T32C-01 INVITED
Research Program of Taiwan Earthquake Research Center (TEC)
With 80 cm/yr plate converging rate that drives a high seismic energy release, about five times higher than California, the lithosphere in Taiwan is more thoroughly illuminated by seismic sources and makes Taiwan an ideal laboratory for active tectonic research. The 1999 M7.6 Chi-Chi, Taiwan earthquake produced a 100-km long surface rupture, with a slip more than 10 m and a record-breaking PGV. With extensive seismological, GPS, and other geophysical instrument networks that have collected extremely valuable data for scientific research. It holds more than 60% global strong-motion data for near-field (< 20 km) large (> M7.5) events. A Taiwan Earthquake Research Center (TEC) is established, following the SCEC model with generous governmental funding. TEC's current research programs are described and international research collaborations are welcomed.
T32C-02
Interseismic crustal deformation of Taiwan: A new insight with constraints of block model on 7-year GPS results
The island of Taiwan lies at the junction of the Eurasian and Philippine Sea plates. The convergence between these two plates forms the Longitudinal Valley, which has long been regarded as one of the major collisional suture zones, to the east and a fault-and-thrust belt to the west across the island. Based on recent published geological map and relevant studies, several major active faults are currently acting in Taiwan. In eastern Taiwan, the Longitudinal Valley fault, the most dominant fault within the suture zone, is locked along the northern section and is creeping at shallow depth but locked at deeper part in the southern section. In western Taiwan, active faults imbricate and propagate to the west above a proposed major Taiwan detachment Lots of the active thrust faults and tear faults in western Taiwan are seismogenic. The locking and creeping of the active faults around Taiwan as well as the locking of the subduction zones along the Ryukyu and Manila Trenches constrain the interseismic deformation across the Taiwan region and accumulate strain which may be released during future large earthquakes. In order to characterize interseismic deformation around Taiwan region, we analyze GPS data from 1990-1997. This time span excludes the effects of large earthquakes in the century, especially the Chi-Chi earthquake of 1999. For better understanding interseismic coupling and fault slip, we construct a three-dimensional block model across the Taiwan region to quantify kinematically consistent estimates of block motions and fault slip rates. The model combined elastic half-space and block motions based on the backslip model to describe the GPS velocities and locking faults. The GPS constrained block model provides estimates of present-day fault slip rates and seismic potential within the entire Taiwan region.
T32C-03
Double coupled faulting associated with the 2006 Mw=6.1 Peinan earthquake, Eastern Taiwan
Based on measurements of a dense geodetic network including 59 GPS stations and six leveling routes, we characterized the pre-, co- and post-seismic deformation of the 2006 Mw=6.1 Peinan earthquake in eastern Taiwan. The focal mechanism of the main shock and the distribution of the aftershocks indicated that the earthquake resulted from left-lateral strike-slip faulting with a NNE striking fault plane. However, the geodetic measurements showed that two conjugated strike faults appeared to simultaneously reactivate during the earthquake. In addition to the aforementioned NNE-striking fault, an E-W trending fault with right-lateral movement seemingly occurred co-seismically across the middle of the Peinanshan massif near the epicenter. Our half- space elastic dislocation modeling indicated that the slip distributions on the causative faults are far from homogeneous, in terms of slip amount and direction. In particular, the southern part of the NNE fault revealed a significant thrust component in addition to the left-lateral motion. Geologically, we interpret that the NNE-striking faulting represented a predominant left-lateral shear occurred along the eastern edge of the Central Range while colliding with the obliquely converging Luzon arc. As for the E-W trending faulting, it represented a transfer fault zone separating the Longitudinal Valley fault, the major plate boundary fault, into the northern Chihshang fault segment and the southern Luyeh and Lichi faults system. The pre-seismic measurements showed a pre- seismic creep in the northern part of the NNE-trending fault and an interesting back-slip with opposing movement in the southern part of the fault. We also observed that the earthquake triggered movements as aseismic creep and major aftershock rupturing along the nearby two major thrust faults, the Luyeh and Lichi fault, respectively.
T32C-04 INVITED
Neotectonics of Tainan Tableland in Southwestern Taiwan Based on Geodetic Measurements and SAR Interferometry
The D-InSAR technique is applied to detect the active fault-related folding structure of the Tainan tableland near the deformation front in SW Taiwan by using ERS SAR images during 1996-2000. The Tainan tableland is located in-between a blind fault in the west and the Houchiali fault in the east, thus the Tainan tableland is interpreted as a pop-up structure in a fold-thrust belt at active tectonic margin. Interferometric processing of six SAR images reveals the average slant range deformation (SRD) as ~12.5 mm/yr. The uplift rate is higher in eastern Tainan tableland than that in western Tainan tableland, and it increases from west edge of Tainan tableland and decreases across the Houchiali fault. The campaign-mode GPS data set from 1999 to 2007 indicate an average horizontal movement of 12±4 mm/yr in the direction of N44°W for the Tainan tableland with respect to western coastline. Furthermore precise leveling surveys across Tainan tableland show an uplift rate of ~14 mm/yr for the benchmarks on the tableland. Based on the 2-D analytical solution with the constraint of he inferred fault geometry, the slip rate along the inferred Tainan fault is ~16 mm/yr, ~10 mm/yr along the Houchiali fault, and ~25 mm/yr along the inferred Chungchou fault. Consequently we propose that active deformation of the Tainan Tableland is likely resulted from the freely slipping of the Tainan fault and the Houchiali fault. The locking depth should be located on the deeper part of décollement, eastern of the Chungchou fault. In addition, the combination of D-InSAR, GPS data and the precise leveling data reveals that the short-term deformation rate is larger than long-term deformation rate, which implies that a destructive seismic event could occur in the eastern Tainan area.
