利用非线性台阵叠加方法快速追踪2015年4月25日尼泊尔M_W7.9地震破裂能量中心运动轨迹

Rapidly tracking the rupture energy center of the M_W7.9 Nepal earthquake by using nonlinear array stacking method

本文应用一种非线性台阵叠加方法,通过对地震记录瞬时相位信号的延迟叠加来估计信号间的一致性程度,用于在震后快速追踪大地震破裂能量中心相对于震中运动轨迹.基于此方法,文中利用欧洲地震台网不同频率段的数据对2015年4月25日尼泊尔M_W7.9地震的破裂轨迹进行了追踪,并对成像结果的不确定程度进行了估计,且通过理论地震图计算,验证了该结果的可靠性.研究结果表明,该事件为东南向发展的单侧破裂,破裂尺度至少达到120 km.整个破裂过程主要分为三个阶段,第一阶段破裂在震中附近释放小部分能量;第二阶段破裂向东南方向发展,表现出明显的沿断层倾向辐射频率变化的现象,相对更高频的能量辐射在更深处发生;第三阶段破裂继续向东南方向发展,不同频段得到的能量中心运动轨迹趋于一致,这可能提示了滑移范围的逐渐变窄.同时,推断在主喜马拉雅逆冲断裂的更深部(断层深度大于等于20 km处)也存在破裂的可能.

We apply a nonlinear array beamforming method , which estimates the coherence of the seismic signals by stacking their instantaneous phases, to rapidly trace the migration of rupture energy center relative to the earthquake＇s epicenter after a large earthquake occurred. And then, the slip amount distributed on the whole track is estimated by the linear stacking of waveforms. The broadband recordings from 204 European stations are collected and processed in both lower and higher frequency band respectively for the Nepal MwT. 9 earthquake which occurred on April 25, 2015. The rupture process of this earthquake, inferred from our final tracking result, is a unilateral rupture, extend to at least 120 km toward SEE direction. The whole process could be roughly divided into 3 stages. In the first stage, the rupture nucleated at the hypocenter and the energy released is small. In the following stage, the rupture started to propagate toward the southeast, with significant high frequency radiation energy concentrated on clown dip of the fault, and more low frequency on up dip. In the last stage, the rupture continued propagating to the southeast, and the departed tracks of the energy centers of different frequencies approached each other, which might imply that slip area becomes tapered before rupture stops. Base on the location of the energy center and the isolines of the Main Himalayan Thrust （MHT） faulting, the rupture area on the MHT might span to greater than 20 km in depths . To assess the resultrs reliability we estimate the uncertainty of energy center＇s location and calibrate it by the synthetic test. The assessments are also done in these two frquency bands accordingly. The array response function （ARF） is calculated to quantify the effect of the array covering on the resolution. The gridded distribution map for waveform semblance used in the method for a time window is also given to demonstrate the contraction of the uncertainty ellipse even for low frequency data. In the calibration test, we preset three point sources along the rupture direction, and build the synthetic seismograms by summing these three subevents, and apply the same processing procedures as the one for the observed data, the rupture tracing result of the synthetic recovers the three points successfully.