A comparison of the seismicity and regional geology of the southeastern United States and southeastern Australia

Abstract

The southeastern United States and southeastern Australia are both intraplate regions. They are characterized topographically and geologically by low relief, Paleozoic mountain belts that parallel continental margins formed by rifting. Interestingly, there are also remarkable similarities in the seismicity of the two areas.

To characterize the temporal aspects of seismicity, the recurrence relationship, log N (number of earthquakes per year) versus I_o (maximum intensity), for southeastern Australia was determined to be:

log N = 3.28 - 0.61 I_o; III ≤ I_o ≤ VII

Comparison with a similar relationship for the southeastern United States,

log N = 3.01 - 0.59 I_o; V ≤ I_o ≤ VIII.

(Bollinger, G. A., 1973) indicates that both the level of seismic activity and the distribution of earthquakes by size are comparable in the two regions. Additionally, strain release studies for the two regions, while subject to considerable uncertainty, do support that similarity. Note that the intensity X, 1886 Charleston, South Carolina, earthquake was not included in either the recurrence or strain studies.

Comparison of the spatial patterns of earthquake epicenters in both regions reveals only weak correlations of the seismicity with topography or with regional geology. There is, however, no apparent correlation with major igneous outcrops or with most major through-going faults. Both regions have seismic zones that are transverse or parallel to the regional geologic trends or tectonic fabrics. The parallel zones are usually associated spatially with severely-faulted regions, suggesting a possible causal relationship. For southeastern Australia, such zones exhibit northeast-trending normal faults of Jurassic age while for the southeastern United States, there are northeast-trending Pennsylvanian-Permian thrust faults. However, no such fault concentrations are found in the transverse seismic zones. Thus, the severely-faulted Source region hypothesis does apply uniformly. Indeed, it may be that, in portions of both of these widely separated geographic regions, the seismic activity is associated with deep crustal features that have no obvious surface expression.