ALBERT

Description: The Central United States Seismic Observatory is a vertical seismic array located in southwestern Kentucky within the New Madrid seismic zone. It is intended to describe the effects of local geology, including thick sediment overburden, on seismic-wave propagation, particularly strong motion. The three-borehole array is composed of seismic sensors placed on the surface, in the bedrock, and at various depths within the 585-m-thick sediment overburden. The array’s deep borehole also provided a unique opportunity in the northern Mississippi embayment for the direct geologic description and geophysical measurement of the complete Late Cretaceous–Quaternary sediment column. A seven-layer intrasediment velocity model is interpreted from the complex, inhomogeneous stratigraphy. The S - and P -wave sediment velocities range between 160 and 875 m/s and between 1000 and 2300 m/s, respectively, and their bedrock velocities range between 1452 and 3775 m/s, respectively. Seismometers and accelerometers operate both at the surface and 2 m into bedrock, with strong-motion accelerometers at depths of 30, 259, and 526 m. The array operation has been frequently interrupted by the large hydrostatic pressures on the deeper instrumentation; however, the full array has recorded weak-motion response from 95 earthquakes at local, regional, and teleseismic distances. Initial observations reveal a complex spectral mix of amplification and de-amplification across the array, indicating the site effect in this deep-sediment setting is not simply generated by the shallowest layers. Preliminary horizontal-to-vertical spectral ratio (H/V) experiments show the bedrock vertical and horizontal amplitudes are not equal, a required assumption for site characterization. Further, there are marked differences between spectral ratios from the directly measured transfer function (H/H) and H/V for particular earthquakes. On average, however, the H/H and H/V methods are coincident within a narrow band of frequencies ranging between 0.35 and 1.1 Hz.

Description: Since the mid 1980s small and moderate-sized earthquakes in the Ohio and Wabash River valleys of the central United States (CUS) have been digitally recorded by seismographs, called blast monitors, deployed to detect and characterize vibrations from explosions associated with regional mining and quarrying. Because there were relatively few conventional networked strong-motion and broadband instruments for this area between 1980 and the early 2000s, the more than 200 collected observations have provided a widespread source of digital earthquake ground motions. Additional deployment of networked instrumentation during the last decade and their numerous recordings of the 18 April 2008 Mt. Carmel Illinois earthquake sequence have provided the first effective means for comparing free-field blast-monitor and conventional network ground motion observations. The peak ground motion characteristics for both data sets relative to a common predictive relationship are similar, suggesting that blast monitor observations compliment conventional network data for small and moderate-sized ( M 〈5.5) events in the CUS.

Description: Much of the ground-motion prediction effort in the central United States has been focused on deep (〉〉30 m) alluvial sites, such as those found in the Mississippi embayment. During the past 20 years we have collected over 200 high-resolution, free-field digital velocity records at sites in the Wabash and Ohio River valleys which are more typical of the areas outside the embayment. These ground-motion records are from small to moderate-size earthquakes located in the same area. The magnitudes range between M  3 and M  5.2, but the bulk of the recordings is associated with the 1987 M  4.96 and 2008 M  5.2 southeastern Illinois earthquakes, and the 2002 M  4.5 southwestern Indiana earthquake. We summarize the velocity recordings and site investigations for the 2008 southeastern Illinois earthquake sequence, put the findings into context with the previous observations, and quantify the reduction in ground-motion variability that can be achieved with conventional site-effect characterization techniques. Results show ground-motion characteristics for the 2008 earthquake sequence are consistent with previous observations in the area, beginning with those associated with the 1987 southwestern Illinois earthquake. In addition, site-effect corrections reduced the range of spectral amplitude for frequencies greater than 2 Hz between 40% and 70%, as well as the spectral variation by a factor of approximately four. The data also show that a peak ground velocity of 1.2 cm/s delineates a clear boundary separating Modified Mercalli intensities IV and V. This observation can be useful in scaling ground motions of historical seismicity, as well as predicting the effects of future events. We speculate these quantitative characteristics are likely representative of ground motions throughout most of the central United States with the exception of those in the infrequent thick-sediment-filled areas within major river valleys.

