| Summary: | 碩士 === 國立中央大學 === 地球科學學系 === 107 === The northern part of eastern Taiwan is situated on the plate boundary between the Philippine Sea plate and the Eurasian plate. The Philippine Sea plate subducts northwestward beneath the Eurasian plate with a very complicated plate interaction in this area. At the surface, the Central Range belongs to the Eurasian plate composed with metamorphic rocks, whereas the Coastal Range belongs to the Philippine Sea plate composed of sedimentary and volcanic rocks. These two geological units are separated by the Longitudinal Valley (LV) as a suture zone.
Seismic tomography is one of powerful tools to investigate the subsurface structures. In the past, a few of high resolution seismic tomography have been performed in this area. Taking the advantage of the data sets of Kuo-Chen et al., (2019) deployed a dense seismic array to capture the aftershock sequence of 2018 Mw6.4 Hualien earthquake and another dense seismic array in 2017, we applied a joint inversion of body and surface wave data to get better Vp and Vp/Vs velocity models. The advantage of this method is the complementary strengths of each data set. A total of 2892 local earthquakes and short period surface wave data were used for inversion. As a result, the earthquakes located with our new velocity model are more clustered than those by using the 1-D model, and the depth range of these events are between 5 and 20 km. Based on the focal mechanisms, the focal mechanisms are dominated by normal faults with few strike-slip faults in Hualien City in our 2017 and 2018 data set. The patterns of our velocity models roughly match the geological units at the surface but provide more information at deeper depths. The velocities in the east of LV are relatively lower than those in the west. Also, an interface between the relatively high and low Vp/Vs ratio zones beneath LV. According to the focal mechanism solution and seismic distribution, the interface could show the geometry of the plate boundary at deeper depths. Another pattern can be found that a high-velocity block extends upward from 20km to 5km in the east flank of LV, which could relate to serpentinized rocks in this region.
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