FINAL TECHNICAL REPORT

EARTHQUAKE RECORD OF THE PENINSULA SEGMENT OF THE SAN ANDREAS FAULT, PORTOLA VALLEY, CALIFORNIA: COLLABORATIVE RESEARCH WITH WILLIAM LETTIS & ASSOCIATES, INC. AND U.S. GEOLOGICAL SURVEY

Recipient: William Lettis & Assoc., Inc., 1777 Botelho Drive, Suite 262, Walnut Creek, CA 945961 (925) 256-6070 phone; (925) 256-6076 fax; URL: www.lettis.com

U.S. Geological Society, 345 Middlefield Road, M.S. 977, Menlo Park, CA 940252 (650) 329-5690 phone; (650) 329-5163 fax; URL: www.usgs.gov

Principal Investigators: John Baldwin1 and Carol Prentice2 Email: [email protected]; Email: [email protected]

Contributors: Sean Sundermann William Lettis & Associates, Inc., 433 Park Point Drive, Suite 250, Golden, CO

Steve Thompson William Lettis & Associates, Inc., 1777 Botelho Drive, Suite 262, Walnut Creek, CA

James Wetenkamp San Jose State University, Department of Geology, San Jose, CA

Program Elements: Priority III: Research on Earthquake Occurrence, Physics and Effects

Keywords: Paleoseismology; Recurrence interval; Slip rate; Trench investigations; Event chronology

U. S. Geological Survey National Earthquake Hazards Reduction Program Award Number 05HQGR0073

May 2008

Research supported by the U.S. Geological Survey (USGS), Department of the Interior, under USGS award number 05HQGR0073. The views and conclusions contained in this document are those of the authors and should not be interpreted as necessarily representing the official policies, either expressed or implied, of the U.S. Government.

i

AWARD NO. 05HQGR0073

EARTHQUAKE RECORD OF THE PENINSULA SEGMENT OF THE SAN ANDREAS FAULT, PORTOLA VALLEY, CALIFORNIA

John Baldwin1 and Carol Prentice2 William Lettis & Associates, Inc., 1777 Botelho Dr., Suite 262, Walnut Creek, CA 945961 (tel: 925-256-6070; fax: 925-256-6076; email: [email protected]) U.S. Geological Society, 345 Middlefield Road, M.S. 977, Menlo Park, CA 940252 (tel: 650-329-5690; fax: 650-329-5163; email: [email protected])

ABSTRACT

Previous paleoseismic studies on the Peninsula segment provide evidence of poorly constrained large magnitude earthquakes occurring in the late Holocene (Wright et al., 1999; Hall et al., 2001), and possibly in 1838 (Toppozada and Borchardt, 1998); whereas paleoseismic investigations on the Santa Cruz Mountains and North Coast segments provide moderately well constrained event chronology information over the last approximately 2,000 years (Fumal et al., 2004; Niemi et al., 2004; Kelson et al., 2006). The recent WGCEP (2003) report provides an estimate of mean recurrence values of 378 years for 1906-type events, and 230 years for ruptures occurring only on the Peninsula segment of the northern San Andreas fault. More recent paleoseismic investigations along the Santa Cruz Mountains segment (Fumal et al., 2004) and North Coast segment (Niemi et al., 2004; Zhang et al., 2003) suggest the occurrence of surface- fault rupture at these sites as frequently as every 105 to 250 years. It is unknown if these short recurrence intervals for the two segments represent local, smaller events, or if they could have been associated with through-going events that included the Peninsula segment located between the Santa Cruz Mountains and North Coast segments of the San Andreas fault.

At the Town of Portola Valley site, located on the Peninsula segment of the San Andreas fault, we interpret three, and possibly four, earthquake events within approximately the past 1,000 years. Evidence includes warped marsh and fluvial deposits, and scarp-derived colluvium. From these data, it is permissible to interpret three and possibly four earthquakes that from oldest to youngest include: Event 1 (A.D. 1030 to 1490); Event 2 (A.D. 1260 to 1490); and Event 3 (1906 A.D.). It is permissible to interpret Event 2 for the site as two separate events: Event 2A (A.D. 1260 to 1490) and Event 2B (A.D. 1410 to 1640). The youngest stratigraphic units do not provide the resolution necessary to assess whether or not the historical 1838 rupture, which has been assigned by other workers to the Peninsula San Andreas fault, ruptured the fault at this site. The colluvial wedge that we interpret as forming in response to the 1906 earthquake at the Town of Portola site could represent both the 1838 and 1906 earthquakes. However, there is no evidence by which to infer the occurrence of an earthquake in 1838, nor can we rule out the occurrence of the 1838 earthquake on the San Andreas fault at this site.

