DEPARTMENT OF THE INTERIOR UNITED STATES GEOLOGICAL SURVEY PROCEEDINGS OF THE NATIONAL EARTHQUAKE PREDICTION EVALUATION COUNCIL JUNE 11-12, 1991 ALTA, UTAH by Virgil A. Frizzell, Jr. Open-File Report 92-249 This report is preliminary and has not been edited or reviewed for conformity with U.S.G.S. publication standards. 1992 REPRODUCED FKOK OUST A * » " * * TABLE OF CONTENTS Page Preface v Current Membership of National Earthquake Prediction Evaluation Council vii Proceedings of the meeting 1 References cited 33 Appendices 35 PREFACE The National Earthquake Prediction Evaluation Council (NEPEC) was established in 1979 pursuant to the Earthquake Hazards Reduction Act of 1977 to advise the Director of the United States Geological Survey (USGS) about issuing any formal predictions or other information pertinent to the potential for the occurrence of a significant earthquake. The Director of the USGS is responsible for deciding whether and/or when to issue predictions or other information pertinent to a prediction. A prediction is defined as a statement on the time of occurrence, location, and magnitude of a future significant earthquake including an analysis of the uncertainty of those factors. NEPEC advises the Director concerning the completeness and scientific validity of the available data and on related matters. Duties include the evaluation of predictions made by other scientists, from within or outside of government, rather than issuance of predictions based on data gathered by NEPEC itself. According to its charter, NEPEC, also referred to in this document as the Council, is comprised of a Chairman, Vice Chairman and from 8 to 12 other members appointed by the Director of the USGS. The Chairman may not be a USGS employee and at least one-half of the membership must be other than USGS employees. NEPEC generally functions through the use of working groups organized by the USGS at the request of NEPEC. Working groups often include representatives from private industry, academia, and the USGS. Members of NEPEC who participate in a working group do not vote during NEPEC's evaluation of the results of the working group. After concluding its evaluation, NEPEC presents its recommendations to the Director, who bears ultimate responsibility for a decision concerning issuance of a prediction or other information. The USGS has published the proceedings of previous NEPEC meetings as open-file reports; these reports are available from the USGS Open-File Distribution Center in Denver, Colorado. CP. NATIONAL EARTHQUAKE PREDICTION EVALUATION COUNCIL September 1991 Thomas V. McEvilly, Chairman University of California Robert L. Wesson, Vice-Chairman USGS, Reston Keiiti Aki University of Southern California William H. Bakun USGS, Menlo Park John N. Davies University of Alaska James F. Davis California Division of Mines and Geology James H. Dieterich USGS, Menlo Park Thomas H. Heaton USGS, Pasadena Arch C. Johnston Memphis State University Hiroo Kanamori California Institute of Technology William H. Prescott USGS, Menlo Park Kaye M. Shedlock USGS, Golden Joann Stock Harvard University Ray J. Weldon University of Oregon Virgil A. Frizzell, Jr., Executive Secretary USGS, Reston vii NATIONAL EARTHQUAKE PREDICTION EVALUATION COUNCIL PROCEEDINGS OF THE MEETING OF JUNE 11-12, 1991 Alt a, Utah Council Members Present Thomas McEvilly, Chairman University of California Robert Wesson, Vice-Chairman USGS, Reston William Bakun USGS, Menlo Park John Davies University of Alaska James Dieterich USGS, Menlo Park Thomas Heaton USGS, Pasadena Arch Johnston Memphis State University Hiroo Kanamori California Institute of Technology William Prescott USGS, Menlo Park Kaye Shedlock USGS, Golden Joann Stock Harvard University Virgil Frizzell, Executive Secretary, USGS, Reston Participating Guests M. Lee Allison Utah Geological Survey Walter J. Arabasz University of Utah Ronald L. Bruhn University of Utah Gary Johnson Federal Emergency Management Agency John 0. Langbein USGS, Menlo Park William R. Lund Utah Geological Survey Michael N. Machette USGS, Golden Stuart P. Nishenko USGS, Golden David P. Schwartz USGS, Menlo Park Robert B. Smith University of Utah JUNE 11, 1991 Morning Session T.McEVILLY, Chairman of the National Earthquake Prediction Evaluation Council (NEPEC) , opened the Council meeting by asking members, participants, and guests, to introduce themselves and by outlining the meeting's agenda (see Appendices A and B) . All Members were in attendance except K. Aki, J. Davis, and R. Weldon. R.WESSON, Vice-Chairman of NEPEC, presented an overview of Council activities with an emphasis on the transition from the Chairmanship of Lynn Sykes to that of McEvilly. At the termination of Sykes 1 tenure, NEPEC had completed a probabilistic assessment of the San Francisco Bay Region and had visited a number of areas of the country (Northern California, Southern California, the Pacific Northwest, and Alaska) that had been perceived as having a level of hazard that warranted attention. NEPEC had intended to visit the Wasatch area, but the press of business