T13F-01 13:40h
Present-day Crustal Deformation in the Fold and Thrust belt of Southwestern Taiwan
A converging rate of 80 mm/yr is observed across the Taiwan arc-continent collision zone, and approximately half of the convergence is accommodated in the fold and thrust belt of southwestern Taiwan. A dense continuous GPS array of about 40 stations was established in the area and put into operation in 1998-2001. Furthermore, more than 100 densely-deployed campaign-surveyed stations have been occupied annually since 1998. The GPS data collected from these continuous and campaign-surveyed stations for the time period between 1998 and 2003 were utilized to study the present-day crustal deformation in southwestern Taiwan. With respect to Penghu, the velocity field in the Chiayi-Tainan area shows crustal motion with maximum rates of 35-40 mm/yr in the west-northwest to west directions. In the Kaohsiung area, the crustal motion changes the direction to southwest and the rate reaches 45 mm/yr. A significant discontinuity on station velocities was detected between Kangshan and Chiaotou, the surface trace of an unknown right-lateral strike-slip fault may pass through this location. Near-surface creep of 8 mm/yr was also found across the Liuchia fault. GPS observations from several near-fault sites indicate that there is no surface creep on the Chukou, Hsinhua, and Houchiali faults. As a consequence of the collision between the Luzon arc and Chinese continental margin, the Chia-Nan area is undergoing E-W compressional tectonic stress and results in a rapid contracting rate of up to 40 mm/yr. On the other hand, extrusion tectonics is prominent in the Kaohsiung-Pingtung area. Preliminary dislocation modeling studies show the interseismic slip is mainly along a sub-horizontal decollement at a depth of 6-10 km with a slip rate of 45 mm/yr. The model predicts elastic strain accumulation at the western end of decollement, i.e., the active thrust fault system, that will be released in the future earthquakes.
T13F-02 14:00h
Contemporary Deformation and Slip Rates in the Taiwan Fold and Thrust Belt
The GPS velocity field during a seven year time period leading up to the 1999 Chi-Chi earthquake shows horizontal contraction across the Western Foothills fold and thrust belt. In central Taiwan, across the Chelungpu fault, which ruptured during the Chi-Chi earthquake, the velocities are roughly parallel to the plate convergence direction and perpendicular to the trend of the major frontal thrust faults. The shortening rate is nearly uniform across the Western Foothills. In southern Taiwan the horizontal velocities have a large fault-parallel component and the fault perpendicular shortening is not so nearly uniform across the Western Foothills. The shortening is concentrated near the frontal thrusts. To explain these observations, we develop and use kinematic and mechanical earthquake cycle models consisting of faulting in an elastic lithosphere overlying a viscoelastic asthenosphere. In the kinematic model, uniform slip is prescribed on the faults at a constant rate. In the mechanical model, the faults slip in response to shear stresses in the lithosphere. We show that the GPS velocity field in Central Taiwan can be explained with creep between earthquakes on a $\sim~10\deg$ dipping d \`{e}collement surface underlying the Western Foothills and the western Central Ranges and periodic earthquakes on the frontal thrusts which are locked interseismically to about 8-10 km depth. The combined slip rate on the frontal thrusts is 30-45 mm/yr. The localized shortening and fault-parallel motion in southern Taiwan cannot be explained with creep on a d \`{e}collement buried 8-10 km deep. Instead, the data are consistent with shallower oblique slip on the thrust faults. Thus the locking depths of the frontal thrust faults may be shallower in southern Taiwan than in northern Taiwan, a result that might have important implications for seismic hazard assessment.
T13F-03 14:15h
Seismogenic structure of the 1935 Hsinchu-Taichung (MGR=7.1) earthquake, Miaoli, western Taiwan
A large earthquake (M$_{GR}$ 7.1) took place in Miaoli on April 21, 1935 and caused severe damage in surrounding area. The associated surface ruptures daylighted the Tuntzuchiao Fault (TTCF), trending NE between the Tachia and Taan River, and the Chihhu Fault (CHF), a separate back thrust trending N-S in the Shihtan area, which is 30 km north from the TTCF. In this study, we try to clarify the structural geometry and further identify the seismogenic structure for this event. The fold-and-thrust belt in Miaoli region is characterized by three major styles of structural interactions of thrusts and folds: the reactivated pre-existing normal faults, the low angle thrust cutting through the shallow strata, and the regional d\'{e}collement at the base of the sedimentary strata in depth. The hypocenter of the 1935 mainshock is located right beneath the middle reach of the Taan River. By subsurface geology, we consider that the reactivation of the pre-existing normal fault preserved in the footwall of the Sanyi Fault may be the seismogenic structure of this earthquake. It currently performs strike-slip in character and extends to the ground surface as the TTCF in the south. Combining the surface rupture and geologic map data, we interpret the CHF as a back thrust located in between two E-W strike-slip faults at both northern and southern ends. With this clear boundary condition, we incorporate the coseismic and interseismic triangulation data and remodel the fault plane geometry of both CHF and TTCF. Despite the two recorded surface ruptures, topography shows no strong evidence to infer a long development history. Therefore we conclude the two faults are either new or with relatively long recurrence interval.
