Earthquake relocations, fault zone geometry and constraints on lateral velocity variations using the joint hypocenter determination method in the Taiwan area

Abstract

Four subsets of earthquakes recorded by an island-wide seismic network from 1991 to 2002 in the Taiwan area are relocated using the joint hypocenter determination (JHD) technique. Relatively large horizontal and vertical shifts are observed during the relocation, which indicates there may be systematic earthquake miss-locations in the Taiwan earthquake catalog due to the over-simplification of complicated earth structures to a simple 1-D velocity model. Generally, earthquakes are more clustered after the JHD relocation. Subsurface structures defined by the relocated seismicity become clearer, while they are not as clear in the initial hypocenters. Significant subsurface structures defined by the relocated seismicity are fault systems beneath the Central Mountain Range, beneath the northern Longitudinal Valley and Coastal Range, beneath the southern Longitudinal Valley and Coastal Range, and beneath the Chao-Chou fault in southern Taiwan. Another set of results from the JHD analysis, P- and S-wave station corrections, provide valuable information on the lateral velocity variations. The JHD station corrections indicate that upper crustal materials in the Central Mountain Range are characterized by higher velocity than those in the Western Foothills and Coastal Plain. The patterns of the observed JHD station corrections are also thoroughly consistent with surface geology observations. Analysis of the four clusters of earthquakes resulted in a remarkable similarity in JHD station corrections, indicating that the overlapping subsurfaces where two or more clusters of rays had traveled through were relatively shallow. These subsurfaces are responsible for the observed patterns of positive and negative JHD station corrections.