Partner organizations: Optim LLC: In-kind Support; Facilities; Collaborative Research; Personnel Exchanges
Optim LLC is a Nevada partnership formed by two UNR Geophysics graduates, Bill Honjas and Sathish Pullammanappallil Ph.D. They are marketing seismic velocity optimization software (SeisOpt), and completed a large DOE-funded investigation at a geothermal field in northern Dixie Valley. Optim LLC volunteered to apply their methods to assist this project in southern Dixie Valley.
Activities and findings:
Research Activities: This project conducted a seismic reflection and gravity experiment in March 1998 to test whether or not part of the 16 December 1954 Dixie Valley Earthquake (Ms=6.8) produced slip on a low-angle normal fault. Our geophysical field work included high- and medium-resolution seismic reflection profiles along Cattle Road from the range-front scarp eastward. Gravity transects were conducted across the valley along Settlement and Cattle Roads and along the scarp from Willow Canyon to Brush Canyon. The high-resolution survey was conducted within 130 m of the rupture with 100 Hz geophones and a sledgehammer source. The medium-resolution line used near-surface explosive sources, 8 Hz geophones, and extended from the rupture to 2.9 km into the basin. Fieldwork involved UNR graduate and undergraduate students, and a drilling and blasting contractor. After data collection, we conducted seismic processing, seismic imaging, velocity optimization, acoustic modeling, and gravity modeling studies. Several of these methods required additional development and adaptation. We presented our results at the 1998 Fall AGU meeting, and are preparing a paper for submission to JGR.
Research Findings: Both high- and medium-resolution profiles define the 1954 rupture surface as dipping at 25-30 degrees to 1.5 km depth. The high-resolution survey shows fault-surface reflections from within 10 m of the surface to 50 m depth; the medium-resolution survey shows fault-surface reflections from 100 m to 1.5 km depth. Reflections subparallel to the fault-surface may be seen in the granite footwall, from 10 to 200 m depth. Also seen are rollover anticlines and buried rupture-graben structures in the sedimentary hanging wall. A vertically coincident refraction off the fault-surface from a long-offset source is also consistent with the presence of a smooth, shallowly dipping fault from 50 to 700 m depth. Gravity results support a shallow basin model, as required by the shallowly dipping boundary normal fault. Our results indicate that slip along a section of the 16 December 1954 Dixie Valley earthquake rupture took place along a fault plane of unusually low dip (25-30 degrees). In this regard, it is the first large historical earthquake for which slip on a low-angle normal fault has been documented.
Research Training: The field work and initial data analysis and modeling were conducted by the Spring semester 1998 Geol 453/653 class, including 10 graduate and undergraduate students in Geology and Geophysics. The course instructor was John Louie; John Caskey organized and partly conducted the field work. Additional lower-division Geophysics majors were hired for the project as field assistants. Class students prepared group term reports explaining the results of individual geophysical techniques. Two students were able to gain enough experience to seek and obtain employment in the Nevada geophysical consulting industry. Graduate student Robert Abbott performed further data processing and modeling, gave the Dec. 1998 presentation at AGU, and is preparing the paper on our results for submission to JGR. This paper will become a chapter of Abbott's Ph.D. thesis.
Education and Outreach: Aside from the AGU presentation, Rob Abbott gave an invited talk to the Nevada Petroleum Society, a group not usually aware of research activities in the UNR Seismological Laboratory. A few months later Rob was recruited for a summer job by NPS members. A public web page at http://www.seismo.unr.edu/ftp/pub/louie/dixie/ makes project results available.
Journal Publications:
http://www.seismo.unr.edu/ftp/pub/louie/dixie/
This page links to presentations, data, and publications under development.
Other specific products:
Contributions within Discipline:
Estimates of extension in the Basin and Range range are commonly above 100 percent (Hamilton, 1978; Wernicke et. al, 1988; Proffett, 1977). Examination of earthquake mechanisms in the western United States reveals the complete absence of large events occurring on normal faults with dips less than 38 degrees (Doser and Smith, 1989). A global study by Jackson (1987) shows a similar limit (approximately 35 degrees) on normal fault dip and reveals that faults are essentially planar and dip steeply down to the brittle-ductile transition. As well, frictional constraints have been used to argue that it is more favorable to create new, steeply-dipping faults than accumulate slip on low-angle normal faults. Given planar faults and minimum fault dip of 38 degrees, simple geometric relations (Jackson and McKenzie, 1983) can be used to show that the maximum extension possible from a single fault system with rotation of dip is 40%. Beyond 40%, extension must be taken up by a new set of high-angle faults cutting the old system, or by aseismic slip on faults which have rotated to a low angle. In contrast, several researchers have compiled observations to argue for the existence of Quaternary seismic slip on low-angle normal faults (Axen et al., 1998; Abers, 1983; Burchfiel et al., 1987, Johnson and Loy, 1992). Also, it has been shown that it is energetically more feasib le to accommodate large amounts extension on normal faults of low dip (Forsyth, 1992). Project results show that slip along a section of the 16 December 1954 Dixie Valley earthquake rupture took place along a fault plane of unusually low dip (25-30 degrees). In this regard, it is the first large historical earthquake for which slip on a low-angle normal fault has been documented. This low-angle normal rupture may represent a relatively rare event, possibly triggered by the M7 Fairview Peak event that preceded it by 4 hours. In the context of Great Basin faulting, however, with typical event intervals in the thousands of years, low-angle normal faulting may be a common basin-building tectonic style.
Previous work in northern Dixie Valley, funded by DOE, showed the geothermal reservoir there is associated with a steeply-dipping normal fault. This project's finding of a shallow-dipping segment of the same fault, 60 km to the south, helps to characterize the tectonic settings conducive to geothermal energy resources.
This project provided practical and research experience to undergraduate and graduate geology and geophysics majors. It helped several to gain employment in Nevada geophysical industries. As well, it funded part of the Ph.D. thesis research of Robert Abbott, a UNR graduate student.
Equipment preparation and maintenance funds from this project helped support a geophysical field facility originally funded by the W. M. Keck Foundation. Foundation funds purchased the equipment, but research projects must maintain the equipment. Keeping the equipment in good order and paying students to learn its use also made the facility available to undergraduate and graduate courses at UNR.
Our proof of a historical low-angle normal-fault rupture affects estimations of seismic hazard in extending areas, because they must include such events at some likelihood. The high-quality seismic data available from this project has spurred Optim LLC to develop additional software products.
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