台灣中央山脈地區地震所引起淺層地殼速度與靜態體積應變擾動之相關性

Abstract

地震引起破裂帶鄰近區域岩石強度下降或是孔隙率增加以及應力和應變狀態的改變，造成區域性彈性介質受到擾動並進一步影響淺層地殼同震速度的變化。 為了探討台灣中央山脈地區2010年M 6.4甲仙地震以及2013年M 6.2南投序列地震斷層破裂引起的地殼應變與岩石強度改變的潛在性原因，我們使用中央氣象局地震觀測網 (CWBSN) 提供的短周期地震儀以及台灣寬頻地震網 (BATS) 提供的寬頻地震儀連續資料，並利用周遭噪訊交互相關函數 (CCF) 來重建經驗格林函數 (EGF)，透過比較疊加長時間與短時間的經驗格林函數表面波到時後尾波的走時偏移來得到地震前後地殼速度隨時間變化的關係。 由頻率範圍0.1-0.8Hz經驗格林函數計算過後的結果顯示，甲仙與南投地震前後均有明顯的速度改變，與測站區域位置進行比較，甲仙地震有較明顯同震速度下降區域為震央西方數公里處餘震活動頻繁地區，同時也為逆衝斷層上盤區域；南投地震明顯同震下降區域則為穿越震源之斷層上盤處，亦是餘震活動頻繁區域。 透過地震鄰近區域一維速度構造加以計算經驗格林函數主要周期3 - 5秒表面波尾波的敏感深度結果顯示，甲仙與南投地震速度變化結果敏感深度皆在10公里之內，暗示所量測到的同震速度變化皆由淺部構造或介質受到地震造成的擾動影響所致。為了探討彈性介質與速度改變的關聯性，我們利用有限斷層滑移分佈資料計算同震靜態體積在空間上的分佈並與速度變化分佈進行比較，發現在淺部地殼同震速度下降區域與靜態體積伸張區域位置結果一致。我們推論量測到的同震速度變化與地震應力造成之淺部地殼靜態體積應變改變結果相關。

Coseismic velocity reduction has been considered to be mediated by perturbations of stress and strain conditions in the crust and/or increased permeability/porosity of fractured rocks within the quake-damaged zones. To investigate potential changes and causes in crustal strains and rock properties of areas impacted by the earthquake ruptures of the two recent large events, Jiasian and Nantou, (Mw > 6 and focal depth > 20 km) occurring in the south Central Range of Taiwan, we construct empirical Green’s functions (EGFs) from cross-correlation functions (CCFs) of continuous ambient noise between available station pairs near the epicenters from the short-period Central Weather Bureau Seismic Network (CWBSN) and the Broadband Array in Taiwan for Seismology (BATS). The temporal variations in seismic velocity perturbations are estimated by measuring the relative time shifts of late-arriving coda waves between short-term and long-term stacked EGFs. The resulting EGFs at 0.1-0.9 Hz show the statistically significant coseismic velocity reduction immediately after both the events. The velocity drop is detected most pronouncedly from the pairs with the interstation paths traversing through the hanging-wall block of the ruptured fault. The sensitivity of surface wave coda arrivals to shear wave speed in the dominant period range of 3-5 s is confined within the depth of 10 km, where the crust mostly experienced the coseismic dilatational strain change induced by the slip distribution from the finite-fault models. Compared with the coseismic slip distribution from GPS data and finite-fault inversion, peak ground velocity, and slip-induced volumetric strain, we suggest the coseismic velocity reduction associated with these two events is plausibly caused by the induced dilatational strain in the shallow part of the crust above the blind thrust ruptures.