Figure 1 illustrates the locations of the Chung-Li ionosonde, the GPS receivers, and the
epicenters of the 144 M ≥ 5.0 earthquakes during 1997 - 1999. The ionosonde observes the
ionosphere within a horizontal region of a radius 500 km from Chung-Li (25.0°N, 121.0°E),
while the GPS network covers an area of 15° - 30°N and 110° - 130°E. Therefore, the two
observations simultaneously monitor the ionospheric volume above the epicenters.