The behaviour of fluids in the crust is key to understanding earthquake occurrence as fluids decrease fault strength. The attenuation of seismic waves may be locally high in fault zones as fluids are intensely distributed in these zones. This study uses a novel, simple approach to examine near-source attenuation in the focal region of intense swarm activity in the Yamagata-Fukushima border region, Japan, which is believed to be triggered by fluid movement following the 2011 Tohoku-Oki earthquake. Near-source attenuation was estimated by determining the decay of amplitude ratios of nearby earthquake pairs with travel time differences precisely quantified using a waveform correlation. In the initial ∼50 d, Q⁻¹ was high, then it significantly decreased to become almost constant for the subsequent period. This pattern is similar to those independently observed for background seismicity rate, b-value, stress drop, and fault strength. These patterns can be attributed to the hypothesis that the swarm was triggered by fluid movement following the 2011 Tohoku-Oki earthquake; the source and seismicity characteristics and the seismic attenuation were altogether affected by the temporal change in pore pressure. The method demonstrated in this study may be a useful tool to detect high pore pressure anomaly at depth and understand its relationship with earthquake occurrence.

中文翻译：

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在2011年东北冲积地震之后的强烈流体驱动地震活动中检测近源衰减的时间变化

随着流体降低断层强度，地壳中流体的行为是理解地震发生的关键。地震波的衰减在断层带可能会很高，因为流体强烈分布在这些断层带中。这项研究使用一种新颖，简单的方法来研究日本山形县-福岛县边境地区群热密集地区的近源衰减，据信这是2011年东北冲积地震之后的流体运动触发的。通过确定附近地震对的振幅比的衰减来估计近震源衰减，并利用波形相关性精确量化传播时间差。在初始〜50 d时，Q ^{− 1}很高，然后显着下降，在随后的时期几乎保持不变。这种模式类似于背景地震活动率，b值，应力降和断层强度的独立观测结果。这些模式可以归因于以下假设：2011年东北冲积地震后，群是由流体运动触发的。孔隙压力随时间的变化完全影响了震源，地震活动特征和地震衰减。这项研究中演示的方法可能是检测深处高孔隙压力异常并了解其与地震发生之间关系的有用工具。