To characterize dynamic groundwater changes, the variation of
groundwater level anomalies is categorized here based on five
characteristics. The first characteristic is the occurrence time of
the anomaly. According to the difference between the occurrence
times of the earthquake and the corresponding groundwater variation,
the groundwater signal is identified as pre-seismic (pr),
co-seismic (co), or post-seismic (po). The second and third characteristics
are based on the shape of the groundwater level fluctuation
triggered by an earthquake. They are oscillatory (O) and step
(S) changes, respectively. The fourth characteristic is the groundwater
recovery (R) behavior after the earthquake. The fifth characteristic
is the direction of the position (P) vector of the
groundwater level from pre-earthquake to post-earthquake, with
u used for an upward movement of the groundwater level, d used
for a downward movement, and o used for no movement.