| Summary: | 碩士 === 國立臺灣大學 === 地質科學研究所 === 106 === We use continuous three-component seismic and volumetric strain waveform data recorded by seismometers and borehole strainmeters, respectively, from CWBSN, BATS and AS of IES, to reconstruct daily variations of empirical Green’s functions (EGFs) from single-station auto-correlation functions (ACFs) and cross-correlation functions (CCFs) between pairs of stations. Coda wave interferometry is then employed to investigate temporal changes in crustal velocity and mechanical properties preceding and following the 2013 Mw 6.2 Ruisui Earthquake that struck the northern segment of the Longitudinal Valley in eastern Taiwan and investigate the possible physical causes for the changes.
By comparing the pre- and post-event coda waves extracted from the ACFs and CCFs, we present a strong case that not only coseismic velocity reduction but also preceding decorrelation of waveforms are explicitly revealed in both the seismic and strain CCFs filtered in the secondary microseism frequency band of 0.1-0.9 Hz. Dispersion analysis of surface waves before coda arrivals shows that signals are dominated in 3-5 s and most sensitive to shear wave velocity at about 3-km depth. We thus compare the obtained interstation path-average velocity perturbations with lateral variations of static volumetric strain induced by coseismic slip distribution and find that the regions showing pronounced coseismic velocity reductions often experience extentional strain changes. In addition to coseismic variations revealed in both the seismic and strain CCFs, quasi-periodic variations only appear in the strain derived EGFs, with a predominant cycle of about 4 months correlating with the groundwater fluctuations observed at Ruisui. During the season of summer monsoon between July and October, the extracted coda signals are anomalously weak and yield irregularly and largely fluctuating variations in the estimated velocity and waveform correlation coefficient. Such phenomena may be associated with the disturbance of very shallow structures affected by significant precipitation amounts and rapid rise of ground water level. The use of borehole seismometer can help prevent this obstruction and obtain more stable and accurate estimates in earthquake-induced velocity and structural perturbation.
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