Evaluation of Seismicity Trends in Kansas to Determine Possible Source Mechanisms Using Focal Mechanism Inversion and Spectral Analysis

Abstract

High b-value, proper stress orientation and low stress drop are three factors that support the suggestion that a majority of events in central Kansas are induced as a result of changes in pore pressure. Furthermore, stress orientation and seismic trends align with regional lineations interpreted from magnetic data. These linear trends are likely influencing seismicity in the northern part of the state and the feature that supports the suggestion that injection of wastewater in an area 90km away is influencing seismicity in Smith, Jewell and Republic counties in Kansas. Kansas seismicity started in 2014 and rapidly increased in frequency, culminating in the largest recorded earthquake in state history on November 12th, 2014. This size event led to a drastic increase in interest and therefore the number of seismic stations distributed in areas previously determined aseismic. This resulted in the discovery of new earthquake clusters located outside historically active seismic areas. The Kansas Geological Survey (KGS) deployed a temporary and permanent station array to significantly improve coverage offered by a temporary network installed by the United States Geological Survey in 2013. This enhanced KGS network along with other stations installed across the state revealed new clusters in Jewell and Saline County, areas that were considered seismically quiescent. The focus of this research is to explore the characteristics of seismic activity clustering in locations outside the high profile area in south-central Kansas with the most proliferate seismic activity (Harper and Sumner Counties). These focus areas include Reno, Salina, and Jewell Counties where new clusters of seismic activity have developed since 2016. These three areas are host to notable clusters and were chosen base on unique and historically inconsistent seismic activity. For each of the three study areas (Jewell, Saline, and Reno) four focal mechanism algorithms determined the focal sphere orientation of 95 events using 34 different stations and the maximum horizontal stress for each area was calculated using a Michael 1984 inversion. Stress orientations in iv Reno County are consistent with those calculated in Oklahoma but rotates almost 90ᵒ further north in Jewell County. Brune stress drop was calculated for 90 events in each of the three study areas to compare with stress drops calculated in the south portion of the state (Harper and Sumner counties) and across the Central United States. Low stress drops throughout the state are consistent with the range of stress drops found for induced events in Oklahoma (Hough, 2014; Sumy et al., 2014) and other parts of the Central United States (Boyd et al., 2017).