利用連續全球衛星定位系統與地震波反演探討集集地震震後地殼變形與甲仙地震同震變形

Abstract

連續全球衛星定位系統觀測結果顯示，相對於澎湖白沙測站(S01R)蘭嶼至海岸山脈間的速度場約為40.5 - 93.6 公厘/年，方位角為307o - 333o；中央山脈與花東縱谷的測站之速度場約為19.0 - 49.3公厘/年，方位角為285o - 318o；至於西部褶皺逆衝代間之測站速度場為14.2 - 45.5公厘/年方位角為284o - 304o。中大型地震對於應變率之影響可經由連續全球衛星定位系統觀測而獲得，關於集集地震(1999, 規模=7.6)後地表位移變化明顯而旋轉變化不大，相對的成功地震(2003, 規模=6.8)後區域性的變化僅在主震附近有較大的旋轉率變化，而更次要的地震，例如: 2005, 規模=5.9宜蘭雙主震及規模=5.6的花蓮地震都僅在區域有重要的影響但對全台灣並無明顯的影響。經由聯合反演重新研究並解算2010年於台灣西南部發生的甲仙地震資料(連續全球衛星定位系統、遠震站及強震站)發現該地震主要由一北311°東 傾角33° 朝東北的逆衝含左移斷層控制指向與旗山轉換帶(CTFZ)有關，另一方面次要的斷層面滑動顯示為一個北20°東的逆衝斷層此為龍船斷層與龍船背斜一帶之構造活動。在甲仙地震時西北-東南的逆衝斷層和應力反演均顯示地震時最大水平應力壓縮方向為東北-西南向此與台灣主要的西北-東南向壓縮方向非常不同。

With respect to Paisha station, S01R, the stations of Coastal Range to Lanyu showed an average displacement of 40.5 - 93.6 mm/yr towards azimuth 307o - 333o. The stations in the Central Range and Longitudinal Valley revealed velocities in the range 19.0 - 49.3 mm/yr towards azimuths 285o - 318o. In western Taiwan, the velocities in the inner fold-and-thrust belt range from 14.2 to 45.5 mm/yr in directions 284o - 304o. Large and medium size earthquakes affect the strain rate pattern revealed by CGPS, albeit in different ways: regional extension and displacement were large and rotations rate were small regarding the Mw=7.6, ChiChi earthquake. In contrast, limited size of affected area, moderate displacement and large rotations rate characterized the 2003, Mw=6.8 Chengkung earthquake. The impact of smaller earthquakes such as the 2005, Mw=5.9 Ilan double earthquakes and the 2005, Mw=5.6 Hualien earthquake was locally significant but regionally minor. A new interpretation is proposed for the 2010 Jiashian earthquake (Mw 6.4) which occurred on March 4, 2010 in southwestern Taiwan. We obtain the slip distribution on a two-faults model by the joint inversion of all three datasets (CGPS, teleseismic, and strong-motion). It corresponds to a N311°E fault dipping 33° to the NE, with left-lateral – reverse oblique slip, likely related to the Chishan Transfer Fault Zone (CTFZ). We find that the earthquake started with a sharp slip patch localized at the hypocenter. The secondary rupture plane is a N20°E thrust fault related to the Lungchuan anticline. Faulting on the main NW-SE plane during the Jiashian earthquake and stress inversions from previous studies both suggest a direction of maximum horizontal compression oriented NE-SW in the area of the earthquake, very different from the dominant NW-SE compression direction in Taiwan.