We provide a probabilistic seismic hazard assessment for the Evansville, Indiana region incorporating information from new surficial geologic mapping efforts on the part of the U.S. Geological Survey (USGS) and the Kentucky and Indiana State Geological Surveys, as well as information on the thickness and properties of near surface soils and their associated uncertainties. The subsurface information has been compiled to determine bedrock elevation and reference depth-dependent shear-wave velocity models for the different soil types. The probabilistic seismic hazard calculation applied here follows the method used for the 2008 U.S. Geological Survey National Seismic Hazard Maps, with modifications to incorporate estimates of local site conditions and their uncertainties, in a completely probabilistic manner. The resulting analysis shows strong local variations of acceleration with 2 percent probability of exceedance in 50 years, particularly for 0.2-second (s) period spectral acceleration (SA), that are clearly correlated with variations in the thickness of unconsolidated soils above bedrock. These values are much greater than the USGS national seismic hazard map values, which assume B/C site conditions. When compared to the national maps with an assumed uniform site D class amplification factor applied, the high-resolution seismic hazard maps have higher amplitudes for peak ground acceleration and 0.2-s SA for most of the map region. However, deamplification relative to the D class national seismic hazard maps appears to play an important role within the limits of the ancient bedrock valley underlying Evansville where soils are thickest. For 1.0-s SA, the new high-resolution seismic hazard maps show levels consistent with D class site response within the limits of this ancient bedrock valley, but levels consistent with B/C site conditions outside.
In this paper, ray theoretical amplitudes and travel times are calculated in slightly perturbed velocity models using perturbation analysis. Also, test inversions using travel time and amplitude are computed. The perturbation method is tested using a 3-D velocity model for Norsar having velocity variations up to 8.0 percent. The perturbed amplitudes are found to be in excellent agreement with the calculated ray amplitudes. Velocity inversions based on travel time and amplitude are next investigated. Perturbation analysis using linearized ray equations is efficiently used to compute amplitude derivatives with respect to model parameters. The trial linearized inversions use smaller velocity variations of 1.7 percent to avoid possible effects due to ray shift, even though the perturbation analysis is valid for larger variations. The trial 2-D inversion results show that linearized amplitude inversions are complementary and not redundant to travel time inversions, even in smoothly varying models.
In this study, we use ambient seismic noise recorded at selected broad-band USArray Earthscope Transportable Array (TA) stations to obtain effective reflection seismograms using noise autocorrelations. In order to best retrieve the body-wave component of the Green's function beneath a station from ambient seismic noise, a number of processing steps are used, including temporal sign-bit normalization, spectral whitening and bandpass filtering. Hourly autocorrelations are stacked for different time periods including one day, one month and one year. On the final stack, different amplitude gain functions are applied, including automatic gain control (AGC), to equalize the correlation amplitudes. The robustness of the resulting ambient noise autocorrelations is first tested on a TA station in Nevada where we are able to identify arrivals similar to those found in an earlier study. We then investigated noise autocorrelations applied to several USArray TA stations in the central U.S., and the results were then compared with reflectivity synthetics for an average crustal model based on CRUST 1.0 where an AGC was used to enhance the later arrivals. Different stacking periods are also investigated in order to find stable correlation stacks.
An interface inversion has been tested and applied to wide‐angle reflection data from the 1986 PASSCAL Ouachita experiment. An interface corrugation with a relief of 10 km and a width of 30 km was successfully imaged in a test of the interface inversion. Velocity‐depth curves derived for shot points 14–19 using the tail‐sum method are similar to an average one‐dimensional velocity model from forward modeling using travel time correlations from southern profiles 14–19. The similarity of these velocity‐depth curves positioned over the southern half of the PASSCAL experiment suggests that crustal structure in this region is approximately laterally homogeneous. Lower crustal velocities are not well resolved due to the recording on most profiles of only small segments of the travel time triplications. A two‐dimensional velocity structure was derived by using the average one‐dimensional velocities for the deeper crustal layers and formally inverting for depth to interfaces. The final inversion model is found to be consistent with previous refraction interpretations south of the Ouachita orogenic trend and concurrent interpretations of the PASSCAL data set. Inversion results for the central and southern portion of the PASSCAL profile indicate a depth of 10–12 km for a midcrustal layer which thins southward from approximately 10 km to about 4 km. A lower crustal layer with an average thickness of 12 km and a Moho depth of approximately 29.5 km are also determined. Interface depths are in agreement with a normal moveout stack of the PASSCAL data set. In particular, the shallowing of the Moho to a depth of 30 km over the northern 50 km of the profile matches previous interpretations of the data set and has been interpreted here and in previous studies as the location of the Paleozoic continental margin. Geophysical studies of the modern Atlantic continental passive margin provide the simplest comparison to the crustal structure derived here. The lower crustal layer found south of the shallowing of the Moho to 30 km beneath the PASSCAL profile is analogous in thickness and position to rift stage lower crust. However, other tectonic interpretations are possible given the uncertainties in velocity and lithology relationships.
