Discriminating Between Induced vs. Tectonic Seismicity From Long-Term History of Fault Behavior in Intraplate Regions

Abstract:

Since 2009 there has been an increase in rate of seismicity in the Central US (CUS), a major fraction of which has been associated with shale gas production and related wastewater injection. Within this context it is important to discriminate between seismic activity that is anthropogenically induced from that arising from natural tectonic deformation. This discrimination is particularly challenging because tectonic strain rates and natural seismicity rates are low in this intraplate region, such that tectonically active faults may display periods of quiescence that are long (100’s to 1000’s of years) relative to the short (10’s of years) instrumental record. In addition, causative faults are unknown with a poor surface expression, both types of seismicity occur on or reactivate ancient faults in the Precambrian basement, and the instrumental seismic record is sparse. While seismicity provides information about the short-term history of deformation on the involved faults, the long-term is missing. Seismic reflection data offer a means by which to interrogate the long-term history of these faults, which can be discriminatory. In this paper we present examples from two regions of the CUS. The first region shows examples of tectonically active faults within the northern Mississippi Embayment south of the New Madrid Seismic Zone, which were imaged by a high-resolution seismic reflection survey along the Mississippi River. The faults deform Quaternary alluvium and underlying sediments dating from Tertiary through Paleozoic, with increasing amount of deformation with formation age, suggesting a long history of activity. The second region shows examples from the North Texas basin, a region of ongoing shale gas exploitation. Here, industry seismic reflection data image basement faults showing deformation of the Precambrian and Paleozoic sequences, and little to no deformation of younger formations. Specifically, vertical offsets, if any, in the post-Pennsylvanian formations are below the resolution of the seismic data (<~7m), far less than one would expect to have accumulated had these faults been active over the long term, as observed on faults in the first region. The lack of recent deformation suggests that current seismicity on North Texas faults may be due to an induced reactivation of inactive faults.