Historical Seismicity (1918-1964) of the Kodiak Island Region

Abstract

I have relocated 35 earthquakes and conducted waveform modeling studies of 12 events located primarily within the Kodiak portion of the 1964 great Alaska earthquake rupture zone and the northeasternmost portion of the 1938 Semidi earthquake rupture zone. These results show that there is considerable similarity between earthquakes occurring before the 1964 mainshock (pre-1964 mainshock seismicity) and after the 1964 mainshock (post-1964 mainshock seismicity). Persistent seismicity has occurred for the past ∼85 years at the southwestern end of the 1964 rupture zone, a region where Global Positioning System (gps)/geodesy studies indicate the plate interface is currently locked. Earthquakes also occurred frequently within the Kennedy Entrance region where the transition between the Kodiak and Kenai block of the subducting Pacific plate occurs. The seismic moment rate for post-1964 mainshock seismicity is about two times larger than for premainshock seismicity. This is in contrast to the Prince William Sound portion of the 1964 rupture zone where post-1964 mainshock moment rates are orders of magnitude lower than pre-1964 mainshock rates and the pattern of seismicity has changed considerably with time. These observations reinforce previous studies that indicate the behavior of plate subduction beneath the Kodiak Island region is much different from that beneath the Prince William Sound region.