Dating and Earthquakes: Review of Quaternary Geochronology and Its Application to Paleoseismology (NUREG/CR-5562)
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Manuscript Completed: January 1998
Date Published: March 1998
J.M. Sowers, J.S. Noller*, W.R. Lettis
William Lettis and Associates, Inc.
1777 Botelho Drive, Suite 262
Walnut Creek, CA 94596
R. McMullen, NRC Project Manager
Division of Engineering Technology
Office of Nuclear Regulatory Research
U.S. Nuclear Regulatory Commission
Washington, DC 20555-0001
NRC Job Code L2211
*Vanderbilt University, P.O. Box 56, Station B, Nashville, TN 37235 NUREG-5562
Quaternary geochronology, or the dating of Quaternary deposits and landforms, is critical to paleoseismology; it provides the means of assessing the rate of deformation and the timing of past displacements. This report provides: (1) a review of twenty-two Quaternary geochronologic methods or groups of methods, (2) a discussion of the application of geochronology to paleoseismology, including twelve case studies, and (3) the results of four original field and laboratory studies. Included in appendices are a report of a workshop that focused on issues of the application of geochronology to paleoseismology, and a reference list of geochronological laboratories.
Quaternary geochronology is a rapidly developing field. Cosmogenic nuclide dating of surface exposure is the most exciting new method and has enormous potential for applications to both Quaternary geology and paleoseismology. The accuracy and precision of the isotopic methods such as K-Ar, radiocarbon, and U-series are improving with the developement of improved instrumentation and analytical techniques. Other types of methods are seeing similar improvements that increase confidence in their use. At the same time, a few existing methods are seeing decreased use as new research has cast new doubts on their reliability, or as better methods are developed. Despite the many methods available and these new advances, obtaining accurate and precise age estimates of Quaternary deposits and landforms remains a challenge.
A limited field and laboratory program was conducted with the overall objective of testing and comparing specific Quaternary dating methods and evaluating their applicability to paleoseismology. In Pleistocene coastal plain sediments near Virginia Beach, Virginia, ages determined by U-series, infrared stimulated luminescence (IRSL), and amino-acid racemization (AAR) methods were generally in agreement, with the greatest precision and accuracy found in the U-series ages. The 36Cl cosmogenic nuclide method was successfully used to date a fault scarp face at Hebgen Lake, Montana, and showed a stepwise decrease in age with height on the scarp face. In South Carolina, we attempted to determine whether luminescence geochronology, specifically optically stimulated luminescence (OSL), could be used to date paleoliquefaction features. The results from this study are still being evaluated. The proposed site at Santa Rosa Island, California, proved unsuitable because the proper geologic materials were not present. Our experience at this site reinforced the importance of thorough geologic and stratigraphic field work.
To encourage the effective application of dating methods to paleoseismology, we recommend increased collaboration between the paleoseismologist and geochronologist, the verification of age estimates by the application of multiple dating methods, that error analysis account for all sources of uncertainty, that studies undergo technical peer review, and that the rapid pace of research in the field of Quaternary geochronology be recognized in the preparation of applicable federal regulations or guidelines.
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