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[MIS08-01] Spatio-temporal Structure of the Ionospheric TEC Anomalies Immediately Before the Three Large Earthquakes in Chile
Keywords: GNSS-TEC, precursor, Chilean earthquake
Here we study three recent large earthquakes in Chile, i.e. the 2010 February Maule (Mw8.8), 2014 April Iquique (Mw8.2), and the 2015 September Illapel (Mw8.3) earthquakes. Fairly large numbers of continuous GNSS stations have been deployed in South America, especially in Chile and Argentine, which enables us to study spatial structure of the observed TEC anomalies. We analyzed raw GNSS data downloaded from data centers of UNAVCO, IGS, RAMSAC, etc., and found clear positive TEC breaks immediately before these earthquakes (see the attached figure). Here we compare the three-dimensional spatial structure of the TEC anomalies with those inferred by Kuo et al.  as the ionospheric response to surface electric charges.
Precursor times (occurrences of TEC breaks) in these earthquakes were, 40 minutes, 25 minutes, and 22 minutes before earthquakes, respectively. Background VTEC reflects the difference in magnetic latitude (Iquique event occurred beneath the equatorial ionization anomaly) and in local time (Maule event occurred during midnight), and were below 5 TECU, ~60 TECU, and ~20 TECU, respectively, for the three cases. As a whole, the increases of the VTEC rates were consistent with absolute VTEC and Mw.
In all the three earthquakes, preseismic positive TEC anomalies appeared to the north of the epicenters (opposite in the northern hemisphere cases). The spatial extent of the positive anomalies also showed Mw dependence, and were ~500 km for 2010 Maule and ~300 km for 2014 Iquique and 2015 Illapel earthquakes (see the attached figure). Negative anomalies (TEC decrease) were found to have started together with positive anomalies (TEC increase) in areas farther from the epicenters. We studied the 3-D spatial structure of these anomalies before the 2015 Illapel event (which had the best station coverage), and found that the positive anomalies appear ~200 km above ground to the north of the epicenter, and that negative anomalies appear at height of ~400 km further to the north of the positive anomalies. These positive/negative anomalies align with the local geomagnetic field, and the inferred structure is consistent with the ionospheric response to surface positive charges by Kuo et al. .
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Figure caption: Distribution of TEC anomalies immediately before the three Chilean earthquakes, i.e. 2010 Maule, 2014 Iquique, and 2015 Illapel events. The anomalies are shown with colors at ground projections of the intersection of the line-of-sight with a thin layer at 200 km altitude.