[1] High-resolution aeromagnetic surveys of the Cascade Range and Yakima fold and thrust belt (YFTB), Washington, provide insights on tectonic connections between forearc and back-arc regions of the Cascadia convergent margin. Magnetic surveys were measured at a nominal altitude of 250 m above terrain and along flight lines spaced 400 m apart. Upper crustal rocks in this region have diverse magnetic properties, ranging from highly magnetic rocks of the Miocene Columbia River Basalt Group to weakly magnetic sedimentary rocks of various ages. These distinctive magnetic properties permit mapping of important faults and folds from exposures to covered areas. Magnetic lineaments correspond with mapped Quaternary faults and with scarps identified in lidar (light detection and ranging) topographic data and aerial photography. A two-dimensional model of the northwest striking Umtanum Ridge fault zone, based on magnetic and gravity data and constrained by geologic mapping and three deep wells, suggests that thrust faults extend through the Tertiary section and into underlying pre-Tertiary basement. Excavation of two trenches across a prominent scarp at the base of Umtanum Ridge uncovered evidence for bending moment faulting possibly caused by a blind thrust. Using aeromagnetic, gravity, and paleoseismic evidence, we postulate possible tectonic connections between the YFTB in eastern Washington and active faults of the Puget Lowland. We suggest that faults and folds of Umtanum Ridge extend northwestward through the Cascade Range and merge with the Southern Whidbey Island and Seattle faults near Snoqualmie Pass 35 km east of Seattle. Recent earthquakes (M W ≤ 5.3) suggest that this confluence of faults may be seismically active today.

SummaryThe seismic design for the Waste Treatment Plant (WTP) on the Hanford Site near Richland, Washington, is based on an extensive probabilistic seismic hazard analysis conducted in 1996 by Geomatrix Consultants, Inc. In 1999, the U.S. Department of Energy Office of River Protection (ORP) approved this design basis following revalidation reviews by British Nuclear Fuels, Ltd., and independent reviews by seismologists from the U.S. Army Corps of Engineers and Lawrence Livermore National Laboratory.In subsequent years, the Defense Nuclear Facilities Safety Board (DNFSB) staff has questioned the assumptions used in developing the seismic design basis, particularly the adequacy of the site geotechnical surveys. The Board also raised questions about the probability of local earthquakes and the adequacy of the "attenuation relationships" that describe how earthquake ground motions change as they are transmitted to the site. The ORP responded with a comprehensive review of the probability of earthquakes and the adequacy of the attenuation relationships. However, the DNFSB remained concerned that "the Hanford ground motion criteria do not appear to be appropriately conservative." Existing sitespecific shear wave velocity data were considered insufficient to reliably use California earthquake response data to directly predict ground motions at the Hanford Site.To address this remaining concern, the ORP provided a detailed plan in August 2004. Key features of this plan included acquiring site-specific soil data down to approximately 500 feet, reanalyzing the effects of deeper layers of sediments interbedded with basalt (down to about 2,000 feet) that may affect the attenuation of earthquake ground motion more than previously assumed, and applying new models for how ground motions attenuate as a function of magnitude and distance at the Hanford Site.This interim report documents the collection of site-specific geologic and geophysical data characterizing the WTP site and the modeling of the WTP site-specific ground motion response.New geophysical data were acquired, analyzed, and interpreted with respect to existing geologic information gathered from other Hanford-related projects in the WTP area. Existing data from deep boreholes were assembled and interpreted to produce a model of the deeper rock layers consisting of interlayered basalts and sedimentary interbeds. These data were analyzed statistically to determine the variability of seismic velocities and then used to randomize the velocity profiles. New information obtained from records of local earthquakes at the Hanford Site was used to constrain site response models. The earthquake ground motion response was simulated on a large number of models resulting from a weighted logic tree approach that addresses the geologic and geophysical uncertainties. Weights were chosen by the working group described in the acknowledgements. Weights were based on the strength or weakness of the available data for each combination of logic tree parameters. Finally, interim design gr...

In 2009 a swarm of small shallow earthquakes occurred within the basalt flows of the Columbia River Basalt Group (CRBG). The swarm occurred within a dense seismic network in the U.S. Department of Energy’s Hanford Site. Data from the seismic network along with interferometric synthetic aperture radar (InSAR) data from the European Space Agency’s (ESA) ENVISAT satellite provide insight into the nature of the swarm. By modeling the InSAR deformation data we constructed a model that consists of a shallow thrust fault and a near horizontal fault. We suggest that the near horizontal lying fault is a bedding‐plane fault located between basalt flows. The geodetic moment of the modeled fault system is about eight times the cumulative seismic moment of the swarm. Precise location estimates of the swarm earthquakes indicate that the area of highest slip on the thrust fault, ∼70 mm of slip less than ∼0.5 km depth, was not located within the swarm cluster. Most of the slip on the faults appears to have progressed aseismically and we suggest that interbed sediments play a central role in the slip process.

No abstract

No abstract