U.S. DEPARTMENT OF THE INTERIOR OPEN-FILE REPORT 03-449 U.S. GEOLOGICAL SURVEY SHEET 1 OF 2

FAULT PERPENDICULAR TRENCHES NORTHEAST SOUTHWEST

EVENT TP-1 FAULT TERMINATIONS ? NORTHEAST SOUTHWEST CHANNEL VI

CHANNEL VI CHANNEL IV Acknowledgements 7-40 PHOTOMOSAICS AND LOGS OF TRENCHES ON TABLE 1 RADIOCARBON DATES CHANNEL V 7-7 We are extremely grateful to the Nature Conservancy for granting access to their property at Thousand Palms Oasis for our paleoseismic investigations. We especially thank Cameron Barrows, the Southern Regional Director of the Center for Natural 7-11 7-10 THE , THOUSAND PALMS 14 13 Lands Management, for his help in making our study possible. LAB # SAMPLE # UNIT MATERIAL C AGE C VALUE 7-45 7-43 STRATIGRAPHIC EARTHQUAKE 7-36 ORGANIC CLASTIC LAYER 7-39 COLUMN LAYER NO. LITHOLOGY HORIZON AA 17729 T1-1A VI charcoal 715 ± 35 -21.7 EVENT TP-2 OASIS, CALIFORNIA References CHANNEL III ROCK AA 17738 T2-16 VI charcoal 1185 ± 70 -24. FAULT TERMINATIONS 7-9 Fumal, T.E., Rymer, M.J., and Seitz, G.G. (2002). Timing of Large Earthquakes since A.D. 800 on the Mission Creek Strand of the San N CHANNEL VI AA 17734 T1-3C Vb soil/charcoal 355 ± 55 -26.6 92 ± 7-8 Andreas Fault Zone at Thousand Palms Oasis, near Palm Springs, California, Bull. Seism. Soc. Am. no.7 TThousandhousand PalmsPalms OasisOasis AA 21918 T10-2 Vb soil/charcoal 285 45 -25.866 EVENT TP-5 poorly sorted sand ± upper AA 17735 T1-4 Va soil/charcoal 500 55 -26.4 FAULT TERMINATION By Sieh, K.E.( 1986). Slip rate across the San Andreas fault and prehistoric earthquakes at Indio, California, EOS, Trans. Am. Geophys. 100100 METERSMETERS w/some gravel ± CHANNEL II 67 AA 21905 T7-7 Va soil/charcoal 345 45 -25.719 Thomas E. Fumal, William T. Frost, Christopher Garvin, John C. Hamilton, Monique Jaasma and Michael J. Rymer Union , 1200. AArearea ooff figurefigure 4 VI a CAMS 63505 T7-40 Va soil/charcoal 480 ± 40 -25* 7-3 Sieh, K.E. and Williams, P.L.(1990). Behavior of the southernmost San Andreas fault during the past 300 years, J. Geophys. Res. 95, CAMS 63506 T7-40 Va humics 420 ± 50 -28* 7-35 TTrenchrench 1144 AA 21917 T10-1 Va soil/charcoal 310 ± 45 -26.241 INTRODUCTION 6629–6645. lower TP-1 horizon CAMS 63507 T7-43 IVe soil/charcoal 450 ± 40 -25* 7-38 CHANNEL II Vogl, R.J. and McHargue, L.T. (1966). Vegetation of California fan palm oases on the San Andreas fault, Ecology, 47, 532–540. silty sand ± CHANNEL II We present photomosaics and logs of the walls of trenches excavated for a paleoseismic study at Thousand Palms Oasis (Fig. 1). The site (within lower CAMS 63446 T7-43 IVe humics 360 40 -28* 7-1 is located on the Mission Creek strand of the San Andreas fault zone, one of two major active strands of the fault in the Indio Hills along the Williams, P. L. (1989). Aspects of the earthquake geology and seismotectonics of the Southern San Andreas and related faults, southern AA 21910 T9-1 IVe soil/charcoal 305 ± 40 -26.433 Channel VI or ± California , P.h.D. thesis, Columbia University, New York, New York . CHANNEL V CAMS 53956 T2-33 IVe soil/charcoal 480 50 -25* 7-2 northeast margin of the (Fig. 2). The Coachella Valley section is the most poorly understood major part of the San Andreas silty sand ± higher) CAMS 53951 T2-33 IVe humics 350 60 -29* CHANNEL I fault with regard to slip rate and timing of past large-magnitude earthquakes, and therefore earthquake hazard. No large earthquakes have