,100,000 people displaced. This disaster provided a horrific reminder of a practical problem: written and instrumental records do not span enough time to warn of the full range of earthquake and tsunami hazards in this region. To address this problem, we have begun to reconstruct the paleoenvironmental record for coastal northwestern Sumatra, utilizing foraminifera and pollen as proxies. The long-term goals of our project are (1) determine the number of great earthquakes and the average recurrence interval; (2) determine if subsidence occurs precursory to the main coseismic subsidence event; (3) determine if there is uplift after the main coseismic subsidence event that may be caused by slow, down dip afterslip on the megathrust and (4) quantify the relative timing of coseismic and interseismic vertical land level changes. We have collected litho-, bio-, and chronostratigraphic data from the northwest coast of Sumatra to resolve both rapid and gradual changes in relative sea level (RSL) and to begin building a taxonomic database of microfossils. The study sites, in Pulot and Seudu, Sumatra, are located approximately 20 km south of Banda Aceh in northwestern Sumatra. The sites are located on the coastal plain in near proximity to the shoreline. Both study sites are estuarine (or former estuarine), low energy depositional environments, which are protected from open-ocean wave attack and minimal fluvial input. Seudu is a site of shrimp ponds partially destroyed by the 2004 tsunami. Pulot is reclaimed agricultural land, probably a former Mangrove swamp. To date, we have found, at both the Pulot and Seudu sites, evidence of two regionally extensive organic-rich buried soils each overlain by a sand to silty-sand clastic deposit. The buried soils date to ~7,000 and ~6,000 cal yr BP and are separated by 1-2 meters of clastic sediment. Each buried soil is interpreted to represent subsidence from a large megathrust earthquake. The dominant mangrove pollen taxa in the organic buried soil strata is Rhizophora, which is replaced by very low counts of the dominate mangrove taxon Bruguira/Ceriops in the overlying clastic unit. Benthic foraminifera were only found in the overlying clastic unit. The foraminifera appear to represent an inner shelf environment, with taxa including several species of Elphidium, Ammonia parkinsoniana, and secondary species including Fissurina sp., and many species of Quinqueloculina and Bolivina. Based on these preliminary identifications of both the foraminifera and pollen, we interpret that the organic-rich buried soil hosted a mangrove environment, the buried soil-clastic contact represents a short-lived relative sea level rise, and the overlying clastic unit was deposited in an inner shelf environment.
U51A-0011
Interplate coupling and slip distribution of the megathrust earthquakes along the southernmost part of the Kuril Trench
A magathrust earthquake in the subduction zone is one of the destructive geohazards along the plate boundary zones. The Kuril trench is known as one of the most active region of the repeated megathrust earthquakes where the Pacific plate is subducting beneath Hokkaido and Kuril Islands with 8 cm/yr. Modern geodetic measurements started in Hokkaido along the southernmost part of the Kuril trench in early 1900's and continuous GPS measurements (GEONET) started in 1994. The geodetic data have recorded the deformation through interseismic, coseismic, and postseismic periods in the subduction zone over a century. Using these data, I estimated the slip distribution of the past large earthquakes and the contemporary coupling distribution on the plate interface. The used geodetic data are acquired by GPS, leveling, triangulation, trilateration, and tide gauge. I collected all available geodetic data including distance measurements which have never been used for deformation analysis. The assumption of the analysis is that the observed deformation is the elastic one due to slip and locking on the plate boundary. The geometry of the plate boundary is approximated by dozens of rectangular faults. The locking boundary is represented by nominal slip opposite to the relative plate motion (N65°W) and slip amount on each rectangular fault is a parameter to estimate. It is assumed that the average velocity at the continuous GPS stations from April 1999 to March 2003 as typical interseismic deformation and estimated coupling distribution on the plate boundary. There are two peaks of strong coupling corresponding the source area of the 1973 M = 7.4 Nemuro-oki and the 2003 M = 8.0 Tokachi-oki earthquakes. Significant coupling reaches the depth of 80 km. The studied earthquakes for slip distribution are the 1952 M = 8.1 Tokachi-oki, the 1973 Nemuro-oki, the 2003 Tokachi-oki, the 2004 M = 7.1 Kushiro-oki earthquakes. The 1952 and 2003 Tokachi-oki earthquakes are estimated to have similar slip distribution. The location of the slip peak (e.g., asperity) is almost identical. I, therefore conclude that the 2003 events are the recurrence of the 1952 earthquake. The coseismic slip of the 2003 earthquake is limited west of the submarine canyon and the postseismic one expanded into the eastern region of the canyon. The slip region of the 1973 Nemuro-oki earthquake locates 100 km east of those of the Tokachi-oki earthquakes. It suggests that the asperities of the Tokachi-oki and Nemuro-oki earthquakes are separated and that afterslip fills their gap. The result of the study can contribute to assess forthcoming large earthquakes causing future geohazards.
