安宁河—则木河断裂带和大凉山断裂带孕震深度研究及其地震危险性

Seismogenic depths of the Anninghe-Zemuhe and Daliangshan fault zones and their seismic hazards

安宁河—则木河断裂带及东侧的大凉山断裂带作为大凉山次级块体西侧与东侧边界,具有发生大地震的活动构造背景.本文意在用有限的形变数据和地震数据两种资料评估大凉山次级块体边界断裂带的孕震深度及其地震危险性.采用弹性半空间模型对安宁河断裂、则木河断裂和大凉山断裂带滑动速率和闭锁深度进行了详细分析;计算了90%、95%和99%不同分位数的小震深度下界值并与GPS得到的闭锁深度进行对比,分析二者异同点.结果显示,安宁河断裂北段闭锁深度为6.2 km,不到90%分位小震震源深度16 km的一半,表明该段在1952年M S63/4地震后,断层逐渐趋于闭锁;而在6~16 km深度主要以小地震和无震滑动两种形式释放能量,存在深部蠕滑运动.大凉山断裂北段在0~10 km范围内完全闭锁,而10~25 km闭锁程度较弱.安宁河断裂南段、则木河断裂、大凉山断裂中段和南段均处于完全闭锁阶段,闭锁深度接近90%分位数小震深度的下界值,标准差约为0.94 km.此外,A、B、C三个剖面的反演结果表明大凉山次级块体的运动自北向南具有顺时针旋转特性,与川滇块体顺时针运动特征吻合.大凉山次级块体北、中、南三段边界断裂及块体内部总的滑动速率分别为9.8 mm·a^-1、8.9 mm·a^-1和8.4 mm·a^-1,呈自北向南递减趋势.大凉山断裂南段布拖断裂和交际河断裂积累的能量分别能够发生一次矩震级为M W7.5的地震,离逝时间已经接近地震平均复发间隔,未来100年大地震的发震概率分别为7.1%和5.9%,应对其地震危险性给予重视.

The Anninghe-Zemuhe and Daliangshan fault zones are the western and eastern boundaries of the Daliangshan sub-block,respectively,both of which lie in a tectonic setting with potential major earthquakes.The purpose of this work is to estimate the seismogenic depths and seismic risks of these two fault zones relying on limited geodetic and seismic data.Firstly,we use a simple 2D elastic dislocation model to infer the slip rates and the locking depths along these fault zones.We calculate the 90%,95% and 99% seismic cutoff depths and compare with those from GPS data.The results show that the locking depth of the north segment of the Anninghe fault is 6.2 km,less than half of 16 km for its 90% seismic cutoff depths.It implies that after the 1952 M S63/4 earthquake,this segment gradually tended to become locked.While within a depth range of 6~16 km,energy was released by seismic and aseismic slip,where exists deep creep.The north segment of the Daliangshan fault is completely locked within a depth range of 0~10 km,while the locking within 10~25 km is relatively weaker.The south segment of the Anninghe fault,the Zemuhe fault,middle and south of the Daliangshan fault are completely locked,of which the locking depths are close to the 90% cutoff depths of small events with a standard deviation of 0.94 km.In addition,inversion of profiles A,B and C reveals that the movement of the Daliangshan sub-block from north to south exhibits clockwise rotation,consistent with the movement feature of the Sichuan-Yunnan block.The slip rates of the north,middle,and south segments of the Daliangshan sub-block are 9.8 mm·a^-1,8.9 mm·a^-1 and 8.4 mm·a^-1,respectively,decreasing from north to south.The accumulated seismic moments of the Butuo fault and Jiaojihe fault,which lie in the south section of the Daliangshan fault,are capable to generate maximum magnitudes of M W7.5 earthquakes,respectively.The elapsed times of the last events of these two faults are close to their earthquake recurrence intervals,and their earthquake occurrence probabilities are estimated to be 7.1% and 5.9%,respectively,which should receive much attention.