CHAPTER 42
Reevaluating Late-Pleistocene and Holocene Active Faults in the Tahoe Basin, California-Nevada
Graham Kent Nevada Seismological Laboratory, University of Nevada, Reno, Reno, Nevada 89557-0174, USA Gretchen Schmauder Nevada Seismological Laboratory, University of Nevada, Reno, Reno, Nevada 89557-0174, USA Now at: Geometrics, 2190 Fortune Drive, San Jose, California 95131, USA Jillian Maloney Department of Geological Sciences, San Diego State University, San Diego, California 92018, USA Neal Driscoll Scripps Institution of Oceanography, University of California, San Diego, 9500 Gilman Drive, La Jolla, California 92093, USA Annie Kell Nevada Seismological Laboratory, University of Nevada, Reno, Reno, Nevada 89557-0174, USA Ken Smith Nevada Seismological Laboratory, University of Nevada, Reno, Reno, Nevada 89557-0174, USA Rob Baskin U.S. Geological Survey, West Valley City, Utah 84119, USA Gordon Seitz &DOLIRUQLD*HRORJLFDO6XUYH\0LGGOH¿HOG5RDG060HQOR3DUN California 94025, USA
ABSTRACT the bare earth; the vertical accuracy of this dataset approaches 3.5 centimeters. The combined lateral A reevaluation of active faulting across the Tahoe and vertical resolution has re ned the landward basin was conducted using a combination of air- identi cation of fault scarps associated with the borne LiDAR (Light Detection and Ranging) three major active fault zones in the Tahoe basin: imagery, high-resolution seismic CHIRP (acous- the West Tahoe–Dollar Point fault, Stateline–North tic variant, compressed high intensity radar pulse) Tahoe fault, and Incline Village fault. By using the pro les, and multibeam bathymetric mapping. In airborne LiDAR dataset, we were able to identify August 2010, the Tahoe Regional Planning Agency previously unmapped fault segments throughout (TRPA) collected 941 square kilometers of airborne the Tahoe basin, which heretofore were dif cult LiDAR data in the Tahoe basin. These data have to trace due to thick vegetation covering rugged a density of 11.82 points per square meter, with alpine terrain. To re ne fault locations beneath approximately 2 points per square meter striking Lake Tahoe, Fallen Leaf Lake and Cascade Lake, 833 834 Applied Geology in California we acquired additional seismic CHIRP imagery unclear (Cashman and Fontaine, 2000; Wesnousky using an Edgetech Subscan system. By combining et al., 2012). Geodetic data suggest that the Tahoe CHIRP and LiDAR imagery, together with new basin accommodates equal amounts of extension multibeam bathymetry collected in Fallen Leaf (on mapped normal faults) and right-lateral shear Lake (Maloney et al., 2013), we were able to iden- (Hammond et al., 2011), but compelling evidence tify previously unknown fault segments, rectify for the strike-slip component is sparse. The gradual those which had been misplaced, and document opening-to-the-north geometry of the Tahoe basin, better continuity along individual fault systems. however, may suggest some form of left-lateral Additionally, we have correlated the timing of accommodation at the northern end of the basin. debris ow and turbidite deposits in Lake Tahoe Prior to acquisition of a basin-wide LiDAR data- near the Stateline–North Tahoe fault (Crystal Bay set, mapping faults in the Tahoe basin was hampered escarpment) and integrated this information into not only by the presence of glacial deposits, but the comprehensive history of Holocene fault rup- also by dense brush and forests, seasonal snow, and ture in the basin. The new indirect evidence of by the lake itself. Lindgren (1911) rst recognized ground shaking as recorded in shallow lacustrine that the basin-bounding faults extend north into the stratigraphy suggests the potential for synchronic- Truckee basin. Additional work by Blackwelder ity of rupture between West Tahoe–Dollar Point and (1933) and Birkeland (1963) continued to study Stateline–North Tahoe faults. Through integration the basin and its associated faults. Hyne et al. of complementary geophysical datasets, we have (1972) performed the earliest marine seismic stud- developed a much-improved model of fault archi- ies in Lake Tahoe, providing the rst views of fault tecture and timing within the Tahoe basin, which architecture at the bottom of the lake, and Gardner presents a much simpler mode of normal faulting et al. (2000) provided further evidence of a right- in this basin in contrast to earlier investigations by stepping, en-echelon normal fault system through Schweickert et al. (2004) and Howle et al. (2012) a comprehensive multibeam survey of Lake Tahoe. that interpreted a much greater density/complexity Schweickert et al. (2004) published the rst com- of faults. Our updated fault map provides a better prehensive fault map of the entire basin showing understanding of the tectonics of the basin, and numerous and complex faults. The faults were par- will help constrain future earthquake magnitudes ticularly abundant near the mapped West Tahoe– and associated seismic hazards, such as ground Dollar Point fault (WTDPF) on the west side of ruptures and tsunamis. the lake, as well as in the northern portion of the lake, coinciding with the Stateline–North Tahoe Introduction and Rationale fault (SLNTF) and Incline Village fault (IVF). Lake Tahoe occupies the westernmost basin within In contrast, faults mapped within lacustrine sedi- the trans-tensional Walker Lane deformation belt ments using a sub-bottom CHIRP pro ler in Lake that abuts the eastern edge of the Sierra Nevada Tahoe, Cascade Lake and Fallen Leaf Lake (Kent microplate (Unruh et al., 2003; Schweickert et et al., 2005; Dingler et al, 2009; Brother et al., al., 2004; Kent et al., 2005, Dingler et al., 2009; 2009; Maloney et al., 2013) presented a simpler Brothers et al., 2009; Maloney et al., 2013; Busby, pattern. These lake-based maps, while clearly 2013). New geodetic surveys suggest that approxi- identifying the WTDPF, SLNTF and IVF, did not mately 7 mm/yr, or about 15 percent of the total exhibit the same complexity or density of second- plate motion between the Paci c plate and the ary faults (Fig. 1), as reported in the onshore fault North American plate, is accommodated along maps by Schweickert et al. (2004) and Howle et this boundary near Lake Tahoe (Bormann, 2013); al. (2012). These observations raise the following the majority of slip in this region of the northern question: why is faulting observed on land so much Walker Lane is accommodated through dextral more complex than that imaged in the lake? In prin- shear, although the exact distribution of this motion ciple, mapping faults and associated structures is (e.g., strike-slip versus block rotation) remains rather straightforward in a lacustrine environment Reevaluating Late-Pleistocene and Holocene Active Faults in the Tahoe Basin, California-Nevada 835
120°10'0"W 120°5'0"W 120°0'0"W 119°55'0"W
39°15'0"N
IVF
