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SIM 3410: Pamphlet for Map of Recently Active Traces of the Rodgers Creek Fault, Sonoma County, California

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SIM 3410: Pamphlet for Map of Recently Active Traces of the Rodgers Creek Fault, Sonoma County, California Map of Recently Active Traces of the Rodgers Creek Fault, Sonoma County, California By Suzanne Hecker and Carolyn E. Randolph Loar Pamphlet to accompany Scientific Investigations Map 3410 2018 U.S. Department of the Interior U.S. Geological Survey U.S. Department of the Interior RYAN K. ZINKE, Secretary U.S. Geological Survey James F. Reilly II, Director U.S. Geological Survey, Reston, Virginia: 2018 For more information on the USGS—the Federal source for science about the Earth, its natural and living resources, natural hazards, and the environment—visit https://www.usgs.gov/ or call 1–888–ASK–USGS (1–888–275–8747). For an overview of USGS information products, including maps, imagery, and publications, visit https://store.usgs.gov/. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government. Although this information product, for the most part, is in the public domain, it also may contain copyrighted materials as noted in the text. Permission to reproduce copyrighted items must be secured from the copyright owner. Suggested citation: Hecker, S., and Randolph Loar, C.E., 2018, Map of recently active traces of the Rodgers Creek Fault, Sonoma County, California: U.S. Geological Survey Scientific Investigations Map 3410, 7 p., 1 sheet, https://doi.org/10.3133/sim3410. ISSN 2329-132X (online) Contents Introduction.....................................................................................................................................................1 Fault Nomenclature .......................................................................................................................................1 Approach and Scope.....................................................................................................................................1 Database Structure and File Formats .........................................................................................................2 Notable Aspects of Revised Mapping ........................................................................................................4 Acknowledgments .........................................................................................................................................5 References Cited (Text and Database) .......................................................................................................5 Figures 1. Shaded relief map of the San Francisco Bay area, California, showing the Rodgers Creek Fault and other significant faults ..................................................................map sheet 2. Maps of the the Rodgers Creek Fault, California, showing relation to regional topography and the main sources of prior fault mapping used in this study ...map sheet 3. Hillshade images derived from 2007 Earthscope lidar dataset illustrating types of geomorphic features used to identify recently active fault traces ....................map sheet Tables 1. Fault-strand database attribute fields .......................................................................................3 2. Fault strands trenched for purpose of paleoseismic study ...................................................3 iii Map of Recently Active Traces of the Rodgers Creek Fault, Sonoma County, California By Suzanne Hecker1 and Carolyn E. Randolph Loar2 Introduction years (Schwartz and others, 1992). More research is needed to clarify other aspects of the fault’s behavior, such as timing The accompanying map and digital data identify of earlier events, lengths of earthquake ruptures, and amounts recently active strands of the Rodgers Creek Fault in and distribution of displacement through time. Aseismic Sonoma County, California, interpreted primarily from shallow fault slip, or creep, a phenomenon documented along the geomorphic expression of recent faulting on aerial many parts of the San Andreas Fault system in northern photography and hillshade imagery derived from airborne California, has been recognized along the Rodgers Creek light detection and ranging (lidar) data. A recently active Fault, from Santa Rosa northward, only relatively recently fault strand is defined here as having evidence consistent (Funning and others, 2007; Lienkaemper and others, 2014; with slip during the Holocene epoch (approximately the McFarland and others, 2016; Jin and Funning, 2017). past 11,700 years). The purpose of the map is to update the fundamental fault dataset for characterizing surface-rupture hazard, siting slip-rate and paleoseismic studies, and study- Fault Nomenclature ing the geometry and evolution of slip. To serve a range of users, the map