| dc.description.abstract | Strong plate convergence in the Taiwan area often produces a lot of earthquakes as well as complicated crustal deformation. However, seismic activity in the Hsinchu area is significantly lower than that in most of other areas where many earthquakes have been taken place in the Taiwan area. In order to improve the understanding of subsurface structures and their geological properties in the Hsinchu area, we deployed a dense micro-seismic network with 25 seismic stations between April 2004 and December 2007. In total, 1938 earthquakes were located in the period of 2.5 years. We used these seismic data to construct one-dimensional, three-dimensional velocity structures and the empirical relationship between MD and ML. We also estimated stress field according to 282 focal mechanisms inverted from the first motions of P-waves. Based on those results, some of interesting results have been obtained. First, low seismicity was largely found beneath the northwestern part of our study area, where has a low P- and S-wave velocity and may be associated with a large scale of the Miocene sedimentary basin. Subsurface structures beneath this part can only be estimated from three-dimensional velocity images without help of seismicity and focal mechanisms. On the other hand, high-angle fault planes of the Hsincheng and Tapingti faults in other parts of the study area can be clearly obtained from seismicity patterns, velocity images and focal mechanisms. Second, some pillar-anomalisms with high velocity are undoubtedly found in inverted velocity profiles. Comparison of the locations between pillar-anomalisms in the upper crust and the Miocene volcanoes near the surface, we ascribe these anomalisms to the intrusive igneous rocks, which were produced of extension environment in the Miocene epoch. Third, significantly spatial variation of stress fields estimated from 282 focal mechanisms shows two active fault systems in the Hsinchu area. One is a wrench fault with a strike of NW-SE; the other is the Raunqiao boundary fault along NE-SW. Finally, the temporal variation of seismic energy released by different faulting mechanisms show a negative correlation between normal and strike-slip sources. Based on the GPS and seismic results, we interpret this phenomenon is might be caused by a deep uplift (deeper than 10 km). As deep strata uplift silently, local stress field predominates over strike-slip faulting. On the contrary, normal faulting might occur in the shallow strata when deep strata uplift actively. Since there was no earthquake in which uplift was found, the deep uplift was a kind of slow slip along the Raunqiao fault plane in the Hsinchu area.
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