2012年北苏门答腊西海域M_W8.6地震的孕震机理及其对地震活动性的影响

Seismogenic Mechanism of the 2012 North Sumatra off West Coast Earthquake and Its Influences on Seismicity

摘要:

2012年4月11日在北苏门答腊的西部海域附近发生了MW8.6特大地震和2小时后的MW8.2强烈地震,这是到目前为止震级最大的板块内部走滑型地震。本文根据该地区的地质构造、地球物理观测资料及研究结果,对这次地震的孕育背景进行了分析,认为本次地震尽管发生于印度洋板块的沃顿海盆内(巽它海沟西南),但该地区断层发育、中强地震活跃、变形情况复杂,特别是印度洋板块在向NW方向运动时速度由南向北逐渐减小,这种差异运动导致在板块内部产生剪切应力,因此在板块内部近SN向的断层及附近区域不断积累弹性应变能;同时2004年的MW9.2地震产生的库仑应力变化对该次地震造成了应力加载,起到了触发作用,最终导致了本次特大左旋走滑型地震的发生。此外库仑应力变化计算表明,MW8.6地震在MW8.2地震的震源区附近的库仑应力增大,增加值超过0.05MPa,因此对MW8.2地震起到了触发作用。计算结果进一步显示,该次地震对其周围陆地影响较小,苏门答腊岛处于地震影响区中,库仑应力降低了0.03MPa,发生强余震的风险下降。主震后的余震活动的空间分布与库仑应力增加区一致。本次MW8.6特大地震后,全球范围内的M≥5.5地震活动性出现了大幅度的上升,而该现象在本世纪以来其他5次M≥8.5的强震之后并未出现;同样,该次特大地震对中国大陆的地震活动性也有一定的影响,震后中国大陆地区M〉4.5地震活动水平明显增高。

Abstract:

On April 11,2012,a large earthquake(MW8.6) and by far the largest strike-slip event ever recorded,struck the west coast of north Sumatra and was followed 2 h later by an MW 8.2 earthquake.A number of issues remain unclear such as the reason for the unusually large strike-slip earthquakes occurring in the intra-oceanic lithosphere,correlations between these two earthquakes,and the manner in which they influence seismicity.Therefore,this paper analyzes the nucleation background of the MW8.6 earthquake according to the region＇s geological environmentand geophysical observation results.Although the earthquake occurred in the Wharton Basin southwest of the Sunda Trench in the Indian Ocean,the strike-slip faults are well developed with complex deformation and frequent seismic activity.In the movement toward northwest of the Indian