VARIABLE WATER LEVEL RESPONSES TO STRESS IN A SANDSTONE AQUIFER OF THE ILLINOIS BASIN
Since deploying the high-resolution system, several notable far-field earthquakes, including a magnitude 6.5 earthquake in Costa Rica on October 24th, 2012, a magnitude 7.7 earthquake in the Queen Charlotte Islands Region on October 28, 2012, and a magnitude 7.4 earthquake offshore of Guatemala on November 7, 2012, generated measurable ground accelerations on the USI campus. A magnitude 3.6 near-field earthquake, associated with the New Madrid fault system, occurred on November 20th, 2012 at a depth of 17 km (10.6 miles) approximately 55 km (34 miles) from USI. Water level data during these events showed no apparent response to seismic stress. The lack of response to earthquakes suggests that we have not yet observed a seismic event of sufficient magnitude or proximity to generate measurable responses, or the Inglefield sandstone does not respond to high-frequency seismic stress as it does to low-frequency tidal stress. Apparently the long period tidal stress allows adequate time for the sandstone to deform while a short period seismic stress occurs too rapidly for a deformation response by the rock body.