Kinematic Analysis of Subsurface Structures of the Northern Longitudinal Valley From Geodetic and Seismic Observations

Abstract:

Longitudinal Valley (LV), extended form Hualien to Taitung between the Central Range (CR) and the Coastal Range in the eastern Taiwan, is considered as a plate boundary formed by the convergence between the Eurasian and Philippine Sea plates. Previous studies reveal ~30 mm/yr shortening in the southern part of the LV; however, many interesting tectonic and geologic features in the northern LV are worth to discuss. Our relocation of M>2.0 background seismicity in the northern LV using HypoDD revealed an east dipping LV fault and a west-dipping lineament beneath the middle Central Range, while the lineament gradually becomes horizontal as extended to the east (Fig. 1). In Oct. 31, 2013, a NNE-strike earthquake of Mw=6.4 occurred near the town of Ruisui (Fig. 1), which is the largest event of the northern LV area since the 1972 M=7.2 earthquake. . The focal mechanism indicates that the earthquake is a high angle thrusting fault dipping to west, consistent with the aforementioned west-dipping seismic lineament beneath CR. In this study, we analyzed GPS data from 38 continuous stations together with ERS and Envisat images processed by PSInSAR (Persistent Scatterers InSAR) technique to study the interseismic and the post-Ruisui ground deformation of the northern LV area (Fig. 2). Our geodetic analysis reveal that the GPS horizontal velocity field decreases toward the north from ~25 mm/yr to <10 mm/yr across the latitude of ~23.5°, with a clockwise rotation of velocity directions from northwest to north and further to east in the Hualien area. In addition, the vertical velocities show subsidence in the most of the area with rates up to 10 mm/yr. Moreover, the mean Line of Sight (LOS) velocity of ERS from 1993-2001 reveals subsidence rates of up to 8 mm/yr in the Longitudinal Valley and an uplift up to 5 mm/yr at the west of the Milun fault (Fig. 2). Besides, the 2004-2008 Envisat data show an uplift of ~3 mm/yr in most of the Milun fault area. A couple of two-dimensional dislocation models across northern LV will be construct to better study the interseismic loading of plausible subsurface active structures of the area and the post-seismic fault slips of the 2013 Ruisui earthquake.