Description: When air blows over water the wind exerts a stress at the interface thereby inducing in the water a sheared turbulent drift current. We present scaling arguments showing that, if a wind suddenly starts blowing, then the sheared drift current grows in depth on a timescale that is larger than the wave period, but smaller than a timescale for wave growth. This argument suggests that the drift current can influence growth of waves of wavelength λ that travel parallel to the wind at speed c. In narrow ‘inner’ regions either side of the interface, turbulence in the air and water flows is close to local equilibrium; whereas above and below, in ‘outer’ regions, the wave alters the turbulence through rapid distortion. The depth scale, la, of the inner region in the air flow increases with c/u*a (u*a is the unperturbed friction velocity in the wind). And so we classify the flow into different regimes according to the ratio la/λ. We show that different turbulence models are appropriate for the different flow regimes. When (u*a + c)/UB(λ) ≪ 1 (UB(z) is the unperturbed wind speed) la is much smaller than λ. In this limit, asymptotic solutions are constructed for the fully coupled turbulent flows in the air and water, thereby extending previous analyses of flow over irrotational water waves. The solutions show that, as in calculations of flow over irrotational waves, the air flow is asymmetrically displaced around the wave by a non-separated sheltering effect, which tends to make the waves grow. But coupling the air flow perturbations to the turbulent flow in the water reduces the growth rate of the waves by a factor of about two. This reduction is caused by two distinct mechanisms. Firstly, wave growth is inhibited because the turbulent water flow is also asymmetrically displaced around the wave by non-separated sheltering. According to our model, this first effect is numerically small, but much larger erroneous values can be obtained if the rapid-distortion mechanism is not accounted for in the outer region of the water flow. (For example, we show that if the mixing-length model is used in the outer region all waves decay!) Secondly, non-separated sheltering in the air flow (and hence the wave growth rate) is reduced by the additional perturbations needed to satisfy the boundary condition that shear stress is continuous across the interface. In a companion paper, we develop a numerical model for the coupled air-water flow with waves of arbitrary speed and in another we examine the detailed energy budget of the wave motions. © 1994, Cambridge University Press. All rights reserved.

Description: We develop a numerical model of the interaction between wind and a small-amplitude water wave. The model first calculates the turbulent flows in both the air and water that would be obtained with a flat interface, and then calculates linear perturbations to this base flow caused by a travelling surface wave. Turbulent stresses in the base flow are parameterized using an eddy viscosity derived from a low-turbulent-Reynolds-number k -ε model. Turbulent stresses in the perturbed flow are parameterized using a new damped eddy viscosity model, in which the eddy viscosity model is used only in inner regions, and is damped exponentially to zero outside these inner regions. This approach is consistent with previously developed physical scaling arguments. Even on the ocean the interface can be aerodynamically smooth, transitional or rough, so the new model parameterizes the interface with a roughness Reynolds number and retains effects of molecular stresses (on both mean and turbulent parts of the flow). The damped eddy viscosity model has a free constant that is calibrated by comparing with results from a second-order closure model. The new model is then used to calculate the variation of form drag on a stationary rigid wave with Reynolds number, R. The form drag increases by a factor of almost two as R drops from 2 ×104 to 2 × 103 and shows remarkably good agreement with the value measured by Zilker & Hanratty (1979). These calculations show that the damped eddy viscosity model captures the physical processes that produce the asymmetric pressure that leads to form drag and also wave growth. Results from the numerical model show reasonable agreement with profiles measured over travelling water waves by Hsu Hsu (1983), particularly for slower moving waves. The model suggests that the wave-induced flow in the water is irrotational except in an extremely thin interface layer, where viscous stresses are as likely to be important as turbulent stresses. Thus our study reinforces previous suggestions that the region very close to the interface is crucial to wind-wave interaction and shows that scales down to the viscous length may have an order-one effect on the development of the wave. The energy budget and growth rate of the wave motions, including effects of the sheared current and Reynolds number, will be examined in a subsequent paper.

Description: A two-dimensional, incompressible, irrotational, linearized flow model is employed in this analysis of two supercavitating, flat-plate hydrofoils in the presence of a free surface. The cavities are taken to have finite lengths, and gravity is neglected. The ensuing boundary-value problem is converted, by means of conformal mapping, to a mixed-boundary-value problem for the complex velocity in the upper half-plane. This altered problem is solved by use of the methods of thin-aerofoil theory and the solution involves digital-computer evaluation of a large number of incomplete elliptic integrals of the first and third kinds. Typical results are presented in graphs, and the results of the present work are compared with Yim's (1964) theory for a single supercavitating body near a free surface.

Description: A series of two- and three-dimensional numerical simulations of transient flow in a side-heated cavity has been conducted. The motivation for the work has been to resolve discrepancies between a flow description based on scaling arguments and one based on laboratory experiments, and to provide a more detailed description of the approach to steady state. All simulations were for a Rayleigh number of 2 x 109, and a water-filled cavity of aspect ratio 1. The simulations (beginning with an isothermal fluid at rest) generally agree with the results of the scaling arguments. In addition, the experimental observations are entirely accounted for by the position of the measurement instruments and the presence of an extremely weak, stabilizing temperature gradient in the vertical. © 1989, Cambridge University Press. All rights reserved.