[1] We use a matched filter technique to detect 41 lowfrequency earthquakes (LFEs) within 700-s of triggered tremor signals in the Southern Central Range in Taiwan during the surface waves of the 2005 Mw8.6 Nias earthquake off the coast of northern Sumatra. The depth distributions of LFEs after double-difference relocations concentrate at the depth range of 12-38 km below the background seismicity and above the Moho depth inferred from receiver function studies. The locations of LFEs are close to the downward extension of the steep-dipping Chaochou-Lishan fault with only modestly high Vp/Vs ratios (1.75-1.85). Our observation indicates that at least portions of triggered tremor consists of many LFEs, similar to ambient tremor observed at other major plate boundary faults. Citation: Tang, C
Many approaches for locating earthquakes . attempt to solve a nonline�r inverse system in .a one-dimensional velocity model. To ensure precise earth quake locations in geologically complex areas, however, a three-dimensional velocity model has to be c()nsidered. In this study, the authors have applied an alternative approximate method based on the dynamic ray theory with a 3-D velocity model to relocate 374 events from 1980 to 1989 recorded bJ.;�: .� :_ . the Ta iwan Telemetered Seismograp• hic Network (TTSN). The solutions to lo cating earthquakes using 1-D and 3-D velocity models are compared. These results show that the root mean square (RMS) of travel time residuals for the 3-D model are much lower than those for the 1-D velocity model. The dis crepancies between the 1-D and 3-D solutions reflect the importance of lateral velocity variations in locating earthquakes in the Ta iwan area. These discrep ancies are larger in focal depths than in epicentral determinations. From the examination of seismicity along the Chaochou fault with both 1-D and 3-D ve locity models, the results here show that the hypocenter determinations from a 1-D model may even lead to an absurd interpretation in seismicity. The presence of the lateral heterogeity of seismic velocity in Ta iwan makes the ne cessity of locating earthquakes with a three-dimensional velocity model. The increases in computation time in using a 3-D velocity model to locate earth quaks can be minimized by applying the dynamic ray tracing technique due to its fast computation capacity and high level of accuracy.
In this study, we adopt a damping least-square inversion method to investigate the Vp structures and Vp/Vs ratios of the crust and upper mantle beneath the Chia-Nan area, Taiwan. Previous studies have shown that, velocity structure can be used as an indicator of the geometry of a fault and the general aspects of tectonics. Therefore, the first goal of this research is to analyze the degree of correlation between the velocity structure and the seismic characteristics with respect to the tectonic implications of the area. The second intention is to study the relationship between the Vp/Vs ratio and the pressure change occurring with crack opening in the deeper crust. Finally, the distribution of Vp/Vs ratios and its association with fault activities is also investigated. Our results indicate that the variations in velocity structure beneath the Chia-Nan area is caused by local geological structures, fault crossing and the existence of the Pei-Kang High Area. We also find that most earthquakes occur in areas that have Vp/Vs gradients varying rapidly. In addition, according to the distribution of the earthquakes, there seems to exist a westdipping fault west of the Chukou fault; however, this prospect needs to be investigated further in a future study.
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