Observations of Normal Modes from 84 Recordings of the Alaskan Earthquake of 1964 March 28

Abstract

We have taken 84 World Wide Standard Seismograph Network recordings of the Alaskan earthquake of 1964 March 28, one of the largest earthquakes in history, and have made an intensive search for the Earth's eigenperiods in their spectra. We have found 61 spheroidal overtones and nine toroidal overtones as well as all but two fundamental modes with angular order less than 46. We have used such theoretical discriminants as polarization, dissipation, group velocity, multiplet width and continuity as a function of total angular order to separate interfering multiplets. Preliminary interpretation of the overtone eigenfrequencies leads us to models of the Earth's core where either the density decreases with depth in the inner core, leaving modes 2S2 and 5S2 with unacceptably large residuals, or there is finite rigidity with a shear velocity of about 3·5 km s−1, and all residuals are acceptably small. We favour the latter interpretation as being the more satisfactory. The average eigenperiod for each multiplet has a relative precision (standard error of the mean divided by the mean) ranging from 1·4 × 10−5 for 6S1, a high Q multiplet, to about 10−3 for the higher frequency toroidal modes, 0T1. The high precision obtained for many high Q overtones, together with their dual ray interpretation as PKIKP, PKJKP, SKIKS, SKJKS. etc. lends support to the interpretation that the inner core is solid.