AN OUT-OF-SEQUENCE THRUST FAULT NEAR THE EPICENTER OF THE 1964 GREAT ALASKA EARTHQUAKE (MW 9.2) AND ITS POTENTIAL AS A SOURCE FOR LOCAL TSUNAMIS

An active out-of-sequence thrust fault, evident in deep-crustal seismic-reflection data collected in Prince William Sound, splays upward from the top of the subducted Yakutat terrane and extends upward to as shallow as 10 km depth. Earthquakes hypocenters cluster along the fault plane, indicating that the fault is active. A simple linear projection of the fault plane toward the surface indicates that the fault would outcrop along the strike of the Patton Bay fault. This fault, along with the nearby Hanning Bay fault, exhibited the only fault-related surface breakage during the 1964 earthquake. Also, the projected surface outcrop of the thrust fault lies just offshore of active thrust faults exposed on Hinchinbrook Island (G.A. Carver, oral commun., 2006), indicating that the deep fault extends to the surface. This out-of-sequence thrust fault is similar to one described from the Nankai Trough and Kii Peninsula, Japan (e.g. Park et al., 2002, Science, p. 1157-1160). The fault in Japan is a suspected source for local tsunamis.

Concerning the 1964 Alaska earthquake, the regional coseismic displacement field, including the Patton Bay fault, was previously determined through the joint inversion of geodetic and tsunami-waveform data (Johnson et al., 1996). Our tsunami modeling builds on these results by incorporating accurate local bathymetry and the deep-fault geometry evident in MCS data. We modeled coseismic displacement and wave propagation first with and then without the out-of-sequence thrust fault to show that movement along the thrust fault generates short-wavelength tsunami waves in comparison to the long wavelengths making up the regional tsunami wavefield. In addition, the part of the thrust fault that extends beneath the deep-water channel between Montague and Hinchinbrook Islands is more effective at tsunami generation than the part of the fault located to the southwest, which is beneath shallow water or on land.

GPS data from Prince William Sound indicates that forearc subsidence following the 1964 earthquake has been concentrated in two areas, instead of being a single large locked region on the plate interface. Because the gap between the two subsidence centers corresponds in location with the edge of the subducted Yakutat terrane, this edge may affect seismogenesis along the subduction interface.