Effects of Driving Stress and Rheology on the Temporal and Spatial Distribution of Faulting Within Intraplate Seismic Zones

Within stable continental regions, present-day seismicity is often highly localized. The reasons are not well understood, but intraplate seismic zones frequently overly ancient failed rift zones. Such zones may be weak relative to their surroundings, thereby explaining the repeated concentration of deformation at these locations over hundreds of millions of years. One example, the New Madrid Seismic Zone in the south-central U.S. produced 3 M ~7.5 events in 1811-1812. Within intraplate weak zones, fault geometry, 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 and the zone's rheological characteristics. If the weak zone is loaded via far-field plate driving forces, stresses concentrate 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 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). Finite element models show that these transient perturbations yield geologically short-lived bursts of seismicity during which deformation rates and earthquake recurrence intervals change with time as stresses are redistributed and relaxed. The spatial distribution of faulting is dependent on 1) the geometry and lateral extent of the weak zone at depth and 2) the lateral distribution of strength in the overlying seismogenic crust. To investigate the latter effect, plastic rheologies are used which permit the formation of faults of arbitrary orientation. Relaxing weak zones increase stress in an area of the upper crust whose lateral extent is equal to that of the underlying weak zone. For a homogeneous elastic layer, strain-rates are highest above the center of the weak zone potentially activating faults at this location. Activation of weak faults at other locations during the initial stages of the relaxation process may significantly alter the spatial distribution of faulting.