is presented in several formats: as an image The nomenclature of the Rodgers Creek Fault system map, as a digital database for use within a geographic infor- has varied over time and with point of view. The fault north mation system (GIS), and as a KML file for visualizing the and south of Santa Rosa is shown on early fault-activity fault using virtual globe software. and geologic maps (for example, Brown, 1970; Huffman Important outcomes of this mapping revision include and Armstrong, 1980) as separate faults (the Healdsburg the following: (1) a northward 17-km increase in the known Fault and Rodgers Creek Fault, respectively), reflecting length of Holocene-active faulting to include most of the discontinuities in the geometry of faulting across the Santa Healdsburg Fault, a structural continuation of the Rodgers Rosa Creek floodplain. However, other fault-activity maps Creek Fault northwest of a bend in the fault at Santa Rosa; and more recent publications have regarded the southern (2) first-time identification of fault strands across the Santa part of the Healdsburg Fault (southward from the vicinity Rosa Creek floodplain in central Santa Rosa (Hecker and oth- of Windsor; fig. 2A, on map sheet) as part of the modern ers, 2016); (3) increases in the known width and complexity Rodgers Creek Fault because of the continuous nature of faulting; and (4) identification of fault splays that project of Holocene faulting (Herd and Helley, 1977; Jennings, toward the Bennett Valley-Maacama Fault system to the east 1994; Hart, 1998a, b; Working Group on Earthquake and toward an active extension of the Hayward Fault to the Probabilities, 2003; U.S. Geological Survey and California south beneath San Pablo Bay, recently mapped by Watt and Geological Survey, 2006). Our revised mapping indicates others (2016) and previously inferred from shallow microseis- that Holocene faulting is indeed continuous across the micity (Lienkaemper and others, 2012). Santa Rosa Creek floodplain (Hecker and others, 2016) and The Rodgers Creek Fault is a principal strand of the San extends along most (at least 80 percent) of the length of the Andreas Fault system north of San Francisco Bay (fig. 1, on Healdsburg Fault, as well as along the Rodgers Creek Fault map sheet) that accommodates 6–10 millimeters per year south of Santa Rosa, suggesting that the two faults operate (mm/yr) of plate-boundary motion (Schwartz and others, as a single, integrated seismic source. To reflect this new 1992). The 30-year mean probability of a magnitude (M)>6.7 understanding, we herein refer to the Holocene-active fault earthquake on the combined Rodgers Creek-Hayward Fault, in its entirety simply as the Rodgers Creek Fault and adapt estimated at 33 percent, is the highest among faults in the the original nomenclature to refer to the sections of the region (Field and others, 2015; Aagaard and others, 2016). fault north and south of Santa Rosa. The most recent surface-rupturing earthquake on the Rodgers Creek Fault was likely between 1715 and 1776 (Hecker and others, 2005), indicating that the elapsed time may have reached or exceeded the poorly constrained average recurrence Approach and Scope of large earthquakes on the fault, estimated to be 130–370 This publication builds upon prior mapping of recently active traces of the Rodgers Creek Fault by Hart (1982, 1992) 1U.S. Geological Survey. and Bryant (1982), prepared under California’s Alquist-Priolo 2Stantec Consulting Services, Inc. program of fault-rupture hazard zoning (fig. B2 , on map sheet). These maps, prepared at a scale of 1:24,000, utilized For this revision, we also incorporated faults identified existing fault mapping, interpretation of fault geomorphology from geologic mapping (fig. B2 , on map sheet) that are within from aerial photography, and limited field inspection to iden- or near the zone mapped as recently active. Geologically tify fault strands that are sufficiently active and well defined mapped faults provide a longer-term context for recent surface to meet the criteria of the Alquist-Priolo regulatory zones. We faulting and, despite lacking geomorphic evidence of recency, also consulted earlier (1970–1971) unpublished 1:24,000- may have been active during the Holocene, as demonstrated scale mapping of recently active traces from the vicinity of by the 2014 M6 South Napa earthquake (DeLong and others, Santa Rosa southward by R.D. Brown of the U.S. Geological 2016). In places where the geomorphically mapped active Survey, as reproduced in Hart (1982, fig. A5 , B). traces lie in the vicinity of these framework faults, we gener- Similar to previous efforts, our mapping of the Rodgers ally show the latter only where they are farther than a few tens Creek Fault relies largely on the interpretation of fault of meters from the more conspicuously active traces, although traces from remote sensing
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