利用TCDP井下地震儀陣列分析車籠埔斷層帶之非均向性

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题名:	利用TCDP井下地震儀陣列分析車籠埔斷層帶之非均向性
作者:	洪瑞駿;Hung, Ruei-Jiun
贡献者:	地球科學學系
关键词:	臺灣車籠埔深鑽計畫;非均向性;車籠埔斷層;尾波交相關法;剪力波分離;Taiwan Chelungpu-fault Drilling Project;Anisotropy;Chelungpu fault;Coda cross correlation;Shear wave splitting
日期:	2017-07-25
上传时间:	2017-10-27 13:16:13 (UTC+8)
出版者:	國立中央大學
摘要:	集集地震(Mw7.6)過後，為了更深入了解該地震之物理特性，台灣車籠斷層埔深鑽計畫(Taiwan Chelungpu-fault Drilling Project, TCDP)於2004年起執行。七部井下地震儀(TCDPBHS)組成之垂直陣列隨後被安裝在跨斷層帶位置以監控區域微地震。本研究利用TCDPBHS陣列探討集集地震後斷層帶的非均向性。方法部分，我們考慮尾波交相關法進行分析。相較於噪訊交相關函數來說，尾波能提供更均勻的散射波場以幫助我們得到更可靠的格林函數。本研究使用陣列最上部的地震儀做為虛擬震源並審慎定義尾波長度進行分析。從走時圖及粒子運動結果證明所得到的交相關波形屬於剪力波後，我們便將尾波以每5˚做水平旋轉並計算交相關以檢視方位角非均向性。其結果發現，斷層帶上磐的剪力波快軸方向大約為130˚，然而在斷層帶-下磐範圍則轉至160˚。此外本研究亦使用剪力波分離法(SWS)進行分析，在使用了94個入射角小於30˚的微地震後，所得到的快軸方向(~110˚)大致上與台灣的板塊聚合方向類似，然而利用該方法並無在斷層帶觀察到旋轉情形，此原因可能是由於震源-測站距離遠大於測站間距，以致無法清楚解析所致; 利用上磐的BHS1及BHS5之剪力波分離得到該地區非均向性程度約為6.4%。大致上來說，利用尾波交相關法幫助我們得到可靠的S波經驗格林函數，且觀察到斷層帶之非均向性，此結果也與前人進行現地量測的結果相似，暗示利用非侵入性的地震波觀測，也能有機會得到小尺度的構造特性。相較於高成本的井測分析，地震波觀測或許也為一可行之方法做為地下構造辨析。;The Taiwan Chelungpu-fault Drilling Project (TCDP) was operated to understand the fault zone characteristics after the 1999 Mw7.6 Chi-chi earthquake. Seven Borehole Seismometers (TCDPBHS) were installed through the identified fault zone (~1.1 km) to monitor the seismic activities and the fault-zone structure properties. This study aims to reveal the fault zone anisotropy after the Chi-chi earthquake. The method used here is coda cross correlation which is feasible for retrieving the reliable empirical Green’s function since coda waves are generated from multi-scattering. We use the top sensor as the virtual source, and the length of coda are also carefully determined before cross correlation. After confirming these cross correlation waveforms are really in presence of S wave Green’s function by examining the traveltime and particle motion analysis, we rotate the coda wave at horizontal components by every 5˚ to obtain the azimuthal anisotropy. Results indicate the fast shear wave direction in the hanging-wall site is about 130˚. However, the fast shear wave direction rotates to 160˚ in the fault zone and the foot-wall-side area. Shear wave splitting method (SWS) from local micro-events is also considered in this study. After checking the data in 2007, 94 events with incidence small than 30˚ are used. The obtained fast shear wave polarizations (FSP) at this area are generally consistent with direction of tectonic convergence (NW-SE) in Taiwan. However, no significant rotation of FSPs recorded at fault-zone station are observed, this might because of the short inter-station distance which unables shear waves to change the polarization. Totally 6.4% of anisotropy is obtained. Coda CCFs can retrieve a stable Green’s function to unveil the fault-zone anisotropy, and this result is also in agree with those identified from in-situ measurement. This means small scaled anisotropy can be revealed via seismic observation. Compare to well-logging, which are usually costly, anisotropy from seismic observation might be a good approach that will be more economical.
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