39°10'0"N SLNTF
39°5'0"N Lake Tahoe
39°0'0"N WTDPF
38°55'0"N
interpreted from Schweickert et al., 2000 interpreted from Gardner et al., 2000 interpreted from Brothers et al., 2009 interpreted from Howle et al., 2012 this study
Figure 1. A schematic diagram of fault density from various studies in the Lake Tahoe basin. The results from our study highlight a simple right- stepping, en-echelon normal fault system (red) that is in stark contrast to several previous fault studies (blue and cyan), where fault densities are dramatically different. Also, previous faults such as the East Tahoe fault (gold, eastern shoreline) have been removed from our current map as no VLJQDWXUHLQHLWKHU/L'$5LPDJHU\RUVHLVPLFSUR¿OLQJZDVIRXQG:HVW 7DKRH'ROODU3RLQWIDXOW :7'3) 6WDWHOLQH±1RUWK7DKRHIDXOW 6/17) DQG Incline Village fault (IVF) are shown. 836 Applied Geology in California such as the Tahoe basin; is there a geologic rea- was further aided by fault exploration within the son for this dichotomy? Perhaps mapping of faults QPS Fledermaus 3-D interpretational environment on land provides a greater integration of time, thus (http://www.qps.nl/display/ edermaus) that allows being more sensitive to recording motion on faults active slope shading of hillsides, and integration with diminished slip-rates relative to the large with other datasets, including multibeam bathy- basin bounding faults? Tahoe and Tioga deposits metric maps and seismic CHIRP pro les. and landforms (Birkeland, 1963) provide excel- Beginning the in summer of 1999, Scripps lent markers for late Pleistocene and Holocene Institution of Oceanography and the University of fault movement, but in many cases may erase or Nevada, Reno and have undertaken several CHIRP inhibit mapping of older histories of rupture on sonar campaigns (1999, 2000, 2002, 2006 and land. Lastly, the mismatch between fault systems 2011) to investigate and then re ne the architecture on land and in the lakes may simply be a misinter- of submerged faults that exist beneath Lake Tahoe, pretation in the geologic processes that shape the Emerald Bay, Fallen Leaf Lake, and Cascade Lake basin, resulting in incongruous maps. To this end, (Kent et al., 2005; Dingler et al. 2009; Brothers et a comprehensive geophysical approach described al., 2009; Maloney et al., 2013). The CHIRP sonar below has been employed in both environments has an effective penetration of approximately to attempt to resolve this issue and provide an 50-70 meters into the lake bottom sediments with updated fault map of late Pleistocene and Holocene sub-meter vertical accuracy. The new CHIRP data rupture in the Tahoe basin, and test whether there is (2011) presented in this manuscript used either an obvious component of right-lateral slip on these 0.7-3 kHz or 1-15 kHz pulses, with a 30 ms swept faults, or newly discovered faults owing to the new pulse. Observations made from the CHIRP pro les capabilities of this LiDAR dataset. include fault locations, fault offset, and the extent of landslide/debris deposits. Methods: In 2010, the Tahoe Regional Planning Agency Observations: (TRPA) commissioned Watershed Sciences to West Tahoe–Dollar Point fault y an airborne LiDAR survey of the Tahoe basin The West Tahoe-Dollar Point fault (WTDPF) (Fig. 2; Figs. 3-6). Our group was involved with bounds the western side of the Tahoe basin, start- data speci cation, contractor selection and data ing in the south near Echo Summit, where it strikes quality control. Although the TRPA commissioned northwestward through Fallen Leaf Lake and the survey for land-use planning purposes, due Cascade Lake, then steps north into Lake Tahoe to the high pulse density of the data, active fault near Emerald Bay, where it bounds the steep west- scarps across the basin could be easily identi ed. ern escarpment offshore of Rubicon. The WTDPF The ight paths and LiDAR instrumentation were then traverses through McKinney Bay, reemerging designed to achieve a pulse density 8 pulses per onshore near Dollar Point. For clarity in discussing square meter. Over 11 pulses per square meter were the WTDPF, it has been divided it into three geo- shot with approximately 2 strikes per square meter metric segments. These are the Fallen Leaf Lake hitting the bare earth. Processing the resulting segment (FLS), the Rubicon segment (RS), and the point cloud supported a 0.5-meter pixel resolution Dollar Point segment (DPS) (Fig. 2). bare earth model of the Tahoe basin (Watershed Sciences, 2010), although data were resampled Fallen Leaf Segment to 1 m grids. The raw LiDAR data were pro- LiDAR imagery of the FLS of the WTDPF is cessed to remove the vegetation signal, resulting shown in a 3-D perspective between Echo Summit in a bare earth model suitable for geologic map- and Fallen Leaf Lake (Fig. 7). As imaged in ping purposes; after identifying faults scarps on Figures 3&7, the fault scarp is clearly visible the processed bare earth data, each trace was then south of Highway 50, where it dies out towards veri ed through a eld investigation. This process Christmas Valley. It continues northward, where Reevaluating Late-Pleistocene and Holocene Active Faults in the Tahoe Basin, California-Nevada 837
120°10'0"W120°5'0"W 120°0'0"W119°55'0"W
Polaris fault Relay N Peak
Incline 39°15'0"N Village Figure 5
Figure 6 IVF D03L08 39°10'0"N
D03L09 Dollar Point Segment Dollar Point McKinney Lake Tahoe Bay
WTDPF SLNTF 39°5'0"N Ellis Peak
Figure 4 39°0'0"N Rubicon Segment
Emerald Rubicon Bay Peak
Cascade Lake 38°55'0"N
Fallen Leaf Fallen Leaf Lake Segment
Freel Peak Figure 3 38°50'0"N Christmas Valley
Figure 2. Regional map of the Lake Tahoe basin that highlights the comprehensive LiDAR coverage of the entire basin, along with the location RIWKHREVHUYHGIDXOWV UHG DQG&+,53SUR¿OHV EOXH GLVFXVVHGLQWKLV SDSHU:HVW7DKRH±'ROODU3RLQWIDXOW :7'3) 6WDWHOLQH±1RUWK7DKRH IDXOW 6/17) DQG,QFOLQH9LOODJHIDXOW ,9) DUHVKRZQ6HJPHQWDWLRQRI WKH:7'3)LVDOVRKLJKOLJKWHG )DOOHQ/HDIVHJPHQW5XELFRQVHJPHQW and Dollar Point segment. Debris from the McKinney Bay slide is evident as the blocky features located near the middle of the lake and are shown in more detail in Figure 5. The boxes outlined on the map show the locations of detailed imagery (Figs. 3-6). Approximate location of the Polaris fault indicated by dashed green line. Locations of several landmarks are also highlighted on this map. 838 Applied Geology in California
120°6'0"W 120°4'0"W 120°2'0"W 120°0'0"W
Lake Tahoe Emerald N Bay 38°58'0"N
Fannette Baldwin Island Beach Tahoe Keys 38°56'0"N Cascade Lake
Fallen Leaf Lake Mt. 38°54'0"N Tallac
Angora Location of 38°52'0"N Lakes Figure 8 Echo Peak EarthquakeEarthquake Lake Flagpole Peak Christmas Valley 38°50'0"N
Figure 3. 'HWDLOHGPDSVKRZLQJWKH)DOOHQ/HDI/DNHVHJPHQWRIWKH:HVW7DKRH± 'ROODU3RLQWIDXOWLQWKHVRXWKZHVWHUQSRUWLRQRIWKHEDVLQ7KHVWUDQGVRIWKH:7'3) are shown in red. Additional landmarks are noted.
