TRIGGERING OF NEW MADRID SEISMICITY BY LATE PLEISTOCENE EROSION

The spatio-temporal behaviour of earthquakes within continental plate interiors is different from that at plate boundaries. At platemargins, tectonic motions quickly reload earthquake ruptures, makingthe location of recent earthquakes and average time between themconsistent with the faults' geological, paleoseismic, and seismichistories. In contrast, what determines the activation of a particularmid-continental fault and controls the duration of its seismicactivity remains poorly understood. Here we argue that the concentrationof magnitude 7 or larger earthquakes in the New Madrid seismic zoneof the central U.S. since the end of the last ice age results fromthe recent, climate-controlled, erosional history of the northernMississippi embayment. We show that the upward flexure of thelithosphere caused by unloading from river incision between 16,000and 10,000 years ago caused a reduction of normal stresses in theupper crust sufficient to unclamp pre-existing faults close tofailure equilibrium. Models indicate that fault segments that havealready ruptured are unlikely to fail again soon, but stress changesfrom sediment unloading and previous earthquakes may eventually besufficient to bring to failure other nearby segments that have notyet ruptured.