- Open Access
Tracking the epicenter and the tsunami origin with GPS ionosphere observation
© The Society of Geomagnetism and Earth, Planetary and Space Sciences (SGEPSS); The Seismological Society of Japan; The Volcanological Society of Japan; The Geodetic Society of Japan; The Japanese Society for Planetary Sciences; TERRAPUB. 2011
Received: 8 April 2011
Accepted: 15 June 2011
Published: 27 September 2011
The global positioning system (GPS) can be used to monitor the seismic perturbation induced by the 2011 off the Pacific coast of Tohoku Earthquake (magnitude 9.0), Japan, on March 11, 2011, and to trace the tsunami across the Pacific Ocean by measuring the changes in the ionospheric total electron content (TEC). We estimate the vertical and horizontal mean speeds of the seismic and tsunami waves using the time and distance of the TEC perturbation, and then, taking into account those determined speeds, trace back to the epicenter and the tsunami origin by applying a 3-dimensional spherical model. The results show that both the tracked epicenter and the tsunami origin are quite close to the epicenter reported by the USGS, with a mean horizontal propagation speed of 2.3 km/s after the earthquake and about 210 m/s after the tsunami. This consistency confirms that the perturbation sources in the ionosphere are due to the earthquake. This implies that the GPS-TEC measurements have the potential to be part of a lower cost, ground-based, tsunami monitoring system.
Since the atmospheric density decreases almost exponentially with altitude, energy conservation implies that the pulse amplitude increases upwards as it propagates into the atmosphere (Calais and Minster, 1995). In the past, many seismo-ionosphere observations based on ionoson-des, high-frequency Doppler sounding systems and ground-based global positioning system (GPS) receivers have been published (Davies and Baker, 1965; Leonard and Barnes, 1965; Row, 1966; Yuen et al., 1969; Tanaka et al., 1984; Calais and Minster, 1995; Afraimovich et al., 2001; Ducic et al., 2003; Artru et al., 2004; Liu et al., 2005, 2011). On the other hand, tsunami waves propagating across long distances in the open ocean can also induce atmospheric gravity waves by dynamic coupling at the surface (Najita et al., 1974; Okal et al., 1999; Artru et al., 2005; Gower, 2005; Blewitt et al., 2006; Liu et al., 2006; Occhipinti et al., 2008; Galvan et al., 2011). The seismic and tsunamigenic signatures in the ionosphere are referred to as seismo-traveling ionospheric disturbances (STIDs) (Liu et al., 2011).
The U.S. Geological Survey (USGS) reported that the origin time of the magnitude 9.0 earthquake was at 05:46:23 UTC; while the epicenter was located at 38.322°N, 142.369°E off the east coast of the Tohoku area, Japan (USGS, 2011). Displacement of the adjacent seabed generated gigantic tsunami waves damaging many coastal communities around the Tohoku area. In this paper, we use data from a total of 17 ground-based GPS stations in the GEONET network in Japan, the IGS network and the Taiwan GPS network to detect the ionospheric perturbation induced by the 2011 Tohoku earthquake and the subsequent tsunami, and then trace their origins using a 3-dimensional spherical model.
2. Observation of the Earthquake
3. Observation of the Tsunami
The seismic ionosphere technique based on a network of GPS receiver-satellite pairs can distinguish the seismic perturbation and tsunami-induced waves in the time-variant TEC difference, where the former tends to a larger amplitude and shorter wavelength whereas the latter tends to a smaller amplitude and longer wavelength as shown in Figs. 1 and 4. For the 2011 Tohoku earthquake, the mean horizontal and vertical speeds of the seismic propagating waves are estimated to be 2.3 km/s; and for the subsequent tsunami, the mean horizontal speed is 210 m/s. Applying a 3-dimensional ray-tracing model and these speeds, we predict the optimal time and location of the earthquake and the tsunami, which confirms that the TEC disturbance can be traced back to their origins.
This research was partially supported by National Science Council and National Space Organization in Taiwan under grant NSC 98-2116-M-008-006-MY3 and NSPO-S-100011, respectively. GPS observation files used in this work were provided by the Geospatial Information Authority of Japan (GSI), Central Weather Bureau of Taiwan (CWB), and International GNSS Service (IGS).
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