AQUIFER STORAGE PROPERTIES FROM GROUNDWATER FLUCTUATIONS INDUCED BY SEISMIC RAYLEIGH WAVES

Accurate aquifer storage properties, storativity S or specific storage Ss, are essential for proper groundwater resource evaluation and management. In confined aquifers, observation wells can be sensitive to large global earthquakes and commonly produce hydroseismograms with centimeter scale water level fluctuations. Regional broadband seismographs can generally provide excellent estimates of Rayleigh wave displacement. To account for the sparse data collection interval of most well recorder data compared to the seismic data, we propose that the mean squared water level fluctuation as measured in the time domain in each well be compared to the mean spectral density (after adjusting to the wavelengths) of the passing Rayleigh wave. For each individual well, Ss can then be calculated from the ratio of these quantities. The Palouse Basin Aquifer Committee (PBAC) maintains a monitoring network of 13 observation wells instrumented for continuous water level monitoring. For aquifers with multiple observation wells, this provides a method to calculate storage properties from an event based stress that can applied uniformly over the areal extent of the aquifer. As an example, we determine storativities using earthquake induced water level fluctuations for the observational wells in the Grande Ronde Aquifer of the Columbia River Basalt Group for comparison with previous estimates from barometric efficiencies, aquifer compressibilities, and aquifer tests. Our results confirm that the storativity in this confined aquifer is very low.