[1] To characterize the present-day vertical displacement field in the active Taiwan orogenic belt, 1843 precise leveling and 199 continuous GPS measurements from 2000 to 2008 are collected and analyzed in this study. Vertical velocities derived from the leveling data are placed in a reference frame of the Chinese continental margin using continuous GPS observations at nearby sites. The leveling and GPS vertical velocities generally reveal a dome-shaped pattern with uplift of ∼0.2-18.5 mm/yr in the interior of the mountain range and subsidence on the flanks of the mountains and coastal plains. Modern uplift rates in the active fold and thrust belt are generally consistent with geologic uplift rates. However, present-day uplift rates in the Central Range are faster than the million-years-averaged exhumation rates. The modern subsidence rates are generally consistent with geologic rates, except for the rates in western coastal areas due to groundwater pumping. Present-day subsidence in the southern Central Range and northern Coastal Range is, however, inconsistent with long-term uplift, which may reflect interseismic elastic strain accumulation across faults. Present-day subsidence in northern Taiwan occurs in a region of postcollisional orogenic collapse. We model the present-day and geologic vertical velocities and published GPS horizontal velocity data across southern Taiwan using a 2-D lithospheric model. The model suggests a combined slip rate of 40 mm/yr on the frontal thrusts and 45 mm/yr on the Longitudinal Valley fault. The model requires an additional source of crustal thickening under the Central Range to match the observed present-day uplift rates.

[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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[1] We present global positioning system (GPS) measurements for the period 1995-2005 at 125 campaign-surveyed sites in northern Taiwan. Based on elastic, rotating block modeling analyses derived from the GPS data, we describe the transitional tectonics from arccontinent (Luzon-Chinese) collision to the converging Ryukyu trench subduction and back-arc opening along the Chinese continental margin. Station velocities relative to station S01R, in the Chinese stable continental margin, were estimated from coordinate time series of each station by using the weighted least squares technique. We found two distinct deformation patterns in two geological areas, which are basically separated by the surface projection of the NW-trending boundary of the subducting Philippine Sea plate across northern Taiwan: (1) a waning collision area to the west and (2) a transition zone to the east. In the waning collision area, the horizontal velocity field shows vectors of 0.3-7.3 mm/yr toward the NW in the foothills and the Hsuehshan Range of northwestern Taiwan. The tectonic blocks represent a significant NW-SE internal contraction along with a small block rotation rate (<3.0°/Myr). The transition zone can be further divided into an outer range and inner range with distinguishing rotation rates and deformation behaviors. In the outer range of the transition zone, velocities of 1.0-7.8 mm/yr from south to north rotating from 008°to 143°is found in the northernmost foothills and the Hsuehshan Range. The tectonic blocks within the outer range are characterized by a coherent rotation (low internal strain rate of <0.10 mstrain/yr) with an angular velocity of $5.1°/Myr, where the Euler pole is located near its southeastern boundary. In the inner range of transition zone, a larger clockwise rotation from west to east, with horizontal velocities of 9.3-41.2 mm/yr from 053°to 146°, are found in the northernmost Central Range. The tectonic blocks of the inner range reveal a remarkable NW-SE internal extension with an ultrarapid clockwise rotation ($47.3°/Myr) where the Euler pole is near the southern boundary of the range close to the collision corner with the colliding Luzon arc. The trench roll-back together with back-arc opening are interpreted to be substantially superposed on the arc-continent collision-induced rotation in the transition zone with particular regard to the inner range of the northeast Taiwan mountain belt.