G. A. Bollinger

A seismotectonic model for the 300-kilometer-long eastern Tennessee seismic zone

Ten years of monitoring microearthquakes with a regional seismic network has revealed the presence of a well-defined, linear zone of seismic activity in eastern Tennessee. This zone produced the second highest release of seismic strain energy in the United States east of the Rocky Mountains during the last decade, when normalized by crustal area. The data indicate that seismicity produced by regional, intraplate stresses is now concentrating near the boundary between relatively strong and weak basement crustal blocks.

Hydroseismicity—A hypothesis for the role of water in the generation of intraplate seismicity

A new hypothesis, termed hydroseismicity, suggests that in crustal volumes with fracture permeability, natural increases in hydraulic head caused by transient increases in the elevation of the water table in recharge areas of groundwater basins can be transmitted to depths of 10–20 km and thereby trigger earthquakes. The flow-path geometry resembles, except for scale, the model familiar to groundwater hydrologists for near-surface flow. Possible trigger mechanisms for hydroseismicity include small increases in fluid pressure at hypocentral depths caused by such transient increases, the dissolution of minerals in water, and the solubility of water in minerals (hydrolytic weakening) that leads to structural weakening. A change in water level of less than 1 m (

Correlations between streamflow and intraplate seismicity in the central Virginia, U.S.A., seismic zone: evidence for possible climatic controls

There is no widely accepted explanation for the origin of intraplate earthquakes. The central Virginia seismic zone, like other seismically active intraplate areas, is a spatially isolated area of persistent, diffuse earthquake activity. We suggested earlier that rainfall plays a key role in the generation of intraplate seismicity (“hydroseismicity”). Observed long-period (10–30 years) changes in streamflow (rainfall) are hypothesized to generate intraplate seismicity by diffusion of pore pressure transients from recharge areas of groundwater basins to depth as deep as the brittle-ductile transition. Streamflow and earthquake strain for a 62-year sample from 1925 to 1987 in the central Virginia seismic zone were cumulated, and a least-squares straight-line fit was subtracted to obtain residuals of streamflow and strain. Residual streamflow was differentiated to obtain the rate of change of residual streamflow. We observed common cyclicities with periods of 10–30 years for residual streamflow and strain. From the one-dimensional diffusion equation, we determined the time response of fluid pressure at depths, ψ, in a hydraulically diffusive crust to an impulsive change in fluid pressure at the surface of a groundwater basin. These responses were convolved with the surface streamflow residuals, or, because of results from reservoir-induced seismicity, with the derivatives of these residuals. Root-mean-square values (rms) of the convolutions were computed for ψ = 5, 10, 15 and 20 km and various values of D. For central Virginia, the number of earthquakes, N, within a crustal slice centered on a depth, ψ, was found to be proportional to the rms value of the convolution, suggesting that the number of intraplate earthquakes generated is directly proportional to the magnitude of the rms changes in fluid pore pressure within the crustal slice. These fluctuations in pore pressure, in concert with stress corrosion and hydrolytic weakening, are hypothesized to trigger intraplate earthquakes in a crust stressed by ridge-push. Crosscorrelation of the residual streamflow convolutions with residual strain, provides a measure of similarity between the streamflow convolution and the strain. Optimum values of crustal diffusivity, D, were considered to be those values for which the Crosscorrelation function looks approximately even. That this occurs only over a reasonable range of diffusivities (near D = 50 km2/year) is consistent with the hypothesized causal relationship between streamflow and intraplate seismicity. Finally, others have shown a clear relationship between the 11-year solar cycle and storm track latitudes in the North Atlantic. Storm tracks over the eastern United States and Canada indicate that north of 50 ° N, the average latitude of storm tracks during December, January, and February is about 2.5 ° farther south at sunspot maximum than at sunspot minimum. A correlation with the 11-year solar cycle was discovered by Labitzke (1987). Significantly, we recognize a clear peak near 11 years in the Fourier spectra of actual (not residual) streamflow in the James River in the central Virginia seismic zone.

The Giles County, Virginia, seismic zone

A well-defined seismic zone recently detected in Virginia has an orientation that is not related to the surrounding geologic structures. The orientation of the zone appears to be related to features below the Appalachian overthrust belt. A damaging earthquake that is important in evaluating seismic hazard in the southeastern United States may have occurred in the zone in 1897.

Forecasting Damaging Earthquakes in the Central and Eastern United States

Analysis of seismograph network data, earthquake catalogs from 1727 to 1982, and paleoseismic data for the central and eastern United States indicate that the Poisson probability of a damaging earthquake (magnitude ≥ 6.0) occurring during the next 30 years is at a moderate to high level (0.4 to 0.6). When differences in seismic wave attenuation are taken into account, the central and eastern United States has approximately two-thirds the likelihood of California to produce an earthquake with comparable damage area and societal impact within the next 30 years.

The Influence of the Coastal Plain Sedimentary Wedge on Strong Ground Motions from the 1886 Charleston, South Carolina, Earthquake

The effect of Atlantic Coastal Plain sediments on ground motion from the 1886 Charleston, South Carolina, earthquake was studied using linear regression analysis and ground motion modeling. Statistical tests applied to 264 Modified Mercalli intensity reports at epicentral distances less than 400 km showed that intensities within the Lower Coastal Plain (near the coast) averaged from 0.5 to 1.0 units less than intensities within other areas over comparable distances. Simulated ground acceleration time histories were generated along two profiles: one trending northeast from the epicenter along the coast and another trending northwest toward the Fall Line. The modeling results (which assume linear behavior) indicate that the thick coastal sediments attenuate high frequency motions (>3-10 Hz), while amplifying lower frequency motions. In contrast, locations on thinner sediments to the northwest experience amplification of high frequency motions. The overall effect is that peak acceleration decays more slowly with distance in the thinning sediments to the northwest of the epicenter than in the thick sediments along the coast.

Seismicity and suspect terranes in the southeastern United States

Evaluation of seismic hazard in the eastern United States is hampered by lack of a strong geologic foundation. If suspect terranes exist in the Appalachians, then geologic and mechanical differences between terranes may provide such a foundation. For example, a published map of suggested terranes is consistent with spatial characteristics of southeastern seismicity and with structural models of seismicity in central and southwestern Virginia and in southeastern South Carolina.