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T13F-04 14:30h
Active Faulting and Pore-Fluid Pressure in the Taiwan Thrust Belt
Pore-fluid pressures significantly in excess of hydrostatic are thought to play an important role in the mechanics of overthrust faulting (Hubbert and Rubey, 1959). However in western Taiwan we argue, based upon a regional analysis of fluid pressures in 76 deep wells, that fluid pressures on the Pliocene Chinshui Shale detachment and ramp of the Chelungpu thrust system that ruptured the surface during 1999 Chi-Chi earthquake (Mw = 7.6) are within the hydrostatic regime and not overpressured. The fluid pressure data are obtained from in-situ borehole pressure measurements (formation tests), from densities of drilling muds, and from analysis of sonic logs using standard petroleum methods show fluid pressures in western Taiwan are stratigraphically controlled, as is typical of clastic sedimentary basins. The analysis provides constraints not only on present-day fluid pressure, but also pressures before uplift and erosion of growing structures which causes a large drop in overpressures. The top of the present overpressured zone is located at Miocene Nankang-Tsouho Formation in the north of Miao-Li, rises to the south to the Nanchuang and Kueichulin Formations in central Taiwan and only reaches the level of the Pliocene Chinshui Shale near Chia-Li (north of Tainan). Therefore the Chelungpu thrust sheet is everywhere in hydrostatic since this thrust runs along the Chinshui shale. This leads us to the conclusion that the static (ambient) pore-fluid overpressure plays no role in controlling fault friction of the Chelungpu thrust. The shallow detachment must be sliding under other mechanisms. Other shallow thrusts penetrated by drilling such as the Hsincheng thrust between Chingtsaohu and Paoshan anticlines and the Luchukeng thrust west of Yunghoshan anticline are also within hydrostatic regime. None of these thrust were ever overpressured as shown by the fossil top of overpressures which is based upon the deviation of porosity controlled shale velocities from the normal compaction trend shown by sonic log data using standard petroleum techniques that show the magnitude of uplift and erosion and the fossil and present pore-fluid pressures. The fossil top of the overpressured zone in several wells drilled through major thrusts and eroded anticlines is at a substantially higher (~1-2 km) stratigraphic level than the present top of fluid pressures, but never reaches the level of the Pliocene Chinshui Shale. It implies uplift and erosion of the active fold-and-thrust belt causes a major drop in fluid pressures in the formerly overpressured zone. Finally, a preliminary estimate of Hubbert-Rubey fluid pressure ratio needed to slide the Chelungpu thrust sheet (and also the Changhua thrust) using normal Byerlee's Law friction is about 0.8 (which is higher than any observed fluid pressures even within the deeper overpressured zone). Therefore the Hubbert and Rubey mechanism of static excess fluid pressure does not appear to be important for major thrusts such as the Chelungpu thrust that slipped in the Chi-Chi earthquake. The many other proposed non-Hubbert-Rubey mechanisms of reduction of fault strength should be considered, including dynamical mechanisms, fluid-pressure transients and non Byerlee coefficients of friction.
T13F-05 14:45h
Investigating The Kinematics Of Shortening Across The Pakuashan Anticline, West Central Taiwan.
The Pakuashan anticline is an active fault-propagation fold which constitutes the most frontal zone of deformation along the Western Foothills of central Taiwan. This blind fault is thought to be responsible for the 1848 Changhua earthquake, M~7.1, and to be currently locked. Previous studies mostly based on balanced cross-sections, have suggested various amounts of total shortening varying from 400m to 5km. The shortening rate was poorly constrained due to the uncertainty on this estimate as well as on the age of initiation of folding. Assessing the seismic hazard associated with this fold and its contribution to crustal shortening across central Taiwan, would require some better constraints on the fold structure and growth rate. To address this issue, field investigations allowed surveying the geometry of several deformed strata and geomorphic surfaces which recorded different cumulated amounts of shortening. These units were dated from Optical Stimulated Luminescence, and yielded ages ranging between ~ 19000 yr to ~ 300000 yr. We combined our data with seismic profiles and well logs which provided useful constraints on the deep structure of the fold and on finite deformation. None of the existing models of fault-propagation folding was found successful in reconciling the fold structure with incremental deformation documented from our study. We therefore used a simple analytical formulation which has been obtained recently from sand-box experiments (Bernard, Avouac, and Dominguez, this session). We derived a kinematic model which predicts a finite deformation consistent with the deep structure of the fold, and incremental folding consistent with the deformation of the shallower sedimentary units and geomorphic surfaces. As an outcome, we estimate cumulative shortening across the fold, age of initiation of folding, and the incremental deformation that occurred since the deposition of the various dated units. Consequently, we were able to derive an estimate of the slip rate on the Changhua thrust fault. This approach proved successful in reconciling all data for the Pakuashan anticline and should therefore be applicable widely to determine the kinematics of fault-propagation-folds.