Williams, P. L., McGill, S. F, Sieh, K. E , Allen, C. R and Louie, J. N. (1988). Triggered slip along the San Andreas fault after the 8 July AA 21909 T7-11 IVd soil/charcoal 535 ± 50 -25.962 7-4 occurred for more than three centuries, the longest elapsed time for any part of the southern San Andreas fault. In spite of this, the Working 1986 North Palm Springs earthquake, Bull. Seism. Soc. Am. 78, 1112–1122. V b clay w/organics CAMS 63508 T7-45 IVd humics 400 ± 30 -28* ± Group on California Earthquake Probabilities (1995) assigned the lowest 30-year conditional probability on the southern San Andreas fault Working Group on California Earthquake Probabilities (1995). Seismic hazards in southern California: probable earthquakes, 1994 to AA 21911 T9-2 IVd soil/charcoal 660 45 -26.062 silty sand CAMS 53958 T9-2 IVd soil/charcoal 400 ± 60 -25* EVENT TP-5 2024, Bull. Seism. Soc. Am. 85, 379–439. FAULT TERMINATION to the Coachella Valley. Models of the behavior of this part of the fault, however, have been based on very limited geologic data. MISSION CREEK STRAND CAMS 53953 T9-2 IVd humics 310 ± 50 -29* ± The Thousand Palms Oasis is an attractive location for paleoseismic study primarily because of the well-bedded late Holocene CAMS 53959 T9-2 IVd roots 150 180 CHANNEL I OF SAN ANDREAS FAULT ZONE V a ± TRENCH 9 SOUTHEAST WALL sedimentary deposits with abundant layers of organic matter for radiocarbon dating necessary to constrain the timing of large prehistoric AA 17731 T2-1 IVd soil/charcoal 830 65 -27.5 silty sand AA 21893 T2-35 IVd soil/charcoal 520 ± 45 -26.528 earthquakes. Previous attempts to develop a chronology of paleoearthquakes for the region have been hindered by the scarcity of in-situ 14C- AA 21920 T10-6 IVd soil/charcoal 520 ± 45 -26.450 TRENCH 7 SOUTHEAST WALL dateable material for age control in this desert environment. Also, the fault in the vicinity of Thousand Palms Oasis consists of a single trace AA 21908 T7-10 IVc soil/charcoal 515 ± 45 -26.941 cobbly sand ± 5 4 3 2 1 0 that is well expressed, both geomorphically and as a vegetation lineament (Figs. 2, 3). Results of our investigations are discussed in Fumal et Figure 3. Aerial view of Thousand Palms Oasis and vicinity showing location of the paleoseismic site (box). Note expression of the Mission AA 21919 T10-5 IVc soil/charcoal 520 45 -26.337 TP-2 horizon CAMS 63444 T7-36 IVb charcoal 390 ± 40 -25* METERS Creek strand of the San Andreas fault as a geomorphic break in slope (left of oasis) and as vegetation lineament (right of oasis). Location silty sand al. (2002) and indicate that four and probably five surface-rupturing earthquakes occurred along this part of the fault during the past 1200 CHANNEL IV CAMS 64147 T7-36 IVb charcoal/humics 410 ± 40 -28* 4 3 2 1 of Trench 14 that exposed the fault at the base of the hill southeast of the site is indicated. IV e ± 0 years. The average recurrence time for these earthquakes is 215 ± 25 years, although interevent times may have been as short as a few gravelly sand CAMS 63503 T7-39 IVb soil/charcoal 560 40 -25* NORTHEAST ± METERS decades or as long as 400 years. Thus, although the elapsed time since the most recent earthquake, about 320 years, is about 50% longer than ? IV d CAMS 64143 T7-39 IVb humics 430 40 -28* coarse sand AA 21907 T7-9 IVb soil/charcoal 375 ± 45 -26.229 SOUTHWEST NORTHEAST SOUTHWEST THOUSAND PALMS TRENCH SITE IV c the average recurrence time, it is not