Airborne LiDAR analysis and geochronology of faulted glacial moraines in the Tahoe-Sierra frontal fault zone reveal substantial seismic hazards in the Lake Tahoe region, California-Nevada, USA James F. Howle1,†, Gerald W. Bawden2,†, Richard A. Schweickert3,†, Robert C. Finkel4,†, Lewis E. Hunter5,†, Ronn S. Rose5,†, and Brent von Twistern6,† 1U.S. Geological Survey, P.O. Box 1360, Carnelian Bay, California 96140, USA 2U.S. Geological Survey, 3020 State University Drive East, Modoc Hall, Suite 4004, Sacramento, California 95819, USA 3University of Nevada, Reno, Department of Geological Sciences, 1900 Greensburg Circle, Reno, Nevada 89509, USA 4University of California, Berkeley, Earth and Planetary Science Department, 371 McCone Hall, Berkeley, California 94720, USA 5U.S. Army Corp of Engineers, 1325 J Street, Sacramento, California 95814, USA 6P.O. Box 5401, Incline Village, Nevada 89450, USA ABSTRACT moment magnitudes (Mw) range from 6.3 ± fault zone; it demonstrates that the Tahoe-Sierra 0.25 to 6.9 ± 0.25. A close spatial association frontal fault zone is an important seismic source We integrated high-resolution bare-earth of landslides and active faults suggests that for the region. airborne light detection and ranging (LiDAR ) landslides have been seismically triggered. Bare-earth airborne light detection and imagery with fi eld observations and modern Our study underscores that the Tahoe-Sierra ranging (LiDAR) imagery has revolutionized geochronology to characterize the Tahoe- frontal fault zone poses substantial seismic geomorphic mapping in densely vegetated, Sierra frontal fault zone, which forms the and landslide hazards. mountainous, and otherwise inaccessible ter- neotectonic boundary between the Sierra rain (Fig. 1). Recent studies of known fault Nevada and the Basin and Range Province INTRODUCTION zones (Hudnut et al., 2002; Frankel et al., 2007; west of Lake Tahoe. The LiDAR imagery Oskin et al., 2007; Prentice et al., 2009; Arrow- clearly delineates active normal faults that The Lake Tahoe basin, California and Ne- smith and Zielke, 2009; Zielke et al., 2010) have displaced late Pleistocene glacial mo- vada, is a tectonically active graben located and discoveries of previously unknown fault raines and Holocene alluvium along 30 km between the Sierra Nevada microplate on the zones in densely vegetated terrain (Haugerud of linear, right-stepping range front of the west and the Basin and Range Province to the et al., 2003; Hunter et al., 2011) have exploited Tahoe-Sierra frontal fault zone. Herein, we east. The Tahoe-Sierra frontal fault zone, west bare-earth airborne LiDAR imagery for fault- illustrate and describe the tectonic geomor- of Lake Tahoe, was recognized over a century zone characterization. Utilizing new bare-earth phology of faulted lateral moraines. We have ago by early geologic investigators (Russell, LiDAR imagery (see GSA Data Repository developed new, three-dimensional modeling 1885; Lindgren, 1896, 1897) from its topo- for a discussion of the LiDAR data used in techniques that utilize the high-resolution graphic expression, but the locations of basin- this study1) we have identifi ed, visualized, and LiDAR data to determine tectonic displace- bounding normal faults have remained elusive mapped faults within late Quaternary glacial, ments of moraine crests and alluvium. The due to diffi cult access and dense vegetative colluvial, and alluvial deposits and granitic statistically robust displacement models cover in mountainous terrain. Schweickert et al. bedrock along 30 km of range-front faults that combined with new ages of the displaced (2000, 2004) mapped the principal faults of the comprise the central portion of the Tahoe-Sierra Tioga (20.8 ± 1.4 ka) and Tahoe (69.2 ± Tahoe-Sierra frontal fault zone along the steep, frontal fault zone (Fig. 2). We analyzed the 4.8 ka; 73.2 ± 8.7 ka) moraines are used to linear range front west of the lake. However, LiDAR imagery using pseudo–sun angles, ver- estimate the minimum vertical separation because complex tectonic geomorphology has tical exaggeration, and oblique perspectives and rate at 17 sites along the Tahoe-Sierra frontal been formed by normal faulting of glacial land- extracted profi les to facilitate geologic interpre- fault zone. Near the northern end of the study forms and because dense vegetation obscures tation and to reveal (and in many cases confi rm) area, the minimum vertical separation rate the morphology, the fault zone has remained 1 is 1.5 ± 0.4 mm/yr, which represents a two- controversial and was omitted from the state GSA Data Repository item 2012192, includes supplementary discussions of the airborne LiDAR to threefold increase in estimates of seismic of California’s seismic hazard risk assessment data, late Pleistocene glacial deposits and stratigra- moment for the Lake Tahoe basin. From this (UCERF2, 2007). This report documents the phy, geochronology of Tioga and Tahoe deposits, study, we conclude that potential earthquake tectonic geomorphology of faulted moraines, Emerald Bay bathymetric data, mathematical