Seismogenic stress field beneath the Tatun Volcano Group, northern Taiwan

The Tatun Volcano Group (TVG) represents the main volcanic center in northern Taiwan and based on various observations it is considered a potentially active volcano. TVG has been monitored since 2003 by a seismic network that consists of eight stations equipped with three-component, short-period and broadband seismometers. In this study, we use waveform data of high frequency earthquakes in order to investigate the stress field orientation beneath the TVG area. The focal mechanisms of 35 selected events have been derived using P-wave polarities and amplitude ratios, assuming a double-couple source. These fault plane solutions and various subsamples derived from them, were subsequently inverted for the best fitting stress tensor using a linear inversion method. The results show stress homogeneity beneath TVG for depths larger than 3 km while the stress tensor is characterized by a subvertical σ1 and a subhorizontal NW-SE trending σ3 axis consistent with the regional stress field in northern Taiwan. On the other hand, Chihsinshan which is an area of vigorous hydrothermal activity, exhibits a localized stress field with horizontal NW-SE trending σ3 axis and NE–SW trending horizontal σ1. Such an axes orientation is likely to be causing opening of microcracks and thus favour the ascent and circulation of fluids in the upper crust. Shear wave splitting measurements seem to confirm these results, showing fast polarization directions along NNE–NE, subparallel to the main fracture system in TVG. Shear wave anisotropy averages about 2.9% and is probably caused by an anisotropic volume of fluid-saturated cracks within the upper 2.5 km of the crust.