Active tectonics of Taiwan orogeny from focal mechanisms of small-to-moderate-sized earthquakes

We use the focal mechanisms of 97 small-to-moderate-sized earthquakes (2.7 less than or equal to M-L less than or equal to 5.7) given by a comprehensi ve focal mechanism study from Rau et al. (1996) to determine the style of f aulting and the state of stress in the active Taiwan orogen. The nature of faulting of the 97 minor earthquakes is characterized by a mixture of rever se, normal, and strike-slip faults. Thirty of the 97 events studied are rev erse faulting events; 24 of them occurred under the Western Foothills and t he Central Range, mostly within a depth range of 10-32 km and a dip angle r ange of 30-70 degrees. The steeply dipping nodal planes and their deep foca l depths demonstrate that the reverse faulting is not confined above a deta chment surface, but occurs in the crystalline basement at high angles. Norm al faulting events are observed under the northern Central Range in both up per and lower crustal levels and under the Western Foothills in the upper c rustal level. The shallow normal faulting under the northern Central Range is probably associated with the uparching of the core of the orogen, causin g the vertical stress to be greater under the orogen than under the lowland s. The cause of the deep normal faulting under the Central Range is enigmat ic. In the Western Foothills, under the flank of the Central Range, the sha llow normal faulting may also be a result of the uparching of the core of t he orogen, while the deep reverse faulting is caused by the horizontal comp ression. The stress tensors estimated are heterogeneous throughout the enti re Taiwan region, with the exception of south-central Taiwan where a nearly homogeneous stress field is observed. Although the spreads of the 95% conf idence region for sigma(1) and sigma(3) are relatively large for most stres s models, they are consistent with the direction of the plate motion of the Philippine Sea plate relative to the Eurasian plate.