VARIABLE WATER LEVEL RESPONSES TO STRESS IN A SANDSTONE AQUIFER OF THE ILLINOIS BASIN

Hydrological systems respond to a variety of stressors. Several years of hourly groundwater levels from a deep-shallow piezometer nest in the Pennsylvanian Inglefield Sandstone of Southern Indiana display water-level responses to barometric pressure and earth tides, but do not have the resolution necessary to detect potential changes from seismicity. Existing data show water-level fluctuations of up to 5.5 cm/day (2.2 in/day) with a pronounced inverse relationship to barometric pressure. Water level fluctuations of 2.7 cm (1.1 in) with periodicities of 12 and 12.4 hours correspond to solar and lunar tidal stressors. To observe potential strain responses due to seismic stress, a pressure transducer with data logger was deployed in the Groundwater Monitoring Laboratory at the University of Southern Indiana (USI) to measure ground water levels every minute. The USI campus is also instrumented with a seismometer as part of the Cooperative New Madrid Seismic Network and the Advanced National Seismic System (ANSS).

Since deploying the high-resolution system, several notable far-field earthquakes, including a magnitude 6.5 earthquake in Costa Rica on October 24^th, 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 20^th, 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.