Summary of geology and conclusions
Since, from the reports received, the shock seems to have been most severe in the northern part of the Valley of Virginia, a very brief summary of the geology of the northern valley region is of some interest in seeking the probable cause of the earthquake. The Valley of Virginia is bounded on the southeast by the Blue Ridge, the central portion of which is composed of pre-Cambrian igneous rocks and on the northwest by the Valley Ridges subprovince of folded sedimentary rocks ranging up to Devonian and Mississippian in age.
The valley maintains an approximate width of twenty miles from the state boundary southwestward to nearly the latitude of Greenville, Augusta County (Map, Plate I). From near the latitude of Strasburg and Riverton to that a short distance south of Harrisonburg, the valley is divided lengthwise by Massanutten Mountain, which is synclinal in structure and composed of sedimentary rock ranging up to and including Devonian in age. The mountain extends southwestward for a distance of about forty-five miles, and divides the valley lengthwise into two narrow valleys which average from five to ten miles in width. The Massanutten syncline, however, which involves the Martinsburg shale (Ordovician) at the surface, continues for a considerable distance both to the northeast and to the southwest of the north and south ends of the mountain proper.
The valley bottom is developed on folded limestone and shales of Cambro-Ordovician age, underlain by quartzites, sandstones, and shales of Lower Cambrian age which, because of their structure and greater resistance, are exposed along the northwest flank of the Blue Ridge. No igneous rocks are known to occur in the valley proper north of the latitude of northern Rockingham County.
The valley rocks are faulted, but in some localities at least the faulting appears to be slight, since the displacement is frequently not great enough to cut one or more formations. Bassler has recognized faulting at Winchester, one of the localities of highest intensity (VI R.-F. scale), during the earthquake of April 9, 1918. He says:5 “Although the full geologic structure in the vicinity of Winchester could not be determined because of lack of continuous exposures, the quarries and other outcrops just west and east of the town indicate that by faulting a band of Lower Ordovician dolomitic limestones has been interpolated between a band of Stones River limestones on the west and argillaceous limestones and shales of Chambersburg and Martinsburg age on the east.”
Faulting occurs at the base of Little North Mountain along the northwest side of the valley, and along the northwest front of the Blue Ridge on the southeast side of the valley a great overthrust fault, which apparently follows the Blue Ridge, has a horizontal displacement in places of at least four miles.
It seems probable, therefore, that the seismic disturbance of April 9, 1918, had its origin in one or more of the faults which characterize the region.