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最近发生的几次地震与洛杉矶盆地邻近,这为提取盆地对地震波传播的影响提供了机会。1987年10月4日发生的惠蒂尔峡谷地震序列中的一次余震(M_L=5.3)和1991年4月28日发生的谢拉马德雷主震(M_L=5.8)在盆地最深处各台站所构成的方位角相近。盆地各台站所得到10月4日余震记录有一显著特点,即S,SS等多重反射波振幅很大。切向分量上其振幅高达直达S波振幅的2倍。在小于25km的这样一个小范围内,需要一个水平方向的地震波速度梯度,使地震射线足够快地发生弯曲,以形成这些多重反射波。本文采用了一种正演模拟方法,用有限差分数字技术得到双力偶点源震源机制。以洛杉矶盆地东端一个近代地质横断面为依据的模型生成的震相比在地震记录中所看到的震相要多。而以顶部几层中低剪切速度的倾斜分层为依据的较简单的模型,与资料拟合得更好。为了符合直达P波、直达S波和第一个多重S波之间的到时差,要改变地震速度、深度、层的倾斜等输入参数。尽管第一个多重波的相位与资料不符,但直达S波和第一个多重S波初动脉冲的到时、振幅模拟得很好。除了在邻近盆地边缘的距离内,盆地模型中加进陡峭的西部边界,对合成波形没有多大影响。在盆地最深部分SS波的振幅是最大的,比直达S波大2倍。从东北到西南,尾波持续时间从8s增加到20多秒。谢拉马德雷主震发生在惠蒂尔峡谷地震序列的东北部25km左右。惠蒂尔峡谷的模型被扩展到这一距离范围,在惠蒂尔地震和谢拉马德里地震震中之间设置一浅盆地,以模拟圣加布里埃尔沉积盆地。虽然由深盆地边缘产生的震相仍然控制着合成波形,但该模型产生了一个长的尾波。这一研究表明:穿过深沉积盆地,频率达1Hz的若干特定震相可以用二维地震波速度模型来解释。
The recent occurrence of several earthquakes adjacent to the Los Angeles basin provides an opportunity to extract the impact of the basin on seismic wave propagation. One aftershock (M_L = 5.3) in the Huitier Canyon earthquake sequence on October 4, 1987 and the Sierra Ledere MST (M_L = 5.8) on April 28, 1991 at the stations in the deepest basin Constitute the azimuth similar. The aftershocks recorded on October 4 in each station of the basin have a remarkable characteristic that the amplitudes of multiple reflected waves such as S and SS are large. Tangential component of its amplitude up to 2 times the amplitude of direct S wave. Within such a small range of less than 25 km, a horizontal seismic velocity gradient is required to cause the seismic rays to warp fast enough to form these multiple reflected waves. In this paper, a forward modeling method is used to obtain the dual-dipole source source mechanism by finite-difference digital technique. The model based on a modern geological cross-section at the eastern end of the Los Angeles basin produces more earthquakes than is seen in the seismogram. The simpler model based on the sloping stratification of the top few layers with low shear rates fits better with the data. In order to match the arrival time of the direct P-wave, direct S-wave and the first multiple S-wave, input parameters such as seismic velocity, depth, layer tilt, etc., are to be changed. Although the phase of the first multiple wave is not consistent with the data, the amplitudes are well simulated at the arrival of the direct S wave and the first multiple S wave initial pulse. The addition of a steep western boundary to the basin model, except within the distance from the edge of the basin, has little effect on the resultant waveform. In the deepest part of the basin SS wave amplitude is the largest, two times larger than the direct S wave. From northeast to southwest, the coda duration increased from 8 s to more than 20 s. The Sierra Sela main shock occurred about 25 km northeast of the Whittiquai earthquake sequence. The Whittier canyon model was extended to this range of distances by setting up a shallow basin between the Whittier and Semaraderic epicenter to simulate the San Gabriel sedimentary basin. Although the phase produced by the edge of the deep basin still controls the composite waveform, the model produces a long coda wave. This study shows that a number of specific seismic phases passing through the deep sedimentary basin up to 1 Hz can be explained by a two-dimensional seismic velocity model.