[1] The 2003 Chengkung earthquake (Mw 6.8) provided diagnostic evidence for a source model showing the deformation process of the seismogenic Chihshang fault in eastern Taiwan. The aftershocks show a fault-bend at a depth of 18 km. Coseismic ground displacements recorded by strong-motion records allow us to deduce instant rupturing of this event. Our resulting model shows a fault length of $33 km and dip-slip dominant rupture on fault-plane deeper than 18 km. Estimated coseismic displacements constrain two fault planes: one at 5 -18 km depth dipping 60°E and 18-36 km depth dipping 45°E. The uppermost fault-plane of the Chihshang Fault (0 -5 km) did not break immediately after the main shock; however, it may have a major role in after-slip and even interseismic ground deformation. The Taiyuan basin developed in the hanging wall is a geomorphic feature consistent with and adequately explained by coseismic ground displacements.

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[1] We characterize the kinematics of modern crustal deformation in Taiwan and evaluate the potential for large earthquakes by computing tectonic block motions and fault slip rates from 531 GPS horizontal velocities. These new GPS velocity field indicates that lateral extrusion in the southern transition from collision to subduction is primarily achieved by motion along several major reverse faults and internal distortion of blocks. The northern transition is characterized by asymmetric opening of the Okinawa trough and collision-induced rotation between the Ryukyu trench and Okinawa trough. We suggest that the differences in style of deformation in northern and southern Taiwan are a result of differences in trenchward motions between the overriding plate and forearc sliver. Along-strike variations in basin thickness and the presence of foreland basement obstacles in central Taiwan result in clockwise rotation with sinistral motion on faults and counterclockwise rotation with dextral motion on faults north and south of the obstacle, respectively. In eastern Taiwan, high slip rate of ∼43 mm/yr on the southern Longitudinal Valley fault (LVF) is responsible for the full collision of Taiwan orogeny. E-W syn-orogenic extension in the southern Central Range has been inferred by our model. Patches with high slip rate deficits on the LVF and the Chelungpu fault from our model, respectively, mainly correspond to the source areas of the 1951 M 7.1 Longitudinal Valley earthquake sequence and of the 1999 M w 7.6 Chi-Chi earthquake.

we characterized the surface deformation in Taiwan after the M w = 7.6 Chi-Chi earthquake of 21 September 1999. In continuous GPS (CGPS) data, the maximum coseismic deformation of the Chengkung earthquake and Ilan double earthquakes reached 165.5 ± 0.5 mm and 35.4 ± 0.5 mm in horizontal displacement and 181.7 ± 1.1 mm and 12.6 ± 1.5 mm in vertical displacement, respectively. With respect to Paisha station, S01R, the stations of the Coastal Range and Lanhsu showed an average displacement of 40.5-93.6 mm/yr with directions of 307°-333°. The stations in the Longitudinal Valley and Central Range revealed velocities in the range 19.0-49.3 mm/yr with directions of 285°-318°. In western Taiwan, the velocities in the inner fold-and-thrust belt range from 14.2 to 45.5 mm/yr with directions of 284°-304°. Extensional strain affects the Ilan and Pingtung plains near belt tips, revealing lateral extrusion toward the adjacent subduction zones. Extensional strain also affects the southern Central Range because of the rapid uplift related to the southward propagating collision process. Large and medium size earthquakes affect the strain pattern revealed by CGPS, albeit in different ways: regional extension and displacement were large and rotations were small regarding the M w = 7.6 Chi-Chi earthquake. In contrast, the limited size of the affected area, moderate displacement, and large rotations characterized the 2003 M w = 6.8 Chengkung earthquake. The impact of smaller earthquakes such as the 2005 M w = 5.9 Ilan double earthquakes and the 2005 M w = 5.6 Hualien earthquake was locally significant but regionally minor. The CGPS data provide a snapshot of the deformation that is generally consistent with the long-term history of the collision but should not be directly extrapolated because thrust deformation is migrating along the tectonic boundary. Regarding the Chi-Chi earthquake, the new CGPS data show that the Chi-Chi hanging wall is still recognizable as a kinematic block, whereas in the previous pattern the hanging wall was not discernable.

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