Water level in wells often fluctuates in response to changes of volumetric strains caused by tectonic deformation or by passage of seismic waves. Continuous monitoring of water level fluctuations in two wells, coded HLC-05 and HLC-03, in the Hualien area of eastern Taiwan has been made since 2002 by digital recorders with high sampling rates at 1- and 6-second intervals. The data thus far show that water level fluctuations are often induced by earthquakes. The results can be summarized as follows: 1. Observable water level fluctuations can be induced by earthquakes of magnitude M ≥ 0.43 + 2.39 log₁₀D, where D is the hypocenter distance from the well; 2. the peak water level fluctuation (PWL) is linearly proportional to either the peak ground velocity (PGV) or peak ground displacement (PGD) on logarithmic scales. However, no clear trends with peak ground acceleration (PGA) are found. The empirical relationships for local earthquakes between PWL and PGV, and between PWL and PGD, respectively, as obtained by regression are as follows:

For the HLC-05 well:

ln(PWL) = 0.83 × ln(PGV_H) - 3.22,
ln(PWL) = 0.97 × ln(PGV_V) - 2.27,
ln(PWL) = 0.80 × ln(PGD_H) - 1.70,
ln(PWL) = 0.91 × ln(PGD_V) - 0.83,

For the HLC-03 well:

ln(PWL) = 0.73 × ln(PGV_H) - 3.29,
ln(PWL) = 0.73 × ln(PGV_V) - 2.65,
ln(PWL) = 0.53 × ln(PGD_H) - 2.44,
ln(PWL) = 0.57 × ln(PGD_V) - 1.93,

where the subscripts H and V denote the horizontal and the vertical component of ground motion, respectively. Similar relationships for teleseismic earthquakes are as follows:

For the HLC-05 well:

ln(PWL) = 1.01 × ln(PGV) + 2.04,
ln(PWL) = 0.95 × ln(PGD) + 0.50.