Shallow Seismicity in Stable Continental Regions

A worldwide compilation of well-constrained fault ruptures and focal depths of earthquakes reveals that the Earth9s crust in many stable continental regions (SCRs) is characterized by a bimodal depth distribution with a very shallow upper crustal component. The distributions can vary in a) depth of the modes and b) strength of bimodality, probably due to intracrustal boundaries, differences in frictional and rheological properties, heat-flow densities, strain-rates, and tectonic forces or forces stemming from the surface. Overall, SCR ruptures and SCR earthquakes are confined within the upper third (0–10 km) and/or the lower third of the crust (20–35 km), while the midcrust (10–20 km) tends to be aseismic. Historical data indicate that some SCRs show very well-developed bimodal distributions of focal depths ( e.g. , North Alpine foreland basin in Europe, Kachchh basin in India), while others show weak to no developed bimodal distributions ( e.g. , Charlevoix seismic zone, New Madrid seismic zone in the central United States). Moreover, many large SCR earthquakes ( M w 4.5–8.0) nucleate on reverse faults and close to the surface ( m b e.g. , ground motion).