The time constant variations of slow slip events in the south Alaska subduction zone

Slow Slip events (SSEs), episodic periods of slip on the plate interface that are very slow in comparison to earthquakes, but much faster steady plate motions, have been identified at several subduction zones. One characteristic shared by all of these events is that they occur near or downdip of the base of the seismogenic zone as defined by the slip pattern of great earthquakes. A large slow slip event occurred at the Alaska subduction zone during 1998-2001 [Ohta et al., 2006, EPSL]. The SSE occurs downdip of the Prince William Sound asperity, on a section that accumulated some slip deficit before and after the SSE. During the SSE, >20,000 sq. km of the plate interfae slipped > 10cm, for a cumulative moment magnitude of Mw=7.2. Slip during this event was accompanied by significant non-volcanic tremor, which located in the same area as the slip. The area of the SSE did not have significant slip in the 1964 earthquake, compared to the slip of the main asperity. Our previous work did not attempt to determine the time history of slip in the SSE, because of a lack of continuous GPS data, but a reanalysis of the data that includes a few new sites allows us to estimate the approximate time history. Freymueller and Ohta (2007) found the short-term abnormal displacement in the south Alaska region during 2002-2006. The end of 2005 event may have the biggest event in the short-term abnormal displacements. We assumed the abnormal deformation as short-term SSE at the plate interface. The slip of short-term event reaches a maximum around 12-14 mm at plate boundary depths of 25-45 km, which corresponded to the transition zone of the plate interface. The short-term event fault located the neighboring of the long-term one during 1998-2001 investigated by Ohta et al., (2006.EPSL). The SSEs may play key role of strain accumulation in subduction zone. We will more precisely compare the displacement pattern and time history of the large SSE to the displacements to evaluate these smaller events actually represent SSEs on the same part of the plate interface.