Tomographic imaging of a seismic cluster in northern Taiwan and its implications for crustal fluid migration

After the occurrence of the 1999 magnitude 7.3 Chi–Chi earthquake, a cluster of NE-SW trending earthquakes, almost distributed along the surface trace of the Lishan fault, has been detected in northern Taiwan. From the spatiotemporal distribution of hypocenters based on cluster analysis, the Lishan Fault cluster (LFC) can quantify the influence of crustal fluids on the occurrence of seismicity after the 1999 Chi–Chi earthquake. In order to investigate the possible causes of the seismicity clusters and their relationship to the movement of fluids through part of the collision/subduction system, high-resolution 3-D tomographic images of the crust are determined under the entire region of northern Taiwan by inverting a large number of arrival time data of P-waves and S-waves. The results of seismic tomographic inversion indicate that the LFC extends down to about 10 km depth and seems to be distributed in or around the low Vp, high Vp/Vs zones. This temporal expansion of the focal area during the first week may be attributed to fluid diffusion. The b-value of the LFC is about 0.98 close to the average value, 1.0 of the entire Taiwan region. Our tomographic images demonstrate a series of relatively high Vp/Vs anomalies dipping to the east which seems to form a fluid upwelling conduit beneath the Central Range. We thus suggest that the Lishan Fault might play a role of an active fluid conduit, transferring additional fluids of the Philippines Sea plate along the east-dipping fault zone into the upper crust.