Depth Dependent Focal Mechanisms of Volcanic Deep Low‐Frequency Earthquakes in Northeast Japan
Depth Dependent Focal Mechanisms of Volcanic Deep Low‐Frequency Earthquakes in Northeast Japan
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DOI：
10.1029/2021jb022666
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发表时间：
2021-11
期刊：
影响因子：
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通讯作者：
G. Oikawa;N. Aso;J. Nakajima
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文献类型：
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作者：
G. Oikawa;N. Aso;J. Nakajima
日本东北部的深部低频地震（LFE）发生在活火山下方 10-40 公里深处。这些火山 LFE 辐射低频地震波，尽管震级较低 (M < 2)，但大部分能量为 2-8 Hz。尽管许多先前的研究已经获得了具有非双偶分量的各种震源机制，并提出了与岩浆相关的物理过程，但普遍的物理过程却知之甚少。因此，我们利用S/P振幅比综合确定了日本东北部26个火山区的264个LFE的震源机制。许多获得的解决方案都具有较大的双耦合分量和较小的补偿线性矢量偶极子分量。这种震源机制可以通过弯曲断层上的剪切滑移来解释。我们观察到，在 15-25 公里深度处，零轴的倾角小至 20-30°，而在 25-35 公里深度处，零轴倾角变得更大且变化更大。我们解释说，地壳中部的区域应力场相对均匀，而在莫霍面附近，由于热应力或与潜在岩浆过程相关的其他影响，区域应力场可能会发生变化。此外，在定量研究了2008年岩手宫城地震和2011年东北地震中静应力变化可能引发LFE的情况后，我们得出结论，部分地区大地震引起的大幅应力变化可能引发了LFE。
Deep low‐frequency earthquakes (LFEs) in Northeast Japan occur beneath active volcanoes at depths of 10–40 km. These volcanic LFEs radiate low‐frequency seismic waves, with most energy at 2–8 Hz, despite their low magnitudes (M < 2). Although many previous studies have obtained various focal mechanisms with non‐double‐couple components and suggested physical processes related to magma, the universal physical process is poorly understood. Therefore, we comprehensively determined the focal mechanisms of 264 LFEs for 26 volcanic regions in Northeast Japan using S/P amplitude ratios. Many of the obtained solutions have large double‐couple components with small compensated linear vector dipole components. Such source mechanisms can be explained by shear slip on the bending fault. We observed that the plunge of the null axis is as small as 20–30º at depths of 15–25 km, whereas it becomes larger and more varied at depths of 25–35 km. We interpreted that the regional stress field is relatively homogeneous in the middle of the crust, whereas it may be altered near the Moho discontinuity due to thermal stress or other effects related to potential magmatic processes. In addition, after quantitatively investigating the possible triggering of LFEs due to static stress changes in both the 2008 Iwate‐Miyagi earthquake and the 2011 Tohoku earthquake, we concluded that LFEs might have been triggered by substantial stress changes caused by large earthquakes in some regions.