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Browsing by Author "Bollinger, Gilbert A."

Now showing 1 - 7 of 7
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    A comparison of the seismicity and regional geology of the southeastern United States and southeastern Australia
    Hunter, Stephen Allen (Virginia Tech, 1981-05-14)
    The southeastern United States and southeastern Australia are both intraplate regions. They are characterized topographically and geologically by low relief, Paleozoic mountain belts that parallel continental margins formed by rifting. Interestingly, there are also remarkable similarities in the seismicity of the two areas. To characterize the temporal aspects of seismicity, the recurrence relationship, log N (number of earthquakes per year) versus Io (maximum intensity), for southeastern Australia was determined to be: log N = 3.28 - 0.61 Io; III ≤ Io ≤ VII Comparison with a similar relationship for the southeastern United States, log N = 3.01 - 0.59 Io; V ≤ Io ≤ VIII. (Bollinger, G. A., 1973) indicates that both the level of seismic activity and the distribution of earthquakes by size are comparable in the two regions. Additionally, strain release studies for the two regions, while subject to considerable uncertainty, do support that similarity. Note that the intensity X, 1886 Charleston, South Carolina, earthquake was not included in either the recurrence or strain studies. Comparison of the spatial patterns of earthquake epicenters in both regions reveals only weak correlations of the seismicity with topography or with regional geology. There is, however, no apparent correlation with major igneous outcrops or with most major through-going faults. Both regions have seismic zones that are transverse or parallel to the regional geologic trends or tectonic fabrics. The parallel zones are usually associated spatially with severely-faulted regions, suggesting a possible causal relationship. For southeastern Australia, such zones exhibit northeast-trending normal faults of Jurassic age while for the southeastern United States, there are northeast-trending Pennsylvanian-Permian thrust faults. However, no such fault concentrations are found in the transverse seismic zones. Thus, the severely-faulted Source region hypothesis does apply uniformly. Indeed, it may be that, in portions of both of these widely separated geographic regions, the seismic activity is associated with deep crustal features that have no obvious surface expression.
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    Disaggregated Seismic Hazard and the Elastic Input Energy Spectrum: An Approach to Design Earthquake Selection
    Chapman, Martin C. (Virginia Tech, 1998-06-25)
    The design earthquake selection problem is fundamentally probabilistic. Disaggregation of a probabilistic model of the seismic hazard offers a rational and objective approach that can identify the most likely earthquake scenario(s) contributing to hazard. An ensemble of time series can be selected on the basis of the modal earthquakes derived from the disaggregation. This gives a useful time-domain realization of the seismic hazard, to the extent that a single motion parameter captures the important time-domain characteristics. A possible limitation to this approach arises because most currently available motion prediction models for peak ground motion or oscillator response are essentially independent of duration, and modal events derived using the peak motions for the analysis may not represent the optimal characterization of the hazard. The elastic input energy spectrum is an alternative to the elastic response spectrum for these types of analyses. The input energy combines the elements of amplitude and duration into a single parameter description of the ground motion that can be readily incorporated into standard probabilistic seismic hazard analysis methodology. This use of the elastic input energy spectrum is examined. Regression analysis is performed using strong motion data from Western North America and consistent data processing procedures for both the absolute input energy equivalent velocity, (Vea), and the elastic pseudo-relative velocity response (PSV) in the frequency range 0.5 to 10 Hz. The results show that the two parameters can be successfully fit with identical functional forms. The dependence of Vea and PSV upon (NEHRP) site classification is virtually identical. The variance of Vea is uniformly less than that of PSV, indicating that Vea can be predicted with slightly less uncertainty as a function of magnitude, distance and site classification. The effects of site class are important at frequencies less than a few Hertz. The regression modeling does not resolve significant effects due to site class at frequencies greater than approximately 5 Hz. Disaggregation of general seismic hazard models using Vea indicates that the modal magnitudes for the higher frequency oscillators tend to be larger, and vary less with oscillator frequency, than those derived using PSV. Insofar as the elastic input energy may be a better parameter for quantifying the damage potential of ground motion, its use in probabilistic seismic hazard analysis could provide an improved means for selecting earthquake scenarios and establishing design earthquakes for many types of engineering analyses.
