The density and Q-factor models based on the new data on the nutations and overtones of the free oscillations of the earth: 1. The analysis of the new GSN data for the Sumatra, Tohoku, and Okhotsk earthquakes

Abstract

We discuss the problem of the ambiguity of gravity inversion, i.e., finding the depth distribution of density and the depth and frequency dependences of the Q-factor from the entire set of the present-day seismic and astrometric data on the travel times of seismic waves, the periods and attenuation factors of the free oscillations of the Earth, as well as the amplitudes and phases of the forced nutations. In the first part of the paper, we present the new and more accurate determinations of the periods and Q-factors for the fundamental tones and overtones of the spheroidal and toroidal oscillations of the Earth, which have periods longer than 3 min. These determinations are based on analyzing the signals from the Sumatra, Tohoku, and Okhotsk earthquakes of magnitude 9, which were recorded by the stations of the Global Seismographic Network (GSN) in Obninsk and Kazakhstan. It is shown that, although the Okhotsk earthquake had a lower magnitude than the other quakes analyzed (since its seismic source was extremely deep (about 600 km)), the amplitudes of the overtones excited by this event are significantly higher than the amplitudes of the overtones caused by the Sumatra and Tohoku events of magnitude 9. Moreover, the amplitudes of the overtones from the Okhotsk earthquake exceed the amplitudes of the overtones of the free oscillations caused by the other seismic events of magnitude 9 that occurred in the second half of the 20th century. Due to this, the data on the Okhotsk Sea earthquake are of utmost importance for the solution of the inverse problems of reconstructing the vertical profiles of Q-factor in the ultra-low frequency (ULF) range and for reconstructing the vertical distribution of density. Based on the new empirical data, we obtained new and more accurate estimates for the periods and attenuation factors of the free oscillations of the Earth.