Examination of interplate coupling in the southern Alaska subduction zone, based on the GPS data

We reexamined the interplate coupling in terms of backslip rate beneath the Kenai Peninsula in southern Alaska with a 2-dimensional GPS time series during three different time periods. Zweck et al., (2002) investigated the variation in coupling between the Pacific and North American plates, and they estimated interplate coupling as a scalar quantity. With this background, we estimate back-slip distribution on the plate interface as a vector by applying geodetic inversion method devised by Yabuki and Matsu'ura (1992) based on the GPS velocities data. We divided into three time periods, before 1997, 1998-2001 and after 2001. Freymueller et al (2000) suggested that a transient crustal deformation, slow slip event, occurred in this region during 1998-2001. The second time period coincides with a slow slip event on the plate interface. We inverted GPS velocities to infer the interplate coupling before 1998 and after 2001. Our results show that a maximum back slip rate of 60mm/year was found at the depth of 15-25km around 148.8W, 60.0N. According to results compiled by DeMets and Dixon (1999), convergence rate of the Pacific plate has a velocity at a rate of 57 mm/year for this region. This convergence rate suggests that east coast Kenai Peninsula region has an approximately full locking between the North American and subducting Pacific plate. In the contrast, forward slip appears north of the Kenai Peninsula during 1998-2001. By subtracting the averaged back-slip rate during before 1998 and after 2001, we extracted the abnormal deformation caused by slow slip event. Event has a northeastward direction (azimuth 165N). Slip at a 3-year average rate of up to 45mm/year was inferred north of the Kenai Peninsula (149.0W, 61.5N) at a depth of 25-45 km. Over 3-years the cumulative moment magnitude reaches Mw=7.2.