Earthquake Hazard Studies at GEO-HAZ

(Updated Jan. 14, 2008)

Seismic Hazard Analyses for Critical Facilities

Probabilistic Seismic Hazard Analysis (PSHA) for Folsom Dam, California

GEO-HAZ is part of a 3-person Consultant Review Board engaged by the US Bureau of Reclamation to review the PSHA for Folsom Dam on the American River, California. The main dam consists of a 340-foot high concrete center section flanked by long earthen wing dams extending to high ground at either end. The dam impounds over a million acre-feet of water in Folsom Lake. The dam lies on the western margin of the Foothills Fault System (FFS), an enigmatic zone of short Quaternary (and pre-Quaternary) faults 320 km long and 50 km wide. The FFS is the main seismic source for Folsom Dam, but it’s seismotectonic setting is poorly understood. Therefore, the reviewers created a PSHA logic tree structure that begins with 2 opposing behavioral branches, representing possible strike-slip kinematics versus possible extensional (normal-fault) kinematics. Input values on successive tree branches (seismic source parameters) are then customized for each kinematic scenario.

Seismic Setting of a Nuclear Fuel Reprocessing Facility, New Mexico

GEO-HAZ performed the seismic hazards part of the basic-data collection for the proposed nuclear fuel reprocessing facility near Roswell, New Mexico, to be operated by Global Nuclear Energy Partnership. This effort involved cataloging the historical seismicity within a 320-km radius of the site, which included the Rio Grande rift zone, and categorizing all capable faults as defined by US NRC criteria. Of the 101 faults within a 200-mile radius, only 8 faults exceeded the minimum length specified by Appendix A of 10CFR100. Two of the 8 faults did not exhibit compelling evidence for being capable faults as defined in Appendix A. The other 6 faults were deemed to be capable, and were further characterized from published and unpublished data.

PROBABILISTIC SEISMIC HAZARD ANALYSES FOR FAULT DISPLACEMENT AND VIBRATORY GROUND MOTION AT YUCCA MOUNTAIN, NEVADA

Yucca Mountain Nuclear Waste Repository, Nevada, USA (1996)

GEO-HAZ provided key input for the probabilistic seismic hazard analysis (PSHA) of the proposed U.S. National High-Level Nuclear Waste Repository at Yucca Mountain, Nevada. Along with team members Burt Slemmons and Jon Ake, we created a logic trees for a standard ground motion PSHA for Yucca Mountain, and developed a new technique for performing probabilistic fault displacement hazard analysis (PFDHA). Seismic sources near the Repository include 10 near-field faults, 22 far-field faults (see map below), and 5 areal source zones. See Yucca Mountain PSHA.

Probabilistic Seismic Hazard Analysis (PSHA) for Los Alamos National Laboratory, New Mexico, USA

GEO-HAZ was part of a 2-person Consultant Review Board engaged by Woodward-Clyde Federal Services (now URS Corporation) to review the PSHA for the Los Alamos National Laboratory (LANL). LANL is a federal research facility with 9,000 employees plus approximately 650 contractor personnel, and an annual budget of approximately $2.2 billion. The Pajarito fault adjacent to the Lab is the main seismic source for LANL, but it’s relationship to two nearby antithetic faults (Guaje Mountain fault, Rendija Canyon fault) is poorly understood. Therefore, the reviewers suggested creating a PSHA logic tree structure that began with 5 opposing behavioral branches, representing possible 5 possible interactions scenario between the master fault and the 2 antithetic faults. Input values on successive tree branches (seismic source parameters) were then customized for each behavioral scenario.

For more detail on paleoseismicity of the master Pajarito fault, see Los Alamos National Laboratory, New Mexico (1996-1999)

Quality Assurance Manual for Geologic Studies for Seismic Hazard Assessment, South Africa

Under contract to the Council for Geosciences, Bellville, South Africa, we compiled a 6-chapter Quality Assurance Manual to guide CGS scientists in data collection and interpretation for neotectonic and paleoseismic studies, for siting critical facilities. The chapters were: 1) Geologic Mapping, 2) Mapping of Trench Walls, 3) Measurement of Paleo-Shoreline Indicators, 4) Identifying Faults and Determining Their Origins, 5) Geochronology, 6) Seismic Source Characterization From Geologic Data. These chapters drew on documents written in the 1980s and 1990s by U.S. Geological Survey, U.S. Department of Energy, and U.S. Nuclear Regulatory Commission, but updated them to current scientific standards.

