Seasonally hydrological load and background seismicity in the intraplate collision zone, the Longmen Shan, China

Hydrological modulation of background seismicity provides a way to understand the seismogenic process and interaction mechanism between the hydrosphere and lithosphere. To elucidate the hydrological modulation of background seismicity in the Longmen Shan, the monthly spatio-temporal distribution of terrestrial water storage (TWS) variations is inverted from the seasonal term of continuous GNSS (Global Navigation Satellite System) data and compared with monthly precipitation. In the Longmen Shan, monthly TWS variations are in phase with precipitation variations in the northern area but lag precipitation variation by 2 months in the southern area. The annual amplitude of the inverted TWS is about 0.2 m, which is consistent with the amplitude of precipitation. To investigate the stress perturbations in the crustal interior induced by the hydrological load, monthly Coulomb failure stress (CFS) variation caused by TWS and precipitation load is calculated on a simplified fault model of the Longmen Shan, respectively. The maximum CFS variations of TWS and precipitation are about 2.4 kPa. To obtain the background seismicity, different earthquake declustering methods are employed on a two-decade earthquake catalogue (M >= 2.5). The declustered results are subjected to null hypothesis tests, including assessing whether the earthquake distribution follows a homogeneous Poisson distribution in time or exhibits a temporally homogeneous, spatially heterogeneous Poisson distribution in space and time. The only declustered catalogue derived by the epidemic type aftershock sequence model pass all the tests. Furthermore, the monthly CFS variation and its rate caused by TWS and precipitation are compared with the monthly background seismic events. Notably, a correlation is observed between CFS variation rate due to precipitation and background seismicity. This correlation suggests the presence of long-term hydrological modulation of seasonal background seismicity in the Longmen Shan.