necessarily unprecedented. silty sand AA 21912 T9-3 IVb soil/charcoal 400 ± 45 -26.782 ± IV b CAMS 53960 T9-3 IVb roots 180 50 CHANNEL VI THE THOUSAND PALMS OASIS SITE IV a AA 17732 T2-2 IVb soil/charcoal 605 ± 35 -27.4 3 11 1 poorly sorted sand AA 17733 T2-4 IVb soil/charcoal 950 ± 65 -26.8 EVENT TP-2 N CAMS 53955 T2-30 IVb soil/charcoal 470 ± 50 -25* FAULTING? EVENT TP-1 CHANNEL VI Methodology 12 III e CHANNEL III CAMS 53950 T2-30 IVb humics 460 ± 60 -29* FISSURE Our paleoseismic site is located in an active wash area between two groves of native California fan palms (Washingtonia filifera) (Fig. CHANNEL III silty sand AA 21895 T2-37 IVb soil/charcoal 525 ± 45 -26.801 MARGIN III d AA 21894 T2-36 IVb soil/charcoal 560 ± 45 -27.055 3). This area is sparsely vegetated with predominantly inkweed (Suaeda torreyana ramosissima) and arroweed (Pluchea sericea). We AA 21906 T7-8 IVa soil/charcoal 510 ± 45 -25.723 CHANNEL IV 9-1 excavated 8 trenches perpendicular to the fault and 7 trenches parallel to the fault (Fig. 4), in order to resolve slip and evidence for individual METERS CAMS 54060 T9-4 IVa soil/charcoal 510 ± 50 -25* CHANNEL III 10 silty sand TP-3 horizon surface-rupturing earthquakes. The depth of the trenches was limited to less than 3 meters because of a shallow water table on the northeast CAMS 54065 T9-4 IVa humics 350 ± 50 -29* CHANNEL IV side of the fault. 7 CAMS 53961 T9-4 IVa roots Modern AA 21900 T5-5 IV soil/charcoal 520 ± 45 -25.168 9-3 2 III c 9-5 9-2 Trenches were excavated during two field seasons. The initial trenches, T1 and T2, were logged with traditional hand-drawing methods. 4 CAMS 53954 T1-13 IIIe-upper soil/charcoal 600 ± 50 -25* ± 9-4 During the second year, portions of the walls of these trenches, and the walls of all subsequent trenches, were photo-logged (note composite TRENCH 1A fine-medium sand AA 17730 T1-5A IIIe-lower soil/charcoal 695 60 -27.5 ± nature of T1 and T2 logs). For this technique, a 1m-by-0.5m nail and string grid was constructed on the trench walls and each 1m by 0.5m III b AA 21913 T9-5 IIId soil/charcoal 795 45 -26.734 EVENT TP-5 fine sand CAMS 63863 T7-39top IIIc charcoal/humics 690 ± 50 -28* FAULT TERMINATION rectangle was photographed with a 35mm film camera fitted with a 20mm rectilinear lens. The camera was mounted on a "Trench-o-matic", III a CAMS 63504 T1-39top IIIc charcoal 830 ± 30 -25* CHANNEL II ± a frame constructed of aluminum tubing that acts to keep the film plane parallel to and and equidistant from the trench wall for each photo poorly sorted silty sand AA 17736 T1-6B IIIb soil/charcoal 1075 70 -26.9 9-6 9 ± CHANNEL II (Fig. 5). Logging was done on mylar over prints of the photos. Later, the photos were digitized, rectified to the measured string and nail grid AA 17737 T1-7 IIIb soil/charcoal 1050 130 -25.7 3A TP-4 horizon AA 21892 T1-17 IIIa soil/charcoal 920 ± 45 -26.10 II f 9-13 visible on each photo, and mosaiced together. Field linework was then transfered to the photomosaics. CHANNEL II CAMS 54061 T9-6 IIf soil/charcoal 1220 ± 50 -25* ? poorly sorted sand CAMS 54064 T9-13 IIe soil/charcoal 1210 ± 50 -25* 9-10 II e CAMS 54066 T9-13 IIe humics 1060 ± 50 -29* 9-8 9-11 Structure ± 13 gravelly sand AA 21903 T7-3 IId soil/charcoal 1330 50 -25.931 The fault appears to project as a nearly straight, continuous trace from an exposure in Trench 14, excavated at the base of the hill CAMS 63501 T7-35 IId soil/charcoal 