THE 1957 GREAT ALEUTIAN EARTHQUAKE

The 9 March 1957 Aleutian earthquake has been estimated as the third l argest earthquake this century and has the longest aftershock zone of any earthquake ever recorded-1200 km. However, due to a lack of high-q uality seismic data, the actual source parameters for this earthquake have been poorly determined. We have examined all the available wavefo rm data to determine the seismic moment, rupture area, and slip distri bution. These data include body, surface and tsunami waves. Using body waves, we have estimated the duration of significant moment release a s 4 min. From surface wave analysis, we have determined that significa nt moment release occurred only in the western half of the aftershock zone and that the best estimate for the seismic moment is 50-100 x 10( 20) Nm. Using the tsunami waveforms, we estimated the source area of t he 1957 tsunami by backward propagation. The tsunami source area is sm aller than the aftershock zone and is about 850 km long. This does not include the Unalaska Island area in the eastern end of the aftershock zone, making this area a possible seismic gap and a possible site of a future large or great earthquake. We also inverted the tsunami wavef orms for the slip distribution. Slip on the 1957 rupture zone was high est in the western half near the epicenter. Little slip occurred in th e eastern half. The moment is estimated as 88 x 10(20) Nm, or M(w) = 8 .6, making it the seventh largest earthquake during the period 1900 to 1993. We also compare the 1957 earthquake to the 1986 Andreanof Islan ds earthquake, which occurred within a segment of the 1957 rupture are a. The 1986 earthquake represents a rerupturing of the major 1957 aspe rity.