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    Estimation of seismic parameters from multifold reflection seismic data by generalized linear inversion of Zoeppritz equations
    Demirbağ, Mustafa Emin (Virginia Tech, 1990-05-05)
    An inversion method is developed to estimate the P- and S-wave velocities and density ratio of two elastic, isotropic, and homogeneous media separated by a plane, horizontal boundary from P-wave reflection amplitude-versus-offset (AVO) data recorded at the surface. The method has for its basis the inversion of the plane wave Zoeppritz. equations by generalized linear inversion (GLI) and bootstrapping. The Zoeppritz equations are converted into the time-offset domain by using Snell’s law, common mid-point (CMP) geometry, and two-way travel (twt) time. The equations in the time-offset domain have five independent variables that enable estimation of P- and S-wave velocities and density ratio for the upper and lower layers. The linearity and uniqueness of the inversion are investigated by residual function maps (RFMs). The RFMs show closed elliptical contours around the true values of the seismic parameter pairs except in the case of S-wave velocity pair for which the open contours imply a linear correlation. However, the RFMs of S-wave velocities with the other model parameters show well defined minima, indicating the uniqueness of the inverse problem in the absence of noise. The estimation of seismic parameters is constrained by physical considerations and the results are enhanced statistically by bootstrapping to obtain the most likely solutions, i.e., the mode values of the distribution functions of solutions, and the confidence limits of the most likely solutions. The inversion method is tested using model AVO data with and without random noise. The tests show that the model parameters are exactly recovered when offset-to-depth (O/D) ratio 1s about 2 or larger, depending on the contrast among the seismic parameters of the media. The results for small O/D ratios (< 1) diverge from the true values, especially for S-wave velocities, and indicate the importance of the O/D ratio in the AVO data inversion. The parameters are not recovered correctly in the case of noisy model AVO data because of the degrading effect of noise in the inversion. However, the model parameters fall into the confidence limits of the estimated parameters when tight constraints are imposed on the solutions, and the signal to noise (S/N) ratio is high. The inversion method is sensitive to auxiliary parameters such as the root-mean-square (rms) velocity and zero-offset twt time which are used in the adjustments of observed or calculated reflection amplitudes to compensate for the effects of wave propagation. Because the plane wave Zoeppnitz equations define the variation in reflection amplitude with offset for a single boundary, the method is limited to isolated reflections in the CMP gathers. The AVO inversion is applied to field data from the Atlantic Coastal Plain in South Carolina to show the feasibility of the method. The first example is from Charleston, S.C. where the estimated seismic parameters from adjacent CMP gathers are in close agreement demonstrating the stability of the AVO inversion. The second example is a data set that crosses the border fault of the Dunbarton Triassic basin, S.C. For this data set common offset stacked CMP gathers are used to increase the S/N ratio and minimize the surface coupling effects. The inversion results show that the seismic parameters are greater north of the border fault indicating crystalline basement while smaller parameters to the south represent the Triassic basin. P-wave velocities estimated for the crystalline basement (6.4 km/s) and the Triassic basin (4.8 km/s) are in good agreement with the computed refraction velocities and support the interpreted location of the Dunbarton Triassic border fault.
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    Failure of asperities by hydraulically induced fatigue: a model for the generation of intraplate seismicity
    Needham, Daniel L. (Virginia Tech, 1987-08-18)
    A model is proposed in which intraplate earthquakes are triggered by the tensile failure of asperities by subcritical crack growth. The principle subcritical crack growth mechanism is fatigue driven by cyclic tensile stresses within an asperity. Cyclic stresses result from cyclic pore pressure variations which propagate from the Earth's surface because of annual variations in the height of the water table. Asperities are postulated to be porous and permeable masses of saturated host rock hydrologically connected to fluids within an open fracture. Porosity and permeability within asperities are assumed to be due to microcracks within the rock matrix. Tensile stresses within an asperity are due partly to mechanical loads, but pore pressure is the primary Inechanism by which tension is developed and fatigue operates primarily in tension. Fatigue crack growth is enhanced by chemical subcritical crack growth mechanisms such as stress corrosion and these mechatusms are proposed to act in unison. Faulting may be initiated when a population of asperities is either driven to failure by these mechanisms alone or when it is weakened to the point at which it is vulnerable to small applied stresses. Numerical modeling of the interaction of pore pressures and stresses within asperities indicates that a small transient increase of pore pressure on the order of a fraction of a megapascal will increase the tensile stresses within an asperity by a fraction of a megapascal. Consequently, it may be possible for a small increase in pore pressure, due to elevated water table levels, to trigger asperity failure and result in seismicity.
