Program

What’s Shaking, Eastern North America? The Continuing Quest to Understand Intraplate Quakes

Susan Hough | Geophysicist, U.S. Geological Survey (Pasadena)

The year 2025 marks the centennial anniversary of three moderately large North American earthquakes: Quebec, Montana and Santa Barbara. Before Richter’s introduction of the magnitude scale, the differences in shaking distribution led credence to arguments that California did not face inordinately high earthquake hazard compared to the rest of the U.S. A century later, scientists understand the stark difference in wave propagation in eastern North America versus the west. But a half-century after plate tectonics provided an elegant paradigm to understand interplate earthquakes, a paradigm to explain intraplate quakes remains elusive. Detailed investigations have shed new light on key seismic zones, including New Madrid, Charlevoix (Quebec) and Charleston (South Carolina). Paleoliquefaction evidence confirms the existence of long-lived seismic zones, most convincingly at New Madrid. But why does stress (or strain) concentrate in a region subjected to broad, low-strain rate tectonic stresses? And how many unknown seismic zones might be lurking quietly, having remained mute during the short historical record? Hough will discuss the continuing quest to understand intraplate quakes, including the known knowns, the known unknowns and the unknown unknowns.