U51A-0012
Apparent Stress Variation in Response to Seamount Subduction at Nicoya Peninsula, Costa Rica
Seamounts are high relief features seen on many oceanic plates, including the Cocos plate offshore Costa Rica. As these features enter the subduction zone at Middle America Trench, they may perturb interface coupling by changing physical properties of the plate interface. Here we explore the role of seamounts on rupture process of microseismicity along Nicoya Peninsula, Costa Rica. This peninsula lies close to a region where down-going Cocos plate structure varies along-strike of the trench. The Cocos plate has low relief along the north and central parts of Nicoya Peninsula, where the subducting plate was created at East Pacific Rise (EPR). Seamounts dot the plate subducting at the southern tip of the peninsula, where the plate was created at Cocos-Nazca Spreading center (CNS). Given these structural differences, we are able to evaluate possible along-strike variations in earthquake source properties. We use 357 earthquakes from the Costa Rica Seismogenic Zone (CR-SEIZE) project to estimate the effects of seamount subduction on apparent stress (σa). We compute σa, which is a measure of stress drop combined with seismic efficiency, using waveform coda because of its proven stability relative to measurements using direct arrivals. We allow variable source scaling, finding that non-constant scaling provides good fit for our data, suggesting that seismic moment is not proportional to the cube root of corner frequency at ML 0.8 to 4.2. σa values for well constrained data indicate along strike variations between the northern and southern tips of the peninsula. Except in the region of a previously subducted seamount in the Gulf of Nicoya where the mean σa is 1.03 MPa, the southern and central portions of the peninsula have mean σa values 0.79 and 0.89 MPa respectively, while the mean σa value in the northern region is 1.66 MPa. The larger mean σa values in the northern region and in the Gulf of Nicoya implies that the interface is more strongly coupled where there is little relief on the subducting plate or surrounding a subducting seamount.
U51A-0013
Geologic Characteristics of Tsunamigenic Earthquakes in the Central Aleutians
The Aleutian subduction zone is the source region for many great tsunamigenic earthquakes. These earthquakes are not distributed homogeneously along the Aleutian margin, but are clustered into particular segments. Across the Amlia Fracture Zone (AFZ), a change in dip of the subducting plate, seismicity, age of the plate, extensional vs. compressional characteristics of the upper plate, and roughness of the seafloor entering the trench demonstrate a clear change in the structure of the subduction zone between adjacent segments. These changes are also apparent in satellite free-air gravity anomalies and in the height of the outer rise. To the east of the AFZ, where the plate is dipping more steeply than to the west and the upper plate shows extensive extension in the fore-arc, back-arc extension occurs in the Amlia-Amukta Basin and virtually no seismicity is recorded. To the west, the plate is dipping less steeply and the upper plate shows compressional characteristics in the fore-arc. This western Amlia segment has both background seismicity and three recent large earthquakes (1957, 1986, and 1996). These earthquakes have generated local and/or transoceanic tsunamis. In the eastern Amlia segment, no large earthquakes have been recorded and the best known rupture areas of the three largest earthquakes either terminate near the AFZ or show little to no slip in the eastern Amlia section, an observation verified by results of GPS studies (Cross and Freymuller, 2007). These observations demonstrate a strong linkage between observable geologic characteristics and the occurrence of great earthquakes along the Aleutian margin. By systematically studying the geologic setting along global subduction margins, we aim to understand the controlling factors of the great earthquakes and better estimate the seismic and tsunami risk for individual margin segments.
