Seasonal Seismicity in the Lake Biwa Region of Central Japan Moderately Modulated by Lake Water Load Change

Abstract:

Seasonal seismicity in Japan has been attributed to variations in atmospheric pressure, snow loading, 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) for a correlation with the annual hydrological cycle of nearby Lake Biwa. Seismicity rates for three fault zones along the BSFZ, the Katata fault, Hira fault, and Albano fault, show seasonal variations, suggesting a modulated effect from the changing lake level. The lake water storage is estimated using satellite altimetry data from 2002 to 2010. The data show an average seasonal lake level increase of ~1.1 m from December to May. Regional surface mass loading from soil moisture and snow is obtained from the Global Land Data Assimilation System (GLDAS) Noah model to augment the lake mass variation. We compute the changes in crustal stresses and stressing rates induced by 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 layered Earth structure. The results show a 1.2, 1.2, 0.8 kPa seasonal Coulomb stress cycle on the three fault segments. The stress changes are correlated to the local seismicity rate variations, particularly along the Hira and Albano faults. The seasonal variation of lake water storage produces ~60% of the total stress change along the BSFZ, while the snow and soil moisture represent 30% and <10% of total stress changes. Our analysis indicates that seasonal surface hydrological loads modulate the background seismicity rates in central Japan along the BSFZ.