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    Long-period background earth noise as measured in shallow, hand excavated holes
    Dalton, David C. (Virginia Tech, 1988-02-05)
    To facilitate its objective of high-resolution imaging of the Earth's crust and upper mantle, The Program for Array Seismic Studies of the Continental Lithosphere (PASSCAL) has initiated studies into developing instrumentation capable of achieving that goal. The requirements include portable sensors capable of resolving seismic signals to 100 second periods. To test the feasibility of obtaining useful long-period seismic data from a portable array, prototype instruments were installed in hand excavated shallow holes (postholes) at several sites in various geologic settings across the continental United States. Three of the sites were near established seismic vaults and comparisons between posthole installation and vault installation were made. Results from this study indicate that posthole installation of long-period sensors may indeed be feasible: eight out of the 12 sites occupied had long-period background noise levels low enough to resolve 100-second surface waves generated from a magnitude 5.0 earthquake 30 degrees distant from the recording station. At periods less than 10 seconds, background noise recorded from postholes was no more than 3 dB. higher than that recorded in vaults. At 100 seconds, vertical noise was 11 to 16 dB. higher than that recorded in vaults and horizontal noise was 4 to 22 dB. higher. Across all posthole installations, as compared to Peterson’s Low Noise Model, vertical and horizontal noise at 100 seconds averaged 27 and 45 dB. higher, respectively. Sites should be located directly on bedrock, where possible. If this is not possible, they should be in well compacted inorganic soil with a low moisture content. Immediately after installation at a potential site, a noise sample should be analyzed in the field to test the suitability of the site.
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    A seismic reflection study over the Bane anticline in Giles County, Virginia
    Edsall, Robert William (Virginia Tech, 1974-02-15)
    A seismic reflection study over the Bane anticline in southwest Virginia (37° 16' 50" N, 80° 41' 30" W) was made in an attempt to gain information about the thickness of sedimentary rocks in this part of the Appalachian Valley and Ridge province. Eleven seismograms were obtained. In addition to frequency filtering, velocity filtering, and deconvolution, new methods of interpreting RMS velocity analyses were developed and applied. Persistent events at about 1.0 and 2.8 seconds were found, suggesting the presence of large reflection coefficients. On the basis of ultrasonic velocity measurements made on rock samples representative of the Cambrian sedimentary sequence and Precambrian basement rocks believed to underlie the Bane anticline, it was concluded that an unbroken Cambrian sequence overlying the basement could not produce reflection coefficients of the magnitude required, but that a thrust fault could. No definite conclusions about the structure beneath the Bane anticline could be reached, although the reflection at 2.8 seconds and the suggestion of a large reflection coefficient favor the interpretation of a thick, repeated sedimentary section.
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    Velocity Structure of the Subducting Nazca Plate beneath central Peru as inferred from Travel Time Anomalies
    Norabuena, Edmundo O. (Virginia Tech, 1993-12-01)
    Arrival times from intermediate-depth (110-150 km) earthquakes within the region of flat subduction beneath central Peru provide constraints on the geometry and velocity structure of the subducting Nazca plate. Hypocenters for these events, which are beneath the sub-andean and eastern Peruvian basins, were determined using a best-fitting onedimensional velocity-depth model with a 15-station digitally-recording network deployed in the epicentral region. For that model, P-wave travel times to coastal stations, about 6° trenchward, exhibit negative residuals of up to 4 seconds and have considerably more complexity than arrivals at the network stations. The residuals at coastal stations are conjectured to result from travel paths with long segments in the colder, higher velocity subducting plate. Travel time anomalies were modeled by 3-D raytracing. Computed ray paths show that travel times to coastal stations for the eastern Peru events can be satisfactorilymodeled if velocities relative to the surrounding mantle are 6% lower within the uppermost slab (a 6 km thick layer composed of basaltic oceanic crust) and 8% higher within the cold peridotitic layer (which must be at least 44 km thick). Raytracing runs for this plate model show that "shadow zones" can occur if the source-slab-receiver geometry results in seismic rays passing through regions in which the slab undergoes significant changes in slope. Such geometries exist for seismic waves propagating to some coastal stations from sources located beneath the eastern Peruvian basin. Observed first-arrival times for such cases do in fact have less negative residuals than those for geometries which allow for \direct\ paths. Modeling such arrivals as trapped mode propagation through the high-velocity part of the plate produces arrival times consistent with those observed.