Date of this Version
THE MOUNTAIN GEOLOGIST, Vol.. 55, No. 4. , pp. 217-229 (December 2018)
A sudden spike in earthquake events has been observed in central Nebraska. Since April 2018, 26 earthquakes with equivalent moment magnitudes from 2.7 to 4.1 occurred, clustered tightly in Custer County. A similar cluster of 24 earthquakes with equivalent moment magnitudes from 2.6 to 3.7 occurred in Jewell County in northern Kansas in 2017. We have compiled an earthquake database for Nebraska and parts of adjacent states from different sources to determine whether these recent earthquake spikes are consistent with historic seismicity. We identified two historic earthquake clusters occurring in our study area. The first contained 32 events and was active in Red Willow County in southwestern Nebraska from 1977 to 1982. As it coincides spatially with the Sleepy Hollow oil field, it may be related to enhanced oil recovery from that field, although it is also located at the edge of the Chadron-Cambridge Arch. The second historical earthquake cluster is located in Pawnee and Richardson counties in southwestern Nebraska and includes eight earthquakes with equivalent moment magnitudes of 2.3 to 2.8 that occurred in a period from 1982 to 1989 over the Nemaha uplift and appear to be related to the Humboldt fault. We note an increase in both maximum magnitude, as well as in the cumulative seismic moment per cluster with time. We have also used gravity and magnetic fields to map potential basement faults in the study area. Our analysis shows that the two recent earthquake spikes are aligned with the proposed basement faults. Despite this correlation, the cause of this sudden spike in seismicity is not well understood, as the stresses that might reactivate these basement faults are unknown. In addition, both recent clusters are distant from oil and gas operations. More seismic stations are necessary in central Nebraska in order to better detect focal depths and faulting style in the ongoing cluster of earthquakes and investigate possible causes.