Time Domain Reflectometry (TDR) is widely applied for measuring soil water content and electrical conductivity. The method works by analyzing the signal responses under the excitations by a small magnitude electrical pulse, which are used to estimate the properties of the soil sensed by the TDR measurement probe. The commonly used method of TDR signal analysis empirically determines the characteristic reflection points. The soil properties obtained with this method are the average values over length of the TDR probe. The variation of material properties with distance along the probe are generally ignored, which otherwise could provide supplementary information of practical importance. The study of vadose zone is an example where the soil moisture varies with depth. This paper presents an approach for obtaining soil property variation within the probe from TDR measurements. It makes use of a forward model that consists of a frequency domain model for a non-uniform TDR system and a simplified model for soil dielectric permittivity. Inversion analysis was used to determine the geometry and soil properties for each layer within the probe. Both the forward model and the inversion method are first illustrated by numerical examples. They are then applied to analyze data from experimental measurements on layered soils. The preliminary results indicate that the framework provides accurate determination of soil properties, including the thickness of each layer as well as the corresponding soil properties. The capability of obtaining material profiles could further enhance the current TDR-based field monitoring system. It could also be incorporated into the existing cone penetrometers to obtain more information on soil properties.
The dispersive electromagnetic (EM) behavior of soils is strongly related to the mineralogy, soil structure, and pore fluid characteristics. However, time domain reflectometry (TDR) measurements (TDR waveforms) are predominantly used for soil water content and dry density estimation. These two parameters are calculated based on empirical equations that relate them to the soil dielectric permittivity (Ka) and the bulk electric conductivity (ECb). Ka and ECb are obtained in the time domain from few data points of the TDR waveform, disregarding most of the acquired data (usually 2048 data points) that reflects the EM response of the material over a broad frequency range. The complexity of the soil-water interaction in the presence of a time-varying EM field, and the presence of a non-transverse propagation mode in the TDR system limit the characterization of soils by dielectric spectroscopy. This paper presents a semi-empirical method for soil texture identification based on an integrated numerical and experimental analysis of the effect of the EM soil dispersive behavior on TDR waveforms. Evaluation of TDR tests conducted at 20°C on sands, silts, and clays using tap water at different water contents and dry densities shows that a simple time-domain signal processing of the first reflection from the probe section captures the effects of the EM soil-water interaction. Considering that the coefficients of the TDR empirical equations for soil water content and dry density estimation are<br>soil-type dependent, the developed method allows self-calibrating the TDR system. The result of this work provides the basis for making the TDR technique a tool not only for water content and dry density estimation, but also for soil characterization.
Reply| April 01, 1990 Reply to J. Vidale's “Comment on ‘A comparison of finite-difference and fourier method calculations of synthetic seismograms’” C. R. Daudt; C. R. Daudt Department of Earth and Atmospheric Sciences Purdue UniversityWest Lafayette, Indiana 47907 Search for other works by this author on: GSW Google Scholar L. W. Braile; L. W. Braile Department of Earth and Atmospheric Sciences Purdue UniversityWest Lafayette, Indiana 47907 Search for other works by this author on: GSW Google Scholar R. L. Nowack; R. L. Nowack Department of Earth and Atmospheric Sciences Purdue UniversityWest Lafayette, Indiana 47907 Search for other works by this author on: GSW Google Scholar C. S. Chiang C. S. Chiang Department of Earth and Atmospheric Sciences Purdue UniversityWest Lafayette, Indiana 47907 Search for other works by this author on: GSW Google Scholar Bulletin of the Seismological Society of America (1990) 80 (2): 496–497. https://doi.org/10.1785/BSSA0800020496 Article history received: 19 Oct 1989 first online: 03 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Tools Icon Tools Get Permissions Search Site Citation C. R. Daudt, L. W. Braile, R. L. Nowack, C. S. Chiang; Reply to J. Vidale's “Comment on ‘A comparison of finite-difference and fourier method calculations of synthetic seismograms’”. Bulletin of the Seismological Society of America 1990;; 80 (2): 496–497. doi: https://doi.org/10.1785/BSSA0800020496 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search nav search search input Search input auto suggest search filter All ContentBy SocietyBulletin of the Seismological Society of America Search Advanced Search This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not currently have access to this article.
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