in California precluded such a visit. At the outset of McEvilly's tenure, NEPEC prioritized areas needing attention, and the Wasatch area remained a high priority region. The present meeting at Alta, Utah, was delayed because of the need to reevaluate the San Francisco Bay Region in light of the Loma Prieta earthquake of 1989 as well as the need to address the so-called "Browning prediction." T.McEVILLY and R.WESSON agreed that several options were available and suggested that Council Members and guests consider various options during the day's presentation and what sort of document might be used to present NEPEC's response to the Wasatch front, as well as options that would allow NEPEC to help focus attention on the other issues under consideration. R.WESSON also noted that Randall Updike completed his term as Executive Secretary of NEPEC and that an open-file report entitled "1990 Proceedings of the National Earthquake Prediction Evaluation Council" (Updike, 1990) resulted from his efforts. Several Members joined Wesson by congratulating Updike for a job well done. R. SMITH of the University of Utah (UU) presented an overview of the seismotectonics, seismicity, and paleoseismicity of the Intermountain Seismic Belt (ISB) as a background to discussion of the Wasatch front of central Utah (Smith and Arabasz, 1991; See Appendix C) . He described the ISB as a six state region with a linear zone of intraplate tectonism on faults dominated by normal deformation (App. C, fig. 1) . The 7.3 magnitude Borah Peak earthquake, which had scarps 3 to 3.5 m high, focused National attention on the importance of normal faulting earthquakes. The Hegben Lake, Montana, earthquake of 1955, demonstrated that active normal faults may be located along preexisting structures, in that case a Laramide thrust fault. The structures at mouth of Little Cottonwood Canyon, the location of the current meeting, were postulated by G.K. Gilbert to be responsible for the uplift of the Wasatch Range. Those same young structures trend through Salt Lake City. Large earthquakes occurring in populated areas along the Wasatch front would have catastrophic effects on the population and economy of the entire state, as well as throughout the surrounding six state region. The ISB occupies a large part of the w astern U.S., forms a arcuate region 1000 km by 400 km, and is comprised of southern, central, and northern portions (App. C, fig. 1). Earthquakes are generally shallow, occurring less than about 20 km in depth, and are diffusely located without obvious correlation to mapped Quaternary or Holocene faults. The earthquakes are the result of differential strain between a more stable part of the North America plate and the Great Basin. Since with the Borah Peak earthquake of 1983, much has been learned about structures responsible for normal faulting events in the region. Aftershocks were not located along the scarps, but instead on a zone displaced 15 km to :he southwest (App. C, figs. 10, 11) , implying substantial dip to the fault. The main shock occurred in the same zone at about 16+4 km depth. Leveling data is compatible with a dipping planar structure Data from several other areas indicate that such structures become listric at depth. Our model for earthquake hazard analysis is one of planar structures with large events nucleating at crustal depths (App. C, fig. 21), so not only do we have to be aware of the possible effects that may extend to adjacent mountain blocks, but we must carefully consider hazards associated with structures that surface somewhat more distally from the population centers. Quaternary faults, including six historic earthquakes have been scaled comparing Ms versus fault surface length and maximum surface displacement. The scaling law for maximum surface displacement is the same as that of Bonilla and others (1984) and DePolo (1990) (See App. C, refs) . The second law, that of Ms versus surface length, is quite different. If one compares the length of Quaternary faults and fault segments in the ISB to determine possible (magnitudes that might be generated by movement on the faults, one finds that the Wasatch fault zone dominates with post-glacial earthquakes in the magnitude 7.2 to 7.3 range. If our maximum surface displacement scaling law is used, larger events are possible along the Wasatch, with magnitudes of 7.4 to 7.5. This [presents an enigma in some areas which have large scarps with short surface length displacement. Smith emphasized thatj the detailed information needed to analyze faults in this manner! is quite limited. In summary, the model appears to be one of earthquakes nucleating at the brittle/ductile transition along planar structures that are connected to relatively shallow low angle detachments. The uniform pattern of coseismic strain must result from long-term interseismic strain. W.ARAB AS Z of UU presented information on seismic hazards along the Wasatch front. Normalized seismicity rates of earthquake occurrence by area in the ISB is lower by a factor of 4 when compared to the plate boundary in California.