Clustered Seismicity in Alabama: Natural or Anthropogenic?

Abstract

Seismicity in the central and eastern U.S. is typically characterized by low-magnitude earthquakes (M < 4) with a random distribution that occur in areas of low to moderate strain rates. An unprecedented increase in earthquakes in the central and eastern U. S. since 2009 has been attributed to operations related to resource extraction. Since November 2014, an earthquake swarm consisting of 21 earthquakes in northwestern Greene County, Alabama, has been recorded. The magnitudes of these earthquakes are small, less than 3.8, with many earthquakes only lightly felt by residents in the area. The source of the seismicity is unknown. This study intends to investigate the spatiotemporal and waveform characteristics of the Greene Country earthquake cluster to explore its possible source and the physical mechanism responsible for their occurrence. After spatiotemporal and waveform characteristics analysis, anthropogenic activities including underground injection, oil and gas extraction, mining extraction, quarrying operations, and sinkhole formation are likely not the cause of the swarm, even though they cannot be unequivocally ruled out. While recent research has demonstrated an unequivocal link between resource recovery processes and seismicity, regional tectonic processes and geologic structures are also responsible for seismicity. Consequently, this study favors the interpretation that the earthquakes may represent a continuation or southwesterly migration of the Southern Appalachian seismic zone (SASZ), because the Greene County cluster follows a northeasterly trend and earthquake focal mechanisms consistently indicate right-hand lateral motion on NNE-trending planes, which is consistent with previous investigation.