1300 ± 50 -25* 9-12 southeast of the site, to outcrops northwest of the oasis (Fig. 3). Other trenches, however, revealed that this is not the case in the near-surface 5 II d CAMS 63864 T7-35 IId humics 1100 ± 40 -28* poorly sorted sand ± sediments. Faulting is expressed in Trenches 1, 2, 7, and 9 as a relatively simple zone less than 1 m wide, although in Trench 2 there are CAMS 63502 T7-35 IId charcoal 1210 40 -25* II c CAMS 64146 T7-35 IId charcoal/humics 1130 ± 40 -28* several older traces about 4–5 m northeast of the main zone. Faulting was not observed, however, in Trench 1A, which was not logged, nor 6 silty sand CAMS 54062 T9-8 IId soil/charcoal 1290 ± 50 -25* CHANNEL I 8 TP-5 horizon in Trench 11, which together span a zone about 36 m wide across the projection of the fault. Also, faulting was not observed in fault- II b CAMS 54145 T9-8 IId humics 1080 ± 50 -29* II a CHANNEL I perpendicular trenches in the southeastern part of the site (Trenches 8 and 13). The deposits in these areas are 500–1000 years old and should CAMS 54063 T9-8A IId soil/charcoal 1420 ± 60 -25* AA 21901 T7-1 IIc soil/charcoal 1195 ± 45 -25.910 TRENCH 9 NORTHWEST WALL have been offset by 2–3 earthquakes. Because the observed fault traces are parallel to the overall trend of the fault zone rather than at an silty fine sand AA 21915 T9-11 IIc soil/charcoal 1330 ± 45 -26.044 angle to it as the individual segments of an en echelon zone would be, we infer the site is located within a small stepover in the fault. Mission Creek strand of CAMS 53963 T9-11 IIc roots modern Because of the lack of faulting in the northwest and southeast portions of the site, logs of Trenches 6, 8, 11, 12, and 13 are not shown. San Andreas fault zone AA 21914 T9-10 IIc soil/charcoal 1555 ± 45 -26.743 CHANNEL I CAMS 53962 T9-10 IIc roots 100 ± 50 5 4 3 2 1 0 AA 21902 T7-2 IIb soil/charcoal 1395 ± 50 -26.268 METERS Stratigraphy AA 21916 T9-12 IIb soil/charcoal 1530 ± 45 -26.059 NORTHEAST TRENCH 7 NORTHWEST WALL We exposed six major channels of different ages that have been eroded and filled during the past approximately 1600 years. We name the 0102030 Meters CAMS 53964 T9-12 IIb roots 480 ± 80 sandy gravel ± channels I–VI, with Channel I the oldest. Deposits within individual channels are indicated with colored tints on the stratigraphic column AA 21896 T5-1 IIb soil/charcoal 1170 45 -26.484 Contour interval = 20 centimeters CAMS 53957 T2-34 IIb soil/charcoal 1280 ± 50 -25* CHANNEL VI (Figure 6) and photomosaics/logs. These channels are sub-parallel and are oriented at high angles to the fault. The sedimentary fill in each CAMS 53952 T2-34 IIb humics 1200 ± 50 -29* 4 3 2 1 0 channel is characterized by a basal coarse-grained gravel deposit overlain by layers of fluvial and eolian silty sand and clay interbedded with AA 21904 T7-4 IIa soil/charcoal 1215 ± 55 -26.023 Figure 4. Topographic map of the Thousand Palms paleoseismic site showing location of trenches. Heavy red line is location of fault CAMS 63865 T7-38 IIa soil/charcoal 2020 ± 80 -25* METERS thin organic-rich layers. The channeling results in complex stratigraphic relationships with subtle unconformities exposed in fault- SOUTHWEST zone observed in trenches. Note location of the southeast margin of Channel III. This is the oldest feature (about 1ka) exposed in the 2-33 radiocarbon sample CAMS 63445 T7-38 IIa humics 1130 ± 40 -28* perpendicular trenches. Commonly, individual beds are no longer physically continuous across the site. For this reason, we have not ± trenches that can be matched across the fault. It is offset 2.0 ± 0.5 m. Figure 5. The Trench-o-matic. 