The two major types of groundwater
level variation triggered by earthquakes are step and oscillatory
changes, respectively. Oscillatory changes may reflect the energy
propagated by seismic waves, whereas step changes are believed
to be more closely related to the crust deformation caused by
anomalous stress. Usually, step changes occur within a limited distance
from the slipping fault. However, a step change in groundwater
levels may be observed for a distant earthquake. For example,
step changes in the groundwater levels in Taiwan were observed
in six of the installed monitoring wells for the 2008 M7.9
Wen-Chuan Earthquake in China, whose epicenter was about
1900 km away from Taiwan. This was the only earthquake event
for which a step change was observed in the Taiwan area whose
epicentral distance was farther than 1000 km. The mechanism
for this event is still unclear and the epicenter location was far
away from Taiwan, so the data were not included in our analysis.
The detectable distance of the oscillatory change of groundwater
can be very large, on the order of thousands of kilometers,