GPS crustal deformation, strain rate, and seismic activity after the 1999 Chi‐Chi earthquake in Taiwan

Abstract

Using data at 110 continuous GPS stations from 1 January 2003 to 31 December 2005, 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.