EARTHQUAKE DATABASE FOR NEBRASKA AS A CONSTRAIN FOR MIDCONTINENT RIFT RELATED FAULTS

The Midcontinent Rift (MCR) is a failed rift that dates back to the Mesoproterozoic. It covers much of the North American craton and is recognized in gravity and magnetic fields by prominent highs. Regularly occurring small magnitude earthquakes (below 4.5) correspond to the MCR related faults in Nebraska and adjacent states. Our study focuses primarily on the SE part of Nebraska where two well populated cities of Lincoln and Omaha are located. The primary objectives of this project are 1) to compose a comprehensive earthquake database for Nebraska and parts of surrounding states, and 2) to use the earthquake data as constraints for the structure map derived from joint analysis of gravity and magnetic data.

For the earthquake database, we used various sources including the USGS Earthquake Catalog, ISC Bulletin, the North American Moment Tensor from SLU, vintage Nebraska earthquake catalogues and articles from the local newspapers. Up to date, the database includes 228 events from the nineteenth century to the present ranging in magnitude from 0.3 to 5.1 with the predominant focal depth of 5 km. The available focal mechanisms suggest that prevailing stress regime is stretching in SW-NE directions. The general NW-SE trends in earthquake locations are consistent with MCR-related faults known from literature. We used the earthquake data to validate and constrain the subsurface faults interpreted from spatial analysis of gravity and magnetic fields. The integration of earthquake data with potential fields increased the overall confidence of the derived geologic structures in our study area.