Crustal pore pressure, which controls the activities of earthquakes and volcanoes, varies in response to rainfall. The status of pore pressure can be inferred from observed changes in seismic velocity. In this study, we investigate the response of crustal pore pressure to rainfall in southwestern Japan based on time series of seismic velocity derived from ambient noise seismic interferometry. To consider the heterogeneity of the area, rainfall and seismic velocity obtained at each location were directly compared. We used a band-pass filter to distinguish the rainfall variability from sea level and atmospheric pressure, and then calculated the cross-correlation between rainfall and variations in S-wave velocity (Vs). A mostly negative correlation between rainfall and Vs changes indicates groundwater recharge by rainfall, which increases pore pressure. The correlations differ between locations, where most of the observation stations with clear negative cross-correlations were located in areas of granite. On the other hand, we could not observe clear correlations in steep mountain areas, possibly because water flows through river without percolation. This finding suggests that geographical features contribute to the imprint of rainfall on deep formation pore pressure. We further modelled pore pressure change due to rainfall based on diffusion mechanism. A strong negative correlation between pore pressure estimated from rainfall and Vs indicates that the Vs variations are triggered by pore pressure diffusion in the deep formation. Our modelling results show a spatial variation of diffusion parameter which controls the pore pressure in deep formation. By linking the variations in seismic velocity and crustal pore pressure spatially, this study shows that seismic monitoring may be useful in evaluating earthquake triggering processes or volcanic activity.

中文翻译：

---

基于地震速度监测的降雨对地壳孔隙压力的时空影响

控制地震和火山活动的地壳孔隙压力随降雨而变化。孔隙压力的状态可以从观测到的地震速度变化中推断出来。在这项研究中，我们基于从环境噪声地震干涉测量法得出的地震速度时间序列，研究了日本西南部地壳孔隙压力对降雨的响应。为了考虑该地区的异质性，直接比较了在每个位置获得的降雨量和地震速度。我们使用带通滤波器从海平面和大气压力中区分降雨变化，然后计算降雨与 S 波速度 (Vs) 变化之间的互相关。降雨量和 Vs 变化之间的大部分负相关表明降雨补给地下水，这会增加孔隙压力。位置之间的相关性不同，其中大多数具有明显负互相关性的观测站位于花岗岩区域。另一方面，在陡峭的山区，我们无法观察到明显的相关性，可能是因为水在没有渗透的情况下流过河流。这一发现表明，地理特征有助于降雨对深层地层孔隙压力的影响。我们进一步模拟了基于扩散机制的降雨引起的孔隙压力变化。由降雨估算的孔隙压力与 Vs 之间的强负相关表明 Vs 变化是由深层地层中的孔隙压力扩散引发的。我们的建模结果显示了控制深部孔隙压力的扩散参数的空间变化。