U51A-0014
Tsunami Inundation Hazard along the Pacific North American margin
We present the results of a comprehensive tsunami hazard inundation study of the Pacific North American margin with a particular focus on California. This work extends earlier work on probabilistic offshore waveheights and presents a first attempt at probabilistic inundation mapping. Compared to the offshore waveheights this approach is more complicated die to the highly non-linear nature of tsunami inundation, especially when considering local as well as distant sources as in the case of the Cascadia coast. We have included large subduction zones sources from around the Pacific Rim, as well as local offshore faults, that may contribute to localized tsunami hazard. We also considered formal uncertainties in the tsunami waveheights by varying the source sizes according to an expected distribution of waveheight errors. These uncertainties are important in a probabilistic framework, and due to the non-linear inundation process can only be adressed by numerical sampling of the uncertainty distribution. We will present preliminary inundation maps for several return periods of interest and discuss how geological information and geological uncertainties are used and expressed in inundation mapping.
U51A-0015
Geology of the region of Guadalajara, Mexico, and its relationships with processes of subsidence
The city of Guadalajara, Mexico, began an accelerated urban growth in early 1950. During a span of 25 years a large number of gullies were artificially filled, with the aim of incorporating new areas for urbanization, particularly in the areas north and west of the city. These gullies originally formed a complex dendritic-type system, whose evolution may be associated with faults or fracture zones whose current identification are only possible based on escarpments along the Canyon of the Rio Grande de Santiago (CRGS), north of Guadalajara. Reports of affectations documented in the 80's described subsidence in buildings and infrastructure, a process that has been continued during 2008. We present the results of work done in the CRGS, which is a tectonic erosive-depression with an average depth of 500 m and exhibits a sequence of volcanic and sedimentary deposits with rapid lateral facies changes. The stratigraphic column spans a 15 km-long section along the Matatlán-Arcediano road, and, from top to bottom contains: 1) Unconsolidated pumice and tuffs with an average thickness of 12 m; 2) basaltic lavas with average thickness of 60 m; 3) the San Gaspar ignimbrite; 4) fluvial- sedimentary deposits with a thickness of approximately 20 meters that include both sub-rounded and angular volcanic clasts, with sizes up to 0.15 m; 5) a thick sequence of ignimbrites and dacitic lavas. At a depth of 1200 m.a.s.l. in the town of Arcediano, the basal sequence is composed of dacites and andesites with interbedded pumice-rich ignimbrites with 10-20 m thickness. The Rio Grande de Santiago talweg to 1018 m.a.s.l. (apparently the base of the sequence) is formed by andesite lava. In the area of San Gaspar we identified oblique-normal left-lateral faults in lavas, with a strike 191° and a dip 89°. In the Colimilla dam, 1297 m.a.s.l., we observed normal faulting (strike 267° and dip 81°), with 20-30 m jumps with reference to a unit of tephra of 3-10 m thickness. The lavas in this site present deformation, the main shear being parallel to the Rio Verde. At the site of the San Gaspar river the faults have a strike of 285° and a dip of 83v and affect ignimbrites that overlie dacitic lavas. In the area of the Arcediano bridge the normal faulting has a strike 188v and dip of 75° in andesites, and in the pumice-rich ignimbrites a shear direction with strike of 92° and dip of 84° that is parallel to the Rio Verde. During the past two years we identified approximately 1100 cases of sinking with varying magnitude in the urban area of Guadalajara. Some of these can be grouped to form alignments that are oriented with the faults identified in the CRGS region. The process of subsidence can be controlled by structures that affect the pumice sequence laying under the city of Guadalajara, facilitating the movement of groundwater through areas of weakness, removing tuffs and pumice and creating voids that later collapse, affecting buildings and infrastructure in the city.
