LOCALIZED INTRAPLATE WEAK ZONES: SOURCES OF STRESS AND THE SPATIAL DISTRIBUTION OF SEISMIC SLIP

Within stable continental regions, present-day seismicity is often highly localized. The reason is not well understood, but it has been found that intraplate seismic zones frequently overly ancient failed rift zones. Such zones can be considered weak relative to their surroundings, thereby explaining the repeated concentration of deformation at these locations over hundreds of millions of years. One example is the New Madrid Seismic Zone in the south-central U.S. which produced 3 M ~7.5 events in 54 days in 1811-1812.

Within intraplate weak zones, fault geometries, the temporal evolution of earthquake repeat times, and the transient vs. steady-state production of large earthquakes depends on the source of stress that drives seismicity. Rheology and geometry of the weak zone are also important. If the weak zone is loaded via far-field plate driving forces, the likely area for stress concentration is at the weak zone boundary. As a result, major rift bounding faults may be reactivated. The concentration of far-field stress will also be continuous over periods of a few million years (time required for significant changes in plate motion directions) and major earthquakes will be continuously produced.

Alternatively, the stress driving seismicity could derive from weak zone relaxation following local or regional perturbations to the stress field (e.g. fluid effects, thermal effects, and/or gravitational loading due to buoyancy, topography, or other surface loads). Such transient perturbations yield geologically short-lived transient bursts of seismicity within the weak zone. In this case, the spatial distribution of faulting in the seismogenic crust is critically dependent on the geometry and lateral extent of the weak zone at depth. Relaxing weak zones increase stress in an area of the brittle crust whose lateral extent is equal to that of the underlying weak zone, but strain-rates are highest at the center. Unless the weak zone is extremely narrow and/or the rift bounding faults are unusually weak, major bounding faults may not be reactivated. Instead, more optimal faulting geometries may develop above the center of the weak zone. Since the process driving seismicity is transient, earthquake recurrence intervals and the locus of deformation may also change with time as stresses are redistributed and relaxed.