2017年米林6.9级地震震源区速度结构与余震重定位

Imaging velocity structures and aftershock distributions in the source region of the 2017 Mainling M6.9 earthquake

利用西藏自治区林芝地区的固定地震台站与南迦巴瓦流动测震台站在2017年11月18日至2017年11月24日记录到的430个余震的直达波走时数据反演得到了震源区的三维P波速度、S波速度结构,并利用三维速度结构对余震进行了重定位.成像结果显示,米林地震震源区在0~5km深度内存在低地震波速度异常;在5~15km深度内,存在高地震波速度异常,该高速异常致使震源区西南侧的地震波速度高于东北侧.重定位结果中,余震呈条带状以NW-SE走向展布,震源深度具有西南方向深、东北方向浅的特征.主震位于11km深度处、高地震波速异常体顶部,余震主要分布在高地震波速度与低地震波速度过渡的区域.对成像结果的分析表明,震源区浅部的低速异常具有低泊松比的特性,与富石英的沉积变质杂岩体-东久杂岩单元的岩性特征有关;深部的速度结构特征则可能反映了发震断层上盘地震波速度高,下盘地震波速度低的介质特性.余震重定位结果与成像结果联合表明:此次地震发震断层从11km深度处,东久杂岩体下方的高地震波速度异常顶部开始破裂,继而在5~15km深度内发生后续破裂,后续破裂的发生区域正处于喜马拉雅构造单元与冈底斯构造单元接触的形变区内.此外,根据地震波速度计算的泊松比反映了震源区持续的低泊松比特征,暗示此次地震与流体活动并无直接关系.

This work imaged the P-wave and S-wave velocity structures in the source region of 2017 Mainling,Tibet earthquake by inverting arrival times of 430 aftershocks observed at seismic stations in the Linzhi area and temporary stations around Namche Barwa during November 18 th to 24 th,2017.Then the aftershocks are relocated using these 3D velocity structures.Results show low-velocity and high-velocity anomalies at depths 0~5 km depths and 5~15 km,respectively.The latter makes the seismic velocity in the southwest is higher than that in the northeast of the source.The relocated aftershocks are distributed in NW-SE direction,deeper in the southwest and shallower in the northeast.The main shock is at depth 11 km,the top of a high-velocity anomaly body,while the aftershocks are located in a transition zone between high- and low-velocity anomalies.Analysis suggests that the initial rupture of this event started from the top of a high-velocity area below the Dongjiu complex at depth of 11 km,and subsequently the rupture continued in a dramatically deformed area caused by Himalayan unit and Gandisê unit at 5~15 km depths.Additionally,there is no fluid activity involved as implied by the persistent low-Poisson′s ratio derived from calculation of seismic velocities.