DISTRIBUTION OF SEISMICITY ON THE MEGATHRUST: CHARACTERIZING THE SEISMOGENIC ZONE IN THE SHUMAGIN GAP, ALASKA WITH PRECISE EARTHQUAKE LOCATIONS

Abstract

The fortuitous location of the Shumagin Islands above the seismogenic zone in the Alaska-Aleutian subduction zone provides an ideal opportunity to study seismic waves directly from the megathrust interface. Double-difference relative relocation of hypocenters are performed on a catalog of earthquakes from the Shumagin Gap spanning nearly a decade. Relocation results show an abrupt transition in seismicity along the plate interface at 44 km depth, from a distinct 4 – 8 km thick plane to a broad zone of sparse seismicity. In the eastern area of the network, deeply rooted faults appear to cut into the downgoing plate, dipping steeply trenchward. Active faults within the upper plate seem to correlate with the strike of mapped normal faults and a splay fault imaged within an MCS reflection profile in the Eastern Sanak Basin. Further investigation of these splay faults may provide important information relating to tsunamigensis in the Shumagin Gap.