GEOLOGICAL CHARACTERIZATION OF SEISMICITY IN THE ILLINOIS BASIN

The risk for induced seismicity from high volume fluid disposal as a result of hydraulic fracturing has generated public concern about man-made seismic hazards. To better understand the potential for seismic hazards, we have examined the size/frequency distribution (also known as the b-value) of naturally occurring earthquakes in the Illinois Basin. Using data downloaded from the USGS database, a b-value was determined for all earthquakes from the Illinois Basin, from 1973 to 2018. A dip change analysis was completed by regional mapping of the top of the Mississippian Beach Creek Limestone (commonly named Barlow lime) using wireline logs from Illinois. Surface dip changes higher than a specified threshold were interpreted as faults. The same type of b-value analysis that was done on the earthquakes was applied to the interpreted faults by using the fault length/frequency. A comparison of the fault b-value analyses and the earthquake b-value analysis shows a strong correlation, suggesting that the earthquake size/frequency distribution is structurally controlled, and the size of the earthquakes is not determined by the earth stress regime, but rather by reactivation of existing faults and fractures. Because the range of b-values in a region provides clues to the structural style it can be used to support interpretations of faulting styles from other sources, and therefore help refine the history of basin development. This relationship has the potential to quantify the expected earthquake size and frequency that may result after an increase in seismicity induced by human activities and could provide probability estimates for large earthquakes.