The resonant response of the ionosphere imaged after the 2011 off the Pacific coast of Tohoku Earthquake
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We provide here a preliminary analysis of the ionospheric perturbations observed after the 11 March 2011 Tohoku Earthquake using a GPS-derived Total Electron Content (TEC) technique. Such anomalies are routinely observed after seismic events of magnitude Mw = 6 and more. Here, we use the high density and the wide coverage of the Japanese Global Positioning System (GPS) network GEONET to image the ionosphere just after the main shock. We describe ionospheric perturbations with exceptional extension in amplitude and duration. As already seen in earlier events, a first intense signal is observed about 10 minutes after the seismic rupture; the first response consists in two modes: one propagating beyond 3 km/s and the other at nearly 1 km/s. A further analysis of TEC time series of the latter mode near the source shows the typical frequencies of acoustic resonance. Beyond 400 km from the source, both the tsunami induced gravity wave and a third mode are imaged, the latter for the first time. We show that the pattern of this slow (225 m/s ± 10 m/s) and long period gravity wave (1.8 ± 0.2 mHz) is most visible in the North-West of the epicentral area. This description is corroborated by a computation of the normal modes of the solid Earth-atmosphere system.
Key wordsGPS-TEC earthquake ionosphere acoustic resonance
In the case of wide dense GPS networks like in California and Japan, ionospheric waves can be imaged dynamically. In particular, ionospheric signatures of class M = 8 earthquakes were imaged near the source (Heki and Ping, 2005; Astafyeva and Heki, 2009; Astafyeva et al, 2009) and also far from the source (Dučić et al., 2003; Rolland et al., 2011). For seismic sources, two kinds of waves were identified in a close vicinity of an epicenter: waves propagating at the typical sound speed at ionospheric heights (from 0.6 to 1 km/s) and waves propagating beyond 3 km/s. The first kind is related to the acoustic plume emitted by the piston-like effect of the Earth’s surface oscillations on the surrounding atmosphere, and modeled using seismic ray tracing (Heki and Ping, 2005) following Calais et al. (1998), or normal modes (Kobayashi et al., 2009). The second kind of waves is induced by the Rayleigh waves propagating at about 3.5 km/s (Astafyeva et al., 2009; Rolland et al., 2011). See Lognonné et al. (1998) for a more complete theoretical description of these waves and Dautermann et al. (2009) for the case of atmospheric source instead of earthquake excitation.
The high quality data acquired the day of the 2011 To-hoku event will undoubtedly carry on new insights on the mechanisms of generation of these waves as well as information on the seismic rupture itself. We present here ionospheric images observed after the earthquake and focus our paper on the analysis of long duration acoustic waves, the tsunami induced gravity waves and on a third mode emitted by the seismic source. This latter was, to our knowledge, never imaged so widely and with such a high signal to noise ratio in the ionosphere.
2. Data Processing and Observations
30s sampled data were downloaded from the GEONET public ftp site (ftp://126.96.36.199). Following the same methodology as Rolland et al. (2011), a 1 to 10 mHz bandpass Finite Impulse Response (FIR) butterworth filter is applied in order to remove the contributions of the daily ionospheric variability, the satellites motion and the instrumental biases. For representation purpose, we locate the TEC measurement at the intersection of the line-of-sight and an ionospheric thin layer whose height is chosen near the peak of electron density, here 250 km. This allows us to map the observed perturbations in so-called “TEC maps”. A movie of TEC maps from 5:30 to 9:00 UT is made available online at http://ganymede.ipgp.fr/~tohoku/ (movie 1) with a movie of TEC ionospheric perturbations observed offshore Hawaii after this same event (movie 2). See Makela et al. (2011) and Occhipinti et al. (2011) for details on Hawaii observations.
2.1 Acoustic resonance
As seen on satellite-station 15-0979 time series (Fig. 3), the duration of the first and sharp N wave exceeds 10 minutes and is propagating at nearly 1 km/s. A 2 hours long ringing signal follows, characterized by frequencies located at about 3.7 mHz and 4.4 mHz, corresponding to the fundamental and first harmonic of the atmospheric trapped acoustic modes (Lognonné et al., 1998; Kobayashi, 2007).
This wave was observed once at isolated locations after the Sumatra giant earthquake (M = 9.2) and the following tsunami of December 2004 (Choosakul et al., 2009) but never imaged so clearly by GPS-TEC technique. The 3.7 mHz waves have also been reported after convective storms (Georges, 1973). The interaction of the spheroidal surface waves with the atmosphere also leads to high sensitivity of the spheroidal modes 0S27/0S29 and 0S34/0S37, whose frequencies are close to 3.69 and 4.35 mHz, respectively. These modes were excited by a source of the bi-chromatic excitation on the Pinatubo eruption (Kanamori and Mori, 1992; Lognonné, 2009).
