High-resolution aeromagnetic data have recently been acquired for most of Minnesota under the sponsorship of the Minnesota Future Resources Commission, which also provided funds for test drilling of selected anomalies. The new data show that northwest-trending dikes and fractures are much more common in the Archean terranes than previously believed, and several epicenters are near such faults. The northeast-trending Great Lakes tectonic zone (GLTZ) was thought to be the chief source of seismicity in the state, because several epicenters were aligned with it. However, more recent seismic reflection profiling has demonstrated that the GLTZ has a moderate dip to the north. This dip is not compatible with the previously favored focal plane solution for a relatively large earthquake, which apparently confirmed the seismogenic role of the GLTZ. The alternative focal plane solution for this earthquake, however, is compatible with some northwest-trending structures in the region.

We propose that the apparent alignment of epicenters with the GLTZ is related to stress buildup at places where it is intersected by northwest-trending faults, with subsequent movement occurring upon release. A similar model may explain seismicity along the northeast-trending Colorado lineament to the southwest.

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