it crosses Highway 50 and then skirts along the Brothers et al. (2009) inferred that the FLS fault base of Flagpole Peak before climbing the side of trace must be topographically lower (following a Echo Peak. It then traverses across an ephemeral straight line) and lie in the valley at the base of lake (also known as Earthquake Lake), and cuts Flagpole Peak, as more obvious sections near Echo between the Upper and Lower Angora Lakes before Summit and Angora Lakes were already identi ed stepping into Fallen Leaf Lake. Previous work by in their study. Our analysis of the new LiDAR data Reevaluating Late-Pleistocene and Holocene Active Faults in the Tahoe Basin, California-Nevada 839
120°8'0"W 120°6'0"W McKinney Lake Moraine Tahoe McKinney–General N Moraine
General–Meeks Meeks Moraine Bay Right Lateral Movement? 39°2'0"N
Rubicon Meeks Creek Bay Moraine
39°0'0"N
Rubicon Peak
Figure 4. 'HWDLOHGPDSRIWKH5XELFRQVHJPHQWRIWKH:HVW7DKRH±'ROODU3RLQWIDXOW VKRZQLQUHG $ORQJ this section, a steep wall formed by this down-to-the-east normal fault characterizes the western edge of the lake. Locations of moraines near Meeks Bay are highlighted, along with location of potential right-lateral slip offsetting the recessional moraine. 840 Applied Geology in California
120°8'0"W 120°6'0"W 120°4'0"W 120°2'0"W
39°14'0"N Agate Bay N Crystal Bay escarpment 39°12'0"N
Dollar Point
Tahoe City 39°10'0"N Baby Slide
39°8'0"N
SLNTF McKinney Bay 39°6'0"N WTDPF McKinney Bay slide blocks Sugar Pine Point 39°4'0"N
Figure 5. 'HWDLOHGPDSRIWKH'ROODU3RLQWVHJPHQWRIWKH:HVW7DKRH±'ROODU3RLQWIDXOW VKRZQLQUHG 7KH:HVW7DKRH±'ROODU3RLQWIDXOWLVVHHQFXWWLQJWKURXJK0F.LQQH\%D\ and continuing across the Tahoe shelf, before trending north-northwest and back onto ODQG7KHEOXHOLQHVUHSUHVHQWWKH6WDWHOLQH±1RUWK7DKRHIDXOWIURPLWVREVHUYHGVRXWKHUQ terminus as it trends northward. The development of a scarp can be observed in the northerly trending portion of this fault. The extent of the McKinney Bay landslide (<60,000 years before present; Kent et al., 2005) is shown outlined in light blue shading. Large slide blocks are VKRZQRQWKHHDVWVLGHRIWKLV¿JXUH7KH%DE\6OLGHDVGHVFULEHGLQ6PLWKHWDO LVVKRZQLQSLQN:HVW7DKRH'ROODU3RLQWIDXOW :7'3) DQG6WDWHOLQH±1RUWK7DKRHIDXOW 6/17) DUHVKRZQ Reevaluating Late-Pleistocene and Holocene Active Faults in the Tahoe Basin, California-Nevada 841
120°0'0"W 119°58'0"W 119°56'0"W
Relay Peak N
39°18'0"N
Incline Lake
39°16'0"N
Incline Village
IVF
39°14'0"N Crystal Bay
SLNTF
Figure 6. Detailed image of the northern portion of the Tahoe basin, which highlights the Incline Village fault ROLYHJUHHQ DQGQRUWKHUQWHUPLQXVRIWKH6WDWHOLQH±1RUWK7DKRHIDXOW/RFDWLRQVRI5HOD\3HDNDQG&U\VWDO %D\DUHDOVRVKRZQ6WDWHOLQH±1RUWK7DKRHIDXOW 6/17) DQG,QFOLQH9LOODJHIDXOW ,9) DUHVKRZQ 842 Applied Geology in California
Echo Lake N ~1 km Earthquake Lake Angora Lakes
Highway 50 Fallen Leaf Lake
Figure 7. $VORSHVKDGHEDUHHDUWK/L'$5LPDJHRIWKHVRXWKHUQH[WHQWRIWKH:HVW7DKRH±'ROODU Point fault from its southern terminus in Christmas Valley through to its descent into Fallen Leaf Lake. 7KHVFDUSLVFOHDUO\YLVLEOHLQWKH/L'$5LPDJHU\QHDU(FKR6XPPLWZKHUHLWWKHQWUHQGVXSZDUG along the mountain front before cutting between the Upper and Lower Angora Lakes. indicates that portions of this fault are not con ned North of Angora Lakes, the WTDPF scarp con- to the base of the range front, but instead locally tinues through the southern end of Fallen Leaf climb upslope. Though previous movement on Lake, where it forms a large escarpment that is the fault is undoubtedly responsible for Christmas imaged in the multibeam sonar bathymetric data- Valley at the base of Flagpole Peak, the most recent set and vertical CHIRP cross-sections (Maloney movement appears to be encroaching westward, as et al., 2013; Brothers et al., 2009). Curiously, a second branch of the fault appears to form is evidenced by fault scarps observed upslope. It onshore just northwest of Fallen Leaf Lake and should also be noted that there are several large ste- continue towards Baldwin Beach at Lake Tahoe povers along the fault, including a nearly one-half (Figs. 2&3). It is dif cult to determine how kilometer right step near Angora Lakes. or if this branch continues south and connects A photo of the fault scarp as observed on the up with the main strand of the fault. The main ground between the Upper and Lower Angora branch traverses near Mt. Tallac, where it crosses Lakes is shown in Figure 8. At this location, breaks Cascade Lake (Fig. 3&9). The FLS is imaged as in slope across the fault scarp may suggest multiple two distinct faults in Cascade Lake CHIRP data (potentially two or three) slip events with an offset (Maloney et al., 2013). The main fault trace may of approximately two meters per event. There is continue a bit farther northwest, through a sad- dle in the moraine, and into Emerald Bay, where an obvious temporal component to weathering, as there is weak evidence from CHIRP pro ling of the top of the scarp has more weakly de ned scarp stratal divergence towards the south (Maloney faces, suggesting longer exposure to the elements. et al., 2013). However, clear evidence of a scarp The top and bottom offset surfaces appear to be in the LiDAR data is lacking between Cascade related to Tioga-aged glacial outwash mantling a Lake and Emerald Bay. Maloney et al. (2013) recessional moraine, which resulted in a very at suggests that the fault may continue on the east surface that de nes the total throw across the fault side of Fanette Island before stepping some 1.5 scarp with a maximum height of ~6 meters. km towards the precipitous western edge of Lake Reevaluating Late-Pleistocene and Holocene Active Faults in the Tahoe Basin, California-Nevada 843
Tahoe that de nes the RS of the WTDPF. There is data. This segment, extending from the mouth of weak evidence for fault scarps continuing onshore Emerald Bay to the mid-section of McKinney Bay, to the northwest from southern Emerald Bay in is characterized by a steep, submerged escarpment, the LiDAR data. Landslide scarps are observed approximately 400 meters in height, and offsets a approximately four kilometers north of Emerald small fan delta at the base of the steep wall (Fig. 10). Bay (Fig. 10). These features exhibit geomor- The fault scarp terminates abruptly at McKinney phology typically associated with landslides, such Bay, where the lakeshore shifts sharply to the west. as arcuate head scarps, slump blocks and glide This termination is a result of the McKinney Bay planes. slide (Gardner et al., 2000), which has altered the Rubicon Segment shoreline and reduced the height of the fault-related The RS of the WTDPF is not observed on land, escarpment. This megaslide (Fig. 5) is thought to with the exception of a small slice of the fault and be roughly 60 ka in age (Kent et al., 2005). Since corresponding scarp observed on a at bench near this catastrophic event, smaller slides sourced to the northern entrance to Emerald Bay, which can the west continue to mask the development of a be observed in the LiDAR dataset (Figs. 3&10). continuous fault scarp (Smith et al., 2013). In the Continuing north, the fault trace plunges into the LiDAR data, recent (Holocene or younger) fault- lake and follows the steep western wall of Lake ing does not appear to offset the onshore moraines Tahoe, which is observed in bathymetric and CHIRP between Emerald Bay and McKinney Bay
Figure 8. Photograph showing the Fallen Leaf segment of the West Tahoe–Dollar Point fault scarp, where the fault cuts between the Upper and Lower Angora Lakes. Location of the photograph is shown in Figure 3. 844 Applied Geology in California
N
~1 km
Mt. Tallac
Cascade Lake Emerald Bay
Fallen Leaf Lake
Recessional End Moraines
Baldwin Beach Figure 9. $VORSHVKDGHEDUHHDUWK/L'$5LPDJHRIWKH:HVW7DKRH±'ROODU3RLQWIDXOWQHDUWKHYLFLQLW\RI0W Tallac is shown. The fault scarp is clearly visible in the LiDAR imagery, where it cuts along the face of Mt. Tallac and then crosses near the southwestern shore of Cascade Lake. North of Fallen Leaf Lake, the fault also splays to the east and forms a second, fairly continuous trace toward Baldwin Beach on the shores of Lake Tahoe.
