精確的地震定位是探討發震構造的重要依據，而利用具有高相似度波形的相似地震提供的相對時間差，更可以消除走時誤差、達到高精度定位的目的，唯相似地震群並不常見。前人研究指出，在台灣東部的池上斷層，發現有重複地震的出現，指示此區域亦為相似地震群的潛在區域。本研究利用在池上地區發現的相似地震群，進行波形相關（Waveform Cross-Correlation）的定位方法，討論在此地震網形的池上地區，不同定位方法是否呈現不同的發震構造特徵？而波形相關定位法是否能達到更好的定位精度？花東縱谷位處於歐亞大陸與菲律賓海板塊之交界，地質構造活躍、地震頻繁處，台東池上地區為其上地表變形最明顯的區域之一。大地測量資料顯示，池上斷層地表有30 mm/yr的地表變形量；而自1991年起有2起規模大於5、1起規模大於6的地震，表示板塊相對運動在此區所累積的能量以潛移及地震方式釋放。本研究選取自1991年1月1日至2006年8月31日ML ≧ 2.0的地震（5236筆事件），先使用三維地震重新定位，定位後之走時殘差均方根（root-mean-square, RMS）由原先0.33降至0.26；再使用雙差分定位（Double-difference location）進行走時及波形相關度比對。其中波形相似度高於0.7的事件共1979筆，佔背景地震活動數目的38％；這些相似地震群集中在池上地區的北段（23.1°N以北），深度分佈約在10-25 km。挑選一組由12筆地震組成的相似地震群進行誤差分析，結果顯示利用波形相關定位方法，平均水平及垂直誤差由地震目錄的1025 m、925 m降為27 m、30 m。將相似地震群與背景地震做聯合重新定位結果則可呈現整個研究區域的發震構造特徵，池上斷層由北到南的構造特徵略有差異。而利用相似地震群單獨定位的結果，在平行斷層走向剖面發現近垂直的線性構造、與滑移方向幾近平行（rake約70°）。此線性特徵表示相似地震群除了在斷層幾何形貌的最佳化描述外，更可反映斷層滑移方向。

High-precision earthquake location hold the key to the understanding of deep fault deformation as well as the better assessment of earthquake hazards. Earthquakes with similar waveform provide very accuarte determination of differential travel times of seismic phases, which can largely reduce the location incertainty casued by original travel-time picking error. These particular group of earthquakes called similar event group, however, are not common and mostly found in the creeping fault zone. The Chihshang fault in eastern Taiwan characterized by a large numbe of small earthquakes has been known to undergo 2-3 cm/yr surface creep. It gives an good opportunity for simialr events search. Using 5236 ML≧2.0 earthquakes from the 1991-2006 Central Whether Bureau Seismic Network catalog, we identify the similar events by the maximum waveform cross-correlation of higher than 0.7 in a time window of 10 sec after P-arrival. We identified 1979 similar events in the Chihshang area, which represnts 38% of all the ML≧2.0 in the starting catalog. These similar events are clustered in the north half of the Chihshang fault, at a depth range of 10-25 km. Location obtained from the differential times by wavefrom cross-correlation function show a large improvement in location accuracy. Median standard errors are reduced from 1025m to 27m in horizontal direction and from 925m to 30m in vertical direction. In the along-strike cross-section, the relocated similar events reveal a slip-parallel pattern, which is consistent with ~70° rake determined by focal mechanisms results. Consequently, the similar events identified in this study allow us to obtain accurate earthquake location. The relocated seismicity defines a SE-dipping, listric narrow fault zone containing alignment of hypocenters parallel to the fault slip direction.

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