Seismo-ionospheric signatures prior to M >= 6.0 Taiwan earthquakes

This paper examines variations of the greatest plasma frequency in the iono sphere, foF2, recorded by the Chung-Li ionosonde (25.0 degrees N, 121.1 deg rees E) before M greater than or equal to 6.0 earthquakes during 1994-1999. The 15-day running median and the associated inter-quartile range are util ized as the reference and the upper or lower bounds to monitor the ionosphe ric foF2 variations for finding seismo-ionospheric signatures (precursors) of the earthquakes. It is found that precursors, in the form of the recorde d 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 2 1, 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 precu rsors 1, 3, and 4 days prior to the Chi-Chi earthquake.