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Shallow Seismicity in Stable Continental Regions

A worldwide compilation of well-constrained fault ruptures and focal depths of earthquakes reveals that the Earth's 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 (< 5 km). Almost 80% of the seismic moment density of shallow SCR ruptures is released in the uppermost 7 km of the crust. However, focal depths of instrumentally recorded major SCR earthquakes (3.5 < m_b < 6.2) and their aftershocks in the northeastern United States and adjacent Canada, for example, suggest systematic overestimates of hypocentral depths of 88 ± 30% (standard mean ± standard mean error), probably due to sparse instrumental coverage. If error estimates for shallow SCR earthquakes, in particular, are of systematic and not of statistical origin, preconceived assumptions of focal depths within the midcrust might have region-specific implications for understanding SCR seismogenesis and for earthquake hazard estimations (e.g., ground motion).