T32C-05 INVITED
Strength and Mechanics of the Taiwan Thrust Belt
The central Taiwan thrust belt is an important laboratory for study of fault and crustal mechanics because its structure is well constrained and it is the site of the well-instrumented 1999 Chi-Chi earthquake (Mw 7.6) and the post-Chi-Chi TCDP scientific drill holes. Here we summarize recent results on crustal and fault strength within this active thrust belt, based on critical-taper wedge mechanics and published TCDP temperature anomalies and stress measurements. These constraints indicate that the major faults are exceedingly weak with effective friction of στ/ ρ gz ≈ 0.03-0.1, whereas the deforming crust containing them is strong (σ1-σ3)/ ρ gz ≈ 0.6-1. Thus there is an order of magnitude difference between fault strength and crustal strength στ/(σ1-σ3)≈ 0.1. Furthermore, the extreme fault weakness is not caused by regional ambient high pore-fluid pressures as classically proposed by the Hubbert and Rubey because petroleum bore-hole data show that the fluid pressures are regionally hydrostatic. These results underline the outstanding causal questions of fault weakness and crustal strength.
T32C-06
Determination of tectonic shortening rates from progressively deformed flights of terraces above the Chelungpu and Changhua thrust ramps, Taiwan
The Chelungpu and Changhua thrust ramps in central Taiwan show contrasting hanging-wall structural geometries that suggest different kinematics, even though they involve the same stratigraphic section and basal detachment. The Chelungpu thrust shows a classic fault-bend folding geometry, which predicts folding solely by kink-band migration, whereas the hanging wall of the Changhua thrust demonstrates the characteristic geometry of a shear fault-bend folding, which predicts a progressive limb rotation with minor kink-band migration. We test the kinematic predictions of classic and shear fault-bend folding theories by analyzing deformed flights of terraces and coseismic displacements in the Mw=7.6 Chi-Chi earthquake. The Chelungpu terraces shows differences in uplift magnitudes across active axial surfaces that closely approximate the assumptions of classical fault-bend folding, including constant fault-parallel displacement, implying conservation of bed length, and hanging-wall uplift rates that are proportional to the sine of the fault dip. This provides a basis for precise determination of total fault slip since the formation of each terrace and combined with terrace dating gives long- term fault-slip rates for the Chelungpu thrust system. An estimation of the long term fault-slip rate of the Chelungpu thrust in the north Hsinshe terrace yields 15 mm/yr over the last 55 ka, which is similar to the combined shortening rate of 16 mm/y on the Chelungpu and Chushiang thrusts in the south estimated by Simoes et al. in 2006. Evan the coseismic displacements of 3 to 9m in the Chi-Chi earthquake are approximately fault-parallel but have additional transient components that are averaged out over the timescale of terrace deformation, which represents 10-100 large earthquakes. In contrast, terrace deformation in the hanging wall of the Changhua thrust ramp shows progressive limb rotation, as predicted from its shear fault-bend folding geometry, which combined with terrace dating allows an estimation of the long term fault-slip rate of 21 mm/yr over the last 31 ka. A combined shortening rate of 37 mm/yr is obtained for this part of the western Taiwan thrust belt, which is about 45 percent of the total plate-tectonic shortening rate across Taiwan. The Changhua shear fault-bend fold ramp is in the early stages of its development with only 1.7km total displacement whereas the Chelungpu classical fault-bend folding ramp in the same stratigraphy has nearly an order of magnitude more displacement (~14 km). We suggest that shear fault-bend folding may be favored mechanically at low displacement, whereas classical fault-bend folding would be favored at large displacement.
T32C-07
Systematic variations in synorogenic fill architecture and fault offsets along strike across the Puli topographic embayment: Quaternary strain gradients in the central Western Foothills and Taiwanese foreland basin
New balanced cross sections of the Taiwanese foreland show systematic and fundamental variations in the thicknesses of synorogenic foreland basin fill that correlate with the northern and southern boundaries of the Puli topographic embayment. Transitions in structural styles across the northern and southern boundaries of the embayment are closely related to the variations in the foreland basin fill architecture. These variations provide a bounding rheological framework that has resulted in the modern reorganization of strain in and around Puli, where actively deforming terraces within that basin provide evidence that active contraction of the orogen occurs over a broader area across the Puli region than in other parts of Taiwan. The greatest observed thicknesses of Quaternary and late Pliocene synorogenic sediments within the foreland basin are located to the west of the Puli basin and thin markedly where they cross the boundaries of the embayment, especially to the south. The Oligocene and Eocene rocks of the Hsueshan range, which the the modern Puli basin sits on top of, have been translated eastward with respect to Asia by ~11.4km Since the initiation of the Chelungpu Thrust. The Puli Basin itself may have not existed at this point in time; ages of the Puli basin fill, indistinguishable from Toukushan from a physical standpoint, have been determined to be as young as 70 ka in some places. Horizontal offsets across the Chelungpu thrust increase southward from 10.5km at the Tatu river to 11.9km just north of the Choshui river. Similarly, the horizontal offsets across the Changhua thrust also vary systematically along strike within the boundaries of the Puli embayment, from 0.13km at the Tachia river to 4.0km immediately north of the Choshui river, which marks the southern boundary of the topographic embayment. Horizontal offset on the Chelungpu thrust just south of the Choshui drops to 8.8km, while the horizontal offset across the Tungshu Hu/Changhua fault immediately south of the Choshui river drops to 0.5km. This large drop in displacement across the Choshui river suggests that the location of the river itself may be controlled by the observed transition in structural style and marked strain gradient across the southern boundary of the topographic embayment.