Measurement of the Parameter Kappa, and Reevaluation of Kappa for Small to Moderate Earthquakes at Seismic Stations in the Vicinity of Yucca Mountain, Nevada
The parameter kappa was defined by Anderson and Hough (1984) to describe the high-frequency spectral roll-off of the strong motion seismic spectrum. In the work of Su et al., (1996) the numerical value of kappa estimated for sites near Yucca Mountain was small (~20 ms). The estimate obtained from these events has been applied through a rigorous methodology to develop design earthquake spectra with magnitude over 5.0. Smaller values of kappa lead to higher estimated ground motions in the methodology used by the Probabilistic Seismic Hazard Analysis (PSHA) for Yucca Mountain. An increase of 10 ms in kappa could result in a substantial decrease in the high frequency level of the predicted ground motions. Any parameter that plays such a critical role deserves close examination. Here, we study kappa and its associated uncertainties. The data set used by Su et al (1996) consisted of 12 M 2.8 to 4.5 earthquakes recorded at temporary stations deployed after the June 1992 Little Skull Mountain earthquake. The kappa elements of that study were revisited by Anderson and Su (MOL.20071203.0134) and substantially confirmed. One weakness of those studies is the limited data used. Few of these stations were on tuff or on Yucca Mountain itself. A decade of Southern Great Basin Digital Seismic Network (SGBDSN) recording has now yielded a larger body of on-scale, well calibrated digital ground motion records suitable for investigating kappa. We use the SGBDSN data to check some of the original assumptions, improve the statistical confidence of the conclusions, and determine values of kappa for stations on or near Yucca Mountain. The outstanding issues in kappa analysis, as they apply to Yucca Mountain, include: 1. The number itself. The kappa estimate near 20 msec from Su et al. (1996) and Anderson and Su (MOL.20071203.0134) is markedly smaller than is considered typical in California (Silva, 1995). The low kappa value has engineering consequences because when it is applied in ground motions analyses used in PSHA, it contributes to the extreme values of peak ground acceleration that the PSHA predicts. Also, in some areas precarious rock evidence indicates that no such accelerations have occurred. 2. The disagreement among analyses in the value of kappa. Previous reports indicate that smallest earthquakes yield kappa estimate 12-20 msec larger than average values from M3 to M4.5 aftershocks of Little Skull Mountain earthquake. 3. The source of kappa. Classically and in engineering usage, kappa is attributed largely to the upper tens or hundreds of meters at the recording site. However, borehole recordings imply that a significant contribution to kappa originates below several hundred meters depth. Also, when earthquakes are considered from a small source region, a true site effect should be common to all recordings. In fact kappa observations of LSM aftershocks to stations on Yucca Mountain and at network stations appear to vary greatly, as though much of kappa actually derives from near the seismic source. 4. The repository overburden contribution to kappa. PSHA estimated ground motions to a free surface at 300 meters depth with properties of confined rock at that depth. Rock mechanical and borehole estimates suggest that several milliseconds of the total kappa accrue between 300 meters to the surface. If estimates of kappa are small at the surface, little is left to reduce incident ground accelerations from the seismic source to the repository level. 5. The variability of kappa. In most cases parametric estimates of kappa have some range of values that fit the data equally well in a statistical sense, so errors in kappa estimates must be addressed. As noted above, kappa at a station also varies significantly for events from the same source area. 6. Are kappa values from small to moderate magnitude earthquakes appropriately applied to the larger, potentially damaging earthquakes of engineering concern? Put another way, is there a significant magnitude dependence in kappa? Questions 1 and 6 are of primary importance, but we find answers to several others in the course of our study. Data from southern Nevada are capable of resolving only some of these questions. In a global search, we identified data from the Japanese borehole accelerometer array KiKNet as most likely to address the questions of the shallow site structural contribution to kappa and the usefulness of moderate earthquake kappa estimates to predict strong ground motion.
- Research Organization:
- Nevada System of Higher Education (NSHE), University of Nevada, Las Vegas (UNLV)
- Sponsoring Organization:
- USDOE - Office of Civilian Radioactive Waste Management (RW)
- DOE Contract Number:
- FC28-04RW12232
- OSTI ID:
- 920643
- Report Number(s):
- TR-07-007; ORD-FY04-006; TRN: US0800949
- Country of Publication:
- United States
- Language:
- English
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