[1] We examined shear wave splitting in SKS phases from a large event recorded by a temporary array across southern Taiwan. The span extends from the western plain to the east coast. We applied particle motion and cross correlation methods to estimate its polarization directions and delay times. Analysis shows clear evidence of splitting except stations on the east coast. The fast split-shear wave along this transect are approximately parallel to the strike of the mountain belt (NE-SW). The delay times show a shortwavelength variation and are well correlated to the surface geology. The largest split time is about 1.6 sec located at Eastern Central Mountain Range and indicates its possible mantle origin. Implications suggest that anisotropy is related to the collision tectonics which built the Taiwan islands, and that this tectonic compression involved the lithosphere and is characterized by a strong coherent deformation of the upper mantle and the crust.

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[1] The geometrical structure of the responsible faults of the 20 September 1999 Chi-Chi, Taiwan, earthquake (M L = 7.3, M w = 7.6) and its aftershocks can be clearly depicted by well-located hypocenters and focal mechanisms of large aftershocks. The mainshock and two large aftershocks with M L = 6.8 were characterized by thrust faulting along a N-S striking fault plane dipping to the east. The underground structure of the Chelungpu fault, which is probably merging with the decollement beneath the Western Foothills, can be clearly associated with the seismicity pattern and the focal mechanisms of the three largest events. A group of deeper aftershocks including two moderate events (M L = 6.3 and 6.0, respectively) were located to the southeast of the mainshock along a fault plane dipping steeply to the west down to a depth of about 30 km. Our results suggest that the spatial pattern of the aftershocks in the southern part of the source area can be interpreted by a conjugate-fault system. This conjugate-fault system is comprised of the gently east-dipping Chelungpu fault and a steeply west-dipping deeper fault zone.

Despite a moderate magnitude, M w = 6.4, the 5 February 2016 Meinong, Taiwan, earthquake caused significant damage in Tainan City and the surrounding areas. Several seismograms display an impulsive S-wave velocity pulse with an amplitude of about 1 m s -1 , which is similar to large S-wave pulses recorded for the past several larger damaging earthquakes, such as the 1995 Kobe, Japan, earthquake (M w = 6.9) and the 1994 Northridge, California, earthquake (M w = 6.7). The observed PGV in the Tainan area is about 10 times larger than the median PGV of M w = 6.4 crustal earthquakes in Taiwan. We investigate the cause of the localized strong ground motions. The peak-to-peak ground-motion displacement at the basin sites near Tainan is about 35 times larger than that at a mountain site with a similar epicentral distance. At some frequency bands (0.9 -1.1 Hz), the amplitude ratio is as large as 200. Using the focal mechanism of this earthquake, typical "soft" and "hard" crustal structures, and directivity inferred from the observed waveforms and the slip distribution, we show that the combined effect yields an amplitude ratio of 17 to 34. The larger amplitude ratios at higher frequency bands can be probably due to the effects of complex 3-D basin structures. The result indicates that even from a moderate event, if these effects simultaneously work together toward amplifying ground motions, the extremely large ground motions as observed in Tainan can occur. Such occurrences should be taken into consideration in hazard mitigation measures in the place with frequent moderate earthquakes.

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