Latest Quaternary Slip Rates of the San Bernardino Strand of the San Andreas Fault, Southern California, from Cajon Creek to GEOSPHERE, V

Research Paper THEMED ISSUE: Seismotectonics of the San Andreas Fault System in the San Gorgonio Pass Region

GEOSPHERE Latest Quaternary slip rates of the San Bernardino strand of the San Andreas fault, southern California, from Cajon Creek to GEOSPHERE, v. 17, no. X Badger Canyon https://doi.org/10.1130/GES02231.1 Sally F. McGill1,*, Lewis A. Owen2,*, Ray J. Weldon3,*, Katherine J. Kendrick4,*, and Reed J. Burgette5,* 18 figures; 2 tables; 1 set of supplemental files 1Department of Geological Sciences, California State University, 5500 University Parkway, San Bernardino, California 92407-2397, USA 2Department of Marine, Earth, and Atmospheric Sciences, North Carolina State University, Raleigh, North Carolina 27695, USA 3 CORRESPONDENCE: [email protected] Department of Earth Science, University of Oregon, Eugene, Oregon 97403-1272, USA 4U.S. Geological Survey, 525 South Wilson Avenue, Pasadena, California 91106, USA 5Department of Geological Sciences/MSC 3AB, New Mexico State University, P.O. Box 30001, Las Cruces, New Mexico 88003, USA CITATION: McGill, S.F., Owen, L.A., Weldon, R.J., Kendrick, K.J., and Burgette, R.J., 2021, Latest Quater‑ nary slip rates of the San Bernardino strand of the San Andreas fault, southern California, from Cajon Creek ABSTRACT previously published rate of 24.5 ± 3.5 mm/yr at sections of the fault. Most of these models infer slip- to Badger Canyon: Geosphere, v. 17, no. X, p. 1–28, https://doi.org/10.1130/GES02231.1. the southern end of the Mojave section of the San deficit rates (also known as “geodetic slip rates”) of Four new latest Pleistocene slip rates from two Andreas fault (Weldon and Sieh, 1985), suggesting 0–8 mm/yr for the San Bernardino and San Gorgo-

Science Editor: Andrea Hampel sites along the northwestern half of the San Ber- that ~12 mm/yr of slip transfers from the Mojave nio Pass sections of the San Andreas fault. Guest Associate Editor: David D. Oglesby nardino strand of the San Andreas fault suggest the section of the San Andreas fault to the northern Initially, this model of slip partitioning appeared slip rate decreases southeastward as slip transfers San Jacinto fault zone (and other faults) between to contrast dramatically with geologic estimates of Received 30 December 2019 from the Mojave section of the San Andreas fault Lone Pine Canyon and Badger Canyon, with most the slip rate of 24 ± 3.5 mm/yr near Cajon Creek Revision received 19 November 2020 onto the northern San Jacinto fault zone. At Badger (if not all) of this slip transfer happening near Cajon (Weldon and Sieh, 1985), 14–25 mm/yr at Wilson Accepted 23 March 2021 Canyon, offsets coupled with radiocarbon and opti- Creek. This has been a consistent behavior of the Creek, in Yucaipa (Harden and Matti, 1989), and cally stimulated luminescence (OSL) ages provide fault for at least the past ~47 k.y. 14–17 mm/yr at Biskra Palms (Behr et al., 2010 and three independent slip rates (with 95% confidence Fletcher et al. 2010, following upon earlier work by intervals): (1) the apex of the oldest dated alluvial Keller et al., 1982, and van der Woerd et al., 2006; fan (ca. 30–28 ka) is right-laterally offset ~300–400 m ■■ INTRODUCTION site locations are shown in Fig. 1). However, sev- +2.2 yielding a slip rate of 13.5 /−2.5 mm/yr; (2) a terrace eral recent investigations have resulted in additional riser incised into the northwestern side of this allu- The partitioning of slip rate between faults Holocene and late Pleistocene slip rates at sites vial fan is offset ~280–290 m and was abandoned ca. within the southern San Andreas fault system is located southeast of Cajon Creek and northwest +0.9 23 ka, yielding a slip rate of 11.9 /−1.2 mm/yr; and still poorly understood. Elastic modeling of geo- of Biskra Palms, which confirm that the San Ber- (3) a younger alluvial fan (13–15 ka) has been offset detic data suggests that a substantial portion of nardino and San Gorgonio Pass sections of the San 120–200 m from the same source canyon, yielding the slip on the Coachella Valley section of the San Andreas fault slip more slowly than any other section +4.2 a slip rate of 11.8 /−3.5 mm/yr. These rates are all Andreas fault passes northward into the Eastern of the fault zone, with rates of 7–16 mm/yr at Plunge consistent and result in a preferred, time-averaged California shear zone, rather than remaining on Creek (McGill et al., 2013), 8 ± 4 mm/yr at Burro Flats +5.3 rate for the past ~28 k.y. of 12.8 /−4.7 mm/yr (95% the San Andreas fault (Fig. 1) (Becker et al., 2005; (Orozco and Yule, 2003; Orozco, 2004; Yule and Spo- +2.7 confidence interval), with an 84% confidence inter- Meade and Hager, 2005; Spinler et al., 2010; Love- tila, 2010; see also Yule, 2009), 5.7 /−1.5 mm/yr at val of 10–16 mm/yr. At Matthews Ranch, in Pitman less and Meade, 2011; McGill et al., 2015). Likewise, Millard Canyon (Heermance and Yule, 2017), >5.7 Canyon, ~13 km northwest of Badger Canyon, a a substantial portion of the slip on the Mojave ± 0.8 mm/yr at Cabazon (Yule et al., 2001), and landslide offset ~650 m with a 10Be age of ca. 47 ka section of the San Andreas fault appears, in these 4–5 mm/yr at Painted Hill, near Whitewater (Gold et +9.9 yields a slip rate of 14.5 /−6.2 mm/yr (95% confi- models, to extend southward onto the San Jacinto al., 2015). In this paper, we report similarly slow rates dence interval). All of these slip rates for the San fault, leaving a relatively low strain accumulation for two sites near the northwestern end of the San Bernardino strand are significantly slower than a rate on the San Bernardino and San Gorgonio Pass Bernardino strand of the San Andreas fault.

This paper is published under the terms of the Sally McGill https://orcid.org/0000-0001-7176-7055 CC‑BY-NC license. *E-mail: [email protected], [email protected], [email protected], [email protected], [email protected]

GEOSPHERE | Volume 17 | Number X McGill et al. | San Bernardino SAF slip rate Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/doi/10.1130/GES02231.1/5386531/ges02231.pdf 1 by guest on 24 September 2021 Research Paper

118° W 117° W 116° W

40 20 0 40 Kilometers

San Andreas fault

San Andreas fault 1999 Mojave section Eastern California Shear Zone LR: Wa Fig. 1 37 Pa: Figure 2 35.6 ± 6.7 North Frontal fault Restraining SGM CC: 24.5 ± 3.5 SBM segment Pt: 15 ± 4 1992 DC1.9 LC BC: 11.6 ± 1.0 2.5 SB-SAF Cy SA Cucamonga fault Pinto Mountain fault C: >1.7-3.3 Pl: 7-16 WC: 14-25 GT: 6-13 BF: 8 ± 4 34° N MC: 4.2-8.4 34° N NSTB: > 5 to >20 SG Pass PH:4-5 Cb: >5.7 ± 0.8 San Jacinto fault PW: 21.6 ± 2 BP: 12-22 Elsinore fault IH: 2.5 ± 1

Coachella Valley section San Andreas fault +9 Az1: 12 - 5 Az2: 9.5-15.5 Az3: 15 ± 3

RH: 8.9 ± 2

Late Quaternary Faults AW: 5.8 SSR: 1.5 ± 0.4 by recency of movement (yrs) N 1857 CE or younger <15,000 <130,000 33° N 33° N 118° W 117° W 116° W Figure 1. Major faults and fault sections discussed in text, color coded according to recency of movement (U.S. Geological Survey and California Geological Survey, 2018). White circles show locations of latest Pleistocene and Holocene slip-rate sites for the San Andreas and San Jacinto faults, with slip-rate estimates in mm/yr. Smaller white circles show slip-rate sites on the Mill Creek strand of the San Andreas fault. Inset map shows location of Figure 1 within southern California. Box shows location of Figure 2. AW—Ash Wash (Le et al., 2008); Az1—Anza (Rockwell et al., 1990); Az2—Anza (Blisniuk et al., 2013); Az3—Anza (Merifield et al., 1991); BC—Badger Canyon (this study); BF—Burro Flats (Orozco and Yule, 2003; Orozco, 2004; Yule and Spotila, 2010); BP—Biskra Palms (Behr et al., 2010; Fletcher et al., 2010); C—Colton (Wesnousky et al., 1991); Cb—Cabazon (Yule et al., 2001); CC—Cajon Creek (Weldon and Sieh, 1985); Cy—City Creek (1.2 mm/yr: Sieh et al., 1994); DC—Day Canyon (Horner et al., 2007); GT—Grand Terrace (Prentice et al., 1986); IH—Indio Hills (Blisniuk et al., 2021); LC—Lytle Creek (Mezger and Weldon, 1983); LR—Littlerock (Weldon et al., 2008); MC—Millard Canyon (Heermance and Yule, 2017); NSTB—Northern San Timoteo badlands (Morton et al., 1986 ; Kendrick et al., 2002; McGill et al., 2012; Onderdonk et al., 2015); Pa—Pallett Creek (Salyards et al., 1992); Pl—Plunge Creek (McGill et al., 2013); Pt—Pitman Canyon (this study); PW—Pushawalla Canyon (Blisniuk et al., 2021); RH—Rockhouse Canyon (Blisniuk et al., 2010); SA—Santa Ana River (2 mm/yr: Weldon, 2010); SBM—San Bernardino Mountains; SGM—San Gabriel Mountains; SG Pass—San Gorgonio Pass; SSR—southern Santa Rosa Mountains (Blisniuk et al., 2010); Wa—Wallace Creek [inset] (Sieh and Jahns, 1984); WC—Wilson Creek (Harden and Matti, 1989); PH—Painted Hill (Gold et al., 2015).

■■ REGIONAL TECTONIC SETTING the San Bernardino strand, which is the strand with (Fig. 2; Weldon, 1986). The Peters fault strikes east- the strongest geomorphic evidence for Holocene west and connects the San Bernardino strand The Badger Canyon and Matthews Ranch/ activity (Matti and Morton, 1993). southeast of Pitman Canyon to the Glen Helen fault. Pitman Canyon slip-rate sites are on the northwest- The northern end of the San Jacinto fault zone Between Pitman Canyon and Devore, the Tokay Hill ern half of the San Bernardino Mountains section closely approaches the San Bernardino strand of fault diverges southward from the San Bernardino of the San Andreas fault zone (Fig. 1). Within the the San Andreas fault and comprises three strands strand for a length of 2 km, where it ends or is San Bernardino region, four major strands of the with evidence for Holocene activity (Fig. 2). The buried beneath young alluvium. These two struc- right-lateral San Andreas fault zone have been Glen Helen fault is the northeasternmost mapped tures (and possibly others that may be buried under active at various times, along with numerous strand of the San Jacinto fault zone and is located active alluvium between them and the Glen Helen other fault splays (Matti and Morton, 1993). The ~2.0 km southwest of the San Bernardino strand at fault) may serve to transfer slip between the San two oldest strands—the Wilson Creek and Mis- Pitman Canyon (Fig. 2). The San Jacinto fault proper Andreas and San Jacinto fault zones. sion Creek strands—have not been active during and the Lytle Creek fault (another strand within the the time period for which our slip-rate estimates San Jacinto fault zone) are located ~4.1 and 5.6 km are valid (Matti and Morton, 1993). Only the San southwest of the San Bernardino strand at Pitman ■■ METHODS Bernardino and Mill Creek strands are expressed Canyon, respectively. geomorphically in our study area, along with sev- Two additional faults have been mapped We conducted geologic mapping at Badger eral other fault strands and splays of shorter length between the San Jacinto and San Andreas fault Canyon and at the Matthews Ranch landslide in (Fig. 2). The slip rates reported in this paper are for zones within the region of their closest approach Pitman Canyon, as described in more detail in the

