Preliminary survey in aseismic zone, southeastern of Central Ridge

 

Jo-Hao Chen 1 , Chien-Hsin Chang 2 , Horng-Yuan Yen 1

1 Institute of Geophysics, National Central University, Taoyuan, Taiwan

2 Central Weather Bureau, Taipei, Taiwan

Abstract

Located between 120.9 ° E-121.25 ° E and 23.3 ° N-23.7 ° N, the aseismic zone in the south-east part of Central Ridge is an area, shaped as rectangular, parallel to the Longitudinal Valley. From the catalog of CWBSN during 1990 to 2010, we can see that in this area, the frequency of earthquakes is relatively low. However, the structure under eastern of Taiwan, especially in lateral, changes fiercely and the tectonophysics is more complex. Under these circumstances, unknowing the accurate the crust velocity structure, there must exist some errors while locating an earthquake. The first part of this study is comparing three velocity structure models, CWB 1D velocity model, Wu et al. (2007) and Hsieh (2009). By using the data of TAIGER, we attempt to enhance the accuracy of locating earthquake. The other part use the data from the temporary seismic network, composed by a broadband seismic station in Chiahsing as a central, build in 2008 by IES of Sinica, and 6 seismic stations around Chiahsing, build by CWB. In this study, we choose those events which are not recorded by CWBSN in July 2010. First of all, we use the data, from Chiahsing station, that S-P travel time is less than 7 seconds. In addition, these events must agree with one of the conditions below: (1) recorded by at least four stations (2) the seismograms must have more than 6 kinds of phases. The programs for locating earthquakes in this study we use HYPO1D and HYPO3D. The preliminary result shows that for these three velocity models, the deviations are within 5 km, only the N4 point has larger deviation. For average, Hsieh3D09 has the smallest deviation for those three models. This indicates that it is successful for the velocity model which is transformed by density structure. On the other hand, this study picks up 323 events, located in aseismic zone, and re-located by HYPO1D. The result shows that there are earthquakes in aseismic zone; most of them are concentrated on the south-west part of Chiahsing station. Because of the result of using HYPO3D is not quite perfect, so we use the result from 1D model as an initial value for locating program of 3D model. It shows that the quantity is enhanced and the location is different from 1D model in obvious. From the outcomes for re-locating by HYPO1D and Wu3D07, there are some earthquakes toward to the aseismic zone, and it means that different model will affect the result of locating.

References

Y.-M. Wu, et al. Seismic tomography of Taiwan: Improved constraints from a dense network of strong motion stations. JGR, VOL. 112, 2007.

張建興，高密度地震資料分析及其用於台灣中部及東部孕震構造之研究， 國立中央大學地球物理研究所博士論文， 2004 。

謝獻祥，利用重力資料探討台灣及其鄰近地區之三維地下構造， 國立中央大學地球物理研究所博士論文 ， 2009 。