Abstract

The relations between geology and earthquakes remain mostly enigmatic in North America east of the Rocky Mountains. Hypocentral depths and the dips of rupture zones (preferred nodal planes) are among the seismological variables most likely associated with geologic structure in the stable continental region (SCR) of central and eastern North America (CENA). We compiled well-constrained depths and dips for 20 SCR shocks in CENA with magnitudes (M) between 4.9 and 6.8. The appendix lists the values we chose, our reasons for selecting specific values, and the uncertainties associated with each value. The median depth is 10 km, but an aseismic interval from about 15 to 25 km depth separates three midcrustal hypocenters from the others. The median dip of preferred nodal planes is 54°, and the larger earthquakes tend to rupture along shallower dips.

These results combined with geologic evidence lead to four inferences about SCR earthquakes with M≥ 4.9 in CENA. (1) If rupture generally progresses updip, then the tendency for large earthquakes to rupture more shallowly-dipping surfaces than small earthquakes can explain the scarcity of surface ruptures in CENA. (2) The few midcrustal earthquakes might be evidence that mafic or granulite facies rocks are present. (3) CENA seismicity is concentrated in Late Proterozoic and Phanerozoic orogens and extended terranes that rim an older, stable, central craton. Unusually shallow large CENA earthquakes have been observed only in the central craton, where they might cause severe but localized shaking. (4) Earthquakes in the Appalachian orogen tend to be shallower than those of comparable size in the Late Proterozoic rifted continental margin that underlies and borders the northwest side of the orogen. As a result, some Appalachian earthquakes could cause more localized shaking than comparable shocks in the rifted margin.

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