There was only one suspected pre-seismic event, detected on
February 9, 2004, at the Hualien station before an ML 4.3 earthquake.
A 1.5cm upward step change was observed an hour before
the earthquake and the co-seismic response was a 1.0-cm downward
oscillatory change, as shown in Fig. 4. A pre-seismic response
is rarely observed because of the small crust strain in the
pre-earthquake period and the difficulty of defining a groundwater
anomaly due to the large number of earthquakes in Taiwan. Large
earthquakes, more sensitive wells, and high resolution instruments
might facilitate the detection of pre-seismic groundwater level
anomalies.