U51A-0016
Large Potential Tsunami Run-up Along the North Coast of Papua New Guinea, as Seen by 12 Mapped Sector Collapses
We have mapped 12 sector collapses from volcanoes in the Bismarck volcanic arc. Comparing collapse orientations with structures within the volcanoes and volcanic chain, we find that nearly all of the collapses are directed either north or south, with the significant exception of Ritter that collapsed to the west and was directed NNW by the surrounding bathymetry of Sakar and Umboi. Volcanoes in the western part of the arc show dike orientations dominantly N-S, and collapses are dike-parallel, except for the dike – perpendicular collapse of Ritter in 1888 AD. In the eastern part of the arc, dominant dike directions are E-W, and collapses are dike-perpendicular. No relationship is yet seen between the collapse orientation and the known chemistry of the volcanoes. We have also estimated the sizes of the collapse fields and computed the size of potential tsunami generation from these features, based on linear wave theory (Ward and Asphaug, Geophys. J. Int., 153: F6-F10, 2003). We calibrate our computations with the known tsunami run-up of the Ritter collapse, with good success. The location of Madang may have experienced tsunamis exceeding 3 m run-up (and possibly exceeding 10 m, depending on the average thickness of the deposit) as a result of collapse of Ritter, Tolokiwa, Crown, Karkar and Garove. The location of Hoskins would have suffered a major event from Garove's collapse, and even small collapses may have had significant run-up in local areas. Had any of these 12 collapses occurred in modern times, each would affect a presently populated region of the coastline. Aside from Ritter island, the ages of the collapses remain unknown.
U51A-0017
Earthquakes and Volcanic Processes at San Miguel Volcano, El Salvador, Determined from a Small, Temporary Seismic Network
The San Miguel volcano lies within the Central American volcanic chain in eastern El Salvador. The volcano has experienced at least 29 eruptions with Volcano Explosivity Index (VEI) of 2. Since 1970, however, eruptions have decreased in intensity to an average of VEI 1, with the most recent eruption occurring in 2002. Eruptions at San Miguel volcano consist mostly of central vent and phreatic eruptions. A critical challenge related to the explosive nature of this volcano is to understand the relationships between precursory surface deformation, earthquake activity, and volcanic activity. In this project, we seek to determine sub-surface structures within and near the volcano, relate the local deformation to these structures, and better understand the hazard that the volcano presents in the region. To accomplish these goals, we deployed a six station, broadband seismic network around San Miguel volcano in collaboration with researchers from Servicio Nacional de Estudios Territoriales (SNET). This network operated continuously from 23 March 2007 to 15 January 2008 and had a high data recovery rate. The data were processed to determine earthquake locations, magnitudes, and, for some of the larger events, focal mechanisms. We obtained high precision locations using a double-difference approach and identified at least 25 events near the volcano. Ongoing analysis will seek to identify earthquake types (e.g., long period, tectonic, and hybrid events) that occurred in the vicinity of San Miguel volcano. These results will be combined with radar interferometric measurements of surface deformation in order to determine the relationship between surface and subsurface processes at the volcano.