Seismic Source Characterization (Multiple Faults)

Holocene Faulting and Gravitational Spreading in the Yakutat Microplate, southern Alaska

This study is part of the NSF-funded STEEP project (St. Elias Erosion-Tectonics) , a multi-university, multi-disciplinary, 5-year project into the tectonic and erosional processes of the Yakutat microplate. GEO-HAZ led the on-land Paleoseismology effort in the summer of 2005 and 2006, and will be attending the STEEP Workshop on Jan. 24-27, 2008. Our trenching work suggests that the thousands of antislope scarps that riddle the mountain ridges in the Yakutat Block, are mainly flexural-slip fault scarps, rather than sackungs as suggested by others.

Mapping of Quaternary Faults in the Central Uncompahgre Plateau, western Colorado

GEO-HAZ mapped Quaternary faults over an area of six 7.5’ topographic quadrangles(scale 1:24,000) that lie NW of the Bureau of Reclamation’s Ridgeway Dam. This effort required several weeks of original mapping in a densely-forested wilderness area, and then georeferencing all the mapping to a GIS. We discovered multiple late Quaternary fault scarps that form a discontinuous, NW-trending zone at least 32 km long. The zone appears to be an extension of the Busted Boiler and Log Hill Mesa faults, which were previously identified as the closest known Quaternary faults to Ridgeway Dam.

Paleoseismicity of Quaternary Faults near Albuquerque, New Mexico

This multi-year study resulted in the trenching of four faults in the Albuquerque metropolitan area between 1996 and 2002. On the west side of the Rio Grande rift (West Mesa, or Llano de Albuquerque), we trenched the County Dump fault (1996), the Calabacillas fault (1999), and the Zia fault (2000). On the eastern side of the rift (base of the Sandia Mountains) we trenched the Sandia fault in 2002. The end result was confirmation that the “seesaw subsidence” model of Rio Grande rift basins (Smith et al, 2001; http://epswww.unm.edu/facstaff/gsmith/research/Smith%202000%20Santo%20Dom%20GSA.pdf) has also operated in the Albuquerque Basin during the late Quaternary. That is, active fault activity shifts from one side of the rift to the other, in groups of many seismic cycles, for each of the rhomboidal sub-basins and intervening structural highs within the Albuquerque Basin. This pattern has led to the lack of late Quaternary faulting on the Sandia fault, for one thing, a fortunate occurrence for the citizens of Albuquerque.

The 1996 County Dump study was published in 2006 by the New Mexico Bureau of Mines & Geology (http://geoinfo.nmt.edu/publications/circulars/212/)

The 1999 Calabacillas is available here:

Paleoseismicity of Quaternary Faults near Albuquerque, New Mexico (PDF file)—trench Calabacillas fault, 1999

The 2002 Sandia fault report is available from USGS at http://earthquake.usgs.gov/research/external/reports/02HQGR0068.pdf

Neotectonics of Bear Lake Valley, Utah and Idaho

The Bear Lake Valley is a graben dominated by the master East Bear Lake fault zone on the east, and the secondary West Bear Lake fault zone on the west. Each fault is composed of 3 geometric segments. In our studies, we excavated trenches on the most-active-looking segments of each fault zone, the central segments. However, late Quaternary fault scarps also exist on other, untrenched, fault segments, so our study is only a preliminary examination of the neotectonics of this graben. See:

http://ugspub.nr.utah.gov/publications/misc_pubs/MP-03-4.pdf

Seismic Source Characterization (Single Faults)

Late Quaternary Faulting on the East Cache Fault (Utah), southern segment

The East Cache fault zone (ECFZ) is an active normal fault zone in north-central Utah, and trends N-S for 80 km on the eastern margin of Cache Valley (western margin of the Bear River Range). Although the densely-populated central segment of this fault was trenched in the 1980s-90s, the less-populated southern segment was not. However, now population growth has shifted onto the southern segment, and updated seismic hazard info is needed. In December 2007 we assisted in excavating trenches across the western and central traces of the southern segment of the ECFZ. No fault could be located beneath the western trace, which coincides with the highstand shoreline of Lake Bonneville; thus that “fault scarp” is probably entirely erosional rather than tectonic. Three trenches across the central strand also did not expose a fault plane, but suggested the possibility that the east-facing scarp here is erosional on a dipslope in the Salt Lake Formation. Final trenching is planned for Spring 2008 on the eastern (range-front) strand.

