THE RUPTURE PROCESS AND TECTONIC IMPLICATIONS OF THE GREAT 1964 PRINCE-WILLIAM-SOUND EARTHQUAKE

We have determined the rupture history of the March 28, 1964, Prince W illiam Sound earthquake (M(w) = 9.2) from long-period WWSSN P-wave sei smograms. Source time functions determined from the long-period P wave s indicate two major pulses of moment release. The first and largest m oment pulse has a duration of approximately 100 seconds with a relativ ely smooth onset which reaches a peak moment release rate at about 75 seconds into the rupture. The second smaller pulse of moment release s tarts at approximately 160 seconds after the origin time and has a dur ation of roughly 40 seconds. Because of the large size of this event a nd thus a deficiency of on-scale, digitizable P-wave seismograms, it i s impossible to uniquely invert for the location of moment release. Ho wever, if we assume a rupture direction based on the aftershock distri bution and the results of surface wave directivity studies we are able to locate the spatial distribution of moment along the length of the fault. The first moment pulse most likely initiated near the epicenter at the northeastern down-dip edge of the aftershock area and then spr ead over the fault surface in a semi-circular fashion until the full w idth of the fault was activated. The rupture then extended toward the southwest approximately 300 km (RUFF and KANAMORI, 1983). The second m oment pulse was located in the vicinity of Kodiak Island, starting at approximately 500 km southwest of the epicenter and ''tending to about 600 km. Although the aftershock area extends southwest past the secon d moment pulse by at least 100 km, the moment release remained low. We interpret the 1964 Prince William Sound earthquake as a multiple aspe rity rupture with a very large dominant asperity in the epicentral reg ion and a second major, but smaller, asperity in the Kodiak Island reg ion. The zone that ruptured in the 1964 earthquake is segmented into t wo regions corresponding to the two regions of concentrated moment rel ease. Historical earthquake data suggest that these segments behaved i ndependently during previous events. The Kodiak Island region appears to rupture more frequently with previous events occurring in 1900, 185 4, 1844, and 1792. In contrast, the Prince William Sound region has mu ch longer recurrence intervals on the order of 400-1000 years.