Seismicity Studies of the Wadati-Benioff Zone, South-Central Alaska

We have relocated over 7900 earthquakes occurring between 1964-1999 at depths > 25 km in the Anchorage, Alaska region using the Double Difference earthquake relocation technique in order to determine localized variations in the structure of the subduction zone and its state of stress. We do this through examination of hypocenters, determination of focal mechanisms from first motion data for individual events, and inversion of first motion data for stress field orientation for clusters of events. We also relate our results to locations of M > 5.5 earthquakes occurring prior to 1971. Our results suggest deformation related to the projected edge of the downgoing Yakutat block as shown by seismicity in the southwest portion of our study area. In addition, focal mechanisms show consistency with observed linear patterns in cross-sections of seismicity suggesting high angle reverse faulting in the lower plate. A series of magnitude 5.0-5.5 events occurring near Anchorage may be associated with these features. Stress inversion results are consistent with results from prior studies showing downdip extension.