U51A-0018
Determining Volcanic Deformation at San Miguel Volcano, El Salvador by Integrating Radar Interferometry and Seismic Analyses
From the early 1900's to the present day, San Miguel volcano has experienced many small eruptions and several periods of heightened seismic activity, making it one of the most active volcanoes in the El Salvadoran volcanic chain. Prior to 1969, the volcano experienced many explosive eruptions with Volcano Explosivity Indices (VEI) of 2. Since then, eruptions have decreased in intensity to an average VEI of 1. Eruptions mostly consist of phreatic explosions and central vent eruptions. Due to the explosive nature of this volcano, it is important to study the origins of the volcanism and its relationship to surface deformation and earthquake activity. We analyze these interactions by integrating interferometric synthetic aperture radar (InSAR) results with earthquake source location data from a ten-month (March 2007-January 2008) seismic deployment. The InSAR results show a maximum of 7 cm of volcanic inflation from March 2007 to mid-October 2007. During this time, seismic activity increased to a Real-time Seismic-Amplitude Measurement (RSAM) value of >400. Normal RSAM values for this volcano are <50. A period of quiescence began in mid-October 2007, and a maximum of 6 cm of deflation was observed in the interferometry results from 19 October 2007 to 19 January 2008. A clustering of at least 25 earthquakes that occurred between March 2007 and January 2008 suggests a fault zone through the center of the San Miguel volcanic cone. This fault zone is most likely where dyke propagation is occurring. Source mechanisms will be determined for the earthquakes associated with this fault zone, and they will be compared to the InSAR deformation field to determine if the mid-October seismic activity and observed surface deformation are compatible.
U51A-0019
Compound Antidunes: a Key to Detect Catastrophic Volcanic Eruptions
Antidunes are common in pyroclastic flow and surge deposits. However, the compound or nested occurrence of antidunes, where smaller antidunes reside within a larger-scale antidune, has seldom been documented or discussed in both pyroclastic and siliciclastic depositional settings. Without realizing this complexity, the frequency and magnitude of volcanic eruptions estimated from pyroclastic deposits are severely unrealistic. We have documented the Holocene outcrops of the antidune-bearing pyroclastites in Niijima Island, 100 miles SSW of Tokyo, Japan. The pyroclastites were formed by the eruptions in 886 AD Along the Habushiura coast in the southeastern part of the island, these outcrops form up to 50 m high cliffs, and are laterally traceable over 5 km from the volcano crater that shed the pyroclastites in the northward (downcurrent) direction. These pyroclastites were previously interpreted as recording about 30 small eruptions, each forming a 0.5-2 meter thick subhorizontal couplet of pumice (inversely grading) and lithic (normal grading) debris, with cm-m thick antidunes. However, we postulate that each of these couplets does not record a single volcanic eruption, but a much shorter time. These couplets occur between concave-up vertical accretion surfaces, which have both upstream- and downstream-migration components, within a 5-15 meter thick compound antidune (our "rank-1" antidune). Three erosively stacked compound antidunes form the coastal cliffs in the Habushiura coast, and each compound antidune is about ten times thicker than antidunes reported by earlier workers (corresponding to our "rank-2 antidunes" that nest within a rank-1 antidune, and "rank-3 antidunes" that nest within a rank-2 antidune). Hence, the Habushiura cliffs represent only three eruption events (instead of 30 events), but each representing much larger magnitude of eruptions. The geometry of these antidunes is comparable to "sediment waves" or "cyclic steps" of siliciclastic deposits recently reported from the modern deep sea (continental slope) and jökulhlaup (glacial outburst flood on land), and from flume studies. The erosional surfaces that separate rank-1 antidunes and hence individual eruption events are subhorizontal to slightly inclined to the upstream direction, and appear to onlap to the volcano's slope. Similar compound antidunes and erosion surfaces, both in size and geometry, occur within the older (c. 10-20 ka) pyroclastic deposits in Niijima and nearby volcanic islands, even though the chemical, mineral and lithologic compositions of pyroclastites associated with each volcano and eruption are highly variable. The geometry and size of these compound antidunes are remarkably similar to large "dunes" within the subaqueous pyroclastic-flow deposits within the Bay of Naples, associated with the AD 79 Mt. Vesuvius eruptions, recently reported by Italian researchers.
