[1] Using a new empirical relation among earthquake magnitude, seismic energy density and hypocentral distance, we show that the documented water level changes during earthquakes occur across seven orders of magnitude of seismic energy density. Combining this relation with a global data set for water level changes, new data from Taiwan, and laboratory data for saturated sediments under cyclic loading, we show that at least two mechanisms may be important for inducing water level changes. Undrained volumetric change may be the dominant mechanism to cause the abrupt decrease or increase of water level documented in the near field, while an earthquake-enhanced permeability may account for the more gradual and sustained water level changes documented in the intermediate field.

Changes of groundwater levels induced by the M L 7.3 Chi-Chi earthquake on 21 September 1999 were recorded at 157 out of 179 monitoring wells in the Choshui River alluvial fan. Of those, 67 observed large groundwater-level changes, ranging from 1.0 to 11.1 m. These 157 wells are clustered at 64 stations located approximately 2 to 50 km west of the north-south-trending Chelungpu fault. Both oscillatory and steplike changes of water level were observed on the analog records at the time of earthquake, while only steplike changes were observed on the hourly digital records. Coseismic changes of groundwater level were recorded not only in the confined aquifers but also in the partially confined aquifers and the unconfined aquifers. The recovery of water-level changes took minutes to months, depending primarily on hydrogeologic conditions of the confining layers. The sign and magnitude of coseismic water-level change at a well varied with its distance from the fault. The distribution of coseismic water-level changes induced by the ChiChi earthquake indicates that water-level rise predominated in most of the footwall area, whereas water-level fall prevailed in a narrow zone adjacent to the fault trace.

The following is a list of acronyms, initialisms, and abbreviations (including units of measure) used in this document.

[1] The 2008 M7.9 Wenchuan earthquake in Sichuan, China, caused water level to oscillate and undergo sustained changes in Taiwan, $2000 km away from the epicenter. Here we use the responses in three wells recorded at high sampling rate (1 Hz) and the broadband seismograms from a nearby station to document, for the first time, that the major water-level responses associated with Rayleigh waves were preceded by small oscillations that occurred concurrently with S waves and Love waves. We also show that the groundwater flow associated with these small oscillations may be strong enough to remove blockades from sediment pores to enhance aquifer permeability and to facilitate the later major responses.

S U M M A R YEarthquake-related groundwater level changes have been recorded by a dense network of monitoring well stations in Taiwan. At most multiple-well stations, the direction and magnitude of coseismic water level changes vary in wells of different depths. Comprehensive water level data recorded in 209 wells in the vicinity of the seismogenic fault during the 1999 M L 7.3 earthquake demonstrate a preliminary 3-D distribution of coseismic changes. The largest coseismic rise at a station observed typically in a confined gravel aquifer indicates the association of the magnitude of coseismic change with characteristics, rather than depth, of the aquifer. The variation of coseismic changes in the vertical direction implies possible inconsistency between the observed water level changes and the volumetric strains calculated from simple dislocation models. At a given hypocentral distance, the coseismic change in the footwall of the ruptured segment of the fault was much greater than that of the unruptured segment. This phenomenon suggests that fault displacement plays an important role in the generation of coseismic changes. While hypocentral distance correlates well with coseismic rise or fall in the vicinity of the ruptured seismogenic fault, poor correlation is found for coseismic change further from the earthquake epicentre.