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 along the surface trace of the Lishan fault, has been detected in the northern portion of the Central Range
in northern Taiwan. From the spatiotemporal distribution of hypocenters based on cluster analysis, the Lishan
fault cluster (LFC) can quantify the evolution of seismicity as aftershocks of the 1999 Chi-Chi earthquake. The
results of seismic tomographic inversion indicate that the LFC extends down to about 10 km depth and seems
to be distributed in high Vp areas rather than in low Vp areas. This temporal expansion is attributed to fluid
diffusion. Seismic activity in the upper crust tends to be high above broad zone with low Vp in the lower crust. 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, fluid or fluid fluxed a partial melt of the Philippines Sea plate would be released along
the east-dipping conduit and rise gravitationally to the upper crust.