A seismotectonic study of the Southeastern Alaska Region

Abstract

We compare relocations of recent (1973–2005) and historic (1919–1972) earthquakes to geologic and geophysical (gravity, aeromagnetic, and uplift) information to determine the relationship of seismicity to crustal deformation in southeastern Alaska. Our results suggest that along strike changes in the structure of the Pacific plate may control the location of the ends of rupture zones for large earthquakes along the offshore Queen Charlotte fault system in the southern portion of the study area. There is a marked increase in background seismicity in the northern portion of the study area where the Fairweather fault begins to bend toward the northwest and crustal uplift due to glacial unloading exceeds 20 mm/year. Focal mechanisms indicate that thrust and reverse mechanisms predominate in the region of maximum uplift, as might be expected by the decrease in ice sheet thickness. The diffuse nature of seismicity between the Fairweather and Denali faults in the northern study area suggests a complex interaction between plate/microplate interactions and glacial unloading, making it difficult to determine the optimal fault orientation for failure in moderate magnitude (5.5 to 6.5) earthquakes within this region.