[S-SS06] Statistical seismology and underlying physical processes

[SSS06-P08] Seasonal variations in crustal seismicity and surface load

*Taku Ueda¹, Aitaro Kato¹ (1.Earthquake Research Institute, the University of Tokyo)

It has been reported that seismicity rate correlates with phenomena cause temporal changes of stress and fault strength at the surface and shallow subsurface, such as precipitation and irrigation (e.g., Heki, 2003: Amos et al., 2014). For example, in California, previous studies have indicated that seismicity rate varies seasonally in response to stress changes due to annual variations in hydrological loading (e.g., Amos et al., 2014; Johnson et al., 2017). Johnson et al. (2017) calculated the stress change which hydrological loading imposes on the fault plane based on GPS vertical displacement. They showed that seismicity in California is more likely to occur when shear stress increases. The seasonal variations in seismicity in Japan have also been pointed out, and its relationship to snow loading and precipitation has been discussed as the plausible cause (e.g., Heki, 2003; Ueda and Kato, 2019). However, few studies have directly compared the seasonal variations in seismicity with the stress changes brought by surface loading.

In this study, we estimate the spatial and temporal distribution of surface load from the vertical displacement data in the inland Tohoku region and investigate the relationship between the stress change by the surface load and the timing of earthquake occurrence. First, we extract the seasonal variation component from the vertical displacement of the GEONET F5 solution (2017/1/29-2020/5/2) by excluding transient changes such as earthquakes and postseismic deformation using the method proposed by Bedford and Bevis (2018). Then, we calculate the spatial and temporal distribution of surface load using the extracted seasonal variation components according to Johnson et al. (2017). Then, we compute the stress changes at the earthquake occurrence using the observed focal mechanism to quantitatively evaluate the relationship between earthquake occurrence and seasonal stress changes.