35mm-film camera fitted with 20mm rectilinear lens is used to take photos of 1m by 0.5m portions of the AA 21899 T5-4 IIa soil/charcoal 1375 45 -27.140 identified individual clastic layers on the trench logs. We have, however, named distinctive organic layers. Deposition at the site has been AA 21898 T5-3 IIa soil/charcoal 1360 ± 45 -25.123 trench walls. ± relatively rapid, about 0.5 m/100 years. AA 21897 T5-2 IIa soil/charcoal 1465 45 -27.176 Figure 6. Composite stratigraphic column of the Thousand Palms paleoseismic site. AA 21891 T1-16 I soil/charcoal 1670 ± 50 -26.188 EVENT TP-1 We exposed a composite section greater than 8 m thick with more than 40 individual layers, half of which contain abundant organic Photomosaic is overlain with colored lines to mark boundaries between channels on FAULTING matter. The organic material in these layers, which range from a few millimeters to about 20 cm thick, primarily consists of fine charcoal. We this and following figures. Black lines delineate lithologic units within channels. Unit interpret these charcoal-rich layers to be the result of local fires within the oasis. California fan palm oases produce heavy accumulations of numbers for organic layers and lithologic descriptions for interbedded clastic units are CHANNEL IV fuel and are adapted to frequent fires due to both human activity and natural causes (Vogl and McMargue, 1966). The abundance of charcoal- shown in color-coded boxes. Arrows indicate locations of earthquake horizons. rich layers makes possible relatively high-resolution dating of the sedimentary deposits and paleoearthquakes. CHANNEL III CHANNEL V We obtained 80 AMS 14C determinations from samples of charcoal, extracted humic acids, and rootlets from 57 samples from 20

EVENT TP-2 FISSURE CHANNEL II 2-1 2-33 EVENT TP-5 2-4 FAULT PERPENDICULAR TRENCHES FAULT TERMINATIONS NORTHEAST 2-2 2-30 116°30' 116°00'

2-34 MMOJAVEO J A V E DDESERTE S E R T poorly sorted silty sand bioturbated

SSANA N Pinto Mountain Fault BERNARDINB E R N A R D I N O CHANNEL I

MOUNTAINSM O U N T A I N S B M SOUTHWEST

F LITTLEL I T T L E moderately well sorted fine sand SSAN A N 1-1a poorly sorted silty v. fine sand w/ some v. fine gravel BBERNARD c c brown 10YR 4/3 34°00' E R poorly sorted silty sand EVENT TP-1 N c c c c c c bioturbated Miss A c R c FISSURE CHANNEL VI TRENCH 2 SOUTHEAST WALL EXPOSURE 1 Desert D c k k ion Creek Fau INOI c k dk. brown clay /organics Hot Springs N c k Banning Fault O k c silt w/organics c k v. f. sand silt w/organics c c CHANNEL V THOUSAND PALMS gravelly coarse sand c 12 11 8 7 6 4 3 -1 lt v. coarse sand c c c 1-3c c 10 9 5 2 1 0 OASIS SITE c c c METERS SANS A N MMOUNTAIN c v. coarse sand Indio O c c Palm U c sand JJACINTOA C I N T O Hills N Springs T c 1-4 c CHANNEL III MMOUNTAINSO U N T A I N S BISKRA PALMS A coarse sand 1-13 coarse sand I N poorly sorted sand CHANNEL VI SITE S c c sand 1-5a silt/c c some fine gravel dk brown 10YR 4/3 COACH c c SSANTA INDIO c c poorly sorted sand EVENT TP-3 A SITE San Andre c N c c fs c FAULT TERMINATIONS T c c silty fine sand c vfs A c c c c SOUTHWEST ELLA c c c EVENT TP-1 as Faul c c silty sand yellowish brown 10YR 5/4 c c c c c c c c FAULTING RROSA c c c