IMPROVED MAP OF SUBSURFACE STRUCTURES IN SOUTHEASTERN NEBRASKA FROM INTEGRATED ANALYSIS OF EARTHQUAKE AND POTENTIAL FIELDS DATASETS

The Midcontinent Rift (MCR) is a Mesoproterozoic failed spreading center in the middle of the North American craton that behaved in a similar fashion as the East African Rift System does today. The MCR is characterized by pronounced highs in gravity and magnetic fields. We focus our study on the structures associated with the portion of the MCR that is located in SE Nebraska. In this region, there are regular small magnitude earthquakes (below 4.5) associated with MCR-related faults. The study area includes the densely populated cities of Omaha and Lincoln, so a confident seismic hazard assessment is important to the general public. This project aims to supply a comprehensive earthquake database compiled from various sources as well as an improved structure map of the study area in order to better understand distribution and intensity of seismic activity.

The workflow for this study includes the following steps: 1) to compose an earthquake database and 2) to perform a combined analysis of earthquake and potential fields data with intent to locate subsurface faults surrounding the MCR in Nebraska. We compiled a database of earthquakes in Nebraska from various sources including the USGS earthquake database, Harvard Centroid Moment Tensor catalog, historical records from literature, and information from the Kansas and Nebraska Geological Surveys. The database is composed of information such as date, time, latitude and longitude, depth, magnitude or intensity (depending on which is available), and focal mechanisms. Then, we grouped seismic events based on their location, focal mechanism similarities, proximity to known structures, and correlate those groups to known lineaments in observed potential fields. By integrating all of these data together, we increase the confidence of the interpreted faults’ locations on our structure map.

As a result of this project, we intend to 1) develop an inclusive earthquake database of SE Nebraska for future use, 2) boost the confidence level of our structure map by integrating multiple geophysical and geological datasets. These results will help us better understand the overall seismic activity in our study area as well as supply important input information for seismic hazard assessments.