Earth Planets Space, Vol. 58 (No. 2), pp. 159-165, 2006
Y. Otsuka1, N. Kotake1, T. Tsugawa1, K. Shiokawa1,T.Ogawa1, Effendy2, S. Saito3, M. Kawamura3, T. Maruyama3, N. Hemmakorn4, and T. Komolmis5
1Solar-Terrestrial Environment Laboratory, Nagoya University, Japan
2National Institute for Aeronautics and Space, Bandung, Indonesia
3National Institute of Information and Communications Technology, Tokyo, Japan
4King Mongkut's Institute of Technology Ladkrabang, Bangkok, Thailand
5Chiang Mai University, Chiang Mai, Thailand
(Received May 27, 2005; Revised December 3, 2005; Accepted December 16, 2005; Online published February 17, 2006)
We report the response of the ionosphere to the large earthquake that occurred in West Sumatra, Indonesia, at 0058 UT on December 26, 2004. We have analyzed Global Positioning System (GPS) data obtained at two sites in Sumatra and at three sites in Thailand to investigate total electron content (TEC) variations. Between 14 and 40 min after the earthquake, TEC enhancements of 1.6-6.9 TEC units (TECU) were observed at subionospheric points located 360-2000 km north of the epicenter. From the time delays of the observed TEC enhancements, we find that the TEC enhancements propagated northward from the epicenter. The time delays between the earthquake and rapid increases in TEC, which occurred near the epicenter, are consistent with the idea that acoustic waves generated by the earthquake propagated into the ionosphere at the speed of sound to cause the TEC variations. A small TEC enhancement of 0.6 TECU was observed south of the epicenter, while no TEC enhancements were seen east of the epicenter. From a model calculation, we find that this directivity of the TEC variations with respect to the azimuth from the epicenter could be caused partially by the directivity in the response of the electron density variation to the acoustic waves in the neutral atmosphere.
Key words: Ionosphere, earthquake, GPS, total electron content, acoustic wave.