SEISMIC HAZARD AND MIDCONTINENT GEOLOGY

Seismic-hazard analysts use paleoseismology and tectonics to determine likely rates of future large, damaging earthquakes. In most cases, the historical earthquake record is too short to provide reliable estimates of rates of large earthquakes, so paleoseismologists study the geologic evidence of past large earthquakes to extend the historical record into the late Quaternary. However, in most of the North American stable continental region (SCR: east of the Rocky Mountains), sparse seismicity urges the additional use of tectonics. During the 1980’s, A.C. Johnston (Univ. Memphis) and colleagues collated data on damaging earthquakes of the world’s SCRs. Their seminal finding is that large SCR earthquakes occur preferentially in Phanerozoic rifts and passive margins, with the preference being stronger for younger rifting. Accordingly, the USGS national seismic-hazard maps divide the North American SCR into a seismically less active central craton, and a more active continental rim of Phanerozoic aulacogens, passive margins, and orogens. However, except for the well-known Reelfoot rift, additional geologic controls on midcontinent seismicity remain enigmas. Why is the southern Illinois basin so much more active than the rest of the craton? Why is the Gulf Coast, in the continental rim, less active than the Dakotas, in the craton? Why is the 1.1 Ga Midcontinent rift less active than the surrounding craton? Do Archean and Proterozoic terranes differ globally in their seismicity levels? How fast and how strongly do reactivated cratonic faults heal? Answering these and similar questions will likely require collaboration of seismologists with geologists who are familiar with the North American SCR and its global analogues.