Normal-mode splitting observations from the great 1994 Bolivia and Kuril Islands earthquakes: constraints on the structure of the mantle and inner core

On June 9, 1994, a magnitude 8.3 earthquake struck ~650 km below Earth's surface in Bolivia. Four months later, on October 4, a second large earthquake of similar magnitude occurred >60 km below the Kuril Islands. Both events were recorded by more than 80 digital instruments distributed around the globe. An amplitude spectrum of a time series recorded after a big earthquake contains hundreds of easily identificable resonance peaks. Each resonance peak corresponds to a particular normal mode or free oscillation of Earth and contains information about its density and its elastic and anelastic structure. The effect of Earth's rotation, ellipticity, and lateral heterogeneity is to distort the shapes of the resonance peaks; this phenomenon is referred to as splitting. The details of the splitting of a given resonance peak are determined by the mode's sensitivity as a function of depth. By analyzing the fine structure of a large number of resonance peaks, global seismologists attempt to improve our knowledge about Earth's three-dimensional structure. -from Author