2018年斐济MW8.2深震:高频辐射过程和发生原因

2018年8月19日，在斐济东部海域563 km深处发生了一次M_W8.2地震.我们首先挑选位于美国阿拉斯加地区的131个宽频带台站构成台阵，选用垂直分量0.5~2 Hz的高频信号，利用广义台阵反投影技术对这次地震的破裂过程进行了成像，然后基于破裂速度对地震的辐射效率进行了估计.结果表明，这次地震总体上呈单侧破裂，破裂方位在3.0°左右，破裂总长度约51 km，持续时间22 s，平均破裂速度为2.5 km·s^-1.但能量释放有2次高峰，形成两次子事件.第一次为前10 s，峰值在7 s左右，破裂速度为2.9 km·s^-1，辐射效率为45%.第二次为10~22 s，峰值在15 s左右，破裂速度为1.6 km·s^-1，辐射效率为26%.结合震源位置、震源机制、破裂速度以及辐射效率，我们认为这次地震是由于俯冲板块前缘受到下部地幔物质上浮阻力引起的剪切失稳所致，起初板块内部的脆性破裂表现突出，致使辐射效率较高，后来震源处高温高压下的熔融耗散特征逐渐凸现，致使辐射效率下降.

2018 MW8.2 Fiji deep earthquake: high-frequency radiation process and seismic genesis

On August 19, 2018, an M_W8.2 earthquake occurred at a depth of 563 km under the eastern coast of the Fiji island. We select 131 stations from the broadband seismograph network in Alaska, USA, to configure an array and image the rupture process of this event by means of the back-projection technique of generalized array using the vertical components of high-frequency (0.5~2 Hz) signals. The image indicates that this event is a unilateral rupture propagating in azimuth of approximate 3.0°. The rupture is approximately 51 km long with duration time 22 s, resulting in an average rupture velocity of 2.5 km·s^-1. However, the process of the energy release is characterized by two peaks, so two subevents are identified. The first one occurred in the first 10 s, with a peak at around 7 s, during which the rupture velocity was 2.9 km·s^-1 and the radiation efficiency was 45%. On contrast, the second took place in the period of 10~22 s, with a peak at around 15 s, during which the rupture velocity and the radiation efficiency were 1.6 km·s^-1 and 26%, respectively. Combining the hypocenter location, focal mechanism solutions, rupture speed and the radiation efficiency, we suggest that this event was resulted from a shear instability due to the upward floating of the lower mantle substance against the frontier of the subducting block. At the beginning, brittle crack dominated, producing a higher radiation efficiency. However, the following rupture under high temperature and pressure was accompanied with melting, resulting in decease of radiation efficiency.