A Seismic Hazards Overview of the Urban Regions of Nevada: Recent Advancements and Research Directions

Abstract

Nevada is a large western state in theUnited States with a seismic hazard that ranges from moderate to high, depending on location. This article identifies priorities to improve estimates of the seismic hazard in the most urbanized parts of the state, specifically the Reno–Carson City urban area of western Nevada and the Las Vegas urban region of southern Nevada. Collaborative task forces are needed to efficiently realize these priorities. For the Reno–Carson City region in western Nevada, the seismic hazard is high because of strain distributed across several active faults, including normal faults that dip beneath parts of the urban areas. The subsurface geometry and possible connections of these faults remain to be determined. The present large uncertainty in estimates of the slip rates can be reduced by future geological and geodetic studies, including trenching at more than one site per fault and increasing the density of geodetic stations to include multiple stations in the mountain ranges between faults to detect rotations. Adjustments to the ground-motion models for the regional properties of western and southern Nevada could reduce ground-motion uncertainties. Ground-motion simulation research needs an improved 3D velocity model. The seismic hazard in Las Vegas is lower than in Reno. An expanded geodetic network and continued geological studies of the active faults are needed. Uncertainties in the geometry and activity of the Frenchman Mountain and Eglington faults particularly introduce significant uncertainties into the seismic hazard in the Las Vegas basin. The more distant Garlock and Death Valley faults in eastern California impact the hazard in Las Vegas because the Las Vegas basin amplifies long-period ground motion and prolongs its duration, so reliable simulations from these sources are needed. Supplemental Content: Summary of research results presented at the workshop, including expanded discussion of recommendations and references for paleoseismic research sites in the Reno and Las Vegas areas. INTRODUCTION: NEVADA’S SEISMIC HAZARD Nevada is a large western state in the United States. The population, with a growth rate of about 2% per year, passed 3 million in 2017. The population is primarily concentrated into the Reno–Carson City urban area of western Nevada and the LasVegas urban area in southern Nevada (Fig. 1). These urban areas face a high to moderate seismic hazard. A more detailed discussion of the tectonic setting and population at risk is given in the E Section S1 of the supplemental content available to this article. Anderson and Miyata (2006) point out that Nevada has been the second most seismically active state of the lower 48 United States, with historical earthquake rates that are lower than in California but that have easily surpassed the combined rates of earthquakes in the Pacific Northwest states (Washington and Oregon) or the Intermountain seismic zone states (Idaho, Montana, Wyoming, and Utah). The hazard in the Reno– Carson City area is greater than the hazard in most parts of California apart from locations near the San Andreas fault system (Fig. 1). For the purposes of this article, the Las Vegas urban area includes Las Vegas and associated cities, the largest of which are Henderson and North LasVegas. This area is entirely within Clark County (2017 population ∼2:2 million). The Reno– Carson City area is spread across parts of Washoe, Carson doi: 10.1785/0220180357 Seismological Research Letters Volume 90, Number 4 July/August 2019 1577 Downloaded from https://pubs.geoscienceworld.org/ssa/srl/article-pdf/doi/10.1785/0220180357/4790856/srl-2018357.1.pdf by University of Nevada Reno user on 03 December 2019 City, Douglas, and Story counties and includes California communities around the shore of Lake Tahoe (2017 population ∼0:62 million). Both the Las Vegas and Reno–Carson City areas are popular tourist destinations. In 2017, an estimated 42.2 million people visited Las Vegas, and 5 million people visited Reno. The 2018 Working Group on Nevada Seismic Hazards (NWG) met in Reno, Nevada, on 5 and 6 February 2018. The purpose of the meeting was to review ongoing earthquake hazard research in Nevada, discuss technical issues related to earthquake hazards in Nevada, and identify priorities for future research that will reduce uncertainties and improve theU.S. Geological Survey (USGS) National Seismic Hazard Model (NSHM). A list of participants and abstracts of presentations from the meeting are available on the website for the Nevada Bureau of Mines and Geology (see Data and Resources). The meeting was funded by the USGS. The purpose of this article is to provide a concise review of the future directions recommended by the participants. A brief review of the current understanding of the hazard, of the data that are available to assess the seismic hazard, and of important outstanding issues that impact the seismic hazard is given in the accompanying E supplemental content. The current generation of hazard maps by the USGS (Frankel et al., 1996, 2002; Petersen et al., 2008, 2014, 2015) incorporated and represented the state-of-the-art summaries of the hazard, so the documentation associated with the USGS maps is a valuable resource. There have been several previous workshops to review research and set future priorities for seismic hazard studies in the Basin and Range Province. These include several organized by the Utah Geological Survey (Lund, 1998, 2005, 2006, 2012, 2015) and workshops in Reno on geodetic and geologic data sets (Briggs and Hammond, 2009, 2011). Outside of the urban areas, Nevada has been a focal point of extensive studies associated with underground nuclear testing and the seismic hazards at the proposed high-level nuclear waste repository at Yucca Mountain (e.g., Stepp et al., 2001). These studies are outside the scope of this review except to the extent that they have helped understand the processes that affect the hazard in the urban areas. ELEMENTS OF THE NATIONAL SEISMIC HAZARD MODEL Active Faults Major paleoseismology contributions to the NSHM are to identify the location and geometry of active faults and estimate the rupture lengths, magnitudes, slip per event, recurrence interval, and slip rate of the faults, and times of paleoearthquakes (Haller et al., 2015; Moschetti et al., 2015; Petersen et al., 2015). This type of information has also been previously used in the Reno– Carson City area to infer earthquake recurrence rates and develop earthquake planning scenarios (dePolo et al., 1996, 1997). The National Quaternary Fault and Fold Database contains summaries of paleoseismic data but is not updated regularly and lacks up-to-date information in many cases.E Section S2.1 includes maps of the faults in the Reno–Carson City and Las Vegas areas and a summary of published research. Based onWesnousky (2019), only a few faults in western Nevada have been examined at multiple locations, so more work is needed. Considering the large numbers of faults that contribute to the hazard and need eventual improved characterization, it is essential to prioritize the future studies of these faults. The NWG agreed that the first priority is to better understand the faults that are capable of causing damage in the main urban regions. Considering the existing hazard model (Petersen et al., 2014), deaggregation assessment, and the qualitative evaluation of existing fault information (E Table S.4), a set of faults were prioritized for future study; Table 1 identifies these priority fault studies. Geodetic Studies On a large scale, as predicted by Kostrov (1974), the geodetic strain rate is consistent with the earthquake occurrence rate in the Basin and Range (e.g., Anderson, 1979; Pancha et al., 2006). Geodesy made major contributions toward understanding the deformation field of the Great basin,Walker Lane, and Sierra Nevada mountains, as summarized in theE Section S2.2, and should have an increasing importance in the future. The geodetic models are limited by the density of geodetic stations. The resolution in southern and eastern Nevada is significantly lower than in western Nevada. More stations are particularly needed to understand the strain field near Las 120.0°W 122.5°W 32.5°N 35.0°N 37.5°N 40.0°N 117.5°W 115.0°W Reno