an upward oscillation and step but no recovery. The
post-response water level is higher than its original position; thus,
it is recorded as co-OuSuRoPu. Since the sampling rate of the earthquake
monitoring well is 2 min, the co-seismic response in this
study is defined as the groundwater level change within 2 min
after an earthquake occurs, while the post-seismic response is that
after 2 min after an earthquake occurs. Fig. 3(b) shows a co-seismic
anomaly with a downward oscillation but no step or recovery. The
post-response water level returned to its original position. It was
thus recorded as co-OdSoRoPo. Fig. 3(c) shows a co-seismic anomaly
with no oscillation or recovery but with an upward step. The
post-response water level is higher than its original position. It
was thus recorded as co-OoSuRoPu. Fig. 3(d) shows a co-seismic
anomaly with an upward oscillation, a downward step, and a
recovery. The post-response water level is lower than its original
position. It was thus recorded as co-OuSdRdPd. Table 2 summaries
the recorded results.
In the sample events, earthquake induced groundwater level
oscillation is detected at Donher, Sinpu, and Tunwei wells for a
2-min sampling rate, as shown in Fig. 3. This implies that the duration
of oscillation is long enough for being detected under the sampling
rate. The direction of the first motion (up or down) of the
groundwater level does not show the P wave characteristics due
to the sampling rate is large. Therefore, the oscillation phenomenon
cannot provide useful information under the recent sampling
rate. The step variation of groundwater level is detected at
Donher, Tonlo, and Tunwei wells in Fig. 3. The step change indicates
the stress (or strain) situation for the well under the earthquake
effect. An upward step (e.g., Donher and Tonlo wells)
presents a compression situation while a downward step