• Virginia;
  • earthquakes;
  • passive margin

[1] The August 2011 M 5.8 Mineral, Virginia, earthquake that shook much of the northeastern U.S. dramatically demonstrated that passive continental margins sometimes have large earthquakes. We illustrate some general aspects of such earthquakes and outline some of the many unresolved questions about them. They occur both offshore and onshore, reaching magnitude 7, and are thought to reflect reactivation of favorably-oriented, generally margin-parallel, faults created during one or more Wilson cycles by the modern stress field. They pose both tsunami and shaking hazards. However, their specific geologic setting and causes are unclear because large magnitude events occur infrequently, microseismicity is not well recorded, and there is little, if any, surface expression of repeated ruptures. Thus presently active seismic zones may be areas associated with higher seismicity over the long term, the present loci of activity that migrates, or aftershock zones of large prehistoric earthquakes. The stresses causing the earthquakes may result from platewide driving forces, glacial isostatic adjustment, localized margin stresses, and/or dynamic topography. The resulting uncertainties make developing cost-effective mitigation strategies a major challenge. Progress on these issues requires integrating seismic, geodetic, and geological techniques.