117.5°W 117.25°W 117.0°W

1857 rupture

LPC CC Cleghorn fault zone Figure 2. Regional tectonic setting of this study. Yellow circles show slip-rate sites 34.25 Pt 34.25°N presented in this study: BC—Badger Can- PF yon site; Pt—Pitman Canyon (Matthews GHF THF Waterman Canyon fault Ranch) site. White circles show other slip- rate sites: CC—Cajon Creek site (Weldon BC and Sieh, 1985); Pl—Plunge Creek site (Mc- LCF Arrowhead Springs strand Arrowhead fault Gill et al., 2013); WC—Wilson Creek site San Bernardino strand (Harden and Matti, 1989). Fault abbrevia- WmC Mill Creek strand tions: GHF—Glen Helen fault; LCF—Lytle Cucamonga fault Creek fault; PF—Peters fault; THF—Tokay Hill fault. Other geographic locations mentioned in text: LPC—Lone Pine Canyon; San Jacinto fault zone Pl Quaternary Faults WmC—Waterman Canyon. Faults from by recency of U.S. Geological Survey and California movement (years) Geological Survey (2018). 1857<150 CE or younger <15,000 N WC <130,000 <1,600,000

10 5 0 10 Km 34.0 Crafton Hills fault zone 34.0°N 117.5°W 117.25°W 117.0°W

Supplemental Material1 (text, section S1.1). Our atmospheric calibration curve IntCal13 (Reimer et Details of sample selection, processing, and labo- work at the Badger Canyon site made use of eleven al., 2013). Table 1 reports the conventional radiocar- ratory analysis are described in the Supplemental trenches excavated west of Badger Canyon in 2005 bon ages (±1σ) and the calibrated age ranges (95.4% Material (section S1.2, see footnote 1). Field and by CHJ Consultants. The trenches were typically confidence intervals). Within the text, calibrated laboratory measurements are reported in Table S4. ~3 m deep, but some ranged up to 5.5 m deep. radiocarbon ages are referred to in the format of 10Be exposure ages for boulders were calculated These excavations offered an opportunity to view mean calibrated age (in units “ka,” rounded to the using Martin et al. (2017), and ages obtained using the subsurface stratigraphy within the offset alluvial nearest century) ±2σ, for ease of comparison with a variety of other models are reported in Table S5. fans, to collect samples for radiocarbon and opti- the luminescence and cosmogenic ages. Table S1 For discussion and analysis, we use the Lal (1991) cally stimulated luminescence (OSL) dating, and provides latitude, longitude, and depth of each and/or Stone (2000) time-dependent model with a for descriptions of soil profiles. We logged selected radiocarbon sample. local production rate scaled from four sites in the trenches and portions of these trenches at a scale Luminescence samples were analyzed at the Sierra Nevada (Baboon Lakes Moraine, Mount Starr, of 1:120. Relevant logs are presented later in the University of Cincinnati using methods described Greenstone Lake, and Twin Lakes; Martin et al., 2017). paper and in Figures S1 and S2 in the Supplemen- in the Supplemental Material (text, section S1.2, To calculate slip rates, we construct probabil- tal Material. Stratigraphic and map units used in see footnote 1). Ages, with ±2σ uncertainties, are ity density functions (PDFs) for the offset (O) and this study are informal designations for this limited reported in Table 2, with calculation uncertainties age (A) of each geologic feature. These PDFs are geographic region; further information about the and methods used to calculate dose rates explained constrained by quantitative measurements and units, as well as how they correlate to regionally in the footnotes to that table. Table S2 provides shaped by our understanding of the geologic his- defined units, is available in section S2 of the Sup- latitude, longitude, and depth of each OSL sam- tory of the site. We then construct a joint probability plemental Material. ple. We documented soil development for pedons density function for offset and age of each feature, We collected detrital charcoal samples from the associated with Qf1, Q2t-w, Qf2a, and Qf3b at Bad- following McGill et al. (2009). Each cell in the

trenches and from one natural exposure at Bad- ger Canyon (Table S3). Details are described in the two-dimensional joint PDF contains the probability GS2231 Supplemental figure and table captions 14 ger Canyon and submitted them for C dating at Supplemental Material (text, section S1.2). that the offset and age both fall within the range of Figure A1: Logs of trenches WT-9A, WT-10 and WT-11, showing exposures of Qf1, location of 10 OSL samples BC-8 and BC-9 in WT-9A and location of soil profile BC01 in WT-10. See Figure the Center for Accelerator Mass Spectrometry at We collected samples for Be surface exposure offsets and ages spanned by that cell. This prob- 3 legend for description of units not described on the figure itself Lawrence Livermore National Laboratory. We cali- dating from ten boulder tops on alluvial fan surfaces ability is calculated using the following equation:

Figure A2: Logs of trenches WT-7B and WT-3B showing exposures of Qf2 and location of soil brated all radiocarbon ages using OxCal 4.2 (Bronk at Badger Canyon and from the tops of six blocks on profile BC02. See Figure 3 legend for description of units not described on the figure itself. Ramsey, 2009) with the northern hemisphere the Matthews Ranch landslide near Pitman Canyon. p(O,A)p= (O)dOp(A)dA, (1)

Figure A3: Photomosaic of the northern end of trench WT-3C, showing the locations of dated

luminescence sample BC2 and dated radiocarbon sample BC-8 from a ~10-cm-thick sand layer within Qf2. Luminescence sample BC1 could not be dated. View to west. See Figure 3 for TABLE 1. RADIOCARBON AGES FROM THE BADGER CANYON SITE locations of these samples in map-view. Calibrated age# Figure A4: Photomosaic showing the location of dated radiocarbon sample BC-22 from Qf3 in the west wall of trench WT-1B, near the northern end of the trench. See Figure 3 for location of CAMS Sample name δ13C† Fraction ± δ14C ± 14C age§ ± Mean 95.4% Context sample in map-view. Lab. no. (per mil) modern (yrs B.P.‡,**) (yr) (cal. B.P.‡) conf. interval

Figure A5: Photomosaic showing the location of dated radiocarbon sample BC-51 in a natural 127368 BC-15 25 0.8618 0.0031 138.2 3.1 1195 30 1120 1010–1230 Young fill (Qa4) over Qls2 in WT-1A exposure of Qa4. See Figure 3 for location of sample in map-view. View to west.

130978 BC-51 split 1 23.67 0.8489 0.0028 151.1 2.8 1315 30 1240 1090–1320 Qa4, natural exposure in Badger Canyon Figure A6. LiDAR imagery (0.5-m resolution) from the B4 project (Bevis et al., 2005) showing 127369 BC-24 25 0.2104 0.0009 789.6 0.9 12,525 40 14,800 14,450–15,100 Base of colluvium over Qf3a in WT-2 locations of trenches and geochronological samples in the vicinity of Badger Canyon. White 127370 BC-22 25 0.1988 0.0008 801.2 0.8 12,975 35 15,510 15,310–15,710 Qf3b gravel from WT-1B

1 127371 BC-42 25 0.2097 0.0009 790.3 0.9 12,545 35 14,860 14,580–15,120 Qf3b in WT-1A, just south of fault

127376 BC-17 25 0.0900 0.0039 910.0 3.9 19,340 360 23,320 22,490–24,120 Qf2b in WT-1A, north of fault, above Qls2 127372 BC-11 25 0.0911 0.0006 908.9 0.6 19,250 60 23,200 22,950–23,450 Qc2 in WT-1A, north of fault, below Qls2 127377 BC-20 25 0.0812 0.0006 918.8 0.6 20,170 60 24,240 24,020–24,450 ET-2B (east of Badger Canyon) 1 Supplemental Material. Additional details on meth- 127373 BC-8 25 0.0481 0.0007 951.9 0.7 24,380 120 28,420 28,100–28,720 Near apex of Qf2, in WT-3C ods and for descriptions and interpretations of geologic units at the Badger Canyon site. Additional 127374 BC-13 25 0.0365 0.0007 963.5 0.7 26,600 160 30,840 30,580–31,090 Qf2a in WT-1A, north of fault, below Qc2 trench logs and photographs showing locations of 127375 BC-46 25 0.0365 0.0015 963.5 1.5 26,590 350 30,730 30,020–31,280 Qf2a in WT-1A, north of fault, below Qc2 dated samples that were not shown in other figures. *All samples were dated at Center for Accelerator Mass Spectrometry (CAMS), Lawrence Livermore National Laboratory. Tables providing latitude and longitude for all dated †All δ13C values are assumed, with the exception of sample BC-51, for which the δ13C value was measured. samples, as well as field and lab data for terrestrial §The quoted age is in radiocarbon years using the Libby half-life of 5568 years and following the conventions of Stuiver and Polach (1977). nuclide dating and complete soil descriptions. Please #Radiocarbon ages were calibrated with OxCal 4.2 (Bronk Ramsey, 2009) using calibration curve intcal13 (Reimer et al., 2013). visit https://doi.org/10.1130/GEOS.S.14292323 to ac- ‡Cal. B.P. indicates (calibrated) calendar years before A.D. 1950. cess the supplemental material, and contact editing@ **yrs B.P. indicates radiocarbon years before A.D. 1950. geosociety.org with any questions.

TABLE 2. OPTICALLY STIMULATED LUMINESCENCE DATA AND DATING RESULTS Sample Trench number: Altitude Depth Particle size Ua Tha Ka Rba Cosmic-doseb,c Dose-rateb,d ne Mean equivalent dosef OSLg ageh number geologic unit (m asl) (cm) (µm) (ppm) (ppm) (%) (ppm) (G/ka) (G/ka) (Gy) (ka) BC3 WT-1A: Qf3 507 366 125–180 2.96 24.5 1.96 109 0.15 ± 0.01 4.06 ± 0.24 26(28) 47.8 ± 6.8 11.8 ± 1.6 BC4 WT-1A:Qf3 507 381 125–180 2.22 23.5 1.90 108 0.15 ± 0.01 3.78 ± 0.22 28(30) 52.4 ± 12.7 13.9 ± 2.0 BC5 WT-1A: Qf3 515 396 125–180 2.28 20.6 1.79 100 0.14 ± 0.01 3.51 ± 0.20 25(27) 46.7 ± 13.9 13.3 ± 2.2 BC2 WT-3C: Qf2 518 274 90–125 2.06 13.5 1.83 133 0.17 ± 0.02 3.07 ± 0.18 25(30) 63.2 ± 14.6 20.6 ± 3.0 BC6 WT-1A: Qf2a 525 259 90–125 2.25 15.4 1.95 131 0.17 ± 0.02 3.34 ± 0.20 17(19) 74.0 ± 17.2 22.2 ± 3.6 BC8 WT-9: Qf1 502 335 125–180 2.32 21.9 2.10 122 0.15 ± 0.02 3.88 ± 0.23 29(31) 70.3 ± 18.5 18.1 ± 2.8 BC9 WT-9: Qf1 502 335 125–180 2.71 17.9 1.98 108 0.15 ± 0.02 3.60 ± 0.21 15(27) 66.3 ± 20.2 18.4 ± 3.6 aElemental concentrations from nuclide activation analysis of whole sediment measured at U.S. Geological Survey Nuclear Reactor Facility in Denver. Uncertainty taken as ±10%. bEstimated fractional present-day water content 10 ± 5%. cEstimated contribution to dose-rate from cosmic rays calculated according to Prescott and Hutton (1994). Uncertainty taken as ±10%. dTotal dose-rate from beta, gamma, and cosmic components. Beta attenuation factors for U, Th, and K compositions incorporating grain size factors from Mejdahl (1979). Beta attenuation factor for Rb taken as 0.75 (cf. Adamiec and Aitken, 1998). Factors utilized to convert elemental concentrations to beta and gamma dose-rates from Adamiec and Aitken (1998) and beta and gamma components attenuated for moisture content. e Number of replicated equivalent dose (DE) estimates used to calculate mean DE. The number in parentheses is the total number of aliquots measured. These are based on recuperation error of <10%. f Mean equivalent dose (DE) determined from replicated single-aliquot regenerative-dose (SAR; Murray and Wintle, 2000) runs. Errors are 1-sigma incorporating error from beta source estimated at ~±5%. gOptically stimulated luminescence. h ½ Errors incorporate dose-rate errors and 2-sigma standard errors (i.e., 2n1/n ) for DE.