T13F-06 15:00h
Inversion of Coseimic Deformation of Chengkung Earthquake in Eastern Taiwan Revealed by Strong Motion and Continuous GPS Data
The Mw=6.8 Chengkung Earthquake is almost a pure thrust event which is occurred on Dec. 10th, 2003 in the Costal Range of eastern Taiwan. This earthquake is believed to rupture the NNE-striking Chihshang fault in the Longitudinal Valley. The Chihshang fault is the most active section of the Longitudinal Valley fault, which is 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, it is believed that the main shock mainly dislocate the high-angle east dipping Chihshang fault plane. The maximum permanent vertical displacement shown by strong motion data in the hanging wall side is about 18 cm. We analyze 40 strong motion and 6 continuous GPS data around the Chihshang fault to model the fault plane geometry and the distribution of coseismic dislocations. Notably, not only all the stations on the hanging wall of the Chihshang fault were uplifted in this earthquake event, but also the stations on the Longitudinal Valley were still raised till the foothills of the Central Range. Because of the significant uplift on the Longitudinal Valley, we find that the model containing only single Chihshang fault can not well fit the data. Thus we try to investigate the much complex model with an addition subfault plane appended under the Longitudinal Valley. The modeled major Chihshang fault plane exhibits the strike of N20\deg E and dipping of 65\deg S. At the depth of 10 km, the dipping angle declines to 50\deg to the depth of 30 km. For the other subfault connecting to Chihshang fault at depth 10 km, the strike is the same as previous major fault but the dip angle is 50\deg S. 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 displacements by inversion of the two fault geometry model are 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 declining to 1~10 cm on both fault planes. The calculated scalar moment is 1.9 * 10 $^{26}$ dyne-cm, which is quite compatible with the 2.0 * 10 $^{26}$ dyne-cm based on the data of Harvard CMT.
T13F-07 15:15h
Millennial Slip Rate of the Longitudinal Valley Fault, a Major Element of Taiwan's Tandem Suturing
The Longitudinal Valley fault is a principal structure in the ongoing tandem suturing of Taiwan. We have used a fluvial terrace to show that over the past 4,000 years the dip-parallel component of slip on this left-lateral reverse fault has accumulated at a rate in excess of 30 mm/yr. Deformation of the terraces in the hanging-wall block, above the fault, allows us to infer the geometry of the fault plane. The island of Taiwan is being created by a tandem suturing and disengagement of the Eurasian continental margin, the Luzon volcanic arc, and an intervening continental sliver. Along the eastern suture, marked by the narrow, north-south Longitudinal Valley, the Luzon arc is accreting to the metamorphic core of the island, forming the Coastal Range of Taiwan above the east-dipping Longitudinal Valley fault. Near the middle of the range, the antecedent Hsiukuluan River flows from west to east through the range. Multiple generations of fluvial terraces along the course of the river enable the calculation of uplift rates for the Coastal Range. Combination of these with a model of the fault geometry constrains the millennial slip rate of the Longitudinal Valley fault. Radiocarbon dating of charcoal from the terrace deposits shows that the terraces were formed by the incision of the river during the past 8,000 years. A 4,000-year-old terrace that is preserved locally from the fault to a point about halfway through the Coastal Range forms a broad anticline. Assuming that river incision keeps pace with rock uplift, the maximum rock uplift rate is about 30 mm/yr, near the village of Chimei, approximately 4 km east of the fault. The rate decreases to about 20 mm/yr near the fault. Previously published work shows that the uplift rate is only about 6 mm/yr farther east, near the coastline. Using this anticlinal pattern of local rock uplift rate and variations in bedding dips along the Hsiukuluan River, we can infer the subsurface geometry of the Longitudinal Valley fault and a slip rate of about 35 to 42 mm/yr for the past 4,000 years. The horizontal component of the shortening, perpendicular to the fault, would be 17 to 30 mm/yr, comparable to the rate of horizontal shortening across the region measured by GPS. This first constraint on the long-term slip rate of the Longitudinal Valley fault demonstrates that most of the shortening between the Luzon arc and the Central Range of Taiwan is concentrated on this structure. Rates of slip on the Central Range thrust and any offshore structures must be dwarfed by the slip rate of the Longitudinal Valley fault. This demonstrates that over the millennia, the Longitudinal Valley fault embodies most of the seismic hazard along Taiwan's eastern suture.