U51A-0020
Scientific Ocean Drilling to Assess Submarine Geohazards along European Margins
Submarine geohazards are some of the most devastating natural events in terms of lives lost and economic impact. Earthquakes pose a big threat to society and infrastructure, but the understanding of their episodic generation is incomplete. Tsunamis are known for their potential of striking coastlines world-wide. Other geohazards originating below the sea surface are equally dangerous for undersea structures and the coastal population: submarine landslides and volcanic islands collapse with little warning and devastating consequences. The European scientific community has a strong focus on geohazards along European and nearby continental margins, especially given their high population densities, and long historic and prehistoric record of hazardous events. For example, the Mediterranean is surrounded by very densely-populated coastline and is the World's leading holiday destination, receiving up 30% of global tourism. In addition, its seafloor is criss-crossed by hydrocarbon pipelines and telecommunication cables. However, the governing processes and recurrence intervals of geohazards are still poorly understood. Examples include, but are not limited to, earthquakes and volcanic eruptions along the active tectonic margins of the Mediterranean and Sea of Marmara, landslides on both active and passive margins, and tsunamites and seismites in the sedimentary record that suggest a long history of similar events. The development of geophysical networks, drilling, sampling and long-term monitoring are crucial to the understanding of earthquake, landslide, and tsunami processes, and to mitigate the associated risks in densely populated and industrialized regions such as Europe. Scientific drilling, particularly in the submarine setting, offers a unique tool to obtain drill core samples, borehole measurements and long-term observations. Hence, it is a critical technology to investigate past, present, and possible future influences of hazardous processes in this area. The Integrated Ocean Drilling Program (IODP) provides technologically top-level drilling vessels and platforms that can be used by scientists to address global scientific problems, including the causes and processes responsible for submarine geohazards. Both IODP and ECORD (the European Consortium for Ocean Research Drilling in collaboration with the European Science Foundation) support scientific initiatives towards submarine geohazards, because the geological record of geohazards can be read and interpreted only through ocean drilling, combined with a broad array of geophysical, geotechnical, and laboratory studies, to identify structures and deposits associated with hazardous phenomena.
U51A-0021
Sedimentological and Geochemical Characteristics of Turbidites Related to Earthquake Activity in the Sea of Marmara
The Sea of Marmara (SoM) is a tectonically active basin being located on a major continental transform fault boundary between the Eurasian and Anatolian plates. It consists of three transtensional major subbasins in excess of -1250 m and smaller ones with -100 to -200 m forming the E-W elongated gulfs and bays. The major subbasins have steep slopes, especially in the north, with slope angles greater than 18°. The sedimentary infill sequence in the deep basins consists of about 75% turbidite-homogenite units (THU) and 25% hemiplagic sediments, deposited at sedimentation rates of 1 to 3 m/ka. Deposition of most of the THU has been triggered by seismo-tectonic activity that constitute a serious geohazard in the densely populated coastal areas. Identification and dating of the THUs are therefore important in the repeat-time determination of the earthquakes on different fault segments, and thus, for the probabilistic earthquake risk assessment in the region. We studied the sedimentological, physical and chemical characteristics of THUs in several cores recovered from different Marmara basins, and identified the record of the devastating (Mw=7.4) 1999 Izmit earthquake, using digital X-Ray Radiography, XRF Core Scanner, MSCL, stable isotope and grain-size analyses. The units were dated using AMS C-14 and radionuclide methods. THUs are characterized by a relatively thin (commonly mm to several cm thick) sand-silt unit at the base and thick (commonly several tens of cm) homogeneous mud at the top. Digital X-ray radiography indicates that the THUs have multiple sand-silt laminae in the basal unit showing bidirectional foresets and a sharp and often erosional basal contact. These features indicates deposition by a single turbidity current reflecting or deflecting from the opposite slopes. The XRF Core Scanner analysis indicates two specific geochemical anomalies associated with the turbiditites sampled over the active faults: Ca enrichment in the basal coarse part, and Mn depletion within the THU unit. Both of these anomalies can be explained by the anaerobic methane oxidation (AMO) at or near the seafloor. The high upward methane flux towards the seafloor during the crustal deformation and fault rupture and the associated AMO lead to suboxic-anoxic bottom-water conditions in the confined Marmara basins and the formation of the black sulphide patches and carbonate crusts along the active segments of the Main Marmara Fault. This conclusion is supported by several lines of evidence, including O- and C-isotope data on carbonates, oceanographic surveys in the Izmit Gulf before and after the 1999 earthquake, and the Nautile submersible observations in 2007. The sedimentary records of earthquakes identified and dated in the cores can be confidently matched with the historical earthquake records.
