Source mechanism of February 4, 1965, Rat Island earthquake

The Rat Island earthquake of February 4, 1965 (origin time 05h 01m 21.8s, h = 40 km), is one of the largest earthquakes recorded in recent years. On the basis of the radiation patterns and the amplitudes of the great circle Rayleigh and Love waves, the earthquake is found to have the following characteristics: fault plane dip, 18ø; fault plane dip direction, N19øE; rupture propagation direction, N51øW; rupture propagation velocity, 4.0 km/sec; faul't length, 500 km; moment, 1.4 X 10 •ø dynes cm; stress drop, 30 bars; and average dislocation, 2.5 meters. It is interesting to observe that the attitude of the fault plane and the general features of this earthquake are similar to those of the 1963 Kurile earthquake (Kanamori, 1970a) and the 1964 Alaskan earthquake (Kanamori, 1970.b). The first 35 see of P waves were also well recorded at many stations. The wave forms suggest a multiple-event nature of the earthquake for at least the initial 35 sec. By using the relative location method, the events were loca.t.ed progressively south of the initial hypocenter. It seems plausible that the earthquake started off at depth, first propagated southward, and then westward along the Aleutian arc. Owing to the long-period nature of the surface waves, only the average feature of the fault is seen, and P waves reveal some of the detailed initial behavior.