Microseismicity of the Intraplate Northern Wabash Fault System and La Salle Anticlinorium, Illinois Basin, U.S.A.

Abstract

ABSTRACT Within the Illinois basin, the northern Wabash Valley seismic zone (WVSZ) and adjoining La Salle anticlinal belt have produced three moderate‐sized earthquakes ranging from m b 4.7 to m b 5.2 in the past 50 years. However, few smaller events from this area are recorded in regional catalogs. The Wabash fault system extends from the junction of the Rough Creek and Cottage Grove faults in the south along the Wabash River into the La Salle anticlinal belt to the north. These faults extend through the Paleozoic and into the Precambrian basement. Two of these moderate‐sized earthquakes occurred north of the terminus of the Wabash Valley fault system where it meets the La Salle anticline. The La Salle anticline, a Precambrian basement feature, is oriented northwest to southeast. The La Salle anticline creates up to a 750 m uplift in the above Paleozoic strata. This uplift creates faults within the Paleozoic strata and within the La Salle anticline. This study uses a nine‐seismograph phased array near the southern terminus of the La Salle anticline to monitor the microseismicity of the region. Analyzing the seismicity of the region over a six‐month period found 834 events. The vast majority were determined to be mine blasts or otherwise human induced. Two small earthquakes ( M L ∼1.0) located near the La Salle anticline. These earthquakes likely occur on faults associated with the anticline. The lack of microearthquakes suggests the b ‐value of the La Salle region is lower than the typical value of 1. The b ‐value is consistent with other intraplate regions and previous studies of the WVSZ finding values nearer 0.7 or even potentially as low as 0.6.