Subsurface structure in the vicinity of an intraplate earthquake swarm, central Arkansas

Abstract

Over 40,000 events have been recorded in the Arkansas earthquake swarm since its inception in 1982. The earthquakes occur at depths between 3 and 6 km and cluster in a volume of about 25 km 3 beneath the easternmost Arkoma basin, near the town of Enola, Arkansas. A study of proprietary reflection seismic lines reveals that the earthquakes cluster within a graben formed in Mississippian time. This graben is part of a system of steeply dipping normal faults that trends ENE across the region. The regional strikes of the basement faults are not favorably oriented for activation under the regional stress regime. In the swarm area, however, these faults bend to form a 2.5-km long segment trending WNW. The small WNW striking segments are well oriented for left-lateral strike slip and focal mechanisms are consistent with this sense of slip. Additionally, a subset of the most accurately located earthquakes do appear to lie along a WNW trend. The length of the WNW trending fault segments is sufficient to have generated the largest of the swarm events. The reflection data reveal a loss of coherent reflectors within the swarm hypocentral volume. Reflectors above the graben have been uplifted about 30 m in post-Atokan (Pennsylvanian) time. Third order leveling surveys show 14.3 cm of uplift between 1961 and 1986 at a benchmark over the graben relative to a benchmark outside of the graben.