EARTHQUAKE B-VALUES: A KEY TO PAST DEFORMATION OR TO CURRENT STRESS STATE?

The size/frequency distribution (also known as the b-value) of naturally occurring earthquakes in the Illinois Basin was analyzed using data downloaded from the USGS Earthquake database. For comparison, a fault length distribution was developed for a portion of the Illinois Basin where a fault map was developed from dip change analysis on the top of the Mississippian Beach Creek Limestone in the south-central part of the Illinois Basin. Surface dip changes higher than a specified threshold were interpreted as faults. The fault lengths were then used to develop a fault length/frequency “b-value” (b_F) to compare to the earthquake b-value for the same area. The resulting fault b_F is very close to the earthquake b-value (b = 0.96 and b_F = 0.95), suggesting that the earthquake size/frequency distribution is structurally controlled. This indicates that the size of the earthquakes is not fully determined by the earth stress regime as has been suggested by other researchers, but rather, is controlled primarily by reactivation of existing faults and fractures. The b-value for induced seismicity data from a CO₂ injection project in the north-central part of the Illinois Basin was higher than that determined from historical natural seismicity (b=1.0) in the south-central area of the basin. Detailed subsurface fault interpretation in the area of the induced seismicity was associated with large uncertainties due to vertical fault displacements that are near the limit of the reflection seismic resolution. In addition, little natural seismicity is available in this area for comparison. However, much of the deep faulting in this part of the Illinois Basin could be strike-slip in origin, which is consistent with small dip-slip fault displacement and also with the global earthquake data b-value = 1 for active strike-slip earthquake regimes. It is possible that the range of b-values in a region provides important clues to the structural style even when other information about the subsurface is unavailable, making the earthquake the b-value a useful parameter to infer past structural deformation.