N ~1 km
Emerald Bay Landslide Toe
Meeks Bay McKinney Bay
Rubicon Segment
Figure 10. A slope-shade bare earth LiDAR model of the Tahoe- and Tioga-aged moraines south of McKinney Bay merged with swath bathymetry data from Lake Tahoe. Recent (latest Pleistocene or younger) faulting does not appear evident across these moraines. Older bedrock faulting may be present beneath these glacial GHSRVLWVWKRXJKIDXOWLQJRIWKHFKDUDFWHUREVHUYHGLQWKLVVWXG\DORQJWKHVRXWKHUQSRUWLRQRIWKH:HVW7DKRH± Dollar Point fault is not observed here. Note the offset of the fan delta in the bathymetry dataset. Location of landslide/slump features north of Emerald Bay is also shown. Reevaluating Late-Pleistocene and Holocene Active Faults in the Tahoe Basin, California-Nevada 845
(Figs. 4&10). The north-facing slopes on General– help distribute some of the total slip, although due to Meeks and McKinney–General moraines have the short length of this structure, it is likely minor in well developed recessional moraines that are pre- comparison to the transfer of slip to the other north- served on the steep, north-facing slopes and are lin- easterly faults, including the Stateline–North Tahoe ear in form. Older bedrock faulting may be present fault and Incline Village fault. beneath the moraines, but faulting of the character The DPS scarp is also observed onshore in the observed from the southern portion of the basin, LiDAR data. At Dollar Point, the DPS splays into and in other sedimentary areas, is not observed at least three poorly de ned, west-northwest and here. It should be noted that the apparent northern north–northwest trending fault traces (Fig. 2). The segment boundary of the RS or change in relief fault scarps, measured both from the LiDAR and across the scarp is a consequence of the McKinney from eld investigation, are approximately 15 to 20 Bay slide (Fig. 2), in contrast to the nearly 1.5 km m high (likely in Younger or Older Tahoe till based stepover between the FLS and RS of the WTDPF. on weathering) for the west-northwest trending structure and signi cantly higher (~30 m or more, Dollar Point Segment some bedrock) for the more northerly oriented fault North of McKinney Bay, the WTDPF follows sub- scarps. The high offset for both scarps suggests that merged topography, this time through a shallow, they record a multitude of rupture events (Fig. 11). offshore shoal (Figs. 5&11). In this location, the The more northerly oriented features are formed topography has both a more gentle and blunt slope; in more competent rock and may integrate a much instead of forming a steep wall as it does to the south, longer span of time in relation to many other scarps the scarp is less strongly developed to the north. A observed within the basin that are likely reset by secondary splay just to the east near this bench may either Tahoe- or Tioga-aged glaciation.
~1 km 2x Vert. Exag. N
Mt. Watson
Dollar Point
Figure 11. A slope-shade bare earth LiDAR model of the Tahoe basin near Dollar Point is shown. At this location, WKHRULHQWDWLRQRIWKH:HVW7DKRH±'ROODU3RLQWIDXOWFKDQJHVIURPDQRUWKQRUWKZHVWWUHQGWRDSUHGRPLQDQWO\ZHVW QRUWKZHVWWUHQG7KHIDXOWDSSHDUVWREHG\LQJRXWQRUWKZHVWRI'ROODU3RLQWDOWKRXJKLWPD\EHLQÀXHQFHGE\WKH Polaris Fault zone farther to the northwest. 846 Applied Geology in California
Figure 12. &+,53SUR¿OH 'D\/LQH)LJXUH LVVKRZQKLJKOLJKWLQJDZHVW±HDVWWUDYHUVH DFURVVWKH6WDWHOLQH±1RUWK7DKRHIDXOWDQGDQDVVRFLDWHGV\QWKHWLFVSOD\WRWKHZHVW7KH alternating white and dark layers represent various sediment deposits. The thick acoustically WUDQVSDUHQWWXUELGLWHOD\HUODEHOHG2ZDVGHSRVLWHGDSSUR[LPDWHO\\HDUV%3DQGLVOLNHO\ WKHUHVXOWRIDPDMRUUXSWXUHRQHLWKHUWKH6WDWHOLQH±1RUWK7DKRHIDXOW:HVW7DKRH±'ROODU3RLQW IDXOWRUERWK/D\HU-DSSHDUVWRKDYHEHHQGHSRVLWHGDW\HDUV%3ZLWKWKHQRWLRQWKDW ERWKWKH:HVW7DKRH±'ROODU3RLQWIDXOWDQG6WDWHOLQH±1RUWK7DKRHIDXOWPD\EHV\QFKURQL]HG producing indistinguishable turbidite sequences. (inset) Turbidite layers J and O are colored magenta (J) and blue (O) for reference.
Scarp heights along the west-northwest trend- Stateline–North Tahoe fault ing structure decrease to the northwest and we The 1998 bathymetry map of Gardner et al. (2000) do not observe a strongly developed main fault captures most of the Stateline–North Tahoe fault trace in the LiDAR data. This type of geometry (SLNTF) structure, although our 2011 CHIRP is characteristic of what occurs near the ends imagery improves mapping of both the northern of a fault as strike orientation can change dra- and southern extent (Figs. 2, 12&13). This is espe- matically. However, the orientation of the north- cially true to the south, where the fault scarp is sub- northwestward trending DPS scarps is spatially correlative to the newly identi ed Polaris Fault dued due to parity between slip rate and sediment zone (Hunter et al., 2011). Therefore, the pos- deposition, which is observed in CHIRP pro les. sibility exists that the DPS of the WTDPF is These additional data con rm that the SLNTF is transferring slip into the Polaris fault zone, high- an east-dipping normal fault with up to ~24 meters lighting a transition from a mostly normal fault of offset vertical separation across the McKinney system, into one that is more characterized by Bay slide complex. Sediments on the hanging wall right-lateral strike slip faulting. (east) side of the fault gradually tilt and thicken Reevaluating Late-Pleistocene and Holocene Active Faults in the Tahoe Basin, California-Nevada 847
Figure 13. &+,53SUR¿OH 'D\/LQH )LJXUH RULHQWHGVRXWKZHVWWRQRUWKHDVWLVVKRZQ7KHDOWHUQDWLQJZKLWH DQGGDUNOD\HUVUHSUHVHQWYDULRXVVHGLPHQWDWLRQGHSRVLWV7XUELGLWHGHSRVLWVIROORZLQJWKHQRPHQFODWXUHRI6PLWKHW DO DUHODEHOHG$WKLQQHU/D\HU*GHSRVLWHGDSSUR[LPDWHO\\HDUV%3LVWKRXJKWWREHDVVRFLDWHGZLWK DQHDUWKTXDNHUXSWXUHRQWKH:HVW7DKRH±'ROODU3RLQWIDXOW 0DORQH\HWDO $FRXVWLFDOO\WUDQVSDUHQWIHDWXUH imaged along the southwest portion is interpreted as a slide block associated with the McKinney Bay slide. (inset) Turbidite layers J and O are colored magenta (J) and blue (O) for reference. towards the fault, which is a typical geometry of have not been con dently identi ed through either growth faulting; on the footwall (west) side of the the LiDAR data nor eld exploration and mapping. fault, the sediments thin considerably just west The regional published geologic map (Saucedo, of the fault scarp. A representative CHIRP pro- 2005) indicates that the sur cial geologic deposits le across the SLNTF highlights shallow fault in this area are Quaternary unnamed gravels, sand structure centered at a small stepover (Fig. 12). and alluvium of Pleistocene age. It is possible that The alternating light and dark acoustic layers are the fault trace in this area is present, though not typically indicative of turbidite/debris deposition observed in the sedimentary deposits; this may be (Smith et al., 2013). The transparent layers (up due to unconsolidated sediments that do not retain to 2.2 m thick) with little stratigraphic layering fault-related geomorphological features, or to are debris ows, likely sourced from the nearby stream related erosion that may be reworking the Crystal Bay escarpment (Smith et al., 2013). These landscape. It is also possible that the fault does not layers are quite continuous along the strike of the extend north of the lakeshore and that that slip that fault/escarpment as shown in Figure 13. The fault was once accommodated by a longer SLNTF, is itself dips east with a maximum scarp height in this now taken up by the IVF to the east. region of approximately 12 m. The dying strand at Incline Village Fault the stepover, located approximately 200 meters to The Incline Village fault (IVF) is traced in Lake the west of the main trace of the SLNTF, is also Tahoe along a prominent submarine ridge from observed with a smaller, 3-4 m scarp (Fig. 12). the shoreline to full water depth, and is roughly Near Crystal Bay on the north end of Lake parallel to the Stateline–North Tahoe fault (SLNTF, Tahoe, the trace of the SLNTF reaches the shore- Fig. 2). The fault extends north of the lakeshore, par- line, though is not easily traced outside of the lake. allel to the Mt. Rose highway corridor, to slightly Fault–related features, such as a prominent scarp, north of the manmade Incline Lake. In Lake Tahoe, 848 Applied Geology in California
