Thirty-nine construction blasts were monitored by mobile and fixed seismic arrays to develop a crustal velocity model in the Bath County, Virginia locale (June-August, 1982). The results indicate an upper-layer with P and S velocities of 5.45 km/sec and 3.07 km/sec respectively, and a thickness 3 km. Data from the 2 most distant observing stations indicate a second layer with a P velocity of 6.04 km/sec. Based on other studies in Virginia, lower crustal layers with P and S velocities 6.05 km/sec and 3.52 km/sec (11.7 km thickness) and 6.53 km/sec and 3.84 km/sec (36.0 km thickness) and upper mantle velocities of 8.18 km/sec and 4.79 km/sec are assumed. These layers make up the discrete velocity layer model. The observed travel-time data cannot be distinguished from theoretical travel-times calculated from either of two other models utilizing linear increases in velocity with depth. The first of these models uses a linear increase in P velocity from 5.45 km/sec at the surface to 6.53 km/sec at 50.7 km depth. The surface VP/VS ratio of 1. 74 is assumed for this entire layer. Beneath this layer is the mantle with P and S velocities 8.18 km/sec and 4.79 km/sec. The second of these models uses a linear increase in P velocity from 5.45 km/sec at the surface to 6.05 km/sec at 14.7 km depth. The surface VP/VS ratio is again assumed for this layer. Beneath this layer is the 36.0 km thick 6.53 km/sec layer underlain by the mantle.
A test of the locational capabilities of the Bath County Network utilizing construction and quarry blasts was carried out for the 3 different velocity models. All three models gave virtually identical locations. The tests indicate an average of less than 1 km epicentral and depth errors inside the network. On the edge of the network, accuracy degrades to 3 km epicentral error with poor depth control.
The Bath County area is seismically quiescent. Portable seismographs recorded over 3,000 hours of low noise seismic data in June, July and August of 1982 and failed to detect any local earthquake activity. Network monitoring by a permanent 4 station microearthquake network from November 1978 to November 1982 resulted in 11 recorded events. Three of these events were too small to be located. The other 8 events were located using all 3 velocity models. The linear increase in velocity over mantle model was eliminated from further consideration due to poor performance in event location. The other two models gave virtually identical locations. For these two models, the events form an apparent east-west trend. Reliable focal wave polarities and SV/P amplitude ratios, mechanisms, using both P indicate one nodal plane striking east-west and dipping to the south. The other nodal plane, which defines the mode of faulting, is poorly constrained. / Master of Science
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/82660 |
| Date | January 1982 |
| Creators | Todd, Eric Donald |
| Contributors | Geophysics |
| Publisher | Virginia Polytechnic Institute and State University |
| Source Sets | Virginia Tech Theses and Dissertation |
| Language | en_US |
| Detected Language | English |
| Type | Thesis, Text |
| Format | vi, 100, [2] leaves, application/pdf, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
| Relation | OCLC# 9617528 |
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