Low‐frequency acoustic‐gravity waves from coseismic vertical deformation associated with the 2004 Sumatra‐Andaman earthquake (M_w = 9.2)

Abstract

[1] Atmospheric pressure perturbations from the 2004 Sumatra-Andaman earthquake (M w = 9.2) were observed by sensitive microbarographs at several global stations. Among these observations, very low-frequency acoustic-gravity waves ($1.4-2.8 mHz) with a group velocity around 300-314 m/s and amplitudes ranging between $1 and 12 Pa can be clearly identified through data processing at four stations on the Japanese Islands and also at four International Monitoring System (IMS) stations around the Indian Ocean. Assuming several seismic source parameters for this great thrust earthquake, we produce synthetic barograms using a realistic thermal structure in the atmosphere up to an altitude of 220 km. For this modeling, we incorporate the source dimensions in different zones, the expanding velocity of the source region, the vertical displacements of uplift and subsidence, and their time constants. Combinations of these source parameters provide synthetic waveforms consistent with the general features of the observed low-frequency records. The results clearly indicate that the recorded waves may have been generated by large-scale coseismic uplift and subsidence of the sea bottom and associated swelling and depression of the sea surface over the source region extending for 1500 km. The uplift in the south-central zone of the Andaman-Nicobar regions may be substantially larger than in the other zones. The time constant of the coseimic vertical deformation is found to be in the range of 1.0-1.5 min, which may correspond to the time elapsed shortly before the generation of tsunami waves.

Brief communication "Seismic and acoustic-gravity signals from the source of the 2004 Indian Ocean Tsunami"

“…We found the source of the first arriving acoustic-gravity wave about 300 km to the north of the US Geological Survey earthquake epicenter. This confirms the result of Mikumo et al (2008) that sea level changes at the earthquake source cause long period acoustic-gravity waves, which indicate that a tsunami was generated. Therefore, a denser local network of infrasound stations may be helpful for tsunami warnings, not only for very large earthquakes.…”

“…They also identified in the later parts of the records signals originating even from the northern part of the Bay of Bengal. Mikumo et al (2008) identified 500 s acoustic-gravity waves caused by swelling and depression of the sea surface and travelling with a speed of about 300 m s −1 . They used waveform modeling for determining the region of largest displacements of the seafloor.…”

“…There are reports of observations of such waves caused by the Sumatra-Andaman tsunami. LePichon et al (2005) and Mikumo et al (2008) used infrasound arrays of the International Monitoring System (IMS, http://www. ctbto.org/) and barograph stations in Japan for such observations.…”

“…Mikumo et al (2008) concluded from the analysis of travel times and amplitudes of first arriving acoustic-gravity waves with periods of about 400-700 s that these waves are caused by coseismic motion of the sea surface mainly to the west of the Nicobar islands in the open seas. We reanalyzed the acoustic-gravity waves and corrected the first arrival times of Mikumo et al (2008) by up to 20 min. We found the source of the first arriving acoustic-gravity wave about 300 km to the north of the US Geological Survey earthquake epicenter.…”

“…Garces et al (2005) and Le emphasized that infrasound was generated by mountainous islands near the epicenter and by tsunami propagation along the continental shelf to the Bay of Bengal. Mikumo et al (2008) concluded from the analysis of travel times and amplitudes of first arriving acoustic-gravity waves with periods of about 400-700 s that these waves are caused by coseismic motion of the sea surface mainly to the west of the Nicobar islands in the open seas. We reanalyzed the acoustic-gravity waves and corrected the first arrival times of Mikumo et al (2008) by up to 20 min.…”