i

TABLE OF CONTENTS

ABSTRACT...... i 1.0 INTRODUCTION ...... 1 2.0 PENINSULA SEGMENT SEISMOTECTONIC SETTING...... 4 3.0 PORTOLA VALLEY GEOLOGIC SETTINGS...... 7 3.1 Woodside Trace...... 12 4.0 PALEOSEISMIC RESULTS...... 13 4.1 Near Surface Stratigraphy ...... 13 4.1.1 Holocene Fluvial Deposits ...... 13 4.1.2 Holocene Marsh Deposits ...... 14 4.1.3 Undifferentiated Quaternary Alluvium ...... 18 4.1.4 Scarp-Derived Colluvium...... 18 4.1.5 Artificial Fill...... 18 4.2 Trench T-1...... 19 4.3 Trench T-2...... 20 4.3.1 Event Stratigraphy...... 21 4.3.1.1 Scarp-Derived Colluvial Wedges...... 21 4.3.2 West-Tilted Stratigraphy ...... 23 4.4 Age of Deposits ...... 23 5.0 DISCUSSION...... 27 5.1 Event 1...... 27 5.2 Event 2...... 28 5.2.1 Alternative Interpretation of Event 2 Horizon...... 28 5.2.1.1 Event 2A (Unit COL 2)...... 30 5.2.1.2 Event 2B (Unit COL 3)...... 30 5.3 Event 3 -1906 Earthquake ...... 30 6.0 CONCLUSIONS...... 31 7.0 ACKNOWLEDGEMENTS...... 33 8.0 REFERENCES ...... 34

LIST OF TABLES

Table 1. Summary of Radiocarbon Analyses ……………………………………………………..26

ii

LIST OF FIGURES

Figure 1. Regional Fault Map Figure 2. Historical Seismicity Map Figure 3. Site Investigation Map Figure 4. Structure Contour Map Figure 5. Historic 1906 Rupture Figure 6. USGS Trench T-1 South Wall Figure 7. USGS Trench T-2 North Wall Figure 8. USGS Trench T-2 South Wall Figure 9. Preliminary Earthquake Chronology

iii

1.0 INTRODUCTION

An accurate determination of the behavioral characteristics of the northern San Andreas fault, including the timing of past earthquakes and their frequency of recurrence, is critical to probabilistic determinations of the fault’s seismic hazards. Information on fault characteristics and timing can be utilized to develop models on the likelihood of large 1906-type earthquake events recurring in the near future (Working Group on California Earthquake Probabilities [WGCEP], 1999, 2003). Developing a well-constrained earthquake chronology of late Holocene surface rupture along segments of the northern San Andreas fault is critical to understanding the earthquake cycle and possible rupture scenarios. Site-specific paleoseismic studies provide data on earthquake timing, amounts of coseismic rupture and fault slip rate that can be used to develop both segment and regional scale fault behavior models, when the historical record alone is inadequate for this purpose. While some information on the fault characteristics and chronology of both 1906-type multi-segment ruptures and smaller, single segment ruptures is available for the North Coast segment and the Santa Cruz Mountains segment, there remains a data gap between these two bounding segments on the Peninsula segment. The focus of this study is to: (1) constrain the timing of prehistoric surface fault rupture events on the Peninsula segment; and (2) compare this preliminary earthquake chronology for the Peninsula segment to similar event chronologies available for the North Coast and Santa Cruz Mountains segments.

