Fluctuations in the ionosphere related to Honshu Twin Large Earthquakes of September 2004 observed by the DEMETER and CHAMP satellites

Highlights

• Ionospheric disturbances related to large earthquakes observed in satellite data.

• Sudden change in the ion temperature profile along the epicenter-close orbits.

• Lowered O⁺ density were also observed along the epicenter-close orbits.

• Ion velocity data imply the fluctuations were caused by vertical drift above the epicenter.

Abstract

While investigating possible precursory signatures of large earthquakes in the ionospheric data measured by the DEMETER and CHAMP satellites, we found ionospheric disturbances related to large earthquakes (M=7.2 and 7.4) that occurred on September 2004 near the south coast of Honshu, Japan. The satellite data were statistically compared with an empirical model and local averages of the large set of data in the study period. A fluctuation in the electron density above the epicenter was observed roughly 2 weeks before the main earthquakes. Surveys of the space weather and geomagnetic activities suggest that these fluctuations were not caused by changes in space conditions or by a geomagnetic storm. The features were also distinct from well-known natural ionospheric anomalies. In addition, a peak-like profile in the ion temperature and lowered ${\mathrm{O}}^{+}$ density around the region of the epicenter was observed a week before the main earthquakes along the satellite passes whose longitudes are close to the epicenter. The features are more apparent when they are compared with the data more distant from the epicenter, suggesting that the disturbances occur along the geomagnetic field lines. The concurrent measurements of the ion drift velocity suggest the fluctuations were triggered by the vertical plasma drift. The observed anomalies disappeared $\sim 2$ weeks after the quakes. According to the current theories on the seismo-ionospheric coupling, the horizontal electric field at the lower boundary of the ionosphere should have been strengthened by the seismic activity in order for the ionospheric plasma movements above the epicenter and its geomagnetic conjugate regions to trigger the observed ionospheric anomalies.

Introduction

There have been numerous reports and studies on observable precursors to earthquakes (Ondoh, 2003, Pulinets and Boyarchuk, 2004, Oyama et al., 2008, Zhang et al., 2011) and on the underlying mechanisms (Namgaladze et al., 2009, Freund, 2010, Kuo et al., 2011). Even so, there is not yet a unified, indisputable consensus on what causes such anomalies and why they occur before and after seismic disturbances. There also exist some pessimistic views on earthquake-precursor phenomena (Geller et al., 1997, Rishbeth, 2007); some even stating categorically that earthquakes cannot be predicted. Meanwhile, recent achievements in statistical analyses using ground (Fujiwara et al., 2004, Liu et al., 2006) and satellite (Li and Parrot, 2013) observations provided uncontroversial correlation between the seismic activities and the ionospheric disturbances preceding the earthquakes. Ryu et al. (2014) reported seismically intensified EIA (equatorial ionization anomaly) features related to the M8.7 Northern Sumatra earthquake of March 2005 and the M8.0 Pisco earthquake of August 2007. They suggested that some of the large earthquakes near the equatorial region can accompany precursory increase of the electron density in the geomagnetic equator.

According to the method of detection, previously reported precursors to earthquakes can be classified into several categories (i.e., plasma waves, plasma – both ionic and electronic – density and temperature, energetic particles, and infrared emissions). Larkina et al. (1989) reported an anomalous increase in the intensity of low-frequency (0.1–16 kHz) radio-wave emissions detected in the INTERCOSMOS-19 data when the satellite was nearly over the epicenter of an earthquake zone. Based on statistical studies of several cases, they remarked that the increase of ELF/VLF emissions occurs between tens of minutes and hours before and after the quakes. Gokhberg et al. (1989) introduced experimental observations of the super long waves and daily phase and amplitude variations before a number of earthquakes. Later, a theoretical study was followed to explain the detected ELF/VLF emissions over the epicenter region (Sorokin and Chmyrev, 2002). According to Sorokin and Chmyrev (2002), slow variation in the parameters of the lower atmosphere above a seismic zone can initiate short-term disturbances of the plasma and electromagnetic field due to vertical, turbulent transfer of charged aerosols and radioactive particles. Consequently, the interaction between these irregularities in conductivity with the background electromagnetic radiation results in ELF emissions in the ionosphere and ULF oscillations at the surface of the Earth.

