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

Abstract

Atmospheric pressure perturbations from the 2004 Sumatra‐Andaman earthquake (Mw = 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.