model- establishes limiting ages of faulted late Pleisto- ing of planes and vectors, Root-Mean-Square error †E-mails: [email protected]; [email protected]; estimates, modeled tectonic displacements, DR Fig- [email protected]; [email protected]; cene glacial and alluvial deposits, and quanti- ures 1–16 and DR Tables 1–6, is available at http:// [email protected]; ronn.s.rose@usace fi es the minimum vertical separation rate and www.geosociety.org/pubs/ft2012.htm or by request to .army.mil; [email protected]. extension rate along the Tahoe-Sierra frontal [email protected]. GSA Bulletin; Month/Month 2012; v. 1xx; no. X/X; p. 1–15; doi:10.1130/B30598.1; 8 fi gures; 1 table; Data Repository item 2012192. For permission to copy, contact [email protected] 1 © 2012 Geological Society of America Howle et al. AB120° 5′ 0″ W C N ′ 38° 55 N ″ 45 ′ 38° 54 0 150 m Figure 1. Comparison of color aerial photography with airborne light detection and ranging (LiDAR) imagery. (A) Color aerial orthophoto for part of range front along eastern base of Mt. Tallac with geomorphic features obscured by dense forest cover. See Figure 2 for location. (B) Same scene, fi rst-return LiDAR imagery. (C) Same scene, last-return (bare-earth) LiDAR imagery showing range-front fault scarp (between white arrows) cutting colluvium, alluvium, and Tioga moraine. See Mt. Tallac segment for discussion. tectonic features. These features include linear originally recognized in the fi eld, such as low- 26Al terrestrial cosmogenic nuclides (TCN) and scarps in unconsolidated alluvium, colluvium, relief scarps on moraine side slopes and land- optically stimulated luminescence (OSL) age and glacial moraines; triangular facets in uncon- slide deposits. Importantly, the high-resolution dates. We then correlated these new data from solidated glacial till; linear side-slope troughs topography has been used for robust three- the study area with other data from the Lake aligned with scarps in moraine crests; antithetic dimensional quantifi cation of tectonic displace- Tahoe region, geochronologic data from the scarps; closed depressions; hanging-wall gra- ment across individual fault strands. southern Sierra Nevada, and the global marine bens; back-tilted moraine crests; and saddles in isotope stages (MIS). ridgelines. GEOCHRONOLOGY OF During the late Pleistocene, the Crystal Most of the localities and features described FAULTED DEPOSITS Range, southwest of Lake Tahoe, was covered in this paper had previously been studied in by a broad ice cap centered over Rockbound Val- the fi eld prior to acquisition of the LiDAR In the central and southern Sierra Nevada, ley. This ice cap extended east across the Sierran data, and preliminary interpretations had been many detailed studies have presented age limits crest (McAllister, 1936; Wahrhaftig and Curtis, made (Schweickert et al., 2000, 2004; Howle, for the glacial chronology (Gillespie, 1984; Gil- 1965) and fed outlet glaciers into the Tahoe 2000). Despite diffi cult access in steep densely lespie et al., 1984; Phillips et al., 1990, 1996, Basin (Fig. 2). The outlet glaciers emerged from vegetated terrain, our detailed fi eld mapping 2009; Bursik and Gillespie, 1993; Clark and the deep canyons, crossed range-front faults (1:12,000 scale) led to the recognition of nu- Gillespie, 1997). In the northern Sierra Nevada, of the Tahoe-Sierra frontal fault zone, and de- merous scarps cutting glacial moraines and however, modern analytical age determina- posited prominent lateral and medial moraines tectonic geomorphic features such as those just tions are few. Correlations of glacial moraines that extend 2.5–7 km beyond the range front. listed. Dense forest cover in most areas meant based on relative weathering criteria are com- Morainal deposits from the late Pleistocene that conventional aerial photography was of plicated by differences in precipitation, types of Tahoe and Tioga glacial stages have been dis- little use (Fig. 1). boulders, and the effects of wildfi res on boul- placed by the Tahoe-Sierra frontal fault zone Acquisition of the LiDAR data has enabled der spalling due to differences in fuel weights and provide a record of post-depositional strain, bare-earth visualization of key tectonic features (Burke and Birkeland, 1979). which, combined with the glacial chronology, and confi rmation of most features originally To estimate maximum and minimum limiting defi ne tectonic slip rates for the Tahoe-Sierra mapped. The bare-earth LiDAR data have im- ages of glacial deposits displaced along faults in frontal fault zone. See GSA Data Repository for proved and clarifi ed the original mapping and the study area, we established age limits for the descriptions of the Tahoe- and Tioga-age glacial have led to identifi cation of some features not Tahoe and Tioga glaciations using new 10Be and deposits and stratigraphy (see footnote 1). 2 Geological Society of America Bulletin, Month/Month 2012 Tahoe-Sierra frontal fault zone, Lake Tahoe, California, USA 120° 10′ W SV NTFZ DP TC ′ TSFFZ Twin Peaks 39° 10 WC segment Ellis Peak TW BW Figure 2.