Temporal and spatial variation of stress field in Taiwan from 1991 to 2007: Insights from comprehensive first motion focal mechanism catalog

The total amount of 4,761 focal mechanisms was determined based on P-wave first motion polarities from 1991 to 2007 in Taiwan region. This dataset offers us a good opportunity to examine temporal and spatial variability of the stress field. We find that the spatial variations of stress axes are mainly controlled by tectonic structures while the temporal changes are greatly influenced by the Chi-Chi earthquake. The orientation of the maximum horizontal compressive stress axes (SH) shows a general agreement with the direction of plate motion between a depth range of 0–30km. The 20° anticlockwise rotation of SH from the Longitudinal Valley (LV) to western Taiwan is probably caused by the left-lateral motion on the Longitudinal Valley Fault (LVF) that has consumed part of the oblique motion of plate convergence. On the other hand, part of the oblique convergence is transferred into the Central Range and the Hsuehshan Range judging from counterclockwise rotation of SH from east to west and strike-slip faulting in the Hsuehshan Range. Most events with a depth greater than 30km occur offshore eastern Taiwan and the azimuth of SH is close to E–W directed, different from NW–SE directed at shallow depths. This may infer the existence of the transition of lithosphere rheology in offshore eastern Taiwan. The trends of SH in the depth of 0–10km are strongly affected by the coseismic stress change of the Chi-Chi earthquake. In the northern half of the Chi-Chi rupture area, the trends of SH rotate 30° clockwise and the stress ratio increased by a factor of six after the mainshock. The orientations of SH still differ by 30° in 2007 comparing to that in the period before the Chi-Chi earthquake. The variation of SH trend is more diverse in the southern half of the rupture area, showing 20° counterclockwise rotation immediately after the 1999 mainshock followed by a clockwise rotation. The trend of SH returns to the pre-seismic direction of 110° in 2001. These notable changes of SH before and after the Chi-Chi mainshock suggests that the magnitude of background stress in the rupture area is close to the coseismic stress drop. We also recognize a significant 10° counterclockwise rotation of SH in the entire Chi-Chi rupture area between 1991 and 1999 before the earthquake took place. However, to the south of the Chi-Chi rupture, the trends of SH remain little changed before and after the Chi-Chi earthquake.