A large slow slip event and the depth of the seismogenic zone in the south central Alaska subduction zone

We modeled crustal deformation before, during and after a slow slip event (SSE) on the plate interface in southern Alaska. We divided the GPS time series into three time periods: before 1998, 1998 to 2001, and 2001-2002; the first and last periods represent the steady deformation, while the middle period represents the SSE. We found a stationary zone of ∼ 100% slip-deficit in the shallow part of the seismogenic zone, to depths of 25-30 km, which we identify with the main asperity that ruptured in 1964 earthquake. The SSE occurs downdip of the asperity, on a section that accumulated some slip deficit during the steady time periods. During the SSE, > 20,000 sq. km of the plate interface slipped > 10 cm, for a cumulative moment magnitude of M_w = 7.2. The area of the SSE did not have a significant slip in the 1964 earthquake, compared to the slip of the main asperity. We propose that between earthquakes this zone fails repeatedly in SSEs and remains at relatively low shear stress compared to the main asperity up-dip of it.