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提出了一种以小震的记录作为格林函数来成像中强地震破裂增长的反演方法。该方法通过迭代法中同时拟合多台的P、S波形,估计了子事件相对格林函数事件的破裂时间和振幅。我们用小事件作为格林函数反演了发生在帕克菲尔德地区的2次中强地震的加速度图。第1次地震(M=4.6)于1993年11月14日发生在米德尔山,震源深度11km,在下一次帕克菲尔德主震的假定孕震区内。第2次地震(M=4.7)于1994年12月20日发生在米德尔山东南方向约6km处的圣安德烈斯断层上,震源深度9km。该断层段以前没有微震活动,1966年帕克菲尔德地震时也几乎没有同震滑动。这2次地震的反演结果相差很大。1993年地震的平均释放应力为50bar,分布在0.9km~2左右的复杂区域上。1994年地震的平均释放应力只有6bar,分布在一个面积为20km~2的近椭圆形的区域上。2次地震的破裂增长似乎都是间歇性的,破裂形状也相对复杂:此反演方法只要求破裂速度比S波速度要慢得多,并没有用平滑约束条件。
A method is proposed to record the growth of moderately strong earthquakes in the form of a small function of Green’s function. This method estimates the rupture time and amplitude of the relative Green’s function event of the sub-event by fitting P and S waveforms of multiple sets simultaneously in the iterative method. We used small events as Green’s function to retrieve the acceleration maps of two moderate-to-moderate earthquakes in Parkfield. The first earthquake (M = 4.6) occurred on Middel Mountain on November 14, 1993, with a focal depth of 11 km, within the assumed earthquake zone of the next Keystone mainshock. The second earthquake (M = 4.7) occurred on December 23, 1994 in the San Andres Fault about 6 km southeast of Middel Mountain with a focal depth of 9 km. There was no microseismic activity in the fault zone before, and there was almost no coseismic slip in the 1966 Parkfield earthquake. The inversion results of these two earthquakes vary greatly. The average release stress of the 1993 earthquake was 50 bar, distributed over a complex area of about 0.9 km -2. The average release stress of the 1994 earthquake was only 6 bar, distributed over an almost oval area of 20 km ~ 2. The rupture growth appears to be intermittent in both earthquakes and the rupture shape is relatively complicated: the inversion method only requires that the rupture rate be much slower than the S-wave velocity and no smoothing constraints are used.