CHIRP pro les indicate the main fault trace is nor- in this location. Near the vicinity of Incline Lake mal, down-to-the-east; some synthetic east dip- (Figs. 6&14), the fault jogs west and climbs Relay ping faults are observed in the vicinity of a small Peak, away from Incline Lake. Near this location, stepover in fault structure near the shoreline. the scarp is approximately 2-4 meters high, and Dingler et al. (2009) observed an offset boulder offsets likely Tioga-aged deposits. Again, the fault layer that provides evidence for ~14 meters of verti- can be traced on the ground in this location. Near cal deformation across the fault since the Tahoe gla- Incline Lake, there is signi cant historical anthro- ciation. Paleoseismic trenching onshore indicates pological disruption of the ground surface. This the most recent rupture of this fault occurred ~500 disruption includes ground terraces and cuts, which years B.P. (Seitz et al., 2005; Seitz et al., this issue). are evidence of road building and dam develop- The nearshore and onshore component of the ment, and makes tracing the fault more dif cult on the ground; nevertheless, the fault is clearly evi- IVF is a north-northeast striking normal fault that dent in the LiDAR data. continues northward through the community of Incline Village before terminating near the base of Relay Peak, just to the northwest of Incline Lake Discussion: (Fig. 14). The fault scarp is clearly visible through- Fault Density and Architecture out the community, where the fault emerges near The primary purpose of our study was two-fold: the lake, crosses Lakeshore Blvd, and continues (1) to identify the location of active faults in the parallel to South/North Blvd, where it is clearly vis- Tahoe basin, recorded within late Pleistocene– and ible behind the former Incline Village Elementary Holocene–aged deposits, using the latest arsenal of School. At this point, the fault scarp is approxi- sophisticated mapping tools (i.e., LiDAR, seismic mately 8 meters in height, and offsets beach depos- CHIRP pro ling and multibeam sonar); (2) to nd its mantled by some glacial till. potential piercing points that may give constraints North of Incline Village, the fault continues along on any right-lateral (or left-lateral) motion on the the south side of a subsidiary ank of Relay Peak. major normal faults within the basin, and/or newly The fault scarp is clearly evident on the ground discovered faults. The application of airborne
Incline Lake
Mt. Rose Hwy Incline Village Elementary School Incline Village
N
~1 km
Figure 14. A slope-shade bare earth LiDAR model highlighting the Incline Village fault is shown. The southern WHUPLQXVRIWKLVIDXOWH[WHQGVVRPHNLORPHWHUVLQWRWKHODNH2QVKRUHWKH,QFOLQH9LOODJHIDXOWWUDYHUVHVWKURXJK neighborhoods before trending north-northeastward up the Mt. Rose Highway corridor before dying out to the west of the former Incline Lake dam abutment. Reevaluating Late-Pleistocene and Holocene Active Faults in the Tahoe Basin, California-Nevada 849
LiDAR to the Tahoe basin was a critical step in Lakes, it also has a curious behavior of climbing helping to resolve the disparity in fault architecture up the steep hillside near Earthquake Lake. This as reported from terrestrial mapping (Schweickert observation implies some level of reorganiza- et al., 2004) and within lakes using sub-bottom tion of slip as the fault abandoned its position at sonar mapping techniques (e.g., Dingler et al., the base of the range front. In contrast, north of 2009; Brothers et al., 2009). At rst glance, the Fallen Leaf Lake, the FLS of the WTDPF appears lake studies provide what appears to be a relatively to break into two separate strands, one maintain- simple architecture, with a system of three en- ing its position near the edge of the range front, echelon, right-stepping, down-to-the-east normal the other outboard toward Lake Tahoe, where the faults, and little evidence for a multitude of associ- LiDAR data suggests that it enters the lake west ated fault structures. This appears to be in con ict of Baldwin Beach. This level of fault complexity with the Schweickert et al. (2000; 2004) fault map, may play an important role in the transfer of strain and further re nement by Howle et al. (2012), across the relatively large 1.5 km right-stepover which used an earlier Army Corps of Engineers between the FLS and RS of the WTDPF. LiDAR survey that show a plethora of fault struc- Onshore north of Emerald Bay, in the region of tures on land (Fig. 1). These terrestrial fault maps Rubicon and Meeks Bay, through-going structures disagree (in terms of density) with the fault maps similar to those found along the FLS of the WTDPF derived from offshore multibeam bathymetry and are lacking. There is some hint of what appears to seismic CHIRP data. The most notable difference be a fault-like structure emerging from the cen- is along the west shore of Lake Tahoe, where the tral portion Emerald Bay heading to the north, but Tahoe-Sierra frontal fault zone has been mapped careful analysis suggests it may owe its existence as a series of four closely spaced faults with a to mass-wasting of material downslope (Fig. 15). NW-SE orientation near Meeks Bay (e.g., Howle North of these landslide features, the moraines near et al., 2012). Integrated separation across this zone Meeks Bay appear to be devoid of any topographic is reported to be nearly 1.5 mm/yr of down-to-the features that show linearity and/or continuity from east normal motion, which is more than twice the moraine crest to moraine crest. The lead and sec- observed WTDPF rate (Dingler et al., 2009). This ond authors have walked the moraine crests both region of the basin is characterized by three to four north and south of Emerald Bay to understand the prominent lateral moraines from glaciers sourced features mapped by both Schweickert et al. (2004) in the Desolation Wilderness, and are composed and Howle et al. (2012). Unlike faulted moraines of pre Tahoe-, Tahoe– and Tioga–aged material along the FLS of the WTDPF, the slopes of the (Howle et al., 2012). moraines in this area appear devoid of any obvious The most striking observation from the 2010 fault scarps. If through-going structures along the Watershed Sciences bare-earth LiDAR data is Tahoe-Sierra frontal fault system exist as purported the high-resolution imaging of the terrestrial por- by Howle et al. (2012), then it is dif cult to explain tions of the WTDPF and IVF active fault scarps. why they were not obviously imaged in the LiDAR Slopeshade maps derived from digital elevation data, given the clear imaging of ruptures along the models (DEM) clearly highlight fault scarps in a FLS of the WTDPF. variety of terrains that include at lying regions, The most complicating factor along the north- steep hillsides, and across lateral moraines. The ern stretch (Dollar Point segment) of the WTDPF FLS of the WTDPF provides a template for fault is that several splays integrate across more com- identi cation in other regions of the basin, given petent material (e.g., in some cases bedrock), and the multitude of landforms that are offset by this thus, may record a longer period of slip than those segment. Although the WTDPF fault trace south regions that have been “reset” by Tahoe and Tioga of Fallen Leaf Lake appears rather straightfor- glaciation. The observed horsetailing of fault struc- ward in terms of structural complexity, aside from ture, including dramatic changes in strike, are con- a nearly one-half kilometer stepover near Angora sistent with terminations of normal fault systems 850 Applied Geology in California
N ~1 km
Head Scarps
Figure 15. A slope-shade bare earth LiDAR image shows features interpreted to be landslides (shown in green) and their associated head scarps just north of Emerald Bay. The terrain in this area is highly susceptible to landslides.
