Seasonal Seismicity Variations in the Southcentral Alaska Subduction Zone and Its Relationship With Surface Hydrological Mass Changes

We analyzed seismicity variations in the Southcentral Alaska subduction zone and found clear seasonal seismic variability, indicating more earthquakes during the winter season and less earthquakes during the summer season. Only earthquakes deeper than 30 km are analyzed to exclude seismicity due to volcanic activity and shallow faulting activity away from the subduction system. We hypothesize that the seasonal seismicity variations reflect the annual surface hydrological mass oscillations in Alaska. During the fall and winter months, strong precipitation, including accumulated snow and ice, results in gravity increases observable by GRACE and crustal subsidence measured by GPS. The surface hydrological loading causes stress changes on the underthrusting plate interface and on faults in the overriding and subducting plates, and leads to resolvable seismicity rate changes. Our first-order model indicates that the surface load produces positive Coulomb stress change and encourages failure for intra-slab normal faults. A recent seismicity study has shown that nearly all the earthquakes along the 1964 rupture zone and below appear to be normal-faulting events indicating down-slab tension. The increased seismicity rate in southcentral Alaska during the winter months can be explained by a scenario of slip-encouraging stress conditions. To further confirm the result, we will model the time-variable Coulomb stress changes on the plate interface and on faults in the surrounding lithosphere due to the seasonal surface hydrological mass change. The deformation modeling efforts rely on surface mass-load changes constrained from crustal deformation observed by GPS, hydrological mass changes measured by the Gravity Recovery and Climate Experiment (GRACE) and results from the Global Land Data Assimilation System (GLDAS). The spatiotemporal stress changes are compared to seismicity rate variations in southcentral Alaska.