以尾波干涉法探討臺灣近地表地區震波構造的時序變化

Abstract

本篇研究中利用尾波干涉法 (coda interferometry) 研究臺灣近地表地區 (<400 m) 震波構造的時序變化。依據Chen et al. (2017)，淺部地層的經驗格林函數 (Empirical Green’s Functions, EGF) 可藉由中央氣象局在2011年開始佈置的全臺地表-井下地震儀的地震尾波的互相關函數 (Cross-Correlation Function, CCF) 得到，並可用來監測P波速度、S波速度和震波非均向性隨時間的變化。這篇論文中我們收集2013到2018年間發生在臺灣附近約700多筆的地震事件，並對每筆地震事件得到的CCF分別在垂直和水平項進行4-16 Hz和3-8 Hz的帶通濾波 (band-pass filter)，得到P波的EGF和S波的EGF，且為了確保EGF的品質和可靠性，對不同事件得到的EGF進行疊加，得到隨時間變化的EGF。疊加所有事件的平均格林函數得到的P波、S波速度以及震波非均向性與前人研究的淺層地質構造吻合。從EGF的時序變化可以觀察到某些測站在特定的時候P波速度和S波速度會急劇下降，且在震波非均向性中也會有明顯的變化，而這些變化的時間點剛好發生在大地震事件的時間點。我們在這篇論文中主要討論引發這些速度變化的機制，以及和地質構造之間的關係。

We report the temporal changes of the near-surface (<400 m) seismic structure of Taiwan revealed by coda interferometry analysis. Following our earlier work (Chen et al., 2017), the Empirical Green’s Functions (EGF) of primary waves and shear waves extracted from the earthquake coda recorded by the vertical pairs of borehole array, deployed by the Central Weather Bureau, are used to examine the temporal variations of VP、VS and Vs azimuthal anisotropy at the borehole sites. In total, about 700 local events, from 2013 to 2018, are used in this study. The band-passed filter EGF extracted from each single event are stacked over variable time period to ensure the reliability of measurements and the desired temporal resolution. The averaged VP 、Vs and patterns of Vs azimuthal anisotropy are in good agreement with the site geology, the ambient stress and those reported in our early work. Apparent drop in the VP velocities, Vs isotropic velocities and perturbations in Vs azimuthal anisotropy are observed in few representative borehole sites, and we also noticed that such variations are tightly correlated with the occurrence of major earthquakes in Taiwan. We present the analyzed results and discuss the triggering mechanisms, the healing revolution, and their relationship with the site geology.