Y. J. Chuo

Variations of ionospheric total electron content during the Chi‐Chi Earthquake

On 20 September1999 UT (21 Septemberin local time, LT) a large earthquake Mw=7.7 struck central Taiwan nearthe small town of Chi-Chi. The greatest plasma frequency in the ionosphere, foF2, observed by the Chung- Li ionosonde (25.0 ◦ N, 121.2 ◦ E) reveals three clear precur- sors at 1, 3, and 4 days prior to the earthquake. This paper examines the ionospheric total electron content (TEC) ob- served by a network of the global positioning system (GPS) receivers in Taiwan area. It is found that variations in foF2 and overhead TEC recorded at Chung-Li have a similar ten- dency. Combining the data of the network of 13 GPS re- ceivers, time, and spatial variations of TEC prior to the Chi- Chi earthquake are examined. Results show that the equa- torial anomaly crest moves equatorward and its TEC value significantly decreases 1, 3, and 4 days before the earth- quake. A comparison between the disturbed and reference (previous 15-day median) days confirms that TEC decreases significantly around the epicenter in the afternoons of these days. Finally, possible mechanisms are proposed and dis- cussed.

Seismo‐ionospheric signatures prior to M≥6.0 Taiwan earthquakes

This paper examines variations of the greatest plasma frequency in the ionosphere, foF2, recorded by the Chung-Li ionosonde (25.0° N, 121.1° E) before M≥6.0 earthquakes during 1994–1999. The 15-day running median and the associated inter-quartile range are utilized as the reference and the upper or lower bounds to monitor the ionospheric foF2 variations for finding seismo-ionospheric signatures (precursors) of the earthquakes. It is found that precursors, in the form of the recorded foF2 falling below its associated lower bound around 1200–1700 LT, appear 1–6 days prior to these earthquakes. On September 20, 1999 UT (September 21, Taiwan local time) a large Mw=7.7 earthquake struck central Taiwan near the small town of Chi-Chi. We analyzed the foF2 and found three clear precursors 1, 3, and 4 days prior to the Chi-Chi earthquake.

Atmospheric anomalies observed during earthquake occurrences

[1] Appearance of anomalies in the atmosphere before earthquakes (EQs) has been verified, through observation of anomalous transmission of VHF electromagnetic (EM) waves beyond line-of-sight. Anomalous increase of the received intensity for a few minutes - several hours on a day was identified by the previous 15-day running median and its inter-quartile range. The cross-correlation between the EQ occurrences and the anomalies shows that the appearance of anomalies was significantly enhanced within 5 days before M � 4.8 EQs. The one-day average number of the anomaly appearance within 5 days was found 2.4 times larger than that of other days. Through the polarization measurement of the received EM waves, the anomalies were found to occur in the atmosphere. INDEX TERMS: 6904 Radio Science: Atmospheric propagation; 6964 Radio Science: Radio wave propagation; 7223 Seismology: Seismic hazard assessment and prediction.Citation: Fujiwara, H., M. Kamogawa, M. Ikeda, J. Y. Liu, H. Sakata, Y. I. Chen, H. Ofuruton, S. Muramatsu, Y. J. Chuo, and Y. H. Ohtsuki (2004), Atmospheric anomalies observed during earthquake occurrences, Geophys. Res. Lett., 31, L17110,

The ionospheric perturbations prior to the Chi-Chi and Chia-Yi earthquakes

Abstract In this paper we analyze the greatest plasma frequency, foF2, named critical frequency, observed by the Chung-Li ionosonde (25.0°N, 121.l°E) during the period of the Chi-Chi (23.87°N, 1 20.75°E) and the Chia-Yi (23.51°E, 120.4°E) earthquakes. The previous 15-day running mean and the associated standard deviation are utilized to construct the upper or lower bound for detecting the seismo-ionospheric perturbations. It is found that the perturbation appeals in 3–4 days prior to the Chi-Chi earthquake as well as 1–3 days prior to the Chia-Yi earthquake.

Ionospheric foF2 variations prior to strong earthquakes in Taiwan area

Abstract Many studies of the seismo-ionospheric coupling effects have been reported. On 17 July 1998(M=6.2), 20 September 1999 (M=7.3) and 22 October 1999 (M=6.4) three large earthquakes respectively struck Rei-Li, Chi-Chi and Chia-Yi in central Taiwan. The three earthquakes severely damaged structures, heavily changed landforms and disturbed geophysical environments. This paper examines variations of the ionospheric penetration frequency, foF2 , observed by Chung-Li ionosonde station (25.0° N, 121.1° E) several days before the three earthquakes. The mean- and median-based statistical techniques are introduced to investigate the ionospheric electron density prior to the three earthquakes. Results show that the foF2 decrease significantly before the three earthquakes.