ANALYSIS OF FOCAL MECHANISM SOLUTIONS TO INVESTIGATE AN APPARENT ASEISMIC ZONE BENEATH THE SOUTHERN CENTRAL RANGE, TAIWAN

Taiwan is a seismically active and important example of the mountain building process by way of the ongoing arc-continent collision between the Eurasian and Philippine Sea plates. We show an aseismic zone beneath the Southern Central Range of Taiwan, bounded on its western flank by an area accommodating anomalous extension in a region experiencing rapid uplift. Using a catalog of over 8000 seismic events beneath the island of Taiwan and the surrounding area we produced a spatially accurate 3D model of hypocenters which reveals a volume of rock seemingly lacking seismic events beneath the Southern Central Range. This apparent gap in seismicity is elongate in nature, trending generally northeast-southwest along the topographic grain of Taiwan. Map views of the events show a region north of the Southern Cross Island Highway, almost completely devoid of events at depths of 0-40km. Focal mechanism solutions of the events bounding this area were used to produce overlapping strain inversions assuming a micropolar model for crustal deformation. Preliminary results show that the events bounding the aseismic zone on the west are experiencing dominant subhorizontal northeast-southwest extension and lesser instances of northwest southeast extension dissimilar to the plane-strain compression expected subparallel to relative plate motion. The area experiencing extension could be accommodating the continued rapid uplift of the region and the absence of seismic events in the zone beneath the Central Range may indicate the aseismic zone is a) a rigid block with no internal deformation, or b) ductile material without the shear strength required to generate seismic events.