Annual Project Summary: November 18, 1997
The goal of the Southern Appalachian
Cooperative Seismic Network (SACSN) is to provide modern instrumental
coverage of seismicity in the southern Appalachians. Research
objectives for the SACSN are focused on seismic activity in the
southern Appalachian region. Research areas include earthquake
monitoring (in part to maintain continuity of earthquake catalogs),
seismic hazard assessment, studies of the seismotectonics of the
region, earthquake source studies, wave propagation at local and
regional distances, crustal structure studies, and the temporal/spatial
behavior of seismicity. Service objectives of the SACSN include
the publication of an annual seismicity bulletin for the southeastern
United States; the development and maintenance of regional earthquake
catalogs; and informational service to federal/state/local governments,
the engineering community, and the general public.
Most stations of SACSN operated
by Virginia Tech have been upgraded with three-component sensors,
high dynamic range electronics and digital radio telemetry. Calibrated
digital waveform data for local and regional seismic events are
available on the Internet. Real-time data from several Virginia
Tech stations are transmitted to NEIC via the USNSN satellite
telemetry link at Blacksburg, Va.
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 email@example.com. 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
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
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
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.