2020年M_(W)7.7加勒比海地震:反投影成像确定的一次超剪切事件

2020 M_(W)7.7 Caribbean Sea earthquake:A supershear event revealed by teleseismic P wave back-projection method

利用反投影方法,使用美国阿拉斯加台网的远场P波垂直分量地震数据,对2020年1月28日M_(W)7.7加勒比海地震的破裂过程分两个频段(0.2~1.0 Hz,0.5~2.0 Hz)进行成像.结果显示,地震沿西向单侧近线性破裂,破裂规模约为250 km,破裂时长约为98 s,平均破裂速率约为2.55 km·s^(-1).破裂的瞬时速率存在明显波动,在大约53~62 s达到约6 km·s^(-1),超过了所在深度的S波速度,为超剪切破裂.地震在5~15 s,21~30 s和66~79 s存在三个高频能量释放峰值,相应时段破裂速率较低,破裂能量得以在局部区域集中释放.破裂一直延伸至大洋中脊附近受阻挡而停止,转换断层几乎完全破裂,释放了加勒比板块和北美板块之间由于相对运动而长期积累的应力.发震断层特殊的空间几何特征和长期积累的应变能,为此次超剪切破裂事件的发生提供了断层几何和动力条件.

In this paper,vertical component teleseismic P wave data recorded by networks in Alaska,USArray are used to image the rupture process of January 28th,2020 M_(W)7.7 Caribbean Sea earthquake via back-projection analysis.Data in two frequency band(0.2~1.0 Hz and 0.5~2.0 Hz)are used in the imaging process.The results show that the earthquake ruptures unilaterally and linearly to the west.The rupture length is around 250 km and rupture duration is about 98 s result in an average speed of~2.55 km·s^(-1).The instantaneous speed fluctuates obviously and reached~6 km·s^(-1)at 53~62 s,exceed the S wave velocity in this area.The high frequency energy release of this earthquake mainly has three peaks at 5~15 s,21~30 s and 66~79 s,which correspond to the three stages with very low rupture speed,in which the rupture propagation almost stagnates so that the rupture energy release is locally concentrated.The rupture extends to the end of the transform fault near the mid-ocean ridge,then terminated by the blockage of the mid-ocean ridge.The transform fault almost ruptured completely,releasing long-accumulated stresses due to relative motion between the Caribbean and North American plates.The special spatial geometry characteristics and long-term accumulated strain energy of the seismic fault provide geometrical and dynamic conditions for the occurrence of the supershear rupture.