Seasonal Seismicity in the Lake Biwa Region of Central Japan Moderately Modulated by Lake Water Storage Changes

Seasonal seismicity in Japan has been observed and attributed to variations induced by hydrological loading, as well as atmospheric and pore-fluid pressure. We examine the seasonal variations of crustal (<15 km depth) and low magnitude (M < 4.5) earthquakes in central Japan near the Biwako-Seigan Fault Zone (BSFZ), and analyze their correlation with the annual hydrological cycle of nearby Lake Biwa, the largest freshwater lake in Japan. The 2002-2018 lake water storage is estimated using water-level gauge and satellite altimetry data. Regional surface mass loading from soil moisture and snow predicted by the Global Land Data Assimilation System is used to augment the lake mass variation. We compute the stress and stressing-rate changes induced by the surface mass loading from lake storage, snow, and soil moisture at seismogenic depth (10 km) along the BSFZ using a finite element model incorporating a 3D Earth structure. The results show that seasonal surface mass loading generates 1-5 kPa peak-to-peak Coulomb stress change and 2.5-15 kPa/year stressing-rate variations on the fault segments along the BSFZ. Two of the four fault segments present significant correlation between local seismicity rate and loading-derived stress and/or stressing rate changes. Among the hydrological load components, lake water, snow, and soil moisture exhibit different annual phases, and the lake water storage dominates the total annual stress changes. Our analysis indicates that seasonal surface hydrological loads, especially lake water variations, modulate the seismicity rate along the BSFZ in central Japan. Plain Language Summary The mass changes of water and snow on Earth's surface could be one of the explanations for observed seasonal variations in seismicity in Japan. We modeled this seasonal hydrological loading induced stress changes in the Biwa Lake region and fault segments of the Biwak-Seigan Fault zone in central Japan. We calculate the seasonal stress change and stressing rate from different hydrological components, including lake water, soil moisture, and snow. We find the hydrological load induced stressing rate can be as large as 15 kPa/year, and it exhibits a good correlation with seismicity rate for two fault segments in the Biwako-Seigan Fault Zone (BSFZ). Furthermore, the geometry of faults also plays an important role on phase and magnitude of surface load induced stress changes. Among the hydrological load components, lake water, snow, and soil moisture exhibit different annual phases, and the lake water storage dominates the total annual stress changes. Our results indicate that seasonal hydrological loads could modulate the stress state and seismicity rate in BSFZ of central Japan.

Automated summary

The study shows that seasonal seismicity in central Japan is influenced by hydrological loads, especially variations in lake water storage, which dominate the seasonal stress changes. The research emphasizes the importance of understanding the impact of hydrological processes on seismic activity in the region.