2015年尼泊尔MS8.1地震震源区S波三维速度结构与强震发生机理研究

利用2001-2003年期间在2015年4月12日尼泊尔M_S8.1级强震震源区流动地震观测记录到的连续波形数据，提取了5~25 s周期的瑞利波相速度频散曲线，并构建了尼泊尔地震震源区二维瑞利波相速度分布图像.以0.5°×0.5°为网格大小将研究区网格化，采用NA算法反演得到尼泊尔地震震源地区三维S波速度结构.结果显示，在上地壳，以主前锋逆冲断裂带（MFT）为界，其以北地区为高波速异常，而其以南为明显低波速异常；在中地壳，以藏南拆离系（STDS）为界，南北两侧速度结构也存在明显差别，以南地区为明显高波速异常，而以北地区为明显低波速异常.这些结构特征说明，印度板块与欧亚板块碰撞挤压作用形成地幔热物质上涌并造成地壳物质部分熔融，并由此形成了东西向拉张的南北向裂谷.2015年尼泊尔M_S8.1级主震和最大余震均发生于高低波速异常过渡区且偏向高波速异常区，暗示了这样的波速异常区易于积累能量孕育强震.主震和最大余震的南侧均存在明显的低波速异常，与主喜马拉雅滑脱断裂带（MHT）相对应，可能代表部分熔融或深部流体作用于主边界断裂带（MBT）附近的MHT断裂带，降低断层面上的有效正应力，从而触发尼泊尔强震及最大余震的发生.主震与最大余震之间的余震分布于高低波速异常变化较为明显的地区，说明研究区内地震的发生受震源区附近的速度结构控制.

3-D S-wave velocity structure around the 2015 MS8.1 Nepal earthquake source areas and strong earthquake mechanism

On 25 April 2015, a strong M_S8.1 earthquake occurred in Nepal. The USGS determined that its epicenter is located at 28.2°N, 84.7°E, and its focal depth is around 8.2 km. Due to a huge number of injuries and property damages, understanding the mechanism of such a strong earthquake is of great significance to mitigate the seismic hazard. Therefore, using many dense seismic stations from several portable seismic networks and a permanent seismic station LSA in and around the Nepal earthquake source areas, a high-resolution S-wave velocity structure of the crust is inferred through the ambient noise tomography to reveal the mechanism of this strong earthquake.Continuous waveform data are collected from temporal and permanent stations in and around the Nepal earthquake source areas from 2001 to 2003, and the Empirical Green's functions (EGFs) of Rayleigh wave are inferred using the cross-correlation technique. Dispersion curves of Rayleigh wave phase velocity at the periods from 5 to 25 s are measured from radial waveforms (with a signal-to-noise ratio greater than 5) between station pairs as many as possible. Finally, a high-resolution S-wave velocity structure is determined from the dispersion curves of Rayleigh wave phase velocity with the tomography technique.Our results show that, in the upper crust, the Main Frontal Thrusting (MFT) fault zone is an obvious boundary, to the north of which exist obvious high-velocity anomalies, whereas to the south of which exist prominent low-velocity anomalies. In the mid-crust, the Southern Tibet Detachment System is roughly a clear transition zone, to the south of which exist high-velocity anomalies, whereas to the north of which exist low-velocity anomalies. These structural features indicate that the Indian-Asian collision resulted in the hot material upwelling of the upper mantle that leads to partial melting in the crust, which may explain the formation of the north-south trending rifts. The 2015 M_S8.1 Nepal mainshock and its largest aftershock are located in the low-to-high velocity transition zone and bias towards high-velocity anomalies, which may suggest that such areas may be easier to accumulate the energy to generate the strong earthquakes. Obvious low-velocity anomalies to the south of the Nepal mainshock and its largest aftershock, which correspond to the Main Himalayan Thrust (MHT), may indicate the existence of partial melting or fluids around the MHT. Such influences could decrease the effective normal stress on the fault planes of the MHT, which might lead to trigger the mainshock and its largest aftershock. The aftershocks between the mainshock and largest aftershock occurred on the transition zone of high-to-low velocity anomalies. These results indicate that the earthquake generation is controlled by velocity structures around the source areas.