Properties of infrasonic signals due to the 2008 Iwate-Miyagi Nairiku Earthquake
Abstract
We have started to make use of the infrasound data of CTBTO for geophysical researches with a cooperation of Japan Weather Association. An infrasonic signal due to the 2008 Iwate-Miyagi Nairiku Earthquake occurred on June 14, 2008 in Japan (Mw7.1, 39.03N, 140.88E, 7km depth) is detected in the infrasound data obtained at Isumi station (IS30) 417km south from the epicenter (Arai et al., 2008). This infrasound signal consists of two obvious phases. The first arrival phase appears ~ 1 minute after the origin time having peak-to-peak amplitude of ~ 3Pa, and this is considered to be a mixture of sensor movement and air oscillation generated by ground motion at the station (e.g., Watada et al., 2006). The second arrival phase appears ~ 25 minutes after the origin time having amplitude of ~ 1Pa, and this is considered to be acoustic waves propagated in the upper atmosphere directly from the epicentral region according to apparent velocity and azimuth determined by a semblance analysis. We found, in the secondary phase, remarkable three low-frequency pulses having travel times of ~ 1400, ~ 1500, and ~ 1600 seconds, respectively, and duration of ~ 1 minute. We carry out a ray-tracing of an acoustic wave that propagates in a stratified atmosphere, of which the acoustic velocity structure is derived from the NRLMSISE-00 model (Picone et al., 2002). It is found that the travel time and duration of the second pulse, which has the maximum amplitude among three pulses, can be explained by refracted waves launched continuously during the rupture time and going up to the lower thermosphere. It is also found that the remained two pulses can be explained by guide waves propagating between the stratopause and the lower troposphere reflected twice and three times at the boundary. An observation of such remarkable pulses is a rare case in the point that each ray path can be resolved clearly taking into consideration geometry and rupture time of the fault and wind field in the upper atmosphere. We believe that investigations of infrasound data for more events make it possible to clarify eventually such as propagation features of acoustic waves, temperature structure and wind field in the upper atmosphere, and coupling mechanism between solid earth and lower atmosphere in an epicentral region.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2008
- Bibcode:
- 2008AGUFM.S13E..05N
- Keywords:
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- 0350 Pressure;
- density;
- and temperature;
- 2487 Wave propagation (0689;
- 3285;
- 4275;
- 4455;
- 6934);
- 3384 Acoustic-gravity waves;
- 7215 Earthquake source observations (1240);
- 7219 Seismic monitoring and test-ban treaty verification