Ionospheric disturbances of the 2007 Bengkulu and the 2005 Nias earthquakes, Sumatra, observed with a regional GPS network

Abstract

[1] We studied ionospheric disturbances associated with the two large earthquakes in Sumatra, Indonesia, namely, the 2007 Bengkulu and the 2005 Nias earthquakes, by measuring the total electron contents (TEC) using a regional network of global positioning system (GPS) receivers. We first focus on coseismic ionospheric disturbances (CIDs) of the Bengkulu earthquake (M w 8.5). They appeared 11-16 min after the earthquake and propagated northward as fast as~0.7 km/s, consistent with the sound speed at the ionospheric F layer height. Resonant oscillation of TEC with a frequency of~5 mHz continued for at least 30 min after the earthquake. The largest aftershock (M w 7.9) also showed clear CIDs similar to the main shock. A CID propagating with the Rayleigh wave velocity was not observed, possibly because the station distribution did not favor the radiation pattern of the surface waves. This earthquake, which occurred during a period of quiet geomagnetic activity, also showed clear preseismic TEC anomalies similar to those before the 2011 Tohoku-Oki earthquake. The positive and negative anomalies started 30-60 min before the earthquake to the north and the south of the fault region, respectively. On the other hand, we did not find any long-term TEC anomalies within 4-5 days before the earthquake. Co-and preseismic ionospheric anomalies of the 2005 Nias earthquake (M w 8.6) were, however, masked by strong plasma bubble signatures, and we could not even discuss the presence or absence of CIDs and preseismic TEC changes for this earthquake.

Coseismic ionospheric disturbance of the large strike-slip earthquakes in North Sumatra in 2012: Mw dependence of the disturbance amplitudes

“…2d). These signatures are similar to the case of the 2007 Bengkulu earthquake in southern Sumatra (Cahyadi & Heki 2013). Gentle positive peaks at ∼11 UT reflect the passage of the IPP across the southern part of equatorial ionization anomaly (compare Figs 2b and e with c), and are not related to the earthquakes.…”

“…Slant TEC shows U-shaped temporal changes due to the apparent movement of GPS satellites in the sky (and consequent changes of the penetration angles of LOS to the ionosphere). When Cahyadi & Heki (2013) studied CIDs of the 2007 Bengkulu earthquake, sampling intervals of the SUGAR stations (2 min) were not sufficiently short for studies of CID whose typical timescale is 4-5 min. In the 2012 data set, however, most stations employed the sampling interval short enough for such studies (15 s).…”

“…The waves reflect downward and interfere with the upward propagating waves and cause resonant oscillation (Tahira 1995;Shinagawa et al 2007). Acoustic resonance in 3.7 and 4.4 mHz is found in the Earth's background free oscillation (Nishida et al 2000), and these frequencies were identified in post-seismic monochromatic TEC oscillation by GPS-TEC after the 2004 Sumatra-Andaman earthquake (Choosakul et al 2009), the 2011 Tohoku-oki earthquake (Rolland et al 2011b;Saito et al 2011), and the 2007 Bengkulu earthquake (Cahyadi & Heki 2013). We show spectrograms obtained by the BlackmanTukey method using the TEC time-series after the main shock and the largest aftershock in Fig.…”

“…Cahyadi & Heki (2013) studied CIDs of the latter with GPS, but found that severe plasma bubble activities masked the CID of the former. They used GPS data from ∼20 continuous GPS stations of the Sumatra GPS Array (SUGAR) operated by the Tectonics Observatory of Caltech and the Indonesian Institute of Sciences (LIPI).…”

Ionospheric F2-layer Perturbations Observed After the M8.8 Chile Earthquake on February 27, 2010, at Long Distance from the Epicenter

“…Seismic TIDs have been registered at distances of more than 3,000 km from epicenters using ionosonde measurements of the F2-layer critical frequency (foF2; Leonard & Barnes 1965;Hegai et al 2011). The TIDs associated with earthquakes have also been studied using ionospheric total electron content (TEC) data measured by GPS receiver networks (e.g., Calais & Minster 1995;Astafyeva & Afraimovich 2006;Liu et al 2006;Tsugawa et al 2011;Cahyadi & Heki 2013).…”