FAULT LOADING AND EARTHQUAKE OCCURRENCE WITHIN LOCALIZED ZONES OF RELATIVE WEAKNESS

The earth's crust is laterally inhomogeneous. Variations in crustal thickness, thermal structure, and rheology are present. As a result of prior tectonic episodes, repeatedly reactivated, localized zones of relative weakness also exist (e.g. the New Madrid Seismic Zone). Within these weak areas, long-term geologic strain-rates, fault geometries, and earthquake recurrence intervals depend on the rheology and geometry of the weak zone as well as its relation to the material that surrounds it.

The effectiveness of tectonic loading due to far-field plate motions in generating seismicity within localized areas of relative weakness depends on deformation rates and stress levels in the surrounding material. Weak zones embedded within diffuse plate boundaries (e.g. the Basin and Range Province) localize tectonic stresses more readily that those embedded within stable continental interiors. Weak zones embedded within stable, high stress cratons (e.g. India) localize tectonic stresses more readily than those embedded within cratons at average stress levels (e.g. North America).

Seismicity in localized area of relative weakness can also be generated in response to stresses resulting from local and regional processes (e.g. fluid effects, thermal effects, gravitational loading due to buoyancy, topography, or other surface loads, and/or stress concentration due to rheologic and structural heterogeneities). Transient perturbations in these local/regional stresses may lead to geologically short-lived transient bursts of seismic activity within the weak zone.

Finally, because the weak zone is embedded within stronger material, its geometry and lateral extent at depth is an important factor in determining the distribution and geometry of faulting within the seismogenic upper crust. 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. Thus, unless faults bounding the weak zone are unusually weak, more optimal faulting geometries may develop. If the process that is driving the seismicity is transient, the locus of deformation may also shift with time as stresses are redistributed and relaxed.