Along-Strike and Down-Dip Variations in Subduction Zone Slip Deficit: Persistent or Transient? (Invited)

The pattern of elastic deformation at subduction zones depends on the along-strike and down-dip variation in the slip deficit on the plate interface. The location and magnitude of contractional strain is controlled mainly by the down-dip transition from locked (high slip rate deficit) to creeping (low slip rate deficit) behavior of the interface. In southern Alaska, there are dramatic along-strike variations in the depth of this transition, including segments where the locked region is more than 150-200 km wide and at least one segment that may be creeping continuously at all depths. Is this pattern is persistent over time, or does it vary substantially with time? The along-strike variations appear to be persistent over the geodetic record (only 10-15 years), and also with the record of slip in great subduction earthquakes (decades to centuries). In contrast, variations in the position of the downdip transition are observed over short timescales. A large slow-slip events in the upper Cook Inlet region occurred from 1998-2001, during which part of the interface that previously had been locked began to creep in slow slip. More recently, the opposite situation has occurred in lower Cook Inlet, near the town of Homer. Since approximately 2004, part of the interface that had been creeping has locked, making the locked zone wider than before (but still narrower than in the neighboring segments along strike). Other changes in the pattern of deformation here may have occurred in the mid-1990s, based on sparse data. Perhaps the period mid-1990s through 2004 represented a very long slow slip event, or perhaps the frictional behavior of the interface at the down-dip end of the locked zone is very sensitive to small stress changes.