Determining fluid migration and pore pressure change within the Earth is key to understand earthquake occurrences. We investigated the spatiotemporal characteristics of the intense foreshocks and aftershocks of the 2017 M_L 5.3 earthquake in Kagoshima Bay, Kyushu, southern Japan, to examine the physical processes governing this earthquake sequence. Our relocated hypocenters show the foreshocks moved on a sharply defined plane with a steep dip. The mainshock rupture initiated at the edge of the foreshock seismic gap. The size of the foreshock seismic gap is comparable to that of the mainshock estimated from the source corner frequency, suggesting this seismic gap corresponds to the large slip region of the mainshock. The aftershocks migrated upward along several steeply dipped planes with a seismicity pattern that deviated from the typical mainshock–aftershock type. This deviation of seismicity pattern, together with the hypocenter migrations, suggests aseismic processes, such as pore pressure migration and aseismic slip, affected this earthquake sequence. We established the following hypothesis. First, fluids originating from the subducting slab migrated upward and intruded into the fault plane, reducing the fault strength and causing the foreshock sequence and potentially aseismic slip. Second, the mainshock rupture occurred due to the decreased fault strength and the increased shear stress in an area with relatively high strength. Third, pore pressure increase associated with post-failure fluid discharge caused the upward aftershock migration. These observations are consistent with the fault-valve model and show the importance of fluid movement at depth not only in earthquake swarms but also in foreshock–mainshock–aftershock sequences.

中文翻译：

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根据日本南部九州2017年鹿儿岛湾5.3级地震序列的强烈前震和余震估计的断流行为

确定地球内部的流体迁移和孔隙压力变化是了解地震发生的关键。我们调查了2017 M _L强烈前震和余震的时空特征5.3日本南部九州鹿儿岛湾的地震，以研究控制该地震序列的物理过程。我们重新定位的震源显示前震在陡峭倾斜的清晰平面上移动。主震破裂始于前震地震带的边缘。前震的地震缝隙大小与根据震源拐角频率估算的主震的地震缝大小相当，这表明该地震缝隙对应于主震的大滑动区域。余震沿着几个陡倾平面向上迁移，其地震活动模式与典型的主震—余震类型有所不同。地震活动模式的这种偏差以及震源的偏移表明地震过程（例如孔隙压力偏移和地震滑移）影响了该地震序列。我们建立了以下假设。首先，来自俯冲板的流体向上迁移并侵入断层平面，从而降低了断层强度，并导致前震序列和潜在的地震滑动。第二，主断裂破裂是由于断层强度降低和剪切应力增大而引起的，该区域具有较高的强度。第三，与故障后流体排放相关的孔隙压力增加导致了余震向上迁移。这些观察结果与断层阀模型一致，并显示了不仅在地震群中，而且在前震—主震—余震序列中，深部流体运动的重要性。降低断层强度，导致前震序列和潜在的地震滑动。第二，主断裂破裂是由于断层强度降低和剪切应力增大而引起的，该区域具有较高的强度。第三，与故障后流体排放相关的孔隙压力增加导致了余震向上迁移。这些观察结果与断层阀模型一致，并显示了不仅在地震群中，而且在前震—主震—余震序列中，深部流体运动的重要性。降低断层强度，导致前震序列和潜在的地震滑动。第二，主断裂破裂是由于断层强度降低和剪切应力增大而引起的，该区域具有较高的强度。第三，与故障后流体排放相关的孔隙压力增加导致了余震向上迁移。这些观察结果与断层阀模型一致，并显示了不仅在地震群中，而且在前震—主震—余震序列中，深部流体运动的重要性。与故障后流体排放相关的孔隙压力升高引起余震向上迁移。这些观察结果与断层阀模型一致，并显示了不仅在地震群中，而且在前震—主震—余震序列中，深部流体运动的重要性。与故障后流体排放相关的孔隙压力升高引起余震向上迁移。这些观察结果与断层阀模型一致，并显示了不仅在地震群中，而且在前震—主震—余震序列中，深部流体运动的重要性。