where p(O,A) is the joint probability that the offset Bernardino strand of the San Andreas fault zone The older alluvial fans are offset from the source (O) and the age (A) are within a particular offset (34.191°N/117.313°W). The San Bernardino strand region (Badger Canyon) by progressively larger increment (dO) and a particular age increment (dA), displaces several alluvial fan units from the mouth amounts than the younger alluvial fans. Reliable given that p(O) and p(A) are the probability density of Badger Canyon (Fig. 3), forming the basis for age control is lacking for Qf1 (see discussion of functions for offset and age and are assumed to three slip-rate estimates. In this section, we briefly available ages in section S2.3.1 of the Supplemen- be independent of each other. We then sum the describe the geologic units that are relevant to tal Material [footnote 1]), and human disturbance probabilities contained in all the cells in the joint understanding the slip-rate estimates. A more com- prevents reliable estimation of the offset of Qf4. PDF that have offsets and ages that contribute to plete description of these and other geologic units Our three slip-rate estimates are thus derived from a particular range of slip-rate values (R), using the and landforms can be found in the Supplemental Qf2 and Qf3. relationship Material (text, section S2, see footnote 1). On the southwest side of the San Bernardino R = O/A, (2) strand, a series of three latest Pleistocene alluvial Qf2 and Qf2a fans (Qf1, Qf2, and Qf3) and one late Holocene allu- to obtain the probability that the slip rate falls vial fan (Qf4) are present. All of the alluvial fans are Qf2 is a broad alluvial fan with a slightly lower within that range. This allows us to calculate a PDF composed of sandy gravel, with abundant cobbles terrace (Qt2-w) cut into its western side. The and cumulative probability distribution for the slip and common small boulders of felsic and interme- topographic contours on the central and eastern rate, from which we can obtain the mean and 95% diate plutonic rock and gneiss, with lesser amounts portions of the alluvial fan (Fig. 3) are consistent confidence intervals for the slip rate. of other rock types (pegmatite, marble, dolomitic with the typical shape of a single, broad alluvial marble, aplite, sandstone, biotite schistose rock, fan. However, topographic contours, profiles (e.g., and epidote). Both clast sizes and lithologies are Fig. 4), and field observations reveal a ~2.5-m-high ■■ DESCRIPTIONS AND AGES OF OFFSET consistent with Badger Canyon being the source of terrace riser separating the lower, western portion DEPOSITS AND LANDFORMS USED TO all four alluvial fans. Sediment transported within of the Qf2 alluvial fan (Qt2-w) from the remainder of CALCULATE SLIP RATES smaller drainages northwest of Badger Canyon is the alluvial fan. We interpret Qt2-w as an erosional finer grained (mostly sand) and is richer in marble, geomorphic surface cut into Qf2 when the active The Badger Canyon study area is located at dolomitic marble, and aplite clasts than deposits channel migrated to the western side of the alluvial and west of where Badger Canyon crosses the San within Badger Canyon or in the four alluvial fans. fan after Qf2 was deposited.

Qvoc2 Qvoc2 Kmg-u Qvols-m Qvols-m Arrowhead Springs strand SAF Qvoc2 Kmg-u Figure 7 Kmg-u Qf1 Qu Kmg-u

WT-11 Qvof1 Qu Qa4 Qyls-g WT-8 WT-10BC01 Qu Qa4 Qf2b Qa4 WT-6 Qf4 Qf1 BC1 BC8 Qls2 Qa5 BC9 Qu WT-7A Qls2 BC2 WT-3A Qa5 Qf1 WT-9A San Bernardino strand SAF Qls2 Qls2 Kmg-u WT-5 BC7 WT-7B Qa4 Qf1 BC3 BC6 BC02 WT-4 WT-9B BC8 Qa4 BC2 Qf4 BC5 BC-8 WT-3B Qt2-w Figure 4 Kmg-u Qls2 Badger Canyon WT-3C Qu Qa4 Qf4-m Tc Mill Creek strand SAF WT-1C Qc2 WT-1A Qvoc2 BC-51 Qf2 WT-2 BC04

Qf3-m Qf2a Qf4-m BC10 Qvoc2 Qa4 BC9 Figure 12 Tc Qf3a Tc BC03 Qf3b Kmg-u Qf4 BC-22 Qf4 Qf4-m WT-1B

Qa5

Figure 3. (A) Simplified geologic map of the Badger Canyon site including trench, soil profile and sample locations. See section (C) of the figure for explanation of map units and symbols. Sample numbers are only shown for samples not shown in Figures 5 or 8. Optically stimulated luminescence dating sample numbers are labeled with white text; all other sample numbers and soil profile labels are in black text. Short black lines crossing trenches mark locations of faults that were observed in trenches but could not be mapped at the surface. Boxes with solid outlines mark locations of enlarged map figures. Contour interval is 10 m. SAF—San Andreas fault. (Continued on following two pages.)

The riser between Qf2 and Qt2-w is important We propose that the alluvial remnant Qf2a on than radiocarbon ages from the same layer. Within because it forms the basis for one of our slip-rate the northeast side of the fault is correlative with Qf2, a detrital charcoal sample from a ~10-cm-thick estimates. Although the riser has been modified the broad Qf2 alluvial fan on the southwest side of sand layer near the base of trench WT-3C has a 14C by a younger, modern channel that flows along the fault. Each of these units is the most prominent age of 28.4 ± 0.3 ka (sample BC-8 in Table 1). This it for most of its length, the riser itself and the alluvial unit on their respective sides of the fault, sample was located near the apex of the Qf2 fan on two surfaces that it separates (the preserved geo- and alternative correlations that we considered led the southwest side of the fault at a depth of ~2.8 m morphic surface at the top of Qf2 and Qt2-w) are to untenable reconstructions of the geologic history. below the surface. Figure S3 in the Supplemen- clearly visible for the first 35–40 m southwest of Age estimates for Qf2 and Qf2a range from 18 to tal Material (section S2.3.2, see footnote 1) has a the fault. 31 ka, with OSL ages being systematically younger photograph of the sample location within trench.

WT-11

WT-8 WT-10

WT-6

WT-7A WT-9A WT-3A WT-5 WT-7B

WT-4 WT-9B Figure 5 WT-3B

WT-3C

WT-1C WT-1A

WT-2

Figure 8

WT-1B

Figure 3 (continued). (B) 0.5-m-resolution light detection and ranging (LiDAR) imagery from the B4 project (Bevis et al., 2005). Boxes with dashed outlines mark the portions of trenches that are displayed in cross section when the entire trench is not displayed. Contour interval is 10 m. (Continued on following page.)

An OSL sample from the same sand layer yields an over a period of time. This supports our correlation could provide ages that are either too old (e.g., age of 20.6 ± 3.0 ka (sample BC2 in Table 2). On the of Qf2 and Qf2a across the fault. An OSL sample incomplete bleaching of luminescence samples; northeast side of the fault, Qf2a, has similar ages. from the same sand lens within Qf2a, has an age delays between the growth of wood and its deposi- Two charcoal samples from a sand lens exposed of 22.2 ± 3.6 ka (BC6 in Table 2 and Fig. 5), slightly tion within a sedimentary layer as detrital charcoal), in trench WT-1A, have radiocarbon ages of 30.7 older than, but consistent with the OSL age from or too young (e.g., incorporation of detrital charcoal ± 0.7 and 30.8 ± 0.3 ka (samples BC-13 and BC-46 Qf2 on the southwest side of the fault (20.6 ± 3.0 ka), into an older sedimentary layer via bioturbation, or in Table 1 and Fig. 5). These ages are slightly older again supporting the correlation of Qf2a with Qf2. mixing of younger grains into an OSL sample from than the radiocarbon age from Qf2 on the southwest Self-consistent results within each dating tech- an older layer via bioturbation). All of the charcoal side of the fault (28.4 ± 0.3 ka) but are consistent nique suggest one of these methods provides a and OSL samples from Qf2 and Qf2a were col- with that age, given that the fan was likely deposited reliable age, but not both. Either dating technique lected from ~10-cm-thick, moderately well sorted

Figure 3 (continued). (C) Explanation of map units and symbols used in (A).

sand layers, in which any burrows intersected by Qf1 also appear to be anomalously young (see dis- 10Be concentrations in the tops of four boulders the trench wall would have been easily visible and cussion of Qf1 below). (4) Other investigators have on Qt2-w (Fig. 3) range from 11 to 43 ka (samples avoided. Of course, we cannot rule out the possibil- reported anomalously young OSL ages on quartz BC5, BC6, BC7, and BC8 in Tables S2 and S3). We ity that the OSL sample tubes may have intersected from southern California (Lawson et al., 2012; Roder have no radiocarbon or OSL ages from the deposits a burrow behind the trench wall. et al., 2012). Although we favor the 14C ages, we that underlie Qt2-w. The soil development associ- We favor the radiocarbon ages for several rea- calculate separate slip rates using the 14C and OSL ated with the Qt2-w surface (described in trench sons. (1) The systematics of radiocarbon dating ages from Qf2 and report both results. WT-7B) is characterized by rubification, develop- are better understood than for OSL dating. (2) If ment of strong subangular structure, and few, thin we accept the two OSL ages from Qf2 and Qf2a, clay films (Table S3). This degree of soil develop- then we must interpret the three aforementioned Qt2-w ment is comparable to nearby surfaces in the San 14C ages, as well as two 14C ages from younger units Timoteo Badlands chronosequence independently (Qc2 and Qf2b, described below) as being thou- The Qt2-w geomorphic surface is a cut ter- dated as latest Pleistocene (Kendrick et al., 2002). sands of years older than the deposits from which race incised into, and therefore younger than, On the northeast side of the fault, Qf2a (cor- they were collected. (3) The two OSL ages from the Qf2 alluvial fan. Surface exposure ages from related with Qf2) has been incised on its northwest

side, resulting in a terrace riser stepping down from Qf2a to a strath terrace incised into bedrock (Kmg-u Kmg-u and Tc) and later buried by a thin veneer of Qa4. We Qf1 Qu WT-7A correlate the riser between Qf2a and the strath ter- Qa4 WT-3A race below it on the northeast side of the fault, with A Qa4 the riser between Qf2 and the cut-terrace below it WT-7B (Qt2w) on the southwest side of the fault. These two Qt2-w risers have similar heights and orientations, and Qt2-w both are incised into Qf2 or its equivalent (Qf2a) (Figs. 3 and 4).

WT-3C Figure 4. Enlarged geologic map and Qf2 topographic profile (AA′) across the Qc2 A’ 2.7-m-high terrace riser between Qf2 meters and Qt2-w; 10× vertical exaggeration Within trench WT-1A (Figs. 3 and 5), on the 0 40 80 160 on the profile; 2 m contour interval on northeast side of the fault, gravel deposits of the map. This terrace riser is correlated with a terrace riser north of the fault Qf2a are overlain by moderately to poorly sorted, between Qf2a and Qa4. See Figure 3 for mostly massive, sandy colluvium with a pinkish Proﬁle AA' explanation of map symbols. tan color, designated Qc2 (Fig. 5). Lenses of gravel A A’ are present within Qc2, indicating an interfingering 512 511 Qf2 relationship with Qf2a. One charcoal sample from 510 Qc2 has a radiocarbon age of 23.2 ± 0.25 ka (BC-11 509 2.7 m in Table 1). The soil developed within Qc2 exhibits 508 507 Qt2-w a 141-cm-thick argillic horizon with 7.5 YR colors, 506 with very few moderately thick and few thin clay (m) Elevation 505 films (Table S3). The degree of soil development 504 503 suggests an estimated age of ca. 30 ka. 0 50 100 150 200 Distance along proﬁle (m)