(Faulds et al., 2011). Kinematically, changes in Tahoe–Sierra Frontal Fault Zone fault location as well as the progressive opening of Research published in the early 2000’s by Howle the basin may be related to a slight counter-clock- (2000) and Schweickert et al. (2004) commented wise (CCW) rotation or pivot along the eastern that the NW-SE-oriented Tahoe-Sierra frontal fault edge of the Sierra Nevada microplate. This rotation system is a “complex zone which includes numer- would drive the basins to open northward through ous subparallel, steeply east-dipping faults with apparent east-side-down normal displacement” and time as observed in the overall morphology of the “carefully documented sinistral oblique displace- Tahoe basin. This change in microplate kinemat- ments along several faults cutting glacial moraines ics may be relatively recent, based on evidence along Meeks Creek southwest of Meeks Bay.” from the geomorphology of the SLNTF. The scarp Recently, a subset of these authors published an height diminishes abruptly where it steps onshore update to their previous work (Howle et al., 2012) at Crystal Bay. South of the 400 m scarp, on the that provides more detailed evidence (through the oor in Lake Tahoe, scarp height varies from on- use of 2008 Army Corps airborne LiDAR) of nor- order 10 m in height to barely visible due to either mal motion (1.5 ± 0.4 mm/yr) along the frontal diminished slip and/or sedimentation rate outpac- fault system, but there wasn’t any discussion of ing accommodation. One can invoke a long-term sinistral (or dextral) motion along this fault zone. history of diminished slip along the southern extent Instead, Howle et al. (2012) highlighted the tec- of the SLNTF that has greatly muted the fault scarp tonic geomorphology of faulted lateral/medial moraines to include side-slope troughs, back-titled height, or perhaps a model where recent propaga- moraine crests, triangular-faceted moraine scarps, tion of the SLNTF toward the south where it dies and extensional fault-propagation folds in lateral/ mid-lake, helps to “open” the basin northward. In medial moraines (or “blind normal faults”). Their this model, the DPS of the WTDPF has diminish- approach was to use indirect indicators of fault ing slip over this time period, as more and more motion (e.g., back tilting of moraine crests) if slip has been transferred over to the southward obvious geomorphic features such as scarps were propagating SLNTF. lacking. South of Emerald Bay, faults mapped by Reevaluating Late-Pleistocene and Holocene Active Faults in the Tahoe Basin, California-Nevada 851
Howle et al. (2012) are in general agreement with saddle, or depression, that cuts through the lateral this study regarding location, as scarps are promi- moraine that bounds Fallen Leaf Lake to the west. nent, continuous features, cross lateral/medial Assuming a Younger Tahoe age of approximately moraines, and are imaged either atop the crest and/ 70 ka (Howle et al., 2012) and a slip-rate of 0.5 or on the sides of the steepened moraines. mm/year (within the range reported by Brothers North of Emerald Bay, there remains a strong et al. (2009) at Fallen Leaf Lake), then one can disagreement on the existence of the Tahoe–Sierra roughly calculate some level of parity between frontal fault system, which provides two dramati- these observations. Maloney et al. (2013) mapped cally different views on the fault architecture of the FLS of the WTDPF transitioning through the the Tahoe basin: Howle et al. (2012) argue that the southern third of Cascade Lake, where it begins Tahoe–Sierra frontal fault system connects with our to horsetail into a handful of splays upon exit- FLS segment of the WTDPF to provide a longer ing the western shoreline. To the north, the fault frontal fault system at the expense of a shortened moves westward towards Emerald Bay, possibly WTDPF. This disagreement has profound implica- dying out, but not before contributing to a complex tions on fault length, maximum earthquake mag- 25-m notch in the medial moraine (likely modi- nitude, and time evolution of basin formation. Our ed through erosion). Although the fault scarp research suggests the FLS of the WTDPF steps into here is not as clean as seen near Angora Lakes, the lake some 1.5 km northeast, linking up with the dramatic expression of the scarp plunging into the RS of the WTPDF; this stepover zone is made Cascade Lake from the east, combined with CHIRP somewhat more complex by the identi cation of a imagery within Cascade Lake, provides some level second active strand just north of Fallen Leaf Lake of con dence that the fault has interacted with the that trends roughly northwest to north, moving off- medial moraine that separates Cascade Lake and shore just west of Baldwin Beach. Our preferred Emerald Bay to the west. model keeps slip-rates somewhat in check along It is clear that not all notches, changes in slope, the entire 55-km-long WTDPF, from the FLS to the and other indirect structures observed within DPS, with vertical rates ranging somewhere near moraines require faulting to produce the range of 0.4-0.8 mm/yr (Dingler et al., 2009; Brothers et al., geomorphologic expressions observed in glacial 2009). The Howle et al. (2012) model has 2 times terrains world-wide. If more direct indicators for greater vertical slip across the frontal fault system fault movement are missing, or are at least ambigu- than observed on the WTDPF; this observation is ous, how does one determine whether the feature at odds with basin morphology in that the WTDPF in question is due to faulting, or just the imperfect is the basin-bounding fault system responsible construction of a moraine, further modi ed through for much of the accommodation that formed the time by erosion and /or slumping? If these features lake. Thus, for the Howle et al. (2012) model to be are due to coherent faulting, one would expect that valid, it would require the frontal fault system to they might be continuous from ridge crest to ridge be a nascent one, otherwise the shoreline of yet a crest with some level of linearity. Along this trend, deeper lake would be shifted to the west coincident a subsection of the fault might have a well-formed with a long term normal faulting history along the scarp that would aid in its identi cation. Absent Tahoe–Sierra frontal fault system. Also, within a linearity and any noticeable subsections contain- larger Tahoe–Sierra frontal fault system, slip-rates ing an unambiguous scarp, interpretation becomes would nearly triple north of Emerald Bay, and yet much more dif cult. This can be made even more fault signatures across the glacial moraines, as dif cult by the presence of landslides that may manifested by scarps, are much more subdued. produce scarps sourced through the process of The use of indirect evidence for mapping faults mass wasting. across moraines is not without merit. For exam- As previously mentioned, our interpretations ple, a well-exposed fault scarp near the Angora diverge with previous work north of Emerald Bay Lakes parking lot is located within a 35-m-deep as one approaches the moraines that are found near 852 Applied Geology in California
Meeks Bay. Howle et al. (2012) identify a fault several notches down the ridge crest, this provides scarp, identi ed as the Rubicon Peak fault zone, an integrated vertical separation that is about 1.5 emerging near the center of Emerald Bay, then mm/yr. If the older moraines have been misiden- wrapping around the range front. We also observe ti ed and are of Older Tahoe age, then slip-rates a system of scarps that extends some 4 km north would be nearly halved. If the offset facets are of Emerald Bay, but a 1.5-kilometer-wide sec- indeed Younger Tahoe-aged, and the recessional tion may provide clues to their origin. The cen- moraines are Tioga-aged, then the side-wall lin- tral piece is arcuate in shape and has many of the ear ridges of the recessional moraines should have characteristics of slope failure, such as a notice- about one- fth the accrued displacement relative to able head scarp, U-shaped scars, and slumps. the Tahoe-aged features—or about 5 meters (Fig. Other examples of slides are observed both above 16). And yet, these linear features do not appear to this feature, and actually within the slide itself. show that degree of deformation within the LiDAR There are also countless examples of slope failure dataset; at best there’s a few meters of chatter along observed in the LiDAR data between Emerald Bay these linear features. This observation decreases and McKinney Bay, including a handful of large con dence that these indirect indicators of slip are slides near Ellis Peak. Our interpretation is that a useful proxy for fault slip in this particular area. the Rubicon Peak fault zone is more likely associ- The moraines of Meeks Bay, however, do exhibit ated with mass wasting, not movement on a fault. some curious behavior. While slip-rate estimates Moving north, there are several prominent lateral do not seem consistent when comparing Tahoe- and medial moraines near Meeks Bay, includ- and Tioga-related features, nonetheless there are ing Meeks Creek, General–Meeks, McKinney– several identi able scarps across this complex. General and McKinney moraines (after Howle Mass wasting is common to the Tahoe basin, as evi- et al., 2012). Slopes of this moraine complex do denced by submarine slide deposits (Smith et al., not perfectly grade toward the lake, and notches 2013; Maloney et al., 2013), and the McKinney Bay and “back-tilting” of the moraine crest, in several landslide (Gardner et al., 2000), which exempli es places are highlighted by Howle et al. (2012). The largest of the recognized facets is on-order of 25 an extreme geologic event. The quadrant extending m in height (Fig. 16). There is also some evidence from Emerald Bay to McKinney Bay appears very for dextral offset (or apparent offset) of a likely prone to landslides, and one can obviously extend Tioga-aged ridge crest along the General–Meeks this northward where remnants of the McKinney moraine at the location of one of the largest facets Bay slide are sitting at the bottom of the lake. One (Fig. 4), but it is dif cult to extrapolate this fea- possibility that would allow Tahoe-aged features to ture, at least in a right-lateral sense, to either Meeks be offset, while Tioga-aged features remain intact, Creek or McKinney–General moraines. This lack would be a large landslide, potentially associated of continuity gives pause with regards to any sus- with the McKinney Bay failure, where a southern tained fault trend; in the Howle et al. (2012) model, sliver of the failure lurched forward, but only par- the largest facets do not connect from moraine to tially failed. That said, there appears to be a chaotic moraine, causing signi cant swings in slip-rates mixture of offsets on inferred Tahoe- and Tioga- over distances less than 2 km. Instead, some form aged moraines, making it dif cult to piece together of linearity is maintained at the expense of consis- a coherent history of either slip or sliding. tency of slip-rate over short distances. Thus, the only other potential forms of continuity would be Stateline–North Tahoe Fault and the facets and breaks in slopes that might reveal a Synchronicity of Rupture more continuous system below. Again, several fac- Analysis of CHIRP data collected during the sum- ets are 10s of meters in height that would require mer of 2011 indicates that the SLNTF extends far- (locally) an approximate 0.35 mm/yr vertical slip- ther south than previously mapped. A map showing rate if offsetting Younger Tahoe moraines as pos- the trace of the SLNTF as determined from CHIRP tulated by Howle et al. (2012). Summed across pro ling is shown in Figure 2. Additional pro ling Reevaluating Late-Pleistocene and Holocene Active Faults in the Tahoe Basin, California-Nevada 853
N 2x Vert. Exag.
