Comparative Analysis of the Free Oscillations Generated by the Sumatra– Andaman Islands 2004 and the Chile 1960 Earthquakes

Abstract

Abstract The Chile 1960 and Sumatra–Andaman Islands 2004 earthquakes are among the three greatest events that have occurred since the instrumental recording of seismic waves. The two events have similarities in hypocentral depth and focal mechanism but are very different when considering the extension of the fault plane. The recent event has close to 50% greater length than the Chile event, and although the moment magnitude of the Chile event is 0.2 to 0.5 points greater, the magnitude determination of the Sumatra–Andaman depends on the frequency range considered in the calculations (free oscillations with periods > 1000 sec or seismic waves, with periods of 300–500 sec). For the Chile event only the seismic moment for periods of T < 1000 sec is presently available, although the estimate of seismic moment at larger periods is crucial for a comparison of the two events. Our study makes a direct comparison of the free oscillation amplitudes of the two events by analyzing the records of the Grotta Gigante long-base tiltmeters, which have been recording tilt continuously since 1960. The particular mountings and physical dimension of the instruments make them particularly apt to record the torsional free oscillation waves, scarcely observed by standard long-period seismographs and not directly observable by the gravity meters. We determine the singlet frequencies for some of the lower spheroidal and torsional modes and a broad spectrum of multiplet frequencies for the higher modes. After correcting for the decay of the modes, we determine the amplitude ratios of the activated modes for the two events. The amplitude ratios vary between 1.5 and 3. Our results can be used directly for determining the ratios of the seismic moments of the two events over the frequency range 0.3–3 mHz, the only necessary correction being that of the site dependence of the free mode amplitudes.