2007 年寮國地震之發震構造特性與區域構造活動關聯研究

Abstract

接近 70 年無大地震的寮國北部,在 2007 年 5 月 16 日,發生了一起 Mw 6.3的地震,以及長達兩個月的群震。在低度開發、無地震測站亦無大地水準測量資料,同時甚難到達的該區域,關於這些地震的資料十分稀少,地震定位解析度也很差,然而在該區域鄰近的緬甸、泰國也在此地震後發生了 Mw 分別為 6.8、6.1的大地震,顯示此區域存在著活躍的活動斷層。前人已依歷史地震及構造地形特性繪製出泰國-寮國-緬甸境內的活動斷層圖,因此我們知道在寮國北部的大地構造型態主要是發育東北—西南走向的斷層,根據 USGS、Global CMT、ANSS 的地震定位及中國雲南地震觀測局與越南地震局的未發表地震資料,本研究推斷 2007 年5 月 16 日的寮國地震應發生在湄善斷層(Mae Chan fault)上,且震後於同斷層上的餘震活動約持續了兩個月。為了研究寮國北部的構造活動特性,本研究利用 ALOS衛星(Advanced Land Observing Satellite)在 2007 年的記錄的的 PALSAR 1.0 (Phased Array type L-band Synthetic Aperture Radar 1.0)影像,製作2007/2/17-7/5 與2007/2/17-8/20 兩幅干涉影像對,並分析此干涉圖的結果,將同震地表變形訊號以Okada 彈性半空間錯位正演模型,模擬出造成該地表變形的地下斷層錯動模式,再分析此斷層模型錯動的庫倫應力轉移,來解釋 2007 年寮國地震及其群震所指示的構造特性。根據本研究的模擬,2007 年 5 月 16 日寮國地震或可由長 18 公里、寬約 8 公里,帄均錯動量為 0.7 公尺的左剪斷層破裂所引發,由模型估算的滑移潛能為1.008×107 m3,計算出來的地震矩規模為 6.31,與全球地震網的觀測值相當。模擬出來的地表變形投影在衛星視線方向的量值也近似於干涉結果,約為 5 cm。而由正推模型計算的震後庫倫應力結果顯示,同震變形發生的主要斷層段在震後庫倫應力值下降;震後庫倫應力值上升的區域分別在主震發生之斷層段的兩側,而餘震集中在其西側應力上升的斷層段上;其東側庫倫應力上升,卻幾乎無餘震活動,可能暗示了該處大地震發生的機率提高,值得投入更多相關研究,探討其地震周期與斷層活動性質。

Large earthquakes are often accompanied by noticeable surface deformations and damages. In cases where surface ruptures are visible and field investigations are feasible, detailed information about the co-seismic deformations is generally obtained in the field. However, in cases where field evidence for surface deformations are difficult to delineate either due to smaller magnitude of the events, deeper hypocenters, or inaccessibility of the earthquake area, remote sensing observations may provide information about the co-seismic deformations. This study focuses on the 16 May 2007 Mw 6.3 earthquake that occurred in northwestern Laos where information from GPS networks or seismic stations is scarce. The event also occurred in an area which is nearly inaccessible. Therefore, SAR interferometry is a feasible solution in an attempt to understand the co-seismic deformation pattern of the event. In this study, Phased Array type L-band Synthetic Aperture Radar 1.0 (PALSAR 1.0) images of Advanced Land Observing Satellite (ALOS) were used, and they were analyzed by using the Differential Synthetic Aperture Radar interferometry (D-InSAR) method on the GMTSAR software. Two co-seismic pairs were analyzed, 2007/2/17-7/5 and 2007/2/17-8/20, in order to obtain better constraint for the co-seismic deformation patterns. In this study, a model for the subsurface fault slip was constructed from the InSAR results. The earthquake may have occurred on the Mae Chang fault, one of a series of left-lateral faults in the region. The main deformation is induced by an 18 km long, ~8 km wide fault patch with the rupture top is at ~ 4 km of and a 0.7m average slip. The attitude of the fault patch is approximately (N54E, 89N). The co-seismic deformation signal is quite apparent on both interferograms. However, the signal is ~15-20 km away from the epicenter locations of most global earthquake catalogues, and the depth of the epicenter is also different by ~10 km. This implies the global catalogues may have large errors in this region due to poor local constraints. The obtained model, nonetheless, is consistent with tectonic geomorphological observations of the area and the focal mechanism from the Global CMT catalogue. Ultimately, the fault parameters suggested from the forward model were applied to calculate coulomb stress transfer, in an attempt to explain the relationship of the main shock and the distribution of the aftershocks.