In this paper, we examine variations in the critical frequency foF2 recorded
by an ionosonde, and the total electron content (TEC) derived from
a network of 5 ground-based receivers of the global positioning system
(GPS), as well as occurrences of 144 M ≥ 5.0 earthquakes in Taiwan during
1997 - 1999. Results show that the foF2 and TEC yield similar tendencies,
and often concurrently register pronounced decrease anomalies 4 days
before the earthquakes. A detailed investigation of anomalies appearing
before and after the earthquakes confirms significant decreases in the foF2
and TEC to be the pre-earthquake anomalies.