Qls2

We mapped a landslide deposit (Qls2) that plays where similar granitic rocks with blocky fracture topography of the region north of the north-dipping an important role in understanding the history of overlie Potato Sandstone (Tc). This location is at the shear planes, which we interpret as a landslide deposition of the Qf2 alluvial fan and in constrain- north end of the narrow outcrop of Tc just west of deposit (Fig. 7). A possible head scarp is outlined ing the age of the piercing line used for one of our WT-1A in Figure 3. This contact between fractured in Figure 7, demarcating the landslide source region slip-rate measurements. We describe the landslide granitic rock on the north and Potato Sandstone on as an area with more intense gully development. deposit here and provide additional details and dis- the south extends westward along a south-facing Lithologies within the landslide deposit are felsic cuss alternative interpretations in the Supplemental slope that we interpret as the toe of a landslide plutonic rocks that are similar to Kmg in the pro- Material (text, section S2.3.4, see footnote 1). At (see blue contact in Fig. 3). This south-facing slope posed source region for the landslide. 230 m in trench WT-1A (Fig. 5), a sheared contact was intersected by trench WT-4, and two shallowly The age of the Qls2 landslide is determined by striking 270° and dipping 37°N separates Qc2 on the north-dipping shear zones were exposed, separat- the ages of one detrital charcoal sample from Qc2 south from granitic rock with blocky fracture on the ing fractured granitic rock above from sandstone (23.2 ± 0.3 ka) below the landslide and one detrital north (Fig. 6). We interpret this contact as the basal below (J. McKeown, written communication, 2006). charcoal sample from Qf2b (23.3 ± 0.8 ka) above shear of a landslide deposit, as did J. McKeown We agree with McKeown’s interpretation of the the landslide (samples BC-11 and BC-17 in Table 1 (written communication, 2006). A north-dipping, shallowly north-dipping shear planes exposed in and Fig. 5). The two ages are nearly identical, tightly sheared contact is also visible in a natural exposure WT-4 and WT-1A as the basal slip surface at the toe bracketing the age of the landslide to 23.1 ± 0.4 ka, ~20 m west of WT-1A (at N34.19238, W117.31330), of a landslide. This is supported by the hummocky using the sequence modeling function of OxCal

Southwest WT-1C Northeast 10 Qf2b 5 Qls2-w Qls2-s

meters Qf2b 287,78 287,87 0 60 50 40 30 20 10 0 match meters line Southwest WT-1A Northeast 590 590 BC-15 (C-14) toe of landslide Qf2b 585 1.1 ± 0.1 ka 585 BC-17 (C-14) Fig. 6 Qls2-s Qa4 23.3 ± 0.8 ka

meters 580 Qls2-w 580 Qc2 296,75 575 270,37 575 BC-11 (C-14) 23.2 ka ± 0.25 ka landslide

220 230 240 250 260 270 280 290 300 310 320 330 meters

match line Southwest WT-1A Northeast 580 580 Soil pro le BC04 Qc2 575 BC-6 (OSL) 575 22.2±1.8 ka 570 San Bernardino Qc2 570 strand SAF Qf2a 565 30.7 ±0.7 ka (BC-46; C-14) 565

meters scarp colluvium 30.8 ± 0.25 ka Kmg? ? (BC-13, C-14) 560 scarp colluvium ? ? 560 Qf3b 555 14.9 ± 0.3 ka (BC-42, C-14) 555 80 90 100 110 120 130 140 150 160 170 180 190 200 210 meters Figure 5. Logs of trenches WT-1C and of the central portion of WT-1A, from 20 m southwest of the San Bernardino strand of the San Andreas fault (SAF) to 240 m northeast of that strand. Dashed box in Figure 3B outlines the portion of WT-1A that is shown here. The southernmost portion of WT-1A is shown in Figure 8. The northernmost portion of WT-1A is not shown and is not essential to the slip-rate estimates. Thick black lines mark the locations of fault strands. Blue lines mark the basal shear plane of the Qls2 landslide. See Figure 7 for an enlarged view of the landslide. See Figure 3 for explanation of other symbols. Two facies of Qls2 are distin- guished: Qls2-w is composed of fine-grained granitoid rock composed of white feldspar and quartz with a few percent biotite, and Qls2-s is composed of fine-grained plutonic rock pervasively fractured into pebble-sized angular blocks (with no matrix between blocks) and with most fracture surfaces stained dark brown. Box shows location of Figure 6. Darker-colored top of Qf2b in WT-1A (220–260 m) and WT-1C (0–50 m) marks location of redder-colored deposits, presumed to be derived from nearby exposures of Qf1, which has a strong red soil. Thin, grayish-brown unit on top of Qc2 and Qa4 in WT-1A (220–240 m, 280–330 m, and 350–410 m) and on top of Qf2b in WT-1C (38–46 m) is an organic-rich soil (A-horizon). Numbers below faults and landslide toe indicate orientations of these semi-planar features (strike, dip, using right-hand rule). Note that Qls2 has overridden Qc2 and has in turn been buried by Qf2b. Uncolored material within the fault zone includes fault scarp colluvium in upper half of trench and undifferentiated sheared alluvial units in lower half.

Qf3

Ground surface Qf3 comprises alluvial gravel derived from Bad- SW NE ger Canyon. An inflection in the topographic contour lines suggests a possible distinction between the Qc2 Figure 6. View of part of trench western and eastern portions of Qf3 (labeled Qf3a Qls2 WT-1A at 230 m showing north-dip- and Qf3b, respectively, on Fig. 3), with topographic ping shear zone (blue line) beneath Bench toe of landslide deposit composed of contours on Qf3b having a tighter radius of cur- Bench fractured, white, granitic rock (Qls2). vature than those on Qf3a. No age estimates are Landslide mass (Qls2) has overrid- available directly from Qf3a, but one radiocarbon Qc2 den colluvium (Qc2). Each tier of the sample from the base of colluvium that buries Qf3a trench is ~1.3 m high. See Figure 5 for is similar to available ages from Qf3b (see below), Base of trench location of photograph. so we interpret the two units as part of the same Qls2 alluvial fan. We interpret the tighter radius of cur- Bench on vature of topographic contours on Qf3b compared opposite side Base of trench to Qf3a as either (1) reflecting the shape of the final pulse of deposition on Qf3b, and/or (2) as a result of incision of the eastern edge of Qf3b, resulting in truncation of contours that once had a broader (Bronk-Ramsey, 2009) to calculate the 95.4% confi- last of these interpretations, but the uncertainty in radius of curvature. The topographic contours on dence interval on the age of the boundary between the location of any deposits southwest of the fault the surface of both Qf3a and Qf3b indicate that Qc2 and Qf2b based on the bounding 14C samples. that may be correlative with Qf2b does not affect these deposits are not part of the Qf2 fan (Fig. 3), The dissected character of the landslide is consis- any of the slip-rate estimates that we obtain from and the ages discussed below also confirm this. tent with a late Pleistocene age. the Badger Canyon site. The slope on the eastern side of Qf3b, where Qf3

Qf2b A Qvols-m Kmg-u In both trenches WT-1A and WT-1C, the granitic Qvof Qvoc landslide deposits of Qls2 are overlain by alluvial gravel derived from Badger Canyon (Figs. 3 and 5). headscarp We refer to the alluvial deposits on top of the Qls2 landslide as Qf2b. Detrital charcoal sample BC-17, with an age of 23.3 ± 0.8 ka (Table 1), is from this WT-6 unit (Fig. 5). Qf2b Presumably the Qf2b alluvial deposits extended Qf1 Qls2 WT-5 Qls2 Qls2 southward across the fault. They may have formed Qa4 a thin (unmapped) veneer of sediment on top of WT-4 the eastern portion of the Qf2 alluvial fan, but any WT-3B Qa4 Qls2 such veneer would have to be quite thin because Kmg-u WT-1C Qu the topographic contours on the eastern side of the WT-1A Qc2 Qvoc Qf2 alluvial fan still reflect the shape of the orig- Qls2 buried Qls2 buried inal, broad alluvial fan beneath any hypothetical beneath Qf2b beneath Qf2b veneer. Alternatively, the downstream continuation Figure 7. (A) Light detection and ranging (Lidar) image showing landslide Qls2 and its head scarp, and (B) geo- of these deposits may form the western one-third logic map of same area showing relations among the Qls2 landslide deposit and Qf2b, whose ages are used to of the Qf3 alluvial fan (the part mapped as Qf3a), define the slip rate for the past ~23 k.y. See Figure 5 for cross-section view of these relationships. See Figure 3C or they may be buried beneath Qf3a. We favor the for explanation of map patterns and symbols.

abuts Qa4, may be the depositional edge of Qf3, Age estimates from multiple techniques are developed on Qf3b within WT-1A exhibits a cambic although it seems rather steep for this. Alternatively, available for Qf3b. Two radiocarbon ages on detri- B horizon with 7.5 YR colors (Table S3), with a soil this slope may be a degraded terrace riser incised tal charcoal range from 14.6 to 15.7 ka (Table 1). One profile index that suggests an age of ca. 4–5 ka, into the eastern edge of Qf3. of the samples (BC-42, from trench WT-1A; Fig. 8) significantly younger than all of the quantitative Trench WT-1A reveals further complexity in that comes from the portion of Qf3b that lies within the dating techniques. Qf3b buries a remnant of much older alluvium channel between the buried ridge of older alluvium with strong soil development (“Qvof2” in Fig. 8). and the fault, and the other comes from a more dis- Between this shutter ridge and the fault, deposits of tal region of Qf3b (sample BC-22 from WT-1B, Fig. 3 ■■ RECONSTRUCTION OF THE HISTORY OF Qf3b fill a channel, the western wall of which strikes and Fig. S4). Three OSL dates from Qf3b, from both OFFSET FANS AT BADGER CANYON S5E between the two walls of Trench WT-1A. This sides of the buried ridge of older alluvium, range suggests that at some point in time, Badger Creek from 10.2 to 15.9 ka (samples BC3, BC4, and BC5 Figure 9 shows our interpretative reconstruc- incised a channel around the eastern edge of the in Table 2 and Fig. 8). We have no direct radiocar- tion of the alluvial fans offset from Badger Canyon. shutter ridge. Whether or not flow from Badger bon ages from Qf3a, but sample BC-24, from the Uncertainties in the amount of slip needed to restore Creek had previously been diverted northwestward base of the colluvium that buries Qf3a (WT-2; Fig. 8), different parts of the alluvial fans are discussed along the fault by the shutter ridge is unknown. has an age of 14.5–15.1 ka (Table 1). This is similar in the upcoming sections in which slip-rate esti- The presence of Qf3b overlying “Qvof2” in Trench to the ages from Qf3b, suggesting that Qf3a was mates are presented. The deposits of fan Qf1 have WT–1A (Fig. 8) indicates that in the late stages of beginning to be buried by colluvium while the final been offset ~700 m from Badger Canyon (Fig. 9A). aggradation, Qf3b completely overtopped the shut- stage of deposition of Qf3b was continuing. Two Unfortunately, we have no reliable, quantitative ter ridge. These uncertainties in the history of the 10Be surface exposure ages from boulder tops on age estimate for these deposits. Reconstruction of Qf3 alluvial fan are included in the uncertainties of Qf3a range from 15.2 to 21.3 ka (samples BC9 and ~350 m of right-lateral slip restores the Qf2 allu- our slip-rate estimate that is based on Qf3. BC10-top in Tables S2 and S3 and Fig. 3). The soil vial fan in its entirety (including the Qf2 deposits

SW NE 0

BC-24 (C-14) Trench WT-2 Kmg? 14.8 ± 0.3 ka A-horizon 10 colluvium Qc2? OSL-BC-6 Qf2? water Qf3a 22.2 ±1.8 k a not logged San Bernardino ? strand SAF BC-46 (C14) 20 Qf2a 30.7 ± 0.7 ka BC-13 (C14)

meters Trench WT-1A (south end) colluvium Kmg? 30.8 ± 0.25 ka Qf3b Qf2? 30 BC-42 (C-14) colluvium Qvof2 BC-5 (OSL) 14.9 ± 0.3 ka Qf3b 13.3 ±1.1 ka channel wall BC-4 (OSL) BC-3 (OSL) strikes N5W 40 13.9±1.0 ka 11.8±0.8 ka

0 10 20 30 40 50 60 70 80 90 100 110 120 130 meters

Figure 8. Logs of trenches that expose Qf3: WT-2 and the southern end of trench WT-1A. See Figure 5 for central section of WT-1A. Thick black lines mark the locations of faults. Depositional contacts are marked by changes in fill colors with no contact lines shown. See Figure 3 for explanation of other symbols. These two trenches are aligned on the fault zone, with black dashed lines connecting the boundaries of the fault zone between these two trenches. Uncolored material within the fault zone in trench WT-1A includes fault scarp colluvium in upper half of trench and undifferentiated sheared alluvial units in the lower half. SAF—San Andreas fault.

