ASA/CAA '05 Meeting, Vancouver, BC

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Infrasound Associated with the
2004 Sumatra Megathrust Earthquake and Tsunami

Milton Garces-
Pierre Caron
Claus Hetzer
Infrasound Laboratory
University of Hawaii, Manoa
73-4460 Queen Kaahumanu Hwy., #119
Kailua-Kona, HI 96740-2638

Popular version of paper 2aPA1
Presented Tuesday morning, May 17, 2005
Joint ASA/CAA Meeting, Vancouver, BC

Infrasound arrays in the Pacific and Indian Oceans that are part of the International Monitoring System (IMS) recorded three distinct waveform signatures associated with the December 26, 2004 Sumatra earthquake (M9) and tsunami. The infrasound stations observed (1) seismic arrivals (P, S and surface) from the earthquake, (2) Tertiary arrivals (T-phases), propagated along SOFAR channel in the ocean, and coupled back to the ground, and (3) infrasonic arrivals associated with either the tsunami generation mechanism or the motion of the ground above sea level. All signals were recorded by the pressure sensors in the arrays. The seismic and T-phase recordings are due to the sensitivity of the microphones to ground vibration, whereas the infrasound arrivals correspond to dispersed acoustic waves propagated through atmospheric waveguides.

The accompanying WAV sound file plays these three arrivals, in the order, as they were recorded at the infrasound station on Diego Garcia Atoll, Indian Ocean. Infrasound, which by definition is below the 20Hz human hearing threshold, is made audible by speeding it up by 200x, and the relative amplitudes of the three signals have been rescaled to maximize their volume. The arrival of the tsunami was not observed by the infrasound stations. A similar (but not identical) sequence of arrivals was observed at Diego Garcia during the March 28, 2005 Sumatra earthquake (M8.7) and the April 10, 2005 Mentawai earthquakes (M6.7 and 6.5). Based on the analysis of infrasound data from stations in Diego Garcia, Palau, Australia, Madagascar, and Kenya, we propose that:

  1. Submarine earthquakes can produce infrasound. The sound may be radiated by the vibration of the ocean surface or the vibration of land massess near the epicenter.
  2. Infrasound stations can also serve as seismic and t-phase stations for large events.
  3. The observed signals are substantially different, which may be due to source or propagation effects.
  4. The vibration of islands may produce infrasound.
  5. Small and large tsunamis may produce infrasound. The source process is not understood.
We show the prominent features of the signals, present infrasonic source location estimates (Figure 1), and consider whether infrasound may be used in conjunction with other technologies as a discriminant for tsunami genesis.

Figure 1. Candidate infrasonic locations for the December 26, 2004 Sumatra earthquake. One possible location is near the epicenter/tsunami source and another near the highest mountains in Aceh. Different source locations are estimated by using different refraction levels from the source to IMS infrasound stations in Palau and Diego Garcia. Atmospheric temperature and wind profiles specific to the time and location of the event were used.

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