Improving Locations of 2018 Central Nebraska Earthquakes via Joint Inversion of P- and S- Waves’ Arrival Times

Abstract:

In 2018, a sudden swarm of 27 earthquakes occurred in the traditionally aseismic state of Nebraska. The equivalent moment magnitudes of those events ranged from 2 to 4.1. The preliminary analysis of potential fields and earthquake epicenters suggested that the seismicity relates to reactivation of a preexisting, but previously unknown, basement fault system near Arnold, NE (Guthrie et al., 2018; Filina et al., 2018). The epicenters published by the United States Geological Survey (USGS) are spread out in the 10 x 10 km region. Moreover, only four events in the cluster have focal depths different from 5 km- the USGS automatically assigned value representing a large uncertainty. Additionally, the recorded seismograms show significant discrepancies between the observed and USGSestimated P- and S-waves’ arrival times.

The cause of seismicity remains unknown. Only six focal mechanisms were published by USGS; they show contradictive patterns with two suggesting compressional faulting and four showing extensional. No seismic activity in that region has been recorded since 2018. In order to understand the nature of this abrupt earthquake cluster, the responsible fault system should be mapped. Since there is no surface expression, geophysical methods, such as gravity and magnetics, should be utilized. The acquisition of potential fields should be guided by the location of earthquakes that appear to have large uncertainties. The objective of this study is to improve the hypocenters’ locations by inverting the P- and S-waves’ arrival times recorded by the three nearest stations. The resulting better-focused hypocenters should allow for more efficient potential field surveying.

A two-layered subsurface model was assumed for inversion: the 1.1 km thick sedimentary layer over the upper crust. We invert both the hypocenters’ coordinates and origin times for the entire cluster as well as seismic velocities for each layer. Our inversion results in a better match of observed and predicted arrival times and reduces the overall uncertainty of the clustered hypocenters. The earthquakes’ improved locations will ultimately guide the geophysical surveying over the seismicity region in order to map the fault system and comprehend the cause of its sudden reactivation.