Origin of the Anton Scarp, High Plains of Colorado

The Anton Scarp is a 20-25 m-high, 150 km-long scarp that traverses the High Plains of Colorado about 150 km east of Denver. The scarp is not coincident with any known structure and runs perpendicular to the regional slope direction and drainage of the High Plains. In addition, the scarp is not obviously related to differential erosion of local bedrock, being contained within a single formation (the Ogallala Group). If the scarp were created by Quaternary faulting, it would constitute the largest source of seismic hazard to the Denver area. From 2005-2007 we assisted in performing geophysical (GPR), trenching, and drilling investigations to determine the origin of this scarp. These studies indicate that the scarp is NOT a Quaternary fault scarp and is presumably erosional. Based on this work, USGS now classifies this fault as Class C.

Williams Fork Normal Fault Review

The Williams Fork normal fault was discovered in 2002 in a dense pine forest at the foot of the Williams Fork Mountains in central Colorado. This fault is now the northernmost known Quaternary fault associated with the Rio Grande rift zone, where scarps are clearly late Quaternary in age, and trenches show displacement of late Quaternary strata. GEO-HAZ reviewed and interpreted the trench mapping for the Colorado Geological Survey.

East Canyon Fault, Utah, trench review

On another project for the US Bureau of Reclamation, we were part of a team that reviewed and interpreted the mapping of a Bureau trench across the East Canyon fault, Utah. The East Canyon fault is in an area of the “Overthrust Belt” where Sevier-age thrust faults have been reactivated as normal faults in the Neogene. However, there are also salt deposits in the subsurface beneath the East Canyon fault, so it was critical to distinguish whether the trenched fault scarp formed by coseismic faulting, or by aseismic, creeping salt tectonics. Structures (fissures) and stratigraphic units (colluvial wedges) exposed in trench walls suggested rapid displacement, more likely coseismic than aseismic.

Fault Trenching in South Africa

We were fortunate to assist the Council for Geoscience (South Africa’s federal geological survey) in trenching the largest known (and most newly-discovered) late Quaternary fault in South Africa, the Kango fault in the Cape Fold Belt. The southward-dipping, east-west striking 320 km-long Kango fault is the largest segment of the 715 km-long Ceres-Kango-Baviaanskloof-Coega-St.Croix fault system. http://www.icms.com.au/inqua2007/abstract/1184.htm. Preliminary indications are that the ca. 80 km-long fault scarp at the base of the range front formed in a single surface rupture during the latest Pleistocene-earliest Holocene. However, the previous event probably occurred more than 100 ky ago, indicating long recurrence, as is typical of many intraplate faults.

Refine Slip Rates and Segmentation of the Northern Sangre de Cristo Fault, Colorado’s Largest Active Fault

This NEHRP-funded project involved excavating a paleoseismic trench on the Crestone section of the Northern Sangre de Cristo fault north of Hayden Pass, during October 2003. The Crestone section, at 90 km long, is the longest section of Colorado’s most active fault. This trench was dug to see if Holocene and late Pleistocene surface ruptures dated in the central part of the section extended into the northern part.

            Preliminary Results: The trench across the 4.5 m-high scarp on a Bull Lake-age fan exposed evidence for 2 or 3 surface ruptures, compared to an estimated total of 5-6 post-Bull Lake surface ruptures in the center of the segment. A strong soil B horizon was developed atop all colluvial wedges, indicating that the Most Recent Event was pre-Pinedale in age (much older than 35 ka). Therefore, only half (or less) of the surface ruptures affecting the central part of the Crestone section continue north of Hayden Pass, and the 3 post-Pinedale events detected in the central part are not present in our trench. This result suggests that most surface ruptures affecting the Crestone section are diverted onto the Villa Grove fault zone. At this time it is not clear whether a new segment boundary should be placed at Valley View Hot Springs, or at Hayden Pass. But clearly, the 90 km length of this section should be decreased in the National Seismic Hazard Map of USGS. Slip rates will be refined once OSL dating is completed by Glenn Berger at DRI.

Quaternary Faulting and Seismic Source Characterization in the El Paso-Juarez Metropolitan Area; Collaborative Research with the University of Texas at El Paso

This NEHRP-funded project involved re-excavating the paleoseismic trench of Keaton et al (1996) on the East Franklin Mountains fault zone north of Fort Bliss, during March 2003. In addition we cleaned off a good arroyo exposure of the fault about 1.5 miles north of Keaton’s trench. Trench logs for both exposures are now being digitized. Diane Doser and colleagues at UTEP made several geophysical surveys adjacent to the trench.

Preliminary Results: Displacement on this 8.5 m-high scarp is much larger than the scarp height, due to burial of the downthrown block by post-faulting alluvium. We were unable to expose any beds on the downthrown block correlative with those on the upthrown block, indicating a minimum offset of about 13 m on the Jornada II fan surface. If the surface is 25-80 ka, slip rates are at least 0.16 to 0.52 mm/yr. These slip rates will be refined once OSL dating is completed by Glenn Berger at DRI, and cosmogenic exposure age estimates are made.

