Slow slip events and time-dependent variations in locking beneath Lower Cook Inlet of the Alaska-Aleutian subduction zone

We identify a series of abrupt changes in GPS site velocities in Lower Cook Inlet, Alaska, in late 2004, early 2010, and late 2011. The site motions during each time period are nearly linear. The surface deformations inferred from GPS for pre-2004 and 2010-2011 are similar to each other, as are 2004-2010 and post-2011. We estimate the slip distribution on the Alaska-Aleutian subduction plate interface accounting for upper plate block rotations and interpret this toggling between two deformation patterns as caused by transient slip. We find that by allowing negative slip deficit rates (i.e., creep rates in excess of relative plate motion), the data in Lower Cook Inlet are fit significantly better during pre-2004 and 2010-2011, suggesting the occurrence of slow slip events (SSEs) there during those time periods. The earlier SSE lasted at least 9years (observations in that area began in 1995) with M-w similar to 7.8. The latter SSE had almost the same area as the earlier one and a duration of similar to 2years with M-w similar to 7.2. During 2004-2010 and post-2011, the inversions result in only positive slip deficit rates (i.e., locking) in Lower Cook Inlet. Slip rates are nearly constant during the Lower Cook Inlet SSEs, and the events start and stop abruptly. Both of these properties contrast with observations of SSEs in Upper Cook Inlet and elsewhere. The Lower Cook Inlet SSEs are consistent with previously proposed duration-magnitude scaling laws and demonstrate that slow slip events can last as long as a decade.