U51A-0022
Slope Instabilities Along the North Anatolian Fault System in the Sea of Marmara
The characterization of slope stability in the Sea of Marmara has important implications in term of geohazards assessment in the vicinity of Istanbul. The Sea of Marmara is a tectonically active marine basin, located along the continuation of the North Anatolian Fault system, where coexist numerous factors triggering seafloor instability: steep slopes, reaching more than 18° in some places, high sedimentation rates, high seismicity, deep fluid migration processes, and specific paleoceanographic conditions. Several types of mass wasting processes are imaged with high resolution multibeam bathymetric data. The slopes of the Sea of Marmara are characterized by numerous submarine canyons, displaying erosional processes at their heads and flanks, and by large destabilized areas, covering more than 400 km2 in total, showing complex seafloor morphology with evidence of multiple slope failures, normal faulting and hummocky surface. Catastrophic events occur occasionally since related mass wasting deposits, such as turbidites, debris flows, and avalanche deposits are recognized within the basins. Evidences are debris layers in cores, transparent lenses interbedded with the sedimentation visible on the high resolution echosounder profiles, and seafloor visual observations of scree and boulders at the basis of the slope. At both extremities of the Sea of Marmara, in the entrance of the Izmit Basin and in the Western Tekirdag Basin, creeping masses are covering the fault trace. Moreover, in the Northeast Cinarcik Basin, a 32 km2 large sediment failure occurs where the active fault changes its strike, bending towards north. Large slope instabilities with tens of meters large tumbled blocks and occurence of methane seepages also occur on the southern side of the Cinarcik Basin above a deep-seated transtensional shear zone (Géli et al. 2008 EPSL in press). Fluid and gas seepages are often observed at the toe of submarine landslides, where they may result from loading and compaction or associated to high permeability conduits formed as a consequence of mass wasting processes (e.g. avalanche deposits and coarse turbidites). The dating of the largest slope failures and sediments flows indicate that most of the slope instabilities occurred at the end of the last glacial period (12-17 kyr) and may be accounted by sea-level variation and the transition from lacustrine to marine conditions in the Sea of Marmara. Flux of coarse sedimentation through the canyons is also higher over this period.
U51A-0023
Post-Megaslide Slope Stability North of Svalbard, Arctic Ocean
Pre-slide slope configuration is a heavily discussed topic for the evaluation of future slope failures. Post-slide slope stability has been investigated on slopes with slide complexes to assess recurrence times of failure elsewhere, meaning 'after the slide is before the slide'. The focus lays on identification of possible indications of future failure. Following the enormous Hinlopen/Yermak Megaslide 30.000 years ago, the adjacent slopes developed several failure types as consequence of the partial removal of the Hinlopen cross shelf trough mouth fan. The eastern slope is characterised by a number of decollements that facilitate large-scale gravity-driven mass movement. The processes involved include deformation along defined horizons (detachments) at apparently slow speed and creeping within less consolidated sediments. This soft sediment deformation includes turbulent structures like folds on meter-scale. The creeping sediments cover partly the eastern main slide debris of the megaslide within Sophia Basin. The speed of this gravity-driven mass transport can roughly be assessed by the time interval between megaslide and today. The absence of younger undeformed sediments ontop indicates a recent process. These slow slope failure may switch into sliding (e.g. following seismic excitation) as seen along the eastern slope and headwalls to develop into debris flows.