Kmg-u Kmg-u A Kmg-u Qf1 Kmg-u Qf1 Kmg-u 350 m Qf2 Qf1 Tc Tc 700 m restored Qf1 Kmg-u restored Qf1 Qf2Qow3.2a Qf1 Qf2 Qf1 Qf1 deposited Qvoc2 deposited Qf1 Qf2 Qvoc2 Tc Tc Qf1 28-30 ka Qf2 Qvof2 Units shown here were not Qf2 Qf1 deposited yet Units shown here Figure 9. Reconstructions showing were not deposited yet inferred history of fault offset and alluvial fan deposition and incision at the Badger Canyon site. See Figure 3 for ex- Kmg-u Kmg-u planation of symbols. Semi-transparent colors (with light detection and ranging hillshade map showing through and D topographic contour lines plotted) are Qt2-w mapped units that exist today and ex- Qf1 Kmg-u Kmg-u Qf1 Qf1 Qls2 Qf1 isted during the time frame shown in 290 m Tc Kmg-u 290 m Tc the figure (except in the southeastern Qf1 Kmg-u restored Qt2-w Qc2 Qf1 Qc2 Kmg-u corner of the map where a sparse dotted Qf1 restored Qf1 Qc2 Qf1 Qt2-w Qf1 Qc2 pattern has been added to mark units Qt2-w Qvoc2 that exist today but had not yet been de- Qls2 Qvoc2 and Qc2 Tc Qt2-w posited at the time frame shown in the Qt2-w landslide Tc deposited figure). Opaque colors with no contour emplaced lines are used for units that are inferred Qf2 Qf2 Qvof2 Qf2 Qf2 Qvof2 Qt2-w Qt2-w to have existed at the time shown in 23 ka the reconstruction, in order to construct Units shown here 23 ka Units shown here a reasonable geologic history, but may were not deposited yet were not deposited yet have been eroded or buried after that. (Continued on following page.)

Kmg-u Kmg-u

Qf3-m E F Qf2b Qf1 Qf1 Qls2 Qf1 Qls2 Qf2b 290 m Kmg-u 245 m Tc Qf1 restored Qf1 Qf2b restored Kmg-u Qf1 Qf3a Kmg-u Qf2b Qf1 Qt2-w Qc2 Qc2 Qf1 Qf3a deposited Qvoc2 Qf3-m Qt2-w Qvoc2 deposited Qt2-w Tc Tc

23 ka Qf2 Sometime Qf3-m Qf2 Qvof2 Qf2 Qf2 Qvof2 Qt2-w between Not Not Qf2b? depos- 23 -15 ka Qf3a deposited yet ited yet

Kmg-u Kmg-u

G Qf1 Qa4 Qf1 Qf3-m H Qf1 Qf1 Qls2 Qf2b Qls2 Qf2b 160 m Qf1 Qa4 Qf1 Qa4 restored Qf3b Kmg-u Kmg-u Qa4 Qt2-w Qc2 Qa4 Qc2 Figure 9 (continued). Qf3b Qf3-m Qt2-w Qvoc2 Qf2 Qvoc2 deposit- Qf2 Qf2 Tc Qf3-m Tc Qf2 Qvof2 ed Qf3-m Qf4 13-15 ka Qvof2? Qf3b Qf3b Qf3b 85 m restored; Riser between Qf3b and Qf4 cut?

beneath the Qt2-w cut-terrace) to the broad mouth fault. The apparently stronger soil development on Restoring 85 m of slip places the terrace riser of Badger Canyon (Fig. 9B). Sometime after deposi- Qc2 than on Qt2-w may possibly be due to detri- between Qf3b and Qf4 (south of the fault) adjacent tion of Qf2, incision occurred on the west side of the tal clay eroded from the Qf1 remnant north of the to the riser between Qf2a,b and Qa4 (north of the alluvial fan, forming the terrace riser between Qf2 fault and incorporated into the parent material of fault) (Fig. 9H). This may have been the geometry and Qt2-w, southwest of the fault, and between Qf2a Qc2, or might reflect a localized difference in the at the time that this riser was last refreshed. and bedrock (Tc and Kmg-w), northeast of the fault. mineralogy of the parent materials at this location. Reconstruction of 290 m of right-lateral slip aligns We infer that stream flow on Qt2-w halted these two terrace risers across the fault (Fig. 9C). when a landslide (Qls2) blocked the western side ■■ OFFSET MEASUREMENTS AND We interpret the sandy colluvium (Qc2) that bur- of Badger Canyon (Fig. 9D). Transport and depo- SLIP‑RATE CALCULATIONS AT ies Qf2a in WT-1A to be a colluvial wedge that built sition within Badger Canyon immediately shifted BADGER CANYON out from the eastern side of Badger Canyon while back to the eastern side of the canyon, and a thin the locus of fluvial erosion, transport and depo- veneer of alluvium (Qf2b) covered the eastern edge Offset geologic features at the Badger Canyon sition was concentrated on Qt2-w (Fig. 9C). This of the landslide deposit (Fig. 5). The Qf2b alluvial site with reliable age estimates include: (1) the apex interpretation implies that Qt2-w and Qc2 should be gravel presumably extended southward across the of alluvial fan Qf2; (2) the terrace riser between Qf2 about the same age. However, the soil within Qc2 is fault. Figure 9E shows our preferred interpretation, and Qt2-w; and (3) the Qf3 alluvial fan. better developed than that observed within Qt2-w, in which the deposits southwest of the fault that and is closer to, though less developed than, that correlate with Qf2b lie beneath Qf3a. on Qf1 (see Table S3 and Supplemental Material, Qf2b was abandoned when the channel of Bad- Slip Rate Estimated from Qf2 text, section S2.3.1 [footnote 1]). We considered ger Creek incised on the east side of Badger Canyon an alternate interpretation in which Qc2 and the and Qf3a was deposited (Fig. 9F). Deposition of Qf3a We create probability density functions (PDFs) underlying Qf2a alluvial deposits northeast of the and/or Qf3b may have been diverted northwestward to represent the range of possible values and find fault would correlate with the Qf1 alluvial fan south- around a shutter ridge of very old alluvium (“Qvof2” the most likely values for both the offset and the west of the fault, but found this to be untenable in WT-1A, Fig. 8) at some point in time (Fig. 9G). How- age of the Qf2 alluvial fan, and then we use these to because (1) Qf1 is more strongly dissected and ever, the shutter ridge was eventually breached by construct a PDF for the slip rate, using the method more steeply sloping than Qf2a and the overlying a channel that flowed around its east edge (Fig. 9G) described in McGill et al. (2009). To find the loca- Qc2, and (2) this would leave no deposits northeast and finally was overtopped by the last stage of Qf3b tion of the apex of the Qf2 fan, we used a Gold et of the fault that could correlate with Qf2, which is deposition (Fig. 8). On the northeast side of the fault, al. (2015) script that uses the MATLAB function fit- the most prominent alluvial fan southwest of the no remnants of Qf3 are preserved. They were likely circle to fit circles to the topographic contour lines fault, and no deposits southwest of the fault that eroded when Badger Creek incised on the east side, on Qf2 (excluding Qt2-w). We used the nonlinear could correlate with the Qf1 remnant north of the followed by deposition of Qa4 and Qf4. fit option (minimizes geometric error), and we

interpret the centers of the fit circles as represent- location of the source channel, as did Gold et al. Canyon, along the contact between Qf2b with Qf1 ing locations within the source channel of Qf2. In (2015) in their study. Reconstructing 370–430 m of and with the landslide deposit (Qls2). We consider this analysis, we used contours from 476–480 m right-lateral slip (Figs. 10B and 10C) restores the set it equally likely that the offset of Qf2 lies between and 482–502 m above sea level (asl), excluding of inferred fan apex locations to the west and east 370 and 430 m; therefore, our probability density contours 481 and 503–506 m asl due to disruption edges, respectively, of Badger Canyon. The 430 m function for the offset, has a flat top between 370 of the ground surface by dirt roads and excluding restoration (Fig. 10C) places the eastern edge of and 430 m (Fig. 11A). contours 507–518 m asl due to potential vertical Qf2 ~85 m east of the eastern edge of Badger Can- An absolute maximum offset of 460 m for Qf2 warping near the fault. This analysis provides a yon, which is not unreasonable given that alluvial comes from restoring the dated samples from Qf2 range of estimates for the location of the apex of fan sediment can spread out from the mouth of in WT-3C to the extreme east edge of Badger Can- the alluvial fan, based on the center of each of the a canyon. The 370 m reconstruction restores the yon, which is the easternmost location at which circles that were fit to the contour lines (see purple east edge of Qf2 to the east edge of Badger Can- these samples could have been deposited (Fig. 10D). dots north of the fault on Fig. 10A). We interpret the yon in addition to restoring the set of inferred fan This reconstruction is unlikely because it places the linear collection of circle centers to represent the apices to near the west edge of present-day Badger eastern edge of Qf2 120 m east of the eastern edge

Figure 10. (A) Geologic map showing the Badger Canyon site with blue and gray Qf2a circle segments fit to the topographic Qf2a contours on Qf2 (excluding Qt2-w). Pur- Qf2 ple dots north of fault mark the centers of the circles, which we interpret as es- B. 370 m Qf2 timates of the location of the alluvial restored fan apex, relative to the southwest side A. Present of the fault. (B, C) Restorations of the con guration Qf2 alluvial fan, yielding slip estimates of 370–430 m, which bound the range of equally preferred offsets. Purple dots mark the restored locations of the fan apex estimates described in (A). (D) Res- toration of Qf2 460 m, which restores the dated samples from Qf2 in WT-3C to the eastern edge of Badger Canyon. We consider this the maximum possible offset for this fan. Note that in this restoration, more than half of the restored alluvial fan apex estimates (purple dots) Qf2a Qf2a fall outside of Badger Canyon, suggesting that this restoration is unlikely.

Qf2 Qf2 C. 430 m D. 460 m restored restored

0.03 0.01 0.003 Qf2 apex 0.025 0.009 Qf2 apex Qf2 apex 0.008 A 0.0025 0.007 0.002 0.02 0.006 0.005 0.0015 0.015 0.004 0.001 0.003 0.01 0.002 0.0005 0.001 0.005 0 0 Probability density

28000 28500 29000 mm/yr 13.5 250 300 350 400 450 500 0 5 10 15 20 25 30

0.03 0.07 Riser between 0.003 Riser between 0.025 D 0.06 F Qf2 and Qt2-w 0.0025 Riser between E Qf2 and Qt2-w 0.02 0.05 0.002 Qf2 and Qt2-w 0.015 0.0015 0.04 0.01

0.001 0.03 mm/yr 11.9 0.005 0.0005 0.02 0 0

Probability density 0.01 240 250 260 270 280 290 300 22000 23000 24000 0 5 10 15 20 25 30

0.025 0.012 0.0004 Qf3 I 0.01 Qf3 G 0.00035 Qf3 0.02 0.0003 H 0.008 0.00025 0.015 0.006 0.0002 0.01 0.004 0.00015 0.0001 0.002 0.00005 0.005

Probability density 0 mm/yr 0 11.8 80 120 160 200 240 11000 12000 13000 14000 15000 16000 0 5 10 15 20 25 30 0.012 0.004 0.00009 Matthews Matthews 0.01 0.0035 0.00008 Matthews Ranch 0.003 Ranch 0.00007 0.00006 Ranch 0.008 0.0025 landslide landslide 0.00005 landslide 0.002 0.00004 0.006 0.0015 0.00003 0.004 0.001 0.00002 0.0005 0.00001 0.002

Probability density 0

0 mm/yr 14.5 0 400 500 600 700 800 900 1000 20000 40000 60000 80000 5 10 15 20 25 30 Right-lateral offset (m) Age (years BP) Slip rate (mm/yr)

Figure 11. Probability density functions (PDFs) for the offset (A, D, G, and J), age (B, E, H, and K) and slip rate (C, F, I, and L) for the three offset features at Badger Can- yon: Qf2 apex (A–C), riser between Qf2 and Qt2-w (D–F) and Qf3 (G–I), and for the offset landslide at Matthews Ranch in Pitman Canyon (J–L). Vertical, dashed lines in C, F, and I mark the bounds of the 95% confidence interval for each slip-rate PDF.