Boskova et al. (1993) reported significant changes of light ions (${\mathrm{H}}^{+}$ and ${\mathrm{He}}^{+}$) in the ion composition data from the low-altitude INTERCOSMOS-24 satellite. Chmyrev et al. (1997) analyzed COSMOS-1809 satellite data on ELF emissions, plasma density N_e and its variations dN_e, in the ionosphere above the Spitak earthquake zone. They reported small-scale plasma inhomogeneities (${\mathit{dN}}_e/N_e\approx 3–8\%$) with a scale of 4–10 km along the orbit, which were excited along geomagnetic flux tubes connected to the epicentral region, simultaneous with earthquake-related ELF emissions. Shklyar and Truhlík (1998) investigated the role of a quasi-static transverse electric field in modifying the plasma density distribution and pointed out possible mechanism for modifying the ion concentration profiles connected with radioactive releases preceding earthquakes. Hayakawa et al. (2000) reported a correlation between global distribution of seismic activity and ion density variation in the ionosphere, based on a large data base of plasma density measured by the INTERCOSMOS-24 satellite. According to their analysis, a clear correlation was found for daytime (10–16 LT), involving highly disturbed magnetic conditions at altitudes of 500–700 km, though these conditions disappeared at night and during magnetic storms. Oyama et al. (2008) found that the electron temperatures observed by the HINOTORI satellite around the epicenters of several large January 1982 earthquakes that occurred in the Philippines, significantly decreased in the afternoons – several days before and after the quakes. They attributed this fall in electron temperature to the existence of electric fields over the epicenters. Recently, Oyama et al. (2011) presented ion density reductions in the ionosphere associated with a large earthquake (M=7.5) observed by the US satellite DE-2. Electric fields are said to be the most probable candidates for modifications of the ionosphere. Several mechanisms were suggested and three of them remain plausible. They are the stress-activated positive-hole model (Freund, 2002), the radon emanation model (Ondoh, 2003), and the atmospheric gravity wave model (Namgaladze et al., 2009).

There were some reports on disturbances of high energy particle fluxes (Galper et al., 1995, Aleksandrin et al., 2003) called particle bursts, which were correlated with seismic activities. Aleksandrin et al. (2003) analyzed the data of various near-Earth space experiments (MIR station, METEOR-3, GAMMA, and SAMPEX satellites) and reported that particle bursts were observed several hours before strong earthquakes. In these cases, L-shells of particle bursts and corresponding earthquakes practically coincided. Anomalous distribution of visible light and infrared (IR) in images before seismic activity was also reported (Tronin, 1996). There were positive anomalies of the outgoing Earth radiation flux recorded at night time that were associated with the largest linear structures and fault systems of the crust.

The diversity of seismic precursor effects implies that the underlying mechanisms are quite complicated. This indicates that collaborative observations of ground and satellite instruments, with rigorous theoretical support, will be needed to reach unified explanations of the seismic precursor phenomena. There are also diverse approaches of studying the seismo-ionospheric coupling. The representative methods are statistical analysis (Fujiwara et al., 2004, Liu et al., 2006, Li and Parrot, 2013), case-study analysis (Oyama et al., 2008, Oyama et al., 2011, Ruzhin et al., 1998), which we applied in this study, and physical model analysis (Namgaladze et al., 2009, Kuo et al., 2011). The statistical approach is strong in proving that there actually exists a correlation and that the signal or the anomalies are significant in statistical aspect. On the other hand, the case-study and the physical model analyses are suitable for studying the physical background of the phenomena.

As noted above, many issues remain unclear and there were no dedicated satellite observations for the study of earthquakes before the DEMETER satellite (Parrot, 2002). DEMETER is a low altitude, micro-satellite (710 km, 130 kg) in a nearly polar orbit (Cussac et al., 2006). The main scientific objectives of the DEMETER experiment are to study the disturbances of the ionosphere due to seismo-electromagnetic effects and anthropogenic activities (Parrot, 2009). In this study, we report fluctuations in the ionosphere around twin large earthquakes with magnitudes greater than 7.0 observed by DEMETER. Preliminary analysis of the ionosphere concerning these large earthquakes can be found in Parrot et al. (2006). We also analyzed data from the CHAMP satellite (Lühr et al., 2012). This allowed us to measure ionospheric conditions at various altitudes around earthquake zones. Finally, we attempted to understand and explain the observed global fluctuations in the ionosphere before and after the large earthquake observed by both DEMETER and CHAMP, from the perspective of seismo-ionospheric coupling.