Slow Slip Events and rotation of the Peninsula block in Lower Cook Inlet of the Alaska-Aleutian Subduction Zone

Abstract:

We identified a series of abrupt changes in GPS site motions observed in Lower Cook Inlet of the Alaska-Aleutian subduction zone in late 2004, early 2010 and late 2011. The site motions during each interval (1992-2004, 2004-2010 and post-2011) appear to be steady. The deformation rates for 1992-2004 and 2010-2011 are similar to each other, as are 2004-2010 and post-2011. This pattern of toggling between two deformation patterns is due to time-dependent slip on the Alaska-Aleutian subduction plate interface. We modeled slow slip events in the seismogenic zone using the software TDEFNODE to estimate the slip rate deficit distribution on the Alaska-Aleutian subduction plate interface. Then we applied three different measures of the significance of the velocity change to estimate their timing in ~2010 and ~2011. Based on the estimated dates of velocity changes, we divided the GPS time series into the four time periods. Then a weighted non-linear least-squares inversion was applied to estimate the angular velocity of the Peninsula block and estimate the plate coupling variation simultaneously using TDEFNODE (McCaffrey, 2009). Fixing the angular velocity of all the blocks in the block model, we estimated the slip deficit rate distribution on the Alaska thrust for the four time periods with two different limiting ranges of the locking fraction (phi): [-2, 1] and [0, 1]. Negative slip deficit rates identified in period 1 and period 3 fit the data significantly better, which indicate that slip rates are faster than plate convergence rates during these two time periods. And our results suggest that a slow slip event in Lower Cook Inlet lasted at least 9 years (given that our data begin in 1995) with a moment magnitude of Mw = 7.5. Another slow slip event in the same area lasted almost 2 years from the end of 2009 to the end of 2011, with a cumulative moment magnitude of Mw = 6.8. We conclude that slip rates appear to be relatively constant during the time periods with SSEs, but relatively abrupt changes in slip rates between time periods with and without SSEs.