| Summary: | 碩士 === 國立中央大學 === 地球科學學系 === 105 === 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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