of Badger Canyon and places most of the inferred both sides of the fault, as well as the presence of on the northeast side of the fault intersects the fault fan apex locations within the bedrock east of Bad- deposits of similar age on the east side of the riser at the white square (Fig. 12). This projection follows ger Canyon. Nonetheless, it represents an upper on both sides of the fault, support this correlation. a bend in the terrace riser that we suspect is the bound on the offset, and our trapezoidal PDF for the If the correlation of the riser across the fault is result of relatively recent erosion, which should offset therefore tapers to zero at 460 m (Fig. 11A). correct, the following constraints can be placed on therefore be ignored. However, if it existed at the Offset of the Qf2 alluvial fan apex can be no less the amount of offset of the riser. The offset of the time the riser was abandoned, then the offset of the than 290 m, because that amount of slip is required riser is most likely 280–290 m, based on the recon- riser could be as large as 300 m. to restore the (younger) terrace riser between Qf2 struction shown in Figure 9E (290 m) and the 280 m Apart from the erosion just mentioned, it and Qt2-w to its correlative riser northeast of the distance measured between the red star south of the is unlikely that the riser has been significantly fault (see next section). Therefore, our PDF for the fault and the white star north of the fault in Figure 12, modified after it was abandoned, because its offset of Qf2 tapers to 0 at 290 m (Fig. 11A). which mark our preferred projections of the two ter- abandonment was caused by emplacement of the The age of the sand layer 2.8 m beneath the race risers to the fault. An alternate projection of the landslide deposit, which forced active flow within apex of alluvial fan Qf2 is either 28.4 ± 0.3 ka (if the terrace riser southwest of the fault intersects the Badger Creek to shift to the east side of the canyon. radiocarbon age of sample BC-8 is representative of fault at the red circle and allows the offset to be as Flow in Badger Creek never returned to the west the true age; Table 1) or 20.6 ± 3.0 ka (2σ) (if the OSL small as 240 m. This projection would require that side of the alluvial fan. On the northeast side of age, OSL-BC-2, is representative of the true age; the fault slipped faster than the stream could keep the fault, the bedrock (Tc and Kmg) on the west Table 2). Combining the PDF for the radiocarbon refreshing the riser straight across the fault, so that side of the riser has been buried by a thin (<1 m) age (Fig. 11B) with the PDF for offset of the apex of at the time the landslide caused flow on Qt2-w to veneer of sandy alluvium (Qa4), which was derived the alluvial fan (Fig. 11A) yields our preferred slip cease, the riser had a fault-parallel segment, along from small, local drainages. Flow from these small rate of 13.5 mm/yr with a 95% confidence interval of the dashed green projection line southwest of the drainages would not have had the erosive power 11.0–15.7 mm/yr (Fig. 11C). If, instead, we use a PDF fault in Figure 12. An alternate projection of the riser to have significantly modified the riser. for the age of Qf2 using the OSL ages (a Gaussian PDF with a mean of 20,600 years B.P. and a standard deviation of 1500 years), this yields a slip rate of 18.6 mm/yr with a 95% confidence interval of 14.7–24 mm/yr. In a previous section (description Qvof1 Kmg-u Kmg-u of Qf2 and Qf2a), we have explained why we have more confidence in the radiocarbon dates from Qf2 and Qf2a than in the OSL ages from that unit. The Qf2 slip rate using the radiocarbon dates is thus Qf2b our preferred rate. Qf1 Qls2 Qf1 Qt2-w Qa4 Slip Rate Estimated from the Riser between Qf2 and Qt2-w Qa4

Our second slip-rate estimate is based on the Qf2 Qc2 offset of the northwest-facing terrace riser between Qvoc2 Qf2 and an erosional terrace, Qt2-w, cut into it (southwest of the fault) from a northwest-facing Preferred o set 280 m Qf2a Qf3a terrace riser on the northwestern edge of Qf2a and Maximum o set 300 m a strath terrace cut into bedrock northeast of the Minimum o set 240 m Qf3b Qf4 Tc fault (Fig. 9E). Although the different substrates below the erosional terraces on the two sides of Figure 12. Geologic map showing preferred (red and white stars) and limiting (red circle and white square) locations of piercing points for the terrace riser between Qf2 and Qt2-w on the southwest side of the fault the fault make this correlation less certain than the (red star and red circle) and the terrace riser that forms the western edge of Qf2a on the northeast side of correlations used for our other slip-rate estimates, the fault (white star and white square). The preferred projection of the risers to the fault yields an offset the similar orientation and height of the riser on of 280 m. Red dashed lines show alternate projections that allow the offset to be between 240 and 300 m.

The original riser on the northeast side of the the initiation of deposition of Qf3, which occurred Qf3 (Fig. 13). Because samples BC-42 and OSL-BC-5 fault might initially have been located farther to sometime prior to ca. 15 ka. If abandonment of are from a channel that trends S5E, it is unlikely northwest and have been laterally eroded by flow Qt2-w occurred as recently as 15 ka, the slip rate that they were deposited by flow of Badger Creek along the Qt2-w terrace prior to emplacement of could be as high as 19 mm/yr, but no higher. spreading out to the southeast or northwest of the the landslide, but this would not affect our slip-rate mouth of Badger Canyon, as would be required estimate because the age control we are using for if their offset was significantly >200 m or <120 m. this estimate is the age of the landslide (bracketed Slip Rate Estimated from Qf3 Because of the extreme unlikelihood of the offset by dates from Qc2 and Qf2b). The emplacement of being outside this range, we apply an exponentially the landslide ended flow of Badger Creek along this To define the slip rate since the time of deposi- decaying tail to both sides of the plateau in the riser. Therefore, we interpret the age of the land- tion of Qf3, we use the dated radiocarbon and OSL PDF and constrain these two tails such that each slide to be identical with the age of abandonment of samples from Qf3b that are located closest to the contains 5% of the area under the PDF, and 90% the riser. Matching any earlier location of the riser fault (BC42 and OSL-BC-5; Fig. 8 and Tables 1 and of the area lies between 120 and 200 m (Fig. 11A). northeast of the fault with the age of termination 2) because the offset of this proximal part of the We also calculate a PDF for the age of sam- of the flow of Badger Creek along this riser is not alluvial fan can be constrained more tightly than ples BC-42 and OSL-BC-5 (Fig. 11B). This PDF has a appropriate. the offset of more distal portions of the alluvial fan. plateau between the mean ages of these two sam- Our PDF for the offset of this riser is trapezoidal, Samples BC-42 and OSL-BC-5 are from within the ples (13.3 ka for OSL-BC-5 and 14.9 ka for BC-42) with a flat top between 280 and 290 m, tapering to top 1.2 m of Qf3b deposits (Fig. 8), indicating that and tapers to zero at 11.1 and 15.1 ka (the low and zero at 240 and 300 m (Fig. 11D). The channel north- they were deposited during the late stages of Qf3. high ends of the 95% confidence intervals for the west of this riser ceased being an active conduit They also are from the portion of Qf3b that fills a ages of samples OSL-BC-5 and BC-42, respec- for flow from Badger Canyon when the Qls2 land- channel that strikes S5E, which we interpret to have tively). The resulting PDF for slip rate has a mean slide was deposited 23.1 ± 0.4 ka. The riser might breached the buried shutter ridge (Figs. 8 and 9G). of 11.8 mm/yr, yet the broad plateau extending have been abandoned and started accumulating The source channel for Qf3a and Qf3b is con- from ~10 to 13.5 mm/yr indicates that the slip rate offset prior to the landslide, while Badger Creek strained to lie between Qf2a,b and the eastern is equally likely to be anywhere within that range was still flowing on Qt2-w and its correlative strath margin of Badger Canyon. We consider it equally (Fig. 11C). The 95% confidence interval for the slip terrace northwest of the fault. Although we argue likely that the offset since the time of deposition rate is 8.3–16.0 mm/yr. this is unlikely, this possibility is included within of samples BC-42 and OSL-BC-5 is between ~120 our uncertainty bounds for the offset, which allow and 200 m. An offset of 120 m (Fig. 13A) restores the offset to be as small as 240 m. Using the PDFs samples BC-42 and OSL-BC-5 to the west edge of ■■ SLIP RATE ESTIMATED FROM THE shown for the offset and age of abandonment of the source channel location. An offset of 200 m MATTHEWS RANCH LANDSLIDE IN this riser (Figs. 11D and 11E), we estimate a slip rate (Fig. 13B) restores samples BC-42 and OSL-BC-5 to PITMAN CANYON +0.9 of 11.9 /−1.2 mm/yr (Fig. 11F). the east edge of Badger Canyon. Restoration of 120– As a result of the well-defined location of the ter- 200 m slip also places the other samples from Qf3b Geology of the Landslide race riser on both sides of the fault and the tightly (BC-22, OSL-BC-3, and OSL-BC-4) in a position where bracketed age of the landslide (and thereby the age they could have been deposited, and sample BC-24 The Matthews Ranch landslide covers ~1 km2 of abandonment of the riser), this rate has the tight- where it could be deposited within the base of fault across the San Andreas fault and is located in and est constraint on the slip rate (10.8–12.8 mm/yr) of scarp colluvium that buried Qf3a (Figs. 8 and 13). around Pitman Canyon, 2 km northwest of the town any of the rates reported in this paper. However, it To calculate the slip rate and its uncertainty, we of Devore (Figs. 2, 14, and 15). The landslide con- is possible that these constraints underestimate construct a PDF for the offset of samples BC-42 sists of two main facies—a coarse, angular blocky the full uncertainty. There is an unquantifiable (but and OSL-BC-5 from Qf3b as follows (Fig. 11A). The mass that apparently failed catastrophically (Qlsp) likely small) possibility that the two risers do not plateau of this PDF extends from 120 to 200 m, and a less coarse, more colluvial-appearing deposit correlate across the fault or that the two detrital reflecting our judgment that the offset is equally (Qlsg) with blocks from 3 to 10 m; the latter deposit charcoal samples used to constrain the age of the likely within this range, and is very unlikely outside appears to be material that subsequently filled the riser both significantly overestimate the ages of the this range. The values of 120 and 200 m were cho- space between the slide and its breakaway scarp deposits that pre- and post-date the riser. None- sen as the limits of this plateau because any offset and possibly extended across the fault (Fig. 14). The theless, even if both detrital charcoal samples are amount within this range places samples BC-42 southernmost possible remnant, south of Pitman significantly older than the deposits, the landslide and OSL-BC-5 immediately downstream from the Canyon (“Qls-o?” in Fig. 14), appears to be made clearly terminated deposition on Qt2-w prior to location of the 50–60-m-wide source channel for of slightly more weathered material, so is likely to

A Kmg-u Kmg-u Qvof1 Qvof1 Figure 13. Restorations of Qf3 for slip estimates of 120 and 200 m, which bound the range of equally preferred offsets. (A) An offset of 120 m Qf2b Qf2b Qls2 restores samples BC-42 and OSL-BC-5 to the Qls2 Qa4 Qa4 west edge of the source channel location. (B) An Qa4 Qa4 Kmg-u offset of 200 m restores samples BC-42 and Kmg-u OSL-BC-5 to the east edge of Badger Canyon. Qt2-w Qc2 Restoration of 120–200 m slip also places the Qc2 Qf2a Qvoc2 other samples from Qf3b (BC-22, OSL-BC-3, and Qf2a Qvoc2 OSL-BC-4) in a position where they could have Tc been deposited, and places sample BC-24 where it could be deposited within the base of fault Qf2 BC-24 BC-42 Tc 200 m OSL-BC-5 Qf2 BC-24 scarp colluvium that buried Qf3a. See Figure 3 BC-42 restored for explanation of symbols. 120 m OSL-BC-5 restored Qf3a Qf3a Qf4 Qf4 Qf3b Qf3b

be an isolated remnant of an older slide (Qls‑o). laterally. Within the study area, two faults cross Qoa-d, which is older than the landslide. Although Geologic units for the Matthews Ranch site are from the Matthews Ranch landslide ~0.3 and 1.0 km a small outcrop of what may be Qlsg is present on Weldon (1986). northeast of the San Bernardino strand (Fig. 14). top of Qoa-d southwest of the fault, the continually The volume of Qlsg suggests that the breakaway The southern one of these two faults has produced preserved surface of Qoa-d indicates that primary could have expanded following the main landslide minor normal slip (~10 m) within the landslide landslide deposits (Qlsp) Age (ka) here, for if they event, especially on the north and east side. The deposits, but neither of them contributes any lat- had been present and later eroded, the Qoa-d sur- landslide deposit (Qlsp) is extensively eroded, eral offset to the landslide. face would not be preserved. producing some uncertainty in the original extent Figure 15 shows a simplified map (A) and sev- The likely extent south of the fault fits within of the deposit. However, this erosion has locally eral possible reconstructions of the offset landslide the head scarp on the north side of the fault with exposed the base of the landslide, which supports (B–D). The likely extent of the slide (Qlsp) on the 650 m of offset reconstructed along the San Ber- the interpretation that the exposed remnants were south side of the fault (green line on Fig 15), is nardino strand in Figure 15. We estimate that the once a single landslide mass. Southwest of the fault, based on (1) the mapped extent on the east and maximum possible offset could be ~950 m, which the landslide is underlain by Pelona Schist, which is (2) projection of the preserved edge (next to “S” in aligns the landslide masses across the fault except typical basement rock in this region, and by fluvial “Samples” on Fig. 15) to the fault on the west. The for the easternmost finger of the landslide that sediments of Cajon Creek. The primary landslide maximum extent of the landslide on the south side could have flowed around the eastern edge of the (Qlsp) and Qlsg include blocks of foliated granitic of fault (pink line in Fig. 15) assumes: (1) landslide head scarp. This also assumes that the west edge rock with pendants of marble, demonstrating that material that appears to be older on the east side of of the slide did not extend to the head scarp on the landslide source is north of the fault. Pitman Canyon (pink in Fig. 15; “Qls-o?” in Fig. 14) the north side, or more of the landslide is eroded is part of the slide; (2) the landslide may have once from the west edge on south side than we infer. We extended to the knob on west side (but was eroded estimate that the minimum possible offset could Offset Estimate by the stream occupying Perdue Canyon).The east be ~425 m, which aligns the westernmost outcrop edge of the landslide can be no farther east than of the landslide (Qlsp; green on Fig. 15) south of The San Bernardino strand of the San Andreas this maximum extent, because it is limited by a the fault with the top of the head scarp north of fault is the only fault that has offset the landslide continuous gully exposure revealing the surface of the fault.

