Active Tectonics of Taiwan Orogeny From Focal Mechanisms of Small-to-moderate-size Earthquakes

Abstract

We use the focal mechanisms of 97 small-to-moderate-sized earthquakes (207 <= ML <= 5.7) given by a comprehensive focal mechanism study from Rau et al. (1996) to determine the style of faulting 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 reverse, normal, and strike-slip faults. Thirty of the 97 events studied are reverse faulting events; 24 of them occurred under the Western Foothills and the Central Range, mostly within a depth range of 10-32 km and a dip angle range of 30-70°. The steeply dipping nodal planes and their deep focal depths demonstrate that the reverse faulting is not confined above a detachment surface, but occurs in the crystalline basement at high angles. Normal faulting events are observed under the northern Central Range in both upper and lower crustal levels and under the Western Foothills in the upper crustal level. The shallow normal faulting under the northern Central Range is probably associated with the uparching of the core of the orogen, causing the vertical stress to be greater under the orogen than under the lowlands. The cause of the deep normal faulting under the Central Range is enigmatic. In the Western Foothills, under the flank of the Central Range, the shallow normal faulting may also be a result of the uparching of the core of the orogen, while the reverse faulting is caused by the horizontal compression. The stress tensors estimated are heterogeneous throughout the entire Taiwan region, with the exception of south-contral Taiwan where a nearly homogeneous stress field is observed. Although the spreads of the 95% confidence region for σ1 and σ3 are relatively large for most stress models, they are consistent with the direction of the plate motion of the Philippine Sea plate relative to the Eurasian plate.

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