Meeks Creek Moraine
General–Meeks Moraine
McKinney–General Moraine
Figure 16. $VORSHVKDGHEDUHHDUWK/L'$5LPDJHRIWKH0HHNV%D\PRUDLQHFRPSOH[LVVKRZQ:KLWH lines represent the moraine crests and cyan represents other linear features (recessionals) on the north faces of the moraines. Notches observed along the moraine crests are measured, though similar or scaled offsets (through time) are not easily observed within other younger, linear features along the sides of the moraines. Linear continuity between the notches or across all moraines is not readily observed, nor is there evidence for faulting on the slopes or valleys between the moraines. beyond the southern terminus of the mapped fault of fault rupture and landslide events is discussed reveals that the SLNTF appears to step over in Maloney et al. (2013). The Most Recent Event towards the WTDPF; some evidence for discontin- (MRE) slide in Fallen Leaf Lake (Brothers et al., uous offsets of the McKinney Bay slide is observed 2009; Maloney et al., 2013) in lls accommodation in our pro les in this region. Local thickening of created by the MRE earthquake at approximately debris deposits near these steep faces, however, 4.5 ka. This, along with sedimentological relation- highlights that while there is ambiguity in terms ships between the MRE slide directly overlying of which fault produced the necessary ground the MRE stratigraphic horizon, suggest earthquake motion to trigger the slide, there is more con - triggering of this particular landslide. dence in determining the source of the debris layer According to their studies, four landslide deposits itself. The thick, non-re ective layers shown in can be correlated between Fallen Leaf Lake, Cascade Figures 12&13 (see red and blue layers) are land- Lake, Emerald Bay and Lake Tahoe, which suggest slide deposits, and likely sourced from the steep at least four earthquake events. Maloney et al. (2013) Crystal Bay escarpment. These debris layers grade classi es these events as the ~11.5 ka event, ~8 ka into turbidite layers and are likely triggered by past event, MRE event on the RS (5.6 ka), and MRE on earthquakes (Smith et al., 2013). the FLS (~4.5 ka). Although other slide and turbi- Maloney et al. (2013) and Smith et al. (2013) dite deposits are present in both Lake Tahoe and have studied landslide and turbidite deposits as a the moraine-bounded lakes to the south, these four proxy for recent movement on Tahoe basin faults. deposits provide the best evidence for movement on Their research in Lake Tahoe (Smith et al., 2013) the WTDPF. Figure 13 shows a CHIRP pro le par- and moraine-bounded lakes south of Lake Tahoe alleling the Crystal Bay escarpment; debris deposits (Maloney et al., 2013) indicate four temporally dis- lacking re ectivity are clearly visible as thick (meter– crete, synchronous landslide events in Tahoe basin scale) deposits interbedded with thinner lacustrine lakes that are likely the result of earthquakes on deposits. Extrapolation of dated cores 20 and 21 the WTDPF. Evidence for the concurrent timing (Smith et al., 2013) identify the two largest slides to 854 Applied Geology in California be temporally equivalent to Slides O and J in the cen- spaced, subparallel faults that have very similar tral and southern reaches of Lake Tahoe. These slides slip rates. Additionally, each fault must be near have maximum thicknesses of 220 cm and 130 cm the end of its rupture cycle so that the change in for deposits O and J (Fig. 12), respectively. the stress caused by the rst earthquake is enough Although rupture on the SLNTF is observed to initiate a second earthquake on a nearby fault. in the CHIRP data as offset beds, the timing of Scholz (2010) found in his work that triggered rupture events is not well constrained as slip on earthquakes typically occur within 0.1 to 1.0 years this fault outpaces sedimentation to in ll accom- after the triggering event, though this time may be modation. This relationship prevents an easy slightly longer, depending on the system. He sug- identi cation of event chronology in the CHIRP gests it takes a very small stress drop, on the order data (Brothers et al., 2009). It is likely that debris/ of 0.1 to 0.01 MPa, to trigger an earthquake on an turbidite deposits in Lake Tahoe found near the adjacent fault late in its inter-seismic period. Crystal Bay escarpment are sourced from earth- Based on previous work (Kent et al., 2005; quakes on the SLNTF, instead of earthquakes on Dingler et al., 2009), slip rates for the WTDPF the WTDPF, although this latter option cannot be and SLNTF are estimated to be similar, with a slip dismissed. Based on our new data, we recognize rate on the WTDPF of approximately 0.4 to 0.8 that turbidite Deposits J and O (de nitions from mm/yr and on the SLNTF of approximately 0.35 Smith et al., 2013; Maloney et al., 2013) have to 0.6 mm/yr. Because the slip rate on the IVF is enhanced thicknesses near the northern section of much less at approximately 0.1 to 0.2 mm/yr, and the SLNTF, suggesting a local source (i.e., Crystal its rupture timing does not appear to coincide with Bay escarpment). More importantly, debris depos- either the WTDPF or the SLNTF, it is not consid- its sourced near the SLNTF appear to have a simi- ered part of this potential sequence. The subparal- lar chronology to those observed along the RS of lel faults are approximately eight to ten kilometers the WTDPF. This suggests that both the WTDPF apart, making them an ideal system for synchro- and SLNTF share the same or inseparable chronol- nized behavior. Since a historical record of rupture ogy of debris-related turbidite deposits. Perhaps on these faults does not exist, and the spatial reso- Deposits O and J, observed as far south as the lution of our data is insuf cient to identify short southern moraine-bounded lakes, are all related to time lapses (days to ~100 years) between events, rupture on the WTDPF. If true, then it is odd that it is unclear if these faults rupture as a cluster. the SLNTF does not have its own distinct chronol- Nevertheless, the size of debris ow Deposits O ogy of rupture (at least within the temporal reso- and J provide indirect evidence of large and pos- lution afforded by CHIRP pro ling and dating). sibly synchronous rupture. Alternatively, both the WTDPF and SLNTF may rupture synchronously, thus providing little time Conclusions for background sedimentation rates to effectively Prior to the 2010 LiDAR survey, the fault com- separate debris deposits in depth that are close in plexity interpreted by previous workers on land time. appeared higher than that observed in Lake Tahoe. In a discussion of rupture synchronicity, Scholz Previous work in Lake Tahoe (Brothers et al., (2010) explains the timing similarities observed 2009; Dingler et al., 2009) clearly showed evi- in both the Basin and Range province in Nevada dence for relatively simple fault geometries within and the Eastern California Shear Zone. His study the deepest part of the basin. These results were area is especially tting considering the Lake inconsistent with work published by Schweickert Tahoe basin is in uenced by both of these systems. et al. (2004) and Howle et al. (2012), who docu- In his work, Scholz describes clusters of events mented very complex fault geometries on land. and the properties of the faults that make them We argue that the fault geometry on land is less likely to exhibit synchronistic behavior. Typically, complicated than previously thought and that it synchronicity occurs within systems of evenly more consistent with the fault geometry imaged Reevaluating Late-Pleistocene and Holocene Active Faults in the Tahoe Basin, California-Nevada 855 within Lake Tahoe. The combined approach of highlight a very dynamic environment for fault this study clearly shows the locations of the reorganization. Lastly, locally-sourced large major active faults in the Tahoe basin. Between debris flows off of the Crystal Bay escarpment the newly acquired LiDAR data and the addition near the SLNTF have a chronology that is simi- of the new CHIRP and multibeam data, the nor- lar to the large slides sourced off of the Rubicon mal faults have been mapped both onshore and escarpment along the RS of the WTDPF. Given within lakes. the slip rates of each fault and characteristic Analysis of LiDAR data between Emerald return time between ruptures, there exists the Bay and Meeks Bay highlights the roles of gla- possibility