Joint focal mechanism determination with source-region station corrections using short-period body-wave amplitude data
Joint focal mechanism determination with source-region station corrections using short-period body-wave amplitude data
复制标题
使用短周期体波振幅数据进行源区站校正的联合震源机制确定
DOI：
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发表时间：
1999
期刊：
影响因子：
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通讯作者：
A. Hasegawa
中科院分区：
文献类型：
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作者：
Ayako Nakamura;S. Horiuchi;A. Hasegawa
本文提出了一种通过P波、SH波、SV波振幅资料和P波极性资料的反演，同时求出多个小地震震源机制解和短周期体波振幅的源区台站改正的方法。观测到的地震波形除包含震源辐射图外，还包含场地放大和沿其射线路径沿着衰减的影响。频率高于几赫兹的地震图的振幅对地表附近的非均匀结构极其敏感。因此，我们需要详细了解场地放大和衰减的影响，以便利用短周期波形来估计震源机制。然而，目前，我们还不知道详细的地壳结构，其分辨率是进行这种估计所必需的。在目前的研究中，我们假设P波和S波的衰减因子沿着射线路径从震源到每个台站可以表示为震源距离，后方位角和入射角的函数。在假设上，利用P波、SH波、SV波振幅和P波极性，同时确定了多个地震的震源机制解和各台站的震源机制函数中的系数。我们应用本方法对1996年发生在日本东北部中部的大神户地震（M5.9）的170次余震进行了分析。我们获得了许多单独用P波极性资料不能确定震源机制解的微震的震源机制解。几乎所有的震源机制解的P轴都是水平的，并且是东西向的。平均而言，T轴在最浅的深度接近垂直。当深度接近5km时，T轴变为水平，然后逐渐再次变为接近垂直。
Abstract We have developed a method that simultaneously determines focal mechanism solutions of many small earthquakes and source-region station corrections for short-period body-wave amplitudes by inverting amplitude data of P, SH , and SV waves, together with P -wave polarity data. The observed seismic waveform includes the effects of site amplification and attenuation along its ray path in addition to the radiation pattern of earthquake source. The amplitude of seismograms at frequencies higher than a few Hertz is extremely sensitive to heterogeneous structure near the ground surface. Consequently, we need to know in detail the effects of site amplification and attenuation in order to estimate focal mechanisms by using short-period waveforms. However, at present, we do not know the detailed crustal structure with a resolution necessary for this estimation. In the present study, we assume that P - and S -wave attenuation factors along ray paths from hypocenters to each station can be expressed as a function of hypocentral distance, backazimuth, and incident angle. Based on this assumption, we determined focal mechanism solutions of many earthquakes and the coefficients in the function for each station simultaneously, by using P -, SH -, and SV -wave amplitudes and P -wave polarities. We applied the present method to 170 aftershocks of the 1996 Onikobe earthquake ( M 5.9), which occurred in the central part of northeastern Japan. We obtained focal mechanism solutions of many microearthquakes whose mechanism solutions could not be determined by using P -wave polarity data alone. P axes of almost all the obtained focal mechanism solutions are horizontal and oriented in the east-west direction. T axes are, on average, near vertical at the shallowest depth. As the depth approaches 5 km, the T axes become horizontal and then gradually become near vertical again.