Stress Promotion of the 1958 M-w similar to 7.8 Fairweather Fault Earthquake and Others in Southeast Alaska by Glacial Isostatic Adjustment and Inter-earthquake Stress Transfer

摘要

We assess how recent glacial isostatic adjustment (GIA) and inter-earthquake stress transfer modulated the state of stress on major faults in Southeast Alaska, and how these effects may have influenced recent large and moderate earthquakes. The Glacier Bay Icefield has lost >3,000 km(3) of ice since ca. 1770, with ice thinning up to 1.5 km, and the rest of southeast Alaska has been deglaciating since ca. 1900. The resulting GIA response in the solid earth produces present-day surface uplift rates of up to similar to 4 cm/yr, among the fastest on Earth. This rapid deformation occurs directly alongside and atop the Fairweather Fault, which accommodates 4-5 cm/yr of right-lateral motion through slip in frequent large earthquakes, recently including a M-w similar to 7.8 earthquake in 1958. We find that the 1958 earthquake nucleated very close to the location of maximum GIA-induced Coulomb stress increase on the Fairweather Fault between 1770 and 1958 (estimated at similar to 0.2-0.6 MPa), on the only section of fault where GIA-induced stress changes had involved both right-lateral shearing and unclamping. The September 10, 1899 M-w similar to 8.1 Yakutat Bay earthquake also promoted failure on at least the northwest part of the eventual 1958 rupture. In turn, the 1899 and 1958 earthquakes and GIA promoted failure along the St. Elias compressional margin, the site of a M-w similar to 7.4 event in 1979. While rapid tectonic loading is very likely the main driver of earthquakes in southeast Alaska, we estimate that 23 of 30 instrumentally constrained M-w >= 5.0 earthquakes in this region were also promoted by post-1770 GIA.