Seismic Hazards to Infrastructure at Great Sand Dunes National Monument and Preserve, Colorado

GEO-HAZ is involved in a multi-year assessment of paleoseismicity and seismic hazards to the newly-enlarged Great Sand Dunes National Monument. In June 2002 and October 2003 we excavated 4 trenches across strands of the Sangre de Cristo fault near Monument facilities. The October 2003 trench near the Visitor Center revealed that the scarp there is erosional, cut by Medano Creek sometime between the Bull Lake and Pinedale glaciations (150-35 ka).

Quaternary Faulting Confirmed on the Roubideau Creek Fault, Uncompahgre Plateau, Western Colorado

GEO-HAZ performed GPS-based geologic field mapping along the 20 km-long Roubideau Creek fault west of Montrose, Colorado. The structure is actually a down-to-the-north monocline with an amplitude of 60-80 m broken by younger normal fault scarps up to 10 m high. Based on geomorphology of the fault scarps, there have been several Quaternary surface-faulting events in the central part of the fault, east of Roubideau Creek.

Paleoseismology of the Wasatch Fault Zone, Utah

GEO-HAZ has performed several paleoseismic studies on various segments of the Wasatch fault zone. The earliest study (1991) was performed on the Brigham City segment at brigham City, Utah. This study first documented the apparent long (ca. 2200 yr) elapsed time since the Most Recent Event, which currently results in the Brigham City segment having the highest conditional probability of any segment for rupture within the next 100 years (see McCalpin and Nishenko, 1996; http://www.agu.org/pubs/crossref/1996/95JB02851.shtml

The trenching study was published by the Utah Geological Survey in 2002: go to http://ugspub.nr.utah.gov/publications/misc_pubs/MP-02-9WFZ-BrigCity.pdf

This next study was based on the first megatrench excavated in the Wasatch fault zone in 1999, which was visited by attendees at the annual meeting of the Association of Engineering Geologists in Sept. 1999. Study results were later published by the Utah Geological Survey in their Paleoseismology of Utah Series: go to http://ugspub.nr.utah.gov/publications/misc_pubs/MP-02-7WFZ-SLC.pdf

Earlier versions are provided here:

Long Recurrence Records from the Wasatch Fault Zone, Utah (html file)

Annual Project Summary (PDF file)

Paleoseismology of the East Cache Fault Zone, Utah

http://ugspub.nr.utah.gov/publications/special_studies/ss-83.pdf

Seismic Source Characterization (Volcanic Faults)

Interaction of Volcanic and Tectonic Processes in the Past 40,000 Years at Long Valley Caldera

Collaborative Research With GEO-HAZ Consulting, Inc., and U.S. Geological Survey: Interaction of Volcanic and Tectonic Processes in the Past 40,000 Years at Long Valley Caldera (1997)

Seismic source characterization of active faults becomes more complex where there have been both tectonic slip episodes and volcanic slip episodes on a single fault or swarm of faults. This is the case where the Pleistocene Long Valley Caldera interferes with the behavior of the Sierra Nevada Frontal Fault in eastern California, USA. In this case, volcanic-tectonic interaction can be assessed by the slip chronologies from standard volcanic field and geochronology techniques, with those from standard paleoseismic practices such as fault trenching.

Integrated Seismic Microzonation (Ground Motion+ Liquefaction+ Landslides)

Seismic Hazards Mapping at 1:24,000 Scale, Wasatch Front, Utah

Seismic Hazards Mapping at 1:24,000 Scale, Wasatch Front, Utah-- A Digital Pilot Project (html file)

This GIS-based pilot project aimed at developing an inexpensive technique for performing medium-scale seismic microzonation based on existing surface and subsurface data. Seismic microzonation is an areal mapping process, but previous studies have attempted to expand the site investigation approach (based on expensive cone penetrometer, shear wave profiling, or drilling/blow count data) to progressively larger areas. This approach rapidly becomes uneconomic, even for areas much smaller than an entire 7.5’ quadrangle. Our approach first made use of abundant water well logs to make a 3D subsurface model for an entire 7.5’ quadrangle. Second, the geologic units identified in the 3D model were assigned geotechnical parameters based on the limited data set from clustered geotechnical boreholes that sampled these same units. In this way, four 7.5’ quadrangles were microzoned with reasonable data density and distribution based on about 1500 water well logs. This approach lies midway (in detail and expense) between reconnaissance microzonation maps based only on surface geologic maps, and small-area site investigations dependent on expensive drilling or shear wave profiling.

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