[J09-1] Geodesy and seismology general contributions I

[J09-1-06] Rise Time of Coseismic Tectonic Deformation during Megathrust Earthquakes, as estimated from Observed Low-Frequency Acoustic-Gravity Waves

^○Takeshi Mikumo (Kyoto University, Kyoto, Japan)

Unusual low-frequency acoustic-gravity waves with periods up to as long as 14 min have been recorded after four megathrust events. These are the 1964 Alaskan earthquake (Mw=9.2), the 2004 Sumatra-Andaman earthquake (Mw=9.2), the 2011 Off-Tohoku earthquake (Mw=9.0), and the 2010 Maule, Chile (Mw=8.8) earthquake, respectively. These extremely long-period atmospheric waves have been clearly observed at several microbarograph stations located at not only several regional stations but at many International Monitoring System stations even up to a far-field of 6,700 km. The phase and group velocities of these disturbances are found to be between 314 m/s to 364 m/s consistent with so far predicted for acoustic-gravity waves.

These observed low-frequency atmospheric waves may be interpreted as excited by upheaval and depression of the sea surface within the extensive source region of these extremely large thrust earthquakes due to coseismic uplift and subsidence of the sea bottom. We calculate the synthetic waveforms for the propagating atmospheric disturbances due to the coseismic vertical displacements of the sea surface, incorporating a standard sound velocity and density structures up to an altitude of 220 km above the earth's surface. The calculated waveforms for many stations well explain most of the observations. Our analysis suggests the initial coseismic change of the sea surface in the main part of the source region may exceed 4 – 6 meters with the rise time between 2 and 4 min depending on their locations, which is related to the source process of these megathrust earthquakes.