Seismicity, seismic reflection studies, gravity and geology of the central Virginia seismic zone: Part I. Seismicity

The central Virginia seismic zone is an area of persistent, low-level seismicity in the Piedmont province of that state. Its north-south dimension is ∼120 km, and it extends some 150 km in an east-west direction from Richmond to Lynchburg. The results of instrumental monitoring there by a seismic network during the past 5 yr are reported herein.

To provide a useful constraint on geologic structural models, considerable care must be exercised in the location of earthquakes and in the specification of the accuracy of those locations. Our procedures for accomplishing those tasks were as follows. First, a velocity model, specific to the network locale, was developed from seismic-refraction and reflection data. Second, evaluation of the detection capability of the network indicated that the detection of seismic events should be virtually complete down to a magnitude of ∼1.5. Third, the locational capability of the network was assessed by locating blasts (originating from three different quarries operating within the network area) as if they were earthquakes of unknown location. The average calculated locations, when compared to the known blast sites, indicated average differences of ±2 km horizontally and ±4 km vertically, except for the northeastern portion. There, the location capability degrades to fair for epicenters (±4 km) and to poor for estimation of focal depths (±8 km). The resultant decrease in locational capability is probably due to the presence of low-velocity Coastal Plain sediments. Fourth, the location of all of the earthquakes, relative to each other, was tested by use of special, joint-hypocenter calculations. The recalculated locations differed from the original locations in a random manner. On the average, that difference was ∼3 km, and it indicates the absence of serious bias introduced by combining earthquake locations derived from different subnetworks of seismographs.

Ninety-six pre-network (pre-1978) earthquakes (for the most part, felt events with noninstrumental locations) and 34 network-determined hypocenters (1978–1982) (for mostly smaller, not-felt shocks) form the basic data set of this study. The spatial distribution exhibited by the 130 hypocenters is diffuse, both vertically and horizontally. Such a pattern favors multiple, rather than singular, seismogenic structures. Seventy-five percent of the focal depths are in the upper one-third of the crust, at depths of 11 km or less. These multiple, shallow sources will be shown, in Part 4 of this sequence of papers, to be along and above a master detachment fault.