EARTHQUAKE HISTORY, SEISMICITY, AND RELATED TECTONICS IN NEBRASKA

Nebraska experienced two significant earthquakes in 2002, one on June 20 (Richter 3.5) that had an epicenter near Ord and one on November 3 that had an epicenter NNW of O’Neill (Richter 4.3). Although neither earthquake was greatly damaging, both aroused public interest and excited media inquiry. About 50 earlier earthquakes, almost all < Richter 4, have occurred in Nebraska since 1867. Furthermore, a cluster of microearthquakes reported in 1979 in Red Willow County in southwestern Nebraska may have been related to petroleum production. Other microearthquakes were reported at about the same time in southeastern Nebraska along the trend of the Nemaha Uplift, a major N-S structure extending into basement rocks. Nebraska’s earthquake record, when incorporated with geophysical and petrographic data, helps define potential zones of weakness in basement rocks and seismic potential. Larger earthquakes seem to be related to the periodic release of accumulated regional stresses in major structures (Nemaha Uplift, Midcontinent Rift) and around proposed sutures resulting from the Proterozoic (1.8-1.6 Ga) accretion of island arcs across the region. The geographic association of earthquake epicenters with these Proterozoic to Phanerozoic features reflects common crustal precursors rather than the development of new fracture patterns. Although no seismograph stations are currently active in Nebraska, water wells (Lincoln, Aurora, Mead) monitored by the Conservation and Survey Division episodically record abrupt water-level changes (hydroseisms) that appear to coincide with major, distant earthquakes on the North American continent, such as the November 3, 2002 Alaskan earthquake. In general, however, Nebraska’s seismicity is insufficiently characterized and earthquake risk is difficult to evaluate.