| Summary: | 碩士 === 國立中央大學 === 地球科學學系 === 106 === There are two research areas in this study, namely the Ilan Plain and Mindoro Island. Ilan plain in NE Taiwan is on the southwest end of Okinawa trough, which is undergoing a period of transition from arc-continent collision to post-arc rifting with frequent earthquakes. In addition, the magmatic activity associated with the back-arc extension provides a good geothermal environment (Tong et al. 2008). The Qingshui geothermal power plant used underground hot water and steam to generate electricity from 1980 to 1993.
The Philippines archipelago was formed by arc-continent collision between the Palawan Continental Block and Philippine Mobile Belt in the early Miocene. The second study area of this study, Mindoro, is located at the center of the arc-continent collision. The Silbuyan Verde Passage Fault (S.V.P.F.) in the north of Mindoro is a branch of Philippine Fault. We can observe intensive earthquakes around this fault, including the Batangas earthquake sequence that occurred in early April 2017.
The tectonic structure of Ilan Plain is complex, so we set up 10 broadband seismic stations on Ilan plain to detect earthquakes at the end of November 2013. In order to study the seismic characteristics of arc-continent collision, we also set up broadband seismic stations in the northwestern of Mindoro. The match and locate method (M&L, Zhang & Wen, 2015) uses the S wave of the template event to detect small earthquakes with similar waveforms. In this study, we used M&L to detect earthquakes around the Ilan Plain and Mindoro Island. First, we discuss the seismicity of the Qingshui geothermal area and Sanxing fault, then detect the complete Batangas earthquake sequence to further analyze the migration and decay of aftershocks.
In Ilan Plain, there were 404 earthquakes in the original catalogue around Sanxing fault. M&L detected 110 additional events in the same period. We further discuss the frequency-magnitude relationship of earthquake catalogs. The magnitude of completeness (Mc) of original catalog is 0.9 and b value is 1.00. The Mc of detection result catalog is 0.2 and b value is 0.80. The Mc of collective catalog is 0.9 and b value rises to 1.01, indicating that M&L can strengthen the ability of detecting small earthquakes. In addition, according to the distribution of earthquakes in collective catalogue, we can find that the earthquakes are mostly concentrated at the end of Sanxing fault and the dip of fault plane is about 45 degrees to the northeast.
M&L detected 3,155 events during the Batangas sequence and there were 3,222 events in the collective catalogue, which is about 8 times the number of original catalog. According to the seismic distribution of collective catalogue, the dip of 4/4 rupture plane is to the northeast, indicating that the strike of fault plane is 339 degrees. As for the 4/8 aftershock sequence, the spatial distribution is consistent with the strike of 315 degrees.
The amount of earthquakes in collective catalogue is large and complete. Therefore, we can discuss the detection ability of broadband seismic network and the tectonic characteristics of study area based on the attenuation rate of aftershock. We use modified Omori's law and integral form to analyze the attenuation rate of 4/4 and 4/8 aftershock sequences. The average K value is 2619.25, c value is 8.64 and the p value is 1.48. The c value represents the ability of seismic network detecting early aftershock sequences. The c value is 8.64 indicating that M&L can detect complete aftershocks about 8 hours after the main shock. The p value is 1.48, which is large within global average of 0.9-1.5, indicating that the crust temperature of this area is high so the cumulative stress released quickly.
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