Figure 14. Geologic map of the Pitman Canyon area (modified from Weldon, 1986) including the offset Matthews Ranch landslide (Qlsp) and associated subsequent debris (Qlsg). Black square shows location of Figure 15. Map shows the distribution of faults and Quaternary deposits immediately southeast of where Cajon Creek crosses the San Andreas fault. See Weldon and Sieh (1985) for more detailed descriptions and ages for the Quaternary units. Only the San Bernardino strand (labeled “SB strand” in the margins) has significant lateral slip since the late Pleistocene.

Qlsg

Pitman Canyon Perdue Perdue Qlsp Canyon Canyon

Qlsp

Qls-o

Qlsp Qls-o Qlsg

Perdue Canyon Perdue Canyon

Figure 15. Plausible reconstructions of the Matthews Ranch landslide in Pitman Canyon. Yellow and purple lines mark the locations of the top and base of the head scarp, respectively. Green line connects the downhill limit of deposits that are definitely part of the offset landslide (“Qls” in Fig. 14) and marks the narrowest limit of offset landslide deposits south of the fault. Pink line marks the broadest limit of offset landslide deposits south of the fault, assuming that the deposits shown in pink above (and labeled “Qls(-o?)” in Fig. 14) are correlative with “Qls.” Location of the boulders from which the 10Be dating samples were collected is marked with an “X” and the word “samples.”

Age of the Landslide Matthews Ranch 0.5 We sampled a geomorphically pristine remnant MR1 of the Matthews Ranch landslide on its western 0.45 MR2 edge south of the fault (“X Samples” in Fig. 15). The 0.4 MR3 10 Be field and lab measurements and ages from six MR4 blocks on the surface of the landslide deposit are 0.35 Product: MR2-MR6 MR5 shown in Tables S4 and S5 and Figure 16. Three MR1 0.3 of the six blocks (MR2, MR4, and MR6) have ages MR6 Sum: MR2-MR6 that cluster ca. 44–47 ka. If we assume that the ages 0.25 of these three samples represent the age of the Product: MR2-MR6 landslide and that the other samples are outliers 0.2 MR2 MR6 whose ages are affected by inheritance (MR3) or MR4

Probability density Probability 0.15 erosion or exhumation (MR1, MR5), then we may MR5 combine the PDFs for MR2, MR4, and MR6 by mul- 0.1 MR3 tiplying the values for each age increment and then renormalizing the combined PDF (i.e., the “product” 0.05 Sum: MR2-MR6 method of Gold et al., 2017, and DuRoss et al., 2011). 0 This results in an age estimate of 45.3 ka with a 0 10 20 30 40 50 60 70 80 95% confidence interval of 42.6–47.9 ka. Including samples MR3 and MR5 in the combination of PDFs Age (ka) using the “product” method, results in a very sim- Figure 16. 10Be ages on boulders on the Matthews Ranch landslide in Pitman Canyon. For our slip-rate estimate, we ilar mean age (45.5 ka) with only a small increase used samples MR2–MR6 with the probability density functions (PDFs) combined using the sum method (red curve). in the 95% confidence interval (41.9–47.9 ka), due to the minimal overlap of the PDFs for MR3 and MR5 with those of MR2, MR4 and MR6 (Fig. 16). of 8.3–24.4 mm/yr (Fig. 11L), and a 68% confidence On the east side of Badger Canyon, the Mill On the other hand, if we treat only MR1 as an interval of 11.0–18.8 mm/yr. Creek strand separates Cretaceous biotite mon- outlier and consider that any of the other five sam- zogranite (Kmg) from Miocene(?) conglomeratic ples is equally likely to represent the age of the sandstone (Tc) (Miller et al., 2001) informally known landslide, then it is more appropriate to combine ■■ DISCUSSION as the Potato Sandstone. We mapped subtle fault the five PDFs by summing the values for each age scarps within very old alluvium (Qvoc2) along the increment and then renormalizing (i.e., the “sum” Possible Slip-Rate Contributions from Other Mill Creek strand on the east side of Badger Can- method of Gold et al., 2017, and DuRoss et al., 2011). Strands of the San Andreas Fault yon but found no clear lateral offsets of drainages This more conservative method results in the PDF incised into Qvoc2 nor scarps within younger units shown in Figure 11K, with a mean of 47.2 ka and a Our slip-rate estimates for the Badger Canyon within the map area (Fig. 3). On the west side of 95% confidence interval of 30.1–71.1 ka, which is site apply to the San Bernardino strand (Fig. 3). Badger Canyon, the contact between crystalline rock used in our slip-rate estimate. The Mill Creek strand traverses the Badger Canyon and Potato Sandstone appears to be the basal plane study area and merges with the San Bernardino of a bedrock landslide (contact visible where a very strand within the northwestern quarter of Figure 3, narrow strip of Potato Sandstone is exposed and Slip-Rate Calculation ~0.6 km northwest of Badger Canyon (Miller et al., overridden by the landslide, west of trench WT‑1A); 2001). The Mill Creek strand is estimated to have so the Mill Creek strand is likely buried beneath We use a triangular PDF for the offset of the initiated in the latter part of the middle Pleistocene both the landslide (Qls2) and the latest Pleistocene landslide with a peak at 650 m, tapering to zero (Matti and Morton, 1993) and may exhibit Holocene alluvium that buries the landslide (Qf2b). Based on at 425 and 950 m (Fig. 11J). Combining this with displacement locally (McGill et al., 1999). Kendrick these relationships, we conclude that the Mill Creek the PDF for the ages of samples MR2–MR6 com- et al. (2015) argue that total displacement along the strand does not contribute significantly to the latest bined by the sum method (Fig. 11K) yields a slip Mill Creek strand in the San Gorgonio Pass region Pleistocene right-lateral slip on the San Andreas rate of 14.5 mm/yr, with a 95% confidence interval is 7.1–8.7 km over the past ~100 k.y. fault zone at Badger Canyon since the time of Qf2.

Right-lateral offsets are present farther south- contribution of the Arrowhead Springs fault to the San Andreas fault to the San Jacinto fault within east along the Mill Creek strand, however. At City late Pleistocene right-lateral slip rate across the San the 16 km stretch between Cajon Creek and Bad- Creek, ~12.5 km southeast of Badger Canyon, the Andreas fault zone at Badger Canyon is no more ger Canyon (Fig. 18A). While the Cajon Creek and Mill Creek strand may accommodate ~10% of the than a few tenths of one mm/yr. Matthews Ranch rates overlap, the best estimate slip rate across the San Andreas fault zone, for late of the slip rate at the Matthews Ranch site is only Pleistocene deposits (Sieh et al., 1994). Similarly, a couple of mm/yr higher than at Badger Canyon, Weldon (2010) estimates a latest Quaternary rate of Slip Rate as a Function of Time suggesting that much of this ~1 ± 0.5 cm/yr of slip 2 mm/yr for the Mill Creek strand where it crosses transfer may occur within the 3 km between Cajon the Santa Ana River, 22 km southeast of Badger Latest Pleistocene slip rates for the San Ber- Creek and Pitman Canyon (Fig. 18A). It is possi- Canyon. We infer that the slip rate of the Mill Creek nardino strand of the San Andreas fault at Badger ble that the Peters fault and/or the Tokay Hill fault strand decreases between the Santa Ana River and Canyon are very similar over three different time (Fig. 2) accommodate a minor amount of slip trans- +2.2 Badger Canyon. scales: (1) 13.5 /−2.5 mm/yr for the past 28 k.y.; fer from the San Andreas fault to the San Jacinto +0.9 Several other late Quaternary fault strands are (2) 11.9 /−1.2 mm/yr for the past 23 k.y.; and (3) 12.0 fault between Pitman Canyon and Badger Canyon. +4.7 also present within the study areas. Subparallel to /−5.6 mm/yr for the past 11–15 k.y. This suggests a This slip transfer is consistent with the results of and 0.3–1.0 km northeast of the Mill Creek strand at relatively steady rate of strain release in earthquake mechanical modeling of the San Andreas fault Badger Canyon is a 6-km-long fault that is unnamed slip between 28 ka and 11 ka (Fig. 17A). system (e.g., Herbert et al., 2014) as well as with on most maps. In the U.S. Geological Survey To obtain our preferred, time-averaged slip modeling of geodetic data (e.g., Becker et al., 2005; (USGS) Quaternary fault and fold database (U.S. rate at the Badger Canyon site, we sum and renor- McCaffrey, 2005; Meade and Hager, 2005; Loveless Geological Survey and California Geological Sur- malize the PDFs for our three slip-rate estimates and Meade, 2011; McGill et al., 2015). vey, 2018) this fault is considered to be a secondary at that site. The resulting PDF (Fig. 17B) has a Northwest of Cajon Creek, best estimates of strand of the Mill Creek strand, but to avoid confu- mean of 12.8 mm/yr, a 67% confidence interval of Holocene slip rates for the Mojave and Carrizo seg- sion between these two fault strands, we prefer the 11.1–14.8 mm/yr and a 95% confidence interval of ments of the San Andreas fault are ~35–37 mm/yr name Arrowhead Springs fault for this northeastern 8.1–18.4 mm/yr. The PDF is irregular, with obvious (Sieh and Jahns, 1984; Salyards et al., 1992; Wel- strand, as it was referred to in reports generated changes in slope at 10 and 16 mm/yr, with 84% of don et al., 2008), but with large uncertainties for the Metropolitan Water District’s Arrowhead the area under the curve contained within those (see Figs. 1 and 18B). Using the best-estimate of Tunneling Project (e.g., Sholley et al., 2011). This limits. We thus report a preferred, time-averaged 34 mm/yr (range 25–40 mm/yr) for the slip rate of fault is not to be confused with the early to mid- for the past 28 k.y. rate of 12.8 mm/yr with an 84% the Mojave South section of the San Andreas fault dle Quaternary Arrowhead fault (U.S. Geological confidence interval of 10–16 mm/yr. from the Uniform California Earthquake Rupture Survey and California Geological Survey, 2018), Forecast version 3 (UCERF3) (Dawson and Weldon, located just east of and outside of our study area. 2013), an additional ~1 ± 1 cm/yr of slip may trans- The Arrowhead Springs fault projects toward the Distribution of Slip between the San Andreas fer from the Mojave section of the San Andreas San Bernardino strand of the San Andreas fault and Other Faults fault onto the San Jacinto fault (north of Cajon zone near Devil Canyon, ~2.3 km northwest of Bad- Creek) and/or onto other faults. Overall, the San ger Canyon, and separates Mesozoic gneiss of Devil The offset and age of the Matthews Ranch land- Andreas fault slip rate decreases by ~2 ± 1 cm/yr Canyon (Mzdc) on the northeast from Cretaceous slide is consistent with the time-averaged slip rate between the southern Mojave section (34 mm/yr; monzogranite (Kmg) on the southwest (Miller et obtained from Badger Canyon (Fig. 17A). All of Dawson and Weldon, 2013) and Badger Canyon al., 2001). Aligned patches of vegetation, saddles, these slip-rate estimates are also very similar to (~12.8 mm/yr, this paper), and this slip must be and fault-line scarps with south-side-up separa- the 7–16 mm/yr slip rate at Plunge Creek, ~18 km transferred onto the San Jacinto and other faults tion mark the fault trace. The fault truncates early farther southeast along the San Andreas fault of the San Andreas system. or mid-Pleistocene sediments (Miller et al., 2001) (McGill et al., 2013). However, these slip-rate esti- In the Uniform California Earthquake Rupture but does not offset late Holocene deposits in Bad- mates from the San Bernardino strand of the SAF Forecast (UCERF) models 2 and 3 (Wills et al., 2008; ger Canyon (Qa4; Fig. 3). There is no observable are notably slower than the San Andreas fault slip Dawson and Weldon, 2013), the San Bernardino lateral offset of the margins of Badger Canyon at rate at Cajon Creek to the northwest (Fig. 1) (24.5 strand of the San Andreas fault was divided into a this fault, but 3.5 km to the east, the western mar- ± 3.5 mm/yr; Weldon and Sieh, 1985). The decrease North San Bernardino strand and a South San Ber- gin of Waterman Canyon (Fig. 2) is right-laterally in San Andreas fault slip rate between Cajon Creek nardino strand based on early slip-rate estimates deflected by 100–150 m at the northernmost strand and Badger Canyon suggests that ~1 ± 0.5 cm/yr for the Badger Canyon and Matthews Ranch/Pitman of the Arrowhead Springs fault. We infer that the of slip transfers from the Mojave section of the Canyon sites, published in abstract form, which