that these systems may have expe- ciation, tectonics and mass wasting in the evolu- rienced synchronous ruptures (decadal time- tion of topography. Although the moraine crests scale), highlighting a system that may be similar of Meeks Bay show a degree of complexity, to that observed in the Central Nevada Seismic through-going structures are dif cult to identify. Belt in the 1950s. While correlation of potential Tahoe-age trian- gular facets and back-tilted slopes from moraine Acknowledgements crest to moraine crest is somewhat possible, We thank Shane Romsos at the Tahoe Regional Tioga-age recessional moraines preserved on Planning Agency (TRPA) for leading the efforts the north slopes of these moraines do not show to collect the 2010 LiDAR dataset. The LiDAR corresponding offsets (about 1/5th of the dis- campaign was successful in part due to the work placement), which is inconsistent with a fault- of Ramon Arrowsmith (ASU) and Chris Crosby based origin. Instead, we speculate that some (UCSD) in contractor selection. This research of these features may be related to mass move- was funded through USGS NEHRP grants ment associated with the McKinney Bay slide or G13AP00017 and 10HQPA1000. This manuscript other mass wasting events. The few candidate was greatly improved with reviews from Steve piercing points in the basin along faulted lat- Wesnousky, Jeff Unruh and Tim McCrink. eral moraines near Angora and Cascade lakes do not show progressive offset between Tioga- and REFERENCES Tahoe-aged features; instead, likely natural vari- ability of geomorphic shape provide inconsistent Birkeland, P.W., 1963, Pleistocene volcanics and deformation of the Truckee area, north of Lake offsets that are not consistent with any right lat- Tahoe, California, Geol. Soc. America Bull., v. 74, eral motion. p. 1453–1464. New CHIRP data crossing the full expanse Blackwelder, E., 1933, Eastern slope of the Sierra of the SLNTF provide new clues into the total Nevada, 16th Internat. Geological Cong., Guidebook fault length, its dynamics through time, and the 16, p. 81–95. potential for synchronous ruptures between the Bormann, J. M., 2013, New Insights into Strain largest two basin bounding faults. 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Watershed Sciences, 2010, LiDAR Remote Sensing, Simply stated, stratigraphy is the tape recorder of Earth’s Lake Tahoe Watershed, prepared for the Tahoe history. Nevertheless, the delity with which stratigraphy Regional Planning Agency, January 31, 2011. records the dynamic processes of sediment erosion, trans- Wesnousky, S. G., Bormann, J. M., Kreemer, C., port, accumulation, and preservation is still incompletely Hammond, W. C., and J. N. Brune (2012), understood. Although these processes can be examined from Neotectonics, Geodesy, Seismic Hazard in the many perspectives, Driscoll’s research has focused on uncon- northern Walker Lane of Western North America: formity generation and stratigraphic development in tectoni- Thirty kilometers of crustal shear and no strike- cally active settings and sediment input and dispersal along slip? Earth and Planetary Science Letters, 329–330, continental margins. 133–140 Annie Kell, Education Outreach Seismologist at the Nevada Seismological Laboratory, earned a Ph.D. in Geophysics in Graham M. Kent. Dr. Graham Kent, Director of the Nevada 2014. Her research focuses on seismic imaging of shallow Seismological Laboratory, specializes in re ection seismol- crust in regions with active tectonics. Research areas include ogy, mid-ocean ridge dynamics, tectonics of the Gulf of the Salton Sea, Pyramid Lake and Lake Tahoe. After experi- California–Walker Lane transtentional system, Southern encing a large magnitude earthquake in 2010, Annie became California tectonics, development of underwater approaches motivated to help educate Nevadans on earthquake hazards to paleoseismology, and earth science-related high end visu- and safety preparation. The work allows her to continue to alization. Dr. Kent also works with instrumentation, includ- do research in interesting environments, but also to interact ing the development of a 100-instrument ocean-bottom seis- with emergency of cials, school-aged children and the gen- mograph pool (developed at Scripps, PI), and more recently, eral public. upgrading the microwave system in eastern California and Nevada to support digital delivery of “all hazards” to include: Kenneth D. Smith. Dr. Kenneth Smith, Assoc. Director seismic, re and extreme weather data types. of the Nevada Seismological Laboratory, specializes in earthquake studies of the Basin and Range of Nevada and Gretchen C. Schmauder. Dr. Gretchen Schmauder is a eastern California. Dr. Smith manages the Nevada Seismic former Ph.D. student at the University of Nevada, Reno Network, which consists of ~150 seismograph stations and (Nevada Seismological Laboratory), where she mapped fault a regional microwave communications system (with pri- structure at Lake Tahoe using a variety of techniques, includ- mary support through the USGS and the Sate of Nevada); ing airborne LiDAR (Light Detection and Ranging) the Nevada Seismological Laboratory reports the locations on land and underwater CHIRP sub-bottom pro ling. She also and magnitudes of over 10,000 earthquakes per year in used fault architecture and shallow velocity structure derived Nevada and eastern California. He was involved in seismic from REMI techniques to simulate ground motion from hazard studies and earthquake monitoring for the DOE’s large damaging earthquakes in the Lake Tahoe Basin. She Yucca Mountain Project (closed out in 2010). Dr. Smith has since moved on to work at Geometrics Inc., where she is manages the Nevada Seismological Laboratory’s role in Customer Service Business Manager. Dr. Schmauder also has the DOE’s ‘Source Physics Experiment’, being conducted her license as a Professional Geologist. by National Security Technologies on the Nevada National Jillian Maloney is an assistant professor of geology at San Security Site. Diego State University. Using both geologic and geophysi- Robert L. Baskin. Dr. Rob Baskin is currently a Supervisory cal datasets, Maloney seeks to understand tectonic deforma- Hydrologist with the U.S. Geological Survey in Salt Lake tion and sediment processes along continental margins. Her City, Utah. Rob is involved in multiple studies; however, his research has focused on offshore neotectonics, paleoseismol- primary research focuses on the integration of multiple tech- ogy, and landslide dynamics in tectonically active settings. nologies to establish the environmental framework that in u- Maloney has also conducted research on marine geohazards ences microbial bioherm occurrence and distribution in Great and sediment dispersal along passive margins. Salt Lake, Utah. He has conducted multiple collaborative Neal W. Driscoll. Dr. Neal Driscoll is a professor of geol- studies on inland waterbodies with the Scripps Institution ogy and geophysics at Scripps Institution of Oceanography, of Oceanography, University of California, San Diego, and University of California, San Diego. Driscoll’s primary inter- the University of Nevada, Reno, including Great Salt Lake, est is in tectonic deformation and the evolution of landscapes the Salton Sea, Pyramid Lake, Lake Tahoe, Walker Lake and and seascapes. His work primarily focuses on the sediment others. He was involved with early applications of thermal record to understand the processes that shaped the earth. imagery in the study of groundwater in ow into lakes, and 858 Applied Geology in California recently completed his Doctoral dissertation at the University and a Ph.D. from University of Oregon. His graduate research of Utah. He is also a registered professional geologist in the focused on paleoseismology. He completed a postdoctoral fel- State of Utah. lowship at Lawrence Livermore National Laboratory at the Center for Accelerator Mass Spectrometry, where he researched Dr. Gordon Seitz is an Engineering Geologist at the California the dating of earthquakes using C-14 samples. He has partici- Geological Survey in Menlo Park, California. He is licensed in pated in paleoseismic and earthquake investigations worldwide. California as a Professional Geologist and Certi ed Engineering He continues to work on developing offshore paleoseismic Geologist. He received his B.S. from San Diego State University, methodologies using Lake Tahoe as a laboratory.