GEORGE P. WOOLLARD TECHNICAL LECTURE: NEW VIEW OF NEW MADRID: LITTLE MOTION, COMPLEX FAULTS, SMALL HAZARD

GPS studies over 18 years within the New Madrid Seismic Zone show no detectable motion to steadily increasing precision - currently 0.2 mm/yr. The NMSZ is thus deforming too slowly - if at all - to account for large earthquakes in the area over the past ~5,000 years. Hence the recent cluster of large magnitude events does not reflect long-term fault behavior. The GPS data together with increasing evidence for temporal clustering and spatial migration of earthquake sequences in continental interiors indicate the need for a different view on midcontinental earthquakes. Traditionally, intraplate seismic zones have been treated like slowly deforming (< 2 mm/yr) plate boundaries. We expected steady deformation in narrow zones, such that the past rates and locations shown by geology and the earthquake record would be consistent with present deformation shown by geodesy, and predict future seismicity. It now seems more useful to view mid-continent earthquakes as migrating, episodic, and clustered. Instead of occurring quasi-periodically along a single major fault system, they migrate over many faults in a large area. A fault will be active for several earthquake cycles, and then become dormant as others become active. Because deformation can be steady for a while then shift, the past earthquake history can be poor predictor of the future, and hazard assessment based on the recent earthquake record can overestimate risks in regions of recent large earthquakes and underestimate them where seismicity has been quiescent. In this scenario the currently active parts of the NMSZ are the presently most active one of many faults, the recent seismicity are primarily aftershocks of large past events, and the lack of present deformation suggests that the recent cluster of earthquakes has ended. It is useful to view midcontinent earthquakes as a classic complex system controlled by interactions between faults. Although an individual fault taken in isolation acts quasi-periodally, the network of interacting faults gives complex variability in space and time. Initial numerical modeling shows that fault interactions can give rise to such variability without local or time-variable loading, either of which can provide further variability. A complex system view seems likely to lmprove understanding of midcontinent tectonics, the resulting earthquakes, and their hazards.