A Oﬀset as a Function of Age 1000

900 Figure 17. (A) Offsets and ages used to constrain the four slip rates presented in this paper. Black 800 dots mark the means of the probability density 700 MR landslide functions (PDFs) for offset and age shown in Figure 11. Boxes outlined by black lines show 600 the 95% confidence intervals on the offset m) and age estimates. Smaller box with red out- 500 12.8 mm/yr line for Qf3 shows the parameter space that falls beneath the plateaus that form the tops

Oﬀset ( 400 of the trapezoidal PDFs for offset and age of 300 Qf3. This represents the most likely offsets and Qf3 Qf2 apex ages (and equally likely anywhere within the 200 Qf2/Qt2-w box). Vertical red bar for Qf2 marks the range of offsets that we deem most likely (and equally 100 likely anywhere along that bar). Thick, hori- riser 0 zontal, red bar for the Matthews Ranch (MR) 0 10000 20000 30000 40000 50000 60000 70000 landslide marks the 95% confidence interval for the age of the landslide if the “product Age (years) method” is used to combine the PDFs for the five boulder ages. Thinner red bar marks the 68% confidence interval for the age landslide if the sum method is used. (The width of the large black box represents the 95% confidence Combined pdf for Badger Canyon Slip Rate Estimates interval for the age of the Matthews Ranch land- 0.16 slide when the “sum method” is used). Sloping line labeled 12.8 mm/yr marks the mean of the 0.14 combined (via the “sum method”) PDF for the Sum method three slip-rate estimates from Badger Canyon 0.12 (shown in B). The 68% confidence interval for y

i t Product method this rate is 11.1–14.8 mm/yr. The age and offset s

n 0.1 for the Matthews Ranch (MR) landslide are only e

d broadly constrained (see large black box) but are

y consistent with the rates from Badger Canyon.

i t 0.08 i l (B) Black curve shows the PDF for the three slip b

a rates from Badger Canyon combined by the sum b 0.06 method. Gray curve shows the PDF for those r o P 0.04 three rates combined via the “product method.” Vertical dashed lines show the 67% confidence 0.02 interval for the combined PDF obtained by the sum method. 0 6 8 10 12 14 16 18 20 Slip rate (mm/yr)

Figure 18. (A) Slip rate of the San Bernardino A Southern San Andreas Fault Slip Rate strand of the San Andreas fault as a function of distance along strike, showing southeastward 35 decrease between Cajon Creek and Badger Can- yon that we interpret as a result of slip transfer 30 to the San Jacinto fault. See part (B) for expla- ) Cajon r

y Creek nation of symbols. Long-dashed line shows

/ 25 Crafton Hills interpretation in which the slip rate across m Horst & Graben m 20 the greater San Andreas fault zone southeast ( Complex of Badger Canyon is constant along strike, e t

a 15 with lower rates at Burro Flats and Cabazon R

explained by slip on other strands of the San p

i 10

l Andreas fault zone within San Gorgonio Pass. Wilson S Pitman SGP Lower, short-dashed line shows interpretation 5 Badger Creek Canyon Canyon Plunge Burro in which additional slip transfers to the San Ja- Cabezon cinto fault via the Crafton Hills fault (Fig. 1). All 0 Creek Flats Painted Hill rates are averaged to the present. Lighter-gray 0 10 20 30 40 50 60 70 80 90 shading outlines range of measured slip rates. Distance southeast from Cajon Creek (km) The Cabazon site is on the San Gorgonio Pass thrust fault, not the San Bernardino strand. The SGP value is a summary rate for the San Gorgo- nio Pass region, including inferred slip on other faults within the region (Yule, 2009). Other rates Southern San Andreas Fault Slip Rate shown are from the following sources: Cajon 90 Creek (Weldon and Sieh, 1985); Pitman Canyon (this paper); Badger Canyon (this paper); Plunge past < 4 ka LR-Pa 80 San Eastern past 6-8 ka Creek (McGill et al., 2013); Wilson Creek (Harden

) Jacinto California r 70 past 10-16 ka and Matti, 1989); Burro Flats (Orozco, 2004; Yule

y Fault Shear / and Spotila, 2010); Cabazon (Yule et al., 2001); 60 past 20-35 ka Zone Zone m Painted Hill (Gold et al., 2015). (B) Slip rate of the past 40-100 ka m

( 50

San Andreas fault zone as a function of distance past 200-400 ka e t 40 Area enlarged in a along strike from central California to the Coach- a ella Valley. Wa—Wallace Creek (Sieh and Jahns, R 30 PW p Wa 1984); LC-Pa—Little Rock and Pallett Creek (Mat- i l CC

S mon et al., 2005; Weldon et al., 2008; Salyards et 20 SGP al., 1992); CC—Cajon Creek; Pt—Pitman Canyon; 10 Wi Pt BP BC—Badger Canyon; Pl—Plunge Creek; Wi—Wil- BC Pl BFCa PH 0 son Creek; BF—Burro Flats; SGP—San Gorgonio -250 -200 -150 -100 -50 0 50 100 150 200 Pass; Ca—Cabazon; PH—Painted Hill; BP—Biskra Distance southeast from Cajon Creek (km) Palms Oasis; PW—Pushwalla Canyon (Behr et al., 2010; Fletcher et al., 2010).

showed a higher slip rate at the latter site (McGill including 6–13 mm/yr (Prentice et al., 1986), Mountains (Figs. 1 and 2), or is accommodated by et al., 2010). In the final slip rates presented here, the >20 mm/yr (Kendrick et al., 2002), 5–18 mm/yr off-fault deformation in the region; see McGill et al. slip rate for the Matthews Ranch/Pitman Canyon site (McGill et al., 2012), and 12.8–18.3 mm/yr (Onder- (2013) for further discussion. is lower than the previously estimated rate, largely donk et al., 2015). These rates for the San Jacinto Holocene and latest Pleistocene slip rates are due to a change in the estimated 10Be production fault suggest that much, but not all, of the slip that relatively low along the San Andreas fault through rate. Thus, the division of the San Bernardino strand transfers off of the San Andreas fault between San Bernardino Valley and San Gorgonio Pass, into two sections is no longer necessary. Pallett Creek and Badger Canyon transfers onto with rates of 8 ± 4 mm/yr on the San Bernardino Slip-rate estimates for the central San Jacinto the San Jacinto fault. A significant amount of slip strand near Burro Flats (Orozco and Yule, 2003; fault near Anza are constrained to 9.5–15.5 mm/yr (~3–8 mm/yr), however, likely transfers onto other Orozco, 2004; Yule and Spotila, 2010; see also (Blisniuk et al., 2013), with similar rates reported faults, such as the Cucamonga fault, North Fron- Yule, 2009), >5.7 ± 0.8 mm/yr (Yule et al., 2001) and +2.7 by Rockwell et al. (1990). Farther north on the tal fault, the Mill Creek strand of the San Andreas 5.7 ⁄−1.5 mm/yr (Heermance and Yule, 2017) on the San Jacinto fault, slip-rate estimates vary widely fault, and other faults within the San Bernardino San Gorgonio Pass fault zone, 4–5 mm/yr on the

Banning strand at Whitewater (Gold et al., 2015; see 2010). The southeastward termination of the 1857 slip-rate estimates from the northernmost strands Figs. 1 and 18). Near Indio, the slip rate of the San earthquake rupture in Cajon Pass is consistent with of the San Jacinto fault as well as from the Peters Andreas fault zone increases dramatically, likely as the reduced slip rate in the vicinity of Cajon Pass. and Tokay Hill faults will help to further clarify the a result of slip transferring from the Eastern Cali- However, the frequency of earthquakes at Pitman transfer of slip in this complex region. fornia shear zone onto the southern San Andreas Canyon during the past 1000 years is comparable fault zone (Gold et al., 2015) (Fig. 18). Specifically, to that at Wrightwood and Pallett Creek, with seven a slip rate of 24.1 ± 3 mm/yr has been measured on surface-rupturing earthquakes at Pitman Canyon in ACKNOWLEDGMENTS the San Andreas fault at Pushawalla Canyon, with the past 1000 years (Seitz and Weldon, 1994), eight We thank Emiko Kent for processing the 10Be samples from Bad- 21.6 ± 2 mm/yr on the Mission Creek strand and earthquakes at Wrightwood (Scharer et al., 2010), and ger Canyon and Joseph Salazar and Mark Swift for assisting with sample collection. John McKeown provided helpful discussions 2.5 ± 2 mm/yr on the Banning strand (Blisniuk et six at Pallett Creek (Scharer et al., 2011) during that in the field. We thank Carol Prentice, Ryan Gold, and Sean Bemis al., 2021). A similar rate of 14-20 mm/yr has been same time period. Slip per event at Pitman Canyon is for their very helpful reviews, which substantially improved the reported from near the junction of the Banning and thought to be 3–4 m for the most recent two events paper. Danielle Madugo provided help with drafting and editing some of the figures. This research was supported by the South- Mission Creek strands (Behr et al., 2010; Fletcher et (Seitz and Weldon, 1994), comparable to the 2–4 m of ern California Earthquake Center (SCEC), grants 06116, 07141, al., 2010) (Fig. 1). The low slip rates described above slip per event documented over the past 1600 years and 08157. During the period of support, SCEC was funded by for the San Bernardino and San Gorgonio Pass sec- at Wrightwood (Weldon et al., 2002). These studies National Science Foundation Cooperative Agreement EAR- tions of the San Andreas fault are also confirmed by suggest that the late Holocene slip rate of the San 0106924 and U.S. Geological Survey Cooperative Agreement 02HQAG0008. This paper is SCEC contribution number 10001. many geodetic studies (Becker et al., 2005; McCaf- Bernardino strand at Pitman Canyon may be faster +9.9 frey, 2005; Meade and Hager, 2005; Spinler et al., than the late Pleistocene rate of 14.5 /−6.2 mm/yr 2010; Loveless and Meade, 2011; McGill et al., 2015). reported here, or that the late Pleistocene slip rate REFERENCES CITED at Pitman Canyon is closer to the upper end of that Adamiec, G., and Aitken, M., 1998, Dose-rate conversion factors: range. The possibility of complementary temporal Update: Ancient TL, v. 16, p. 37–50. Paleoseismic Implications fluctuations in slip rate between the San Andreas Becker, T.W., Hardebeck, J.L., and Anderson, G., 2005, Con- straints on fault slip rates of the southern California plate and San Jacinto faults has been raised by Bennett boundary from GPS velocity and stress inversions: Geo- The low rates of slip on the San Andreas fault et al. (2004). 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