Relation of seismic rate changes to the timing of ice mass fluctuations and surface temperature variations

In the region of southern Alaska that includes the Malaspina and Bering Glacier systems, large ice fluctuations occur directly above a shallow main thrust zone associated with subduction of the Pacific-Yakutat plate beneath continental Alaska. We have used recent glacier elevation change studies [Muskett et al., 2008, 2009] and GRACE mascon-derived estimates of glacier mass fluctuations [Luthcke et al., 2008] to estimate ice mass variations from 1988 - 2007 for this region. Also, we calculated surface displacement rates, incremental stresses, and change in the fault stability margin of the solid Earth associated with these ice mass variations. In 2002-2006, for instance, the predicted displacement rates of the solid Earth due to average annual changes in ice loads were up to 20 mm/yr for the vertical and 3 mm/yr for the horizontal. To empirically evaluate the influence of short-term ice fluctuations on fault stability, we compared the seismic rate from a reference background time period against other time periods with variable ice or tectonic change characteristics. After AEIC removed probable icequakes, the seismic rate change for the time periods of 1993-1996 and 1997-2001 relative to 1988-1992 was not statistically significant and the tectonic events did not show a strong seasonal dependence. However, we found that the frequency of earthquakes increased significantly in the 2002-2006 time interval relative to the reference time period of 1988-1992. For most months the regional temperatures in 2002-2006 were several degrees warmer than the longer-term average monthly temperatures but recently average monthly temperatures are cooler again. During 2002-2006 more earthquakes occurred in the late summer and fall than during other seasons. Although temperatures are cooler in the fall than during the peak summer temperatures in July, the 2002-2007 GRACE derived estimates of mass change in this region suggest that the maximum ice loss occurs in July but continues through October.