Discriminating Between Induced vs Tectonic Seismicity in Intraplate Regions: the Contribution of the Long-Term History of Fault Behavior

Maria Beatrice Magnani, Michael L. Blanpied, Heather DeShon, & Matthew Hornbach

Published August 15, 2016, SCEC Contribution #6692, 2016 SCEC Annual Meeting Poster #024

Since 2009 there has been an increase in rate of seismicity in the Central U.S. (CUS), a major fraction of which has been associated with shale gas production and related wastewater injection. The surge of seismicity has resulted in an increased seismic hazard in the region, which relies on the discrimination between seismic activity that is anthropogenically induced and that arising from natural tectonic deformation. This discrimination, however, 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 most recent deformation short-term deformation history of the involved faults, the long-term history is missing. Seismic reflection data offer a means to interrogate the long-term history of these faults, a history which can be discriminatory. In this poster we present examples from two regions of the CUS. The first region shows examples of tectonically active faults within the northern Mississippi Embayment imaged by high-resolution seismic reflection data. The faults deform Quaternary alluvium, as well as 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 sequences. Specifically, any vertical offsets in the post-Pennsylvanian formations are below the resolution of the seismic data at these depths (~10 m), far less than expected had these faults accumulated deformation over the long term, as observed in the first region. Assuming that current seismicity recorded in the North Texas Basin is representative of past seismic sequences, a vertical offset equal to or less than 10 m along the currently active faults implies a minimum average recurrence interval of ~150,000 yrs, based on average displacement/sequence derived from cumulative seismic moment calculations. These exceptionally long intervals are at odds with the increasing number of faults reactivated in the North Texas basin since 2008, and suggest that the recent seismicity in the North Texas basin is highly anomalous, and therefore more likely induced than natural.

Key Words
Induced seismicity, Central US, Reflection Seismology

Citation
Magnani, M., Blanpied, M. L., DeShon, H., & Hornbach, M. (2016, 08). Discriminating Between Induced vs Tectonic Seismicity in Intraplate Regions: the Contribution of the Long-Term History of Fault Behavior . Poster Presentation at 2016 SCEC Annual Meeting.

Related Projects & Working Groups
Stress and Deformation Over Time (SDOT)