Monitoring the Increase in Seismicity in South-Central Kansas

There has been a dramatic increase in seismicity in the Midcontinent over the last five years, which appears to be linked to the injection of large volumes of wastewater from oilfield operations. Injection of fluids into deeper formations causes an increase in pore pressure, which can facilitate slip on existing faults oriented optimally to subsurface stress fields. Very little is known about the stresses within the shallow basement in Southern Kansas which has seen an increase in seismicity. The historical average of 21 M>3 earthquakes a year has increased to 188 M>3 reported earthquakes observed in 2011, in the US midcontinent. Earthquake focal mechanisms were analyzed for western Sumner County, south-central Kansas, from May of 2015 to July of 2016. The Kansas Geological Society (KGS) seismometer array in the Wellington Oil Field and the United States Geological Survey (USGS) portable array in southern Kansas were used to locate the earthquakes. These arrays generated a catalog of events from Mw .4 to Mw 3.5. Analyses of focal mechanisms for nearly 200 earthquakes larger than approximately Mw 2.0 were included in the inversion. Earthquakes of this magnitude were recorded on nearly all stations. The larger magnitude events tend to cluster in Northeast-Southwest and Northwest-Southeast lineations. These local, larger earthquakes provide a better understanding of the stresses that are causing the increased seismicity. The stress tensor was calculated for the region to the west of the city of Wellington, KS, in Sumner County. The primary horizontal stress direction is nearly east. This observation is in agreement with well data that estimates the maximum horizontal stress at approximately 75 degrees.