TCDP井下地震儀之觀測與微地震尺度分析; Observation and Scaling of Microevents from TCDP Borehole Seismometers

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题名:	TCDP井下地震儀之觀測與微地震尺度分析;Observation and Scaling of Microevents from TCDP Borehole Seismometers
作者:	林彥宇;Yen-Yu Lin
贡献者:	地球物理研究所
关键词:	井下地震儀;微地震;尺度分析;TCDP;Borehole Seismometer;TCDP;Scaling;Microearthquake
日期:	2009-06-05
上传时间:	2009-09-22 09:56:33 (UTC+8)
出版者:	國立中央大學圖書館
摘要:	TCDP井下地震儀安裝於台中大坑地區為集集地震最大同震變形發生之區域，地震儀記錄到規模最小之地震為MW0.0。地表安裝了10個短週期觀測網幫助精準定位。我們利用地震定位軟體MIMO判斷P波與S波到時、方位角與入射角協助定位。地震定位結果發現台中大坑地區微地震發生最頻繁的區域在10公里深左右的地震活動帶上，靠近地表附近的車籠埔主斷層帶上幾乎沒有任何微地震的發生。連續GPS觀測也證實此區域並沒有震後滑移的情形。這暗示了在車籠埔主斷層帶北部最大同震變形的發生區域累積應力已經完全於集集地震時釋放，因此在間震期完全鎖住（Lock）並沒有任何的滑移。2006年11月、2007年1月、3月及4月進行4次灌水試驗，在TCDP-C井中注入3至5MPa的高壓水，期待能記錄到微地震異常。但我們並沒有發現地震活動度在這些月份中有與灌水試驗相關的異常現象，此現象可能因斷層帶之滲透率太低造成灌入水量太少，影響區域有限。震源參數尺度分析方面，分析了139筆地震規模於Mw0.0~2.5後發現，地震規模與拐角頻率並無相關。規模與破裂半徑亦無相關，但是規模與視應力有顯著的正相關情況。本研究成果與Abercrombie於1995年之研究成果相似有正相關之情況。但Ide和Beroza修正路徑效應得到規模與視應力無相關。本研究目前尚未考慮路徑效應影響，此情況值得未來深入探討。 Microearthquakes with magnitude down to 0.0 were detected by the TCDP BHS. A temporary seismic array with 10 short period seismometers around the TCDP drill site was also installed to incorporate with the TCDP BHS for the precise locations of the microearthquakes. A location software (MIMO) (Oye and Roth, 2003) was used to determine P- and S-wave onset times, incidence and azimuth angles for the locations of the microearthquakes. Regardless of the large co-seismic slip of 12 m at the drill site during the 1999 Chi-Chi earthquake, our preliminary studies show very less seismicity near the drill site from the TCDP BHS recording. The micro earthquakes clustered at a depth of 8-10 km, where the 30 degree dipping of the Chelungpu thrust fault becomes flat to a decollement of the Taiwan fold-and-thrust tectonic structure. As a continuous GPS survey did not observe post-slip at the large slip region, and as no seismicity was observed near the drill site, we suggest that the thrust belt above the decollement during the interseismic period is locked. A Fluid Injection Test (FIT), pumping high pressure fluid into hole B and C with hole A as observation well was carried out at the TCDP boreholes in November 2006, and January, March and April, 2007. Compared with background seismicity in November, 2007, the observation did not show significant correlation of the FIT related seismicity, despite the distinct observations on the arrival of gas and chemical monitoring through FIT. This discrepancy in the physical and chemical observations to the FIT is an odd. It is possible that the fluid rate of FIT experiments is too deficient to trigger micro-earthquakes. The low injected fluid rate might indicate the low permeability of the fault zone. In addition to the FIT related observation, we also try to understand the scaling of the small earthquakes in stress drops and seismic moment. The source parameters of 139 micro-events were examined from the source spectra using Brune ω-square model for a constant Q model. The scaling of the magnitude to the Brune stress drop is a significant positive correlation. However, there has been a debate that this positive relationship might be biased for without Q correction. We had observed 26 clusters repeating events showing similar waveforms. But, those clusters have magnitude difference less than 1.0. It was hard to use empirical Green’s function to remove the path effect to validate the source scaling studies.
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