The Southern Appalachian Cooperative Network:
Virginia Tech Component

Network Operations:

Since 1977, Virginia Tech has operated a calibrated network comprised currently of nine stations (Figure 1). SACSN stations in western Virginia (Giles County subnet) have been upgraded to three-components, with digital radio telemetry and 24-bit digitization. The three station central Virginia subnetwork continues to be recorded digitally using analog telephone telemetry supported by contributions by Virginia Power Company, Inc.

Calibrated waveform data from the stations of the SACSN network operated by Virginia Tech are available via either anonymous ftp vtso.geol.vt.edu, or the world-wide web at http://www.geol.vt.edu/outreach/vtso/. Special waveform data sets recorded by the network and the high-dynamic-range broadband GSETT-2 seismograph system at BLA can be accessed. In addition to the special waveform data sets, triggered event files from the analog telemetered digitizing system are put on the Virginia Tech anonymous ftp account within 20 minutes of the trigger time. Lists of processed data sets can be gotten from finger, e-mail or login to quake@vtso.geol.vt.edu. A description of how to access waveform files, calibration files, and other information was published in Southeastern U. S. Seismicity Bulletin 30 and is also available at our website, or through more direct communication with the authors of this report. Data from the U. S. National Seismic Network station with downhole broadband sensors are available through the USNSN AutoDRM system.

Recent Seismicity in the Southeast:

Figure 2 shows the epicenters of earthquakes in the Southern Appalachian region detected and located by SACSN during the three year period November 1, 1994 to November 1, 1997. The only SACSN stations recording during that period were those operated and maintained by Virginia Tech. Calibrated digital waveform data are available (SAC format) for the events indicated in Figure 2 via the Internet as described above.

Figure 2. Circles show the epicenters of earthquakes in the Southern Appalachian region detected and located by SACSN during the three year period November 1, 1994 - November 1, 1997. Locations for all events shown used arrival-time data from stations operated by Virginia Tech.

For Virginia and West Virginia, the past year (November 1, 1996 to November 1, 1997) has been active, compared to the two previous years. Twenty earthquakes were detected and located in Virginia and West Virginia during the year.

In chronological order, the list includes a (duration) magnitude Md=1.3 event in central Virginia on November 20, 1996, a Md=2.0 shock on February 22, 1997, in West Virginia; a Md=1.8 shock in Central Virginia on March 15, 1997; a possible mining induced earthquake in southwestern Virginia (mblg=2.3) on March 29, 1997; a Md=1.4 shock in Giles County, Virginia, on July 12, 1997; and a Md=1.5 shock in central Virginia on July 24, 1997.

An interesting sequence of shocks occurred in Culpeper, Virginia, on July 27, 1997. At least eleven shocks occurred in the town in a 24-hour period, the largest of which was Md=2.5. The sequence of shocks caused alarm and forced the evacuation of a local business. The residents of Manassas, Virginia were alarmed on September 29, 1997, when an isolated Md=2.5 shock occurred in that town.

The Culpeper and Manassas shocks are the first events located in northern Virginia during the past 20 years of network monitoring, and they aroused considerable local interest, as evidenced by a article appearing in the September 30, 1997, edition of the Washington Post. These events are intriguing because they may be related to a larger-scale pattern of activity in the region in recent years: in both cases, the earthquakes are spatially associated with Mesozoic extensional faulting of the Culpeper basin. These small northern Virginia shocks, as well as other small, but strongly felt earthquakes in recent years in Maryland (Figure 2), indicate that shallow crustal stresses are now being released in a previously quiet region that contains a large urban population and includes the nation's Capitol. Unfortunately, the area lacks modern seismographic coverage.

A magnitude Md=2.3 earthquake occurred a few kilometers north of the town of Galax, in southwestern Virginia on October 30, 1997. A similar (Md=2.1) shock occurred in the same area, approximately 10 hours later. Both shocks were felt by many residents of Galax, Fries, and Ivanhoe, Virginia.

Research Results and Data Products

The Southeastern U. S. Seismicity Bulletin 31 (Southeastern U.S. Seismic Network Operators, 1997) for calendar year 1996 was distributed in August, 1997, to 200 institutions and individuals. The southeastern U.S. region seismicity during 1996 included 31 tectonic (not reservoir related) earthquakes, 2 mining induced earthquakes, and 54 earthquakes associated with reservoirs. Felt reports were received for five of the tectonic earthquakes. The Bulletin contains complete phase arrival time data for all stations recording each tectonic earthquake, as well as much additional information on southeastern U.S. seismicity and network operation. Text versions of the Southeastern U.S. Seismicity Bulletins can be obtained electronically at our website, or by anonymous ftp at vtso.geol.vt.edu.

The CNSS Composite Catalog (http://quake.geo.berkeley.edu/cnss/) now contains the complete listing of instrumentally located tectonic earthquake hypocenters and magnitude estimates for the southeastern U.S. region (Figure 3). Early in 1997, over 1400 southeastern U.S. earthquake hypocenter data parameter sets compiled in Southeastern U.S. Seismicity Bulletins No. 1 through 30 were contributed to the CNSS composite catalog. The CNSS composite catalog listing for the southeastern U.S. region was updated for earthquakes occurring during 1996 (Bulletin 31) in early September, 1997.

A recent Master's Thesis (Student, 1997: see also, Student and Chapman, 1997) examined the effects of different statistical models on probabilistic seismic hazard estimates for Charleston, South Carolina. The main purpose of the study was to make a quantitative comparison of the Poisson and time-dependent characteristic earthquake models assuming 1) the Gutenberg-Richter, and 2) characteristic magnitude distribution functions. Information on how to obtain an electronic copy of this thesis is available at our internet website. The seismicity catalog for the southeastern region (both instrumental and historical) used for hazard calculations performed in the study is also available at the internet website.

Two collaborative studies focusing on basic seismological aspects of the eastern Tennessee seismic zone are now in press: Chapman et al., 1997, examines statistically the spatial aspects of the seismicity and the focal mechanisms, Vlahovic et al., 1998, developed new 1-D and 3-D seismic velocity models, jointly relocates the earthquakes, and examines correlations between the velocity anomalies and the seismicity.

Figure 3. Shaded circles show the epicenters of earthquakes in the southeastern United States for the period 1977 through 1996, detected and located by all regional seismic networks in the region. These data have been compiled in the Southeastern United States Seismicity Bulletins, and are now also contained in the CNSS Composite Catalog (SE authoritative region).

Chapman, M. C. (1997). A statistical analysis of earthquake focal mechanisms and epicenter locations in the eastern Tennessee seismic zone, Bull. Seism. Soc Am, vol. 87, no. 6, in press.

Southeastern U.S. Seismic Network Operators (1997). Southeastern U. S. Seismic Network Bulletin No. 31, (compiled by M. C. Chapman, E. C. Mathena and J. A. Snoke), Virginia Tech Seismological Observatory, Blacksburg, VA, 58 p.

Student, Heather H. (1997), Assessing the Seismic Hazard in Charleston, South Carolina: Comparisons Among Statistical Models, Virginia Polytechnic Institute and State University M.S. Thesis, 102 p.

Student, Heather H. and M. C. Chapman (1997). Seismic hazard estimates for Charleston, South Carolina: A comparison of Gutenberg-Richter and characteristic-earthquake models, Seismological Research Letters, vol. 68, no. 5, p. 822.

Vlahovic, G., C. A. Powell, M. C. Chapman and M. S. Sibol (1998). Joint hypocenter-velocity inversion for the eastern Tennessee seismic zone, Journal of Geophysical Research, in press.