Possible spatial extent of ionospheric GPS-TEC and NmF2 anomalies related to the 1999 Chi-Chi and Chia-Yi earthquakes in Taiwan

Abstract

Scientists analyzed temporal variations of local data recorded in Taiwan and reported anomalous reductions in the ionospheric total electron content (TEC) on days 4 and 3 before (17 and 18 September) the Chi-Chi earthquake (Mw 7.6, ML 7.3), and days 3 and 1 prior to (19 and 21 October) the Chia-Yi earthquake (ML 6.4) in Taiwan. In this paper, we propose a spatial analysis which compares the data recorded inside and outside of the earthquake area around Taiwan to discriminate whether the anomalies are local (earthquake related) or global (non-earthquake related) phenomena. Results suggest that the anomalies appearing day 3 before the Chi-Chi earthquake, and days 3 and 1 before the Chia-Yi earthquake are local (earthquake related) phenomena and the anomaly appearing on 17 September 1999 (day 4 before) might be the result of both global phenomena (i.e., a geomagnetic storm) and the local effect of the Chi-Chi earthquake.