The acoustic trapped modes, computed following Lognonné et al. (1998) are found in the range 3.7–3.8 mHz and 4.35–4.48 mHz (Fig. 4(b)) for angular orders smaller than ℓ = 170 and wavelength larger than about 235 km. They have quality factors Q of about 150 and 20 respectively, meaning that the first ones are less attenuated than the second ones. This can explain why, in the spectrogram for satellite 15 observed by station 0979 (Fig. 3(a)), the 3.7– 3.8 mHz resonance lasts longer than the 4.35–4.48 mHz resonance.
2.2 Atmospheric gravity waves and tsunami pattern
Travel-time diagrams of satellite 15, 21 and 22 show a third type of wave, aligned along the 225 m/s (±10 m/s) slope (magenta plain line). On the snapshots in Fig. 2(c)), these waves appear at about 400 km of epicentral distance. They then propagate as concentric waves (with respect to the epicenter) and are observed more than 2 hours after the main shock. This strong pattern is followed by a lower amplitude signal, with a horizontal propagation speed of about 170 m/s (Fig. 2(b)), associated to the gravity waves forced by the tsunami.
These two waves are both characterized by a much slower vertical propagation than acoustic waves and need about 45 minutes to reach the ionosphere in the pure gravity case (Occhipinti et al., 2008, 2011), which explains why they appear at about 400 km of epicentral distance. Figure 4 shows the likely associated dispersion curves of these two waves, with a first one associated to free atmospheric gravity modes (continuous magenta line) and the second one associated to the tsunami modes (dashed magenta line). Note that the observed wavelengths are ranging typically from 150 to 200 km in the 1–2 mHz bandwidth (Fig. 3(b)). More precise analysis will request correction for Doppler effects associated to the moving ionospheric sounding points and a better modeling of the attenuation effects.
Even if the observations of free gravity waves have already been reported several times after major earthquakes (see Bolt (1964), for the Alaska quake and Mikumo et al. (2008) for a review), the Japanese event reported here provides for the first time dynamic images of the propagation. If already observed at tele-seismic distances (see Rolland et al., 2010 and movie 2 for this event offshore Hawaii), it is also the first time that the tsunami induced gravity waves are observed so close from the epicenter.
We also note that the observed patterns have a clear northwestward directivity. However, previous observations over Japan of acoustic ionospheric waves have shown a directivity in the opposite direction, i.e. towards the SouthEast, due to the effect of the geomagnetic field (Heki and Ping, 2005; Rolland et al., 2011). Indeed the coupling factor between the neutral wave and the geomagnetic field is proportional to the cosine of the angle enclosed by the geomagnetic field vector and the wave vector (Calais et al., 1998). As gravity waves are transverse and acoustic waves are longitudinal, the directivity of coupling effect in opposite directions seems coherent.
3. Discussion and Conclusion
By using GEONET GPS data, we have analyzed the ionosphere response to the Tohoku major earthquake of 11th March 2011. Our GPS-TEC data exhibit the appearance of three different types of waves in the TEC signal over Japan, in addition to the tsunami forced atmospheric waves. The first wave propagates at ~3 km/s and is, most likely, induced by the Rayleigh surface waves. However, part of this fast-propagating wave could also be directly related to the fault propagation. More in depth analysis of high-resolution (1-second) GPS data should help in validating one or both hypotheses. The second slower wave propagates at ~1 km/s, and manifests the acoustic wave directly generated by the earthquake itself. These observations are in a good agreement with previous observations (Astafyeva et al., 2009; Rolland et al., 2011). For the first time in the Japanese area, we also observed appearance of a third type of wave. This third mode is a free gravity wave appearing in the TEC ~45 min after the main shock, from a distance of ~400 km away from the epicenter. Its propagation speed was estimated to be ~225 m/s and frequency of about 1.8 mHz. These features are coherent with the observations of Maruyama et al. (2011) and Tsai et al. (2011) using GPS-TEC technique and the observations of Liu and Sun (2011) in ionogram records. They are corroborated here by normal modes computation.
Further analyses of the very rich dataset presented here must be made, as their unprecedented quality will certainly provide new insights on the mechanisms taking place during the giant seismic rupture of the 2011 Tohoku Earthquake.
This work is supported by French Space Agency CNES. Additional supports were provided by the Campus Spatial Paris Diderot and the United States Office of Naval Research (ONR) globally under contract IONONAMI-N07-25. We thank the operators of the GEONET network for providing high quality data and T. Gabsi for support in data management. We also thank two anonymous reviewers for contructive comments. This is IPGP contribution number 3182.
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