Intraplate seismicity of the New Madrid seismic zone in the central USA superimposed on a topography map with warmer colors indicating higher elevations. The red circles are earthquake locations from local seismic network analyses. The magenta line shows the boundary of the Mississippi embayment structure that, geologically, is an incursion of the Gulf of Mexico coastal plain. Thick black lines show the approximate boundary of the Reelfoot Rift zone, an ancient (approximately 500 million year old) rift that has been subsequently covered by recent Mississippi embayment sediments. Ancient faults of the Reelfoot Rift have presumably been reactivated to form the complex fault structures seen in the distribution of earthquakes. This region has experienced at least four series of large earthquakes over the past 2500 years, including three with magnitudes of 7.0, 7.2, and 7.5 in 1811–1812. (Image courtesy of M.B.Magnani.)
The theory of plate tectonics predicts that most earthquakes result from repeated accumulation and release of strain at the contacts between differentially moving plates. However, plate tectonic motions do not readily account for earthquakes located within the interiors of oceanic and continental plates. These “intraplate” earthquakes present fundamental challenges to understanding how strain accumulates and what the associated seismic potential and hazard are for intraplate regions. For example, large areas of Australia, North America, Asia, and Europe experience intraplate earthquakes, some of which are catastrophically large. Three large earthquakes in North America occurred in 1811–1812 in the Mississippi River valley of the central United States and a large event in 1886 severely damaged Charleston, South Carolina. Some other intraplate earthquakes, such as those in Hawaii, are associated with volcanic processes. Interactions between lithospheric and asthenospheric systems may be the cause of these events, but the mechanisms are not known.
Plate interiors do not appear to experience significant strain over long periods of geological time, nor have GPS measurements detected significant strain rates in areas of past large intraplate earthquakes. However, paleoseismologists, who study historic faulting events, have found geological evidence of repeated large earthquakes over thousands of years in intraplate environments like the New Madrid seismic zone of the central United States. There are many ideas and speculations on why intraplate events occur, but no consensus has emerged on how they are generated or how they continue to occur over many earthquake cycles. Intraplate earthquakes also present significant seismic hazards, especially because they can affect densely populated regions with little preparation for seismic shaking. The existence of intraplate earthquakes remains a deep scientific mystery with strong societal implications that needs to be solved by innovative approaches.
Photographer / Contributor: M.B.Magnani