From the collected data in Taiwan, the groundwater variations
with step changes triggered by earthquakes show that the maximum
detectable distance to the epicenter or hypocenter is about
250 km and that the minimum detectable earthquake magnitude is ML 5.0. For earthquakes that caused oscillatory changes in the
groundwater level, the maximum detectable distance is less than
300 km and the minimum earthquake magnitude is ML 4.3.
However, some data show that oscillatory- and step-type groundwater
level variation can be observed in the installed monitoring
wells as responses to distant earthquake such as the Sumatra
and Wen-Chuan earthquakes, whose epicentral distances were farther
than 3000 and 1900 km from Taiwan, respectively. The
responses to distant earthquakes are beyond the scope of our
analysis