Hydrospheric modulation of stress and seismicity on shallow faults in southern Alaska

Shallow (≤40 km), low magnitude (M ≥ 2.0) seismicity in southern Alaska is examined for seasonal variations during the annual hydrological cycle. The seismicity is declustered with a spatio-temporal epidemic type aftershock sequence model. The removal of aftershock sequences allows detailed investigation of seismicity rate changes as water, snow and ice loads modulate crustal stresses throughout the year. The GRACE surface loads are obtained from the JPL global land and ocean mass concentration blocks (mascon) solutions. The stress changes at a depth of 10 km are calculated using a 1D spherical layered Earth model. To evaluate the induced seasonal stresses of ∼10 kPa, we use >30 yr of earthquake focal mechanisms to constrain the tectonic background stress field orientation and assess the seasonal stress change with respect to the principal stress orientations. The background stress field is assumed to control the preferred orientation of faulting, and stress perturbations are expected to increase or decrease seismic activity on the faults. The number of excess earthquakes is calculated with respect to the background seismicity rates for discrete stress intervals. The results indicate a ∼25% increase in regional seismicity rates that correlate with a ∼3-month time lag following failure-encouraging annual meannormal-stress, differential stress, and least principal compressive stress. No immediate earthquake rate variations are observed in this region-wide analysis. The correlation with a 3-month time lag suggests increased mobility of preexisting fluids at seismogenic depths is varying the pore pressure within fault zones to modulate the seismicity rates throughout the seasonal loading cycle.