A Notable Earthquake Swarm in Alabama: Natural or Anthropogenic

Seismicity in the central and eastern United States is typically characterized by low-magnitude earthquakes (M < 4) that occur in areas of low to moderate strain rates. In November 2014, however, an unusual swarm of earthquakes began in northwestern Greene County, Alabama. The magnitudes of these earthquakes are small, less than 3.8, with many earthquakes only lightly felt by residents in the area. This study investigates the spatiotemporal and waveform characteristics of the Greene Country earthquake cluster to explore its possible source and the physical mechanism responsible for its occurrence. Although the possibility that the swarm is related to wastewater disposal or resource extraction cannot be unequivocally ruled out, our analysis suggests that the swarm more likely represents a continuation or southwesterly migration of the Southern Appalachian seismic zone. Earthquake relocations and calculated focal mechanisms are consistent with slip along northeast-trending faults that support both explanations. However, the large distance between the epicentral area and locations of active wells exceeds that typically associated with induced seismicity and would necessitate a hydrologic connection along regional faults that would allow propagation of a pore-pressure front from the area of resource recovery to the area of earthquake activity