Estimates of the apparent quality factors Q<sub>Lg</sub> (attenuation determined from the spatial decay of Lg waves) and Q<sub>c</sub>; (attenuation determined from the temporal decay of seismic coda waves) are made for the crust of Virginia and its environs. The results are presented in the form Q<sub>(Lg,C)</sub>(f) = Q₀f<sup>N</sup>, where Q₀ = Q<sub>(Lg,C)</sub>(1 Hz) and N represents the frequency dependence. The study area is located in the Appalachian region of Virginia and eastern Tennessee, containing three areas of regionally high seismicity: the central Virginia, Giles County, and eastern Tennessee seismic zones.
The attenuation of the Lg phase was studied using vertical component digital recordings from Virginia Tech Seismological Observatory network stations. The seismic sources were ten regional surface mine explosions and six regional earthquakes. It was determined that Q<sub>(Lg,C)</sub> can be represented by Q₀ = 186, σ<sub>logQ₀</sub> = 0.05, and N = 1.1 ± 0.1 for the frequency band 1-4 Hz.
A site effect corrected estimate of Q<sub>(Lg,C)</sub> was also determined for the study area. This was accomplished using a spectral ratio method in which station site effects and instrument responses are canceled out. For the frequency band 1-10 Hz the site independent apparent quality factor can be represented by Q₀=155, σ<sub>logQ₀</sub> = 0.1, and N=1.2±0.2. Station site factors were estimated using a mean residual technique.
The decay of seismic coda waves across the Giles County, Virginia seismic network was studied to estimate Q<sub>c</sub> for western Virginia. A relatively new spectral method was used. The seismic sources were four local earthquakes. For the frequency band 1-10 Hz, the results can be represented by Q₀= 111, σ<sub>logQ₀</sub> = 0.07, and N = 1.3 ± 0.07 . These values agree with a limited number of results obtained using a bandpass, time domain method which showed Q₀ = 132, N = 1.3.
The results obtained for the Virginia area differ significantly in the 1 - 3 Hz range from those reported in most previous studies of the eastern United States. Previous studies have generally shown 800 ≤ Q₀ ≤ 1000 and 0.3≤ N≤0.5, but many of those results are for much larger regions and determined using different analysis techniques.
Several reasons that could account for the different results include 1) estimates of attenuation may be affected by incorrect geometrical spreading models, 2) the size of the study area may affect the estimates, and 3) estimates of Q<sub>(Lg,C)</sub> made for broad regions may be biased by zones of differing tectonic activity. Of these factors, only the effects of changing geometrical spreading coefficients and scattering models (related to study area) can be quantified. Neither of these affect the results by more than a factor of two.
The high frequency dependence values (N≃1.1) are probably influenced by the lack of definition of higher frequency (≃10 Hz) data at the path distances studied. Future studies should employ more extensive data sets covering a larger geographic area. At greater distances, the attenuation of higher frequency waves may be more easily observable.
The large frequency dependence values are probably indicative of an area where scattering dominates over anelastic attenuation. The folded and thrusted Appalachian provinces may, indeed, be such a region of high scattering. Such a mechanism may also help to explain southeastern United States meizoseismal areas that are small relative to the total felt areas.
Large frequency dependence results for Q<sub>Lg</sub> and Q<sub>c</sub> are relevant with respect to seismic hazard. We do not believe the results are overly biased by station site effects or varying source effects and if they hold for magnitudes greater than those studied here (m < 4.2) , they indicate a greater potential for damage by higher frequency waves to engineering structures in Virginia and its environs than previously assumed. / Master of Science
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/45174 |
| Date | 13 October 2010 |
| Creators | Rogers, Melissa J. B. |
| Contributors | Geological Sciences |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Detected Language | English |
| Type | Thesis, Text |
| Format | xi, 95 leaves, BTD, application/pdf, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
| Relation | OCLC# 18354335, LD5655.V855_1988.R637.pdf |
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