Induced seismicity of central Arkansas

Enola area is known to generate swarms of earthquakes that are clustered in space and time. The remarkable swarm of the early 80s generated thousands of earthquakes many of them were felt by people lived in the area. More recently, a magnitude 4.4 earthquake took place on May 4, 2001 in the area which was followed by thousands of recorded earthquakes. The main shock was felt widely in the region including the city of Little Rock. On February 27, 2011, North Central Arkansas area was shaken by an earthquake of magnitude 4.7. This earthquake was the biggest to be registered in this area for the last three decades.;During the summer of 2009, a three-component 7-element small aperture array was installed in a three kilometers radius area centered at a Deep Six disposal well. The installation of the array was part of the Arkansas Oil and Gas Commission permit requirement to monitor the seismic activities that might be induced due to fluid injection at that well. The injected fluid is the byproduct waste water of the natural gas exploration processes in the Fayetteville Shale formation of North Central Arkansas.;Seismic data was collected for the period between September 2009 and August 2011. The array recorded more than 8000 events which have been analyzed and located. Local magnitude for these events ranges between -1.5 and 4.7. In order to establish a correlation between induced seismicity and fluid injection, we examined the background seismicity, temporal and spatial correlation, local geology, and injection practice. Results of this study show that within the immediate vicinity (5 km radius area) of the Deep-Six well there was no direct correlation between seismicity and fluid injection. The majority of seismic activities which appear to cluster around a NE-SW linear trend were located more than 11 km NW from the Deep Six well in an area between the cities of Greenbrier and Guy. All events of magnitude 2.0 and larger were relocated; the majority of the relocated events seem to be better sited within the NE-SW linear zone between Greenbrier and Guy. The seismic analysis indicates that the activation or reactivation of the seismicity has been triggered by the injection of hydrofracturing waste water in wells in the vicinity of the seismicity. Another three linear features have been determined near the Deep Six well. The seismic events at these linear features have a focal depth less than 3 km, which could be related to either the tectonic activities or fluid injection. At this stage it is difficult to rule out any of these mechanisms because of the short period of recording that ended due to the shutdown of the injection well.;Focal mechanism solutions of 49 events with magnitude of ≥ 3 ML were performed. The results show that the strike-slip movement is the most dominated faulting style in the study area. Focal mechanism solutions were used to derive the compressional stress axes, which indicate that the main direction of the stress was NE-SW.