Shear-Wave Splitting Observed In The Taiwan Area: An Effect Of Actively Convergent Process

Seismic anisotropy is found to be a ubiquitous property that is due to lithospheric deformation from past and present orogenic activity. Some analyses of shear wave splitting derived an accordance between crustal and mantle deformation, which suggested that the mantle plays a major, while may not be dominant, role in orogenies (e.g. Silver, 1996; Savage, 1999, etc). In the case of the Taiwan orogen, nevertheless, the observed splitting parameters tend to be regrouped with varied-scale tectonic frameworks. The previous study from regional and teleseismic shear-wave splitting revealed that the orientation of the upper mantle anisotropy affected by the collisional tectonics and generally submitted to the direction parallel to the strike of the Taiwan mountain belt (Rau, et al., 2000). On the contrary, the splitting parameters evaluated from crustal earthquakes show local-structure dependent for the Taiwan area. In this study, the fast polarization directions and delay times are analyzed from the local earthquakes recorded by both short-period and accelerometric stations at Taiwan. Some larger earthquakes are examined by aspect-ratio method to justify its polarization direction of the leading S wave. The resultant shear-wave splitting parameters from the southwestern Taiwan display about 0.2 second of delay times. The earthquakes located at the northern subduction zone of Taiwan are used to study seismic anisotropy characteristics with varied path propagation. Ultimately, we tend to interpret the seismic anisotropy in the Taiwan area as accommodation of lithospheric deformation provoked by the active convergence of the neighboring plates. Refs: Rau et al., 2000, EPSL, 177, pp177-192. Savage, 1999, Reviews of Geophysics, 37, pp. 65-106. Sliver, 1996, Annu. Rev. Earth Plant. Sci., 24, pp. 385-432.