Page 39 - Fall 2005

The newsletter of
The Acoustical Society of America
Infrasound from the 2004-2005 earthquakes
and tsunami near Sumatra
Infrasound arrays in the Pacific and Indian Oceans that are
From the prominent features of infrasonic arrivals and infra-
Milton Garces, Pierre Caron, and Claus Hetzer,
part of the International
Monitoring System (IMS) of the
Comprehensive Nuclear Test Ban
Treaty (CTBT) recorded three dis-
tinct waveform signatures associ-
ated with the December 26, 2004
Aceh earthquake [Magnitude
(M)9, US Geological Survey
(USGS)] and tsunami. The infra-
sound stations observed (1) seis-
mic arrivals–compressional
motion (primary or P-wave that
arrives first), shear motion (sec-
ondary or S–wave that arrives sec-
ond) and surface waves from the
earthquake, (2) tertiary arrivals (T-
phases that arrive third), propagat-
ed along sound channels in the
ocean and coupled back into the ground, and (3) infrasonic arrivals associated with either the tsunami generation mecha- nism near the seismic source 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 sensi- tivity of the microphones to ground vibration, whereas the infra- sound arrivals correspond to dispersed acoustic waves propagat- ing through atmospheric waveguides. A similar, but not identi- cal, sequence of arrivals was observed at Diego Garcia Atoll (range of ~2860 km) during the March 28, 2005 Nias earthquake (M8.7) and the April 10, 2005 Mentawai earthquakes (M6.7 and 6.5), suggesting that above-water ground motion can generate infrasound in the Sumatra region. In addition, very low frequen- cy infrasound was produced in the Bay of Bengal region, sug- gesting that the interaction of the tsunami with the coastal bathymetry can produce sound.
sonic source location estimates for the selected Sumatra earth- quake and tsunami sequence, we deduce that submarine earth- quakes can produce infrasound. The sound may be radiated by the vibration of the ocean sur- face or the vibration of land masses near the epicenter.
It is also apparent that infra- sound stations can also serve as seismic and T-phase stations for large events. For the three subma- rine earthquakes that we investi- gated, the differences in the observed signals may be due to either source or propagation effects. Although there is a sub-
stantial difference between the information contained in the low (0.02 – 0.1 Hz) and high (0.5-5 Hz) frequency bands of the infrasound range, it does appear that both small (Nias) and large (Aceh) tsunamis may produce infrasound.
The candidate source locations near the epicenter, in con- junction to the unique signal observed at Diego Garcia for the Aceh event, suggests that infrasound may be combined with other technologies as a discriminator for tsunami genesis. Fundamental research is needed on how low-frequency sound from large earthquakes and tsunamis can be utilized in hazard warning and mitigation.
Milton Garces, Pierre Caron, and Claus Hetzer are at the Infrasound Laboratory at the University of Hawaii. This arti- cle is based on paper 2aPA1 at the Vancouver ASA meeting.
Paths of infraound signals from earthquakes.
Echoes 37