The Seasonal Deformation Characteristics And The Adjustment Mechanism Of Hydrological Load On Seismicity In Nepal

The modulation of seasonal hydrological load on seismicity is currently a hot topic in the world.However,the interaction between seasonal non-tectonics and tectonic deformation along the Himalayan plate boundary is still controversial.Studying the modulation effect of seasonal hydrological load on seismicity is of great significance to clarify the relationship between hydrological load and seismicity.Nepal is located in the Mediterranean-Himalayan-South Asia earthquake zone.where seismic activity and plate tectonic activity are frequent.the special geographical location not only causes the characteristics of the terrain in Nepal to be higher in the north,lower in the south,but also a unique monsoon climate.In Nepal,there are many GPS continuously operating reference sites and accumulating a large amount of original observation data,which also provides data support for the study of surface deformation and plate tectonic movement in the region.In this paper,some stations with short observation time and poor data quality are eliminated first,and 38 uniformly distributed stations with good data quality in Nepal are selected.Partially missing data are interpolated and offset corrected.The seasonal signals in the time series of these GPS stations were extracted by the method of least squares fitting,and the seasonal 3D displacement field information of each station was obtained according to the seasonal displacement model.At the same time,in order to study the change of rainfall in Nepal,using the TRMM meteorological satellite data from 2000 to 2018,the changes and distribution characteristics of different seasons and different months in the annual cycle of Nepal were obtained.The results of comparative analysis show that the seasonal variation of rainfall in Nepal is significant,and the vertical displacement of the measuring station is significantly correlated with the seasonal rainfall area.Seasonal rainfall may be the main reason for the seasonal variation of the vertical displacement of the surface.By using the horizontal seasonal displacement to calculate the seasonal changes of the surface strain in this area,it is found that there is a correlation between the surface strain and the hydrologic load.In July,when the monthly average rainfall is the highest,the surface strain contraction is the maximum,and it is inferred that the hydrologic load is the main driving factor for the surface seasonal deformation.Furthermore,the surface strain was converted into coulomb stress on a vertical fault parallel to the Main Frontal Thrust(MFT).The surface strain and coulomb stress were consistent in phase and relative amplitude,and the seasonal variation of coulomb stress was related to surface deformation caused by hydrological load.For the seismic catalogue of Nepal in the past 40 years,three methods including Gardner&Knopoff,Uhrhammer and Gruenthal were used to remove the aftershock cluster,so as to reduce the impact of aftershocks related to the Main shock on the seismic catalogue analysis.The results show that there are more earthquakes in Nepal in the winter than in the summer monsoon,based on the prediction of seasonal displacement,stress strain and rainfall distribution,the rainfall in summer monsoon period disturbs the long-term movement trend of the fault and releases the coulomb stress to a certain extent.Furthermore,it can suppress the seismic activity and affect the time of the earthquake.