poorly sorted sand CALIFORNIA O c c c c c c EVENT TP-4 c S t c c c v. coarse sand c c c CHANNEL II A silty sand pale brown 10YR 6/3 FISSURE or VALLEY c c clay c c c gravelly sand c c c c c CHANNEL III sand c vfs MMOUNTAIN c c c Mecca c c c 1-39t c FILL O c c vf sand vfs c U Hills c c c CHANNEL V N c c c c c 33°30' T c A gravell coarse sand I N SALT CREEK fine sand CHANNEL IV S poorly sorted sand c c SITE grading to c Durmid coarse sand moderately well sorted sand some lenses of v. coares sand yellowish brown 10 YR 5/4 EVENT TP-2 AREA Hill coarse sand c c OF MAP SALTON c c c c FISSURE c c v. fine sand moderately well-sorted sand SEA c c c c c CHANNEL III 0 20 KILOMETERS c c c charcoal-rich c c c c c c v. fine sand c c c v.fine sand Figure 1. Index map showing generalized geology and Quaternary active faults c c c 1-6b c in the vicinity of the southernmost San Andreas fault. Light gray, crystalline rock; c fine sand dark gray, stratified late Cenozoic deposits; white, Quaternary alluvium. TRENCH 1 SOUTHEAST WALL CHANNEL II c = thin organic layer grey Location of paleoseismic sites indicated with black dots. f-c sand FAULT OR FAULT (?) CHANNEL MARGIN

CHANNEL I 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 METERS NORTHEAST TRENCH 2 SOUTHEAST WALL EXPOSURE 2 MMISSION C INI IS N S greyish brown poorly sorted sand boiturbated NORTHEAST D IO N 7 6 5 4 3 2 1 0 -1 IIO O H C REEKR F clay w/organics E E SOUTHWEST METERS H K F IL AAUL silty fine sand U carbonate greyish brown poorly sorted sand L bioturbated L T bioturbated CHANNEL VI S silty sand carbonate SOUTHWEST EVENT TP-1 c c c fine gravelly sand coarse sand c 2-16 FAULTING c CHANNEL VI c c c TTRENCHRENCH c c c clay w/organics TTHOUSANDHOUSAND c c c SITESITE c c c PPALMSALMS c c c fine gravelly sand c OOASISASIS c c medium-coarse sand CHANNEL V c f-c sand poorly sorted sand cobbly sand c c c c c fine-medium sand gravelly sand EVENT TP-1 CHANNEL III fine-coarse sand c medium-coarse sand N c organic vf sand FAULTING c c c O c c coarse sand c Y c c c silty sand c c c c c c silty f-m sand EVENT TP-4 N c A f sand fine sand c c c c FAULTING or C c fine sand c cc c c EVENT TP-3 S fine-coarse sand c c c c c CHANNEL III M c c c c c c c c FAULT TERMINATIONS EVENT TP-2 L c c c c c MARGIN AALMS CANYON c c c c c c c FISSURE P c c c v fine sand c c c fine gravelly sand c c v. fine sand gravelly c CHANNEL II CHANNEL V D c BBANNI c c sand c A NND P c c N A gravelly coarse sand c c N S c INNG F U cobbly c c c c 2-35 c c G O medium-coarse sand sand c c c CHANNEL I F H silty fine sand c 2-36 AAULU TTHOUSA L medium sand T coarse sand 2-37 grading to CHANNEL IV c moderately well sorted fine-medium sand silty fine sand 1-7 c CHANNEL II c c c gravelly coarse sand EVENT TP-5 c c c c c c c fine sand c FAULT TERMINATION c 1-17 c c c well sorted fine-medium sand c c c c c c c c c c c c poorly sorted sand 1-16 c c c c c c c c v fine sand /lenses of v coarse sand c c c c c c c c grey well sorted f-c sand vf sand TRENCH 1 NORTHWEST WALL c = thin organic layer TRENCH 2 NORTHWEST WALL Figure 2. Geologic map of the central Indio Hills, Salton Trough, California. Location of Thousand Palms Oasis trench site shown along the Mission Creek 14 6 strand of the San Andreas fault. Orange, mid-late Pleistocene alluvium; yellow, 15 13 12 11 10 9 8 7 5 4 3 2 1 0 12 11 10 9 8 7 6 5 4 3 2 1 0 late Pelistocene–Holocene alluvium. Geology by M.J. Rymer. METERS METERS