The northern San Andreas fault (north of San Juan Bautista, the reported southern end of the 1906 surface rupture [Lawson, 1908; Prentice and Schwartz, 1991]) has been divided into four major segments that from north to south include the: Offshore and North Coast, Peninsula, and Santa Cruz Mountains segments (WGCEP, 2003) (Figure 1). The 1906 earthquake produced a 435-to 470-km-long surface rupture along the four segments between San Juan Bautista to the south and Point Delgada to the north (WGCEP, 2003; Thatcher et al., 1997) (Figure 1). Ground deformation associated with the 1906 surface- rupture included right-lateral offsets of up to 2 to 3 m (5 to 10 feet) on the Peninsula segment of the San Andreas fault (Dickinson, 1970). Understanding the late Holocene earthquake record of the Peninsula segment of the San Andreas fault is important for assessing seismic hazards and calculating probabilities of large earthquakes in the densely populated San Francisco Bay region. Currently, information on the timing of large surface-fault ruptures on the Peninsula segment is sparse with the exception of a historical 1838 M7 earthquake that could have occurred on this segment (Toppozada and Borchardt, 1998; Hall et al., 1995; Hall et al., 1999). Findings from recent detailed paleoseismic studies on the Santa Cruz Mountains (Fumal et al., 2003) and North Coast segments (Niemi et al., 2003; Zhang et al., 2003; Goldfinger et al., 2003) provide new insight on the long-term rupture behavior of the northern San

1

Andreas fault, and suggest that large magnitude surface-rupturing earthquakes may be more frequent on individual fault segments comprising the northern San Andreas fault than are single continuous ruptures similar to the 1906 rupture length.

Current 30-yr-interval probability estimates for the San Francisco Bay area incorporate a “stress shadow” model in which the Bay area currently is emerging from a seismic quiescence that followed the stress drop associated with the 1906 earthquake (WGCEP, 2003). Understanding the frequency of earthquakes over the last 1,000 to 2,000 years is critical to the understanding of the evolution of the 1906 stress shadow. Currently, there is geologic evidence for at least two pre-1906-like events rupturing the San Andreas fault. Earthquake data from paleoseismic trench sites along the 1906 rupture suggest that a rupture similar in length to the 1906 may have occurred in the middle 1600’s (Prentice, 1989; Schwartz et al., 1998; Baldwin et al., 2000; Hall et al., 2001; Knudsen et al., 2002) and another in AD 1280 to AD 1400 (Figure 1). It is hypothesized that much of the seismic moment released by the San Andreas fault occurs primarily during large events similar to the 1906 earthquake, rather than smaller events confined to the Peninsula segment, such as the 1838 earthquake. Our interpretation of the Portola Valley site provides some evidence on the timing of prehistoric earthquakes on this fault segment. The trenches contain a 1,200-yr-old stratigraphic record providing evidence for the recurrence of earthquakes during this period, including the 1906-event, as well as two or three prehistoric events.

2 125 124 123 122 121

Paleoseismic Sites 1 Cape Shelter Cove Ð Prentice et al., 1999 Mendocino 2 Noyo Canyon - Goldfinger et al., 2003a, b 10 m 3 1 Point Arena Ð Baldwin, 1996; Baldwin et al. 2000 40 14 mm/yr 4 Point Arena Ð Prentice, 1989 0 5 Fort Ross Ð Noller and Lightfoot, 1997; Simpson et al., 1996; Prentice et al., 2001; Muhs et al., 2003; Kelson et al., 2005 6 Bodega Harbor Ð Knudsen et al., 2002

Offshore segment 7 Vedanta Ð Niemi and Hall, 1992; Niemi et al., Maacama Fault 2002; Zhang, 2005 8 2 Bolinas Lagoon Ð Knudsen et al., 2002 9 Filoli Ð Hall et al., 1999 10 3 Grizzly Flat Ð Schwartz et al., 1998 39 11 24.5±6 4 Mill Canyon Ð Fumal et al., 2004 Pt. Arena San Andreas 12 25.5±2.5 Arano Flat Ð Fumal et al., 2004 Symbols

HealdsburgÐRodgers Approximate termination of 1906 surface rupture 5 25.5±2.5 Geological slip rate in mm/yr

Green Valley 6 PACIFIC OCEAN North Coast segment

7 ± Concord 38 24 3 8 Hayward Fault

Calaveras Fault

9 THIS San Gregorio STUDY Fault

Peninsula segment

10

37 11 1906 fault slip (Thatcher 17±4 12 and others, 1997) Fault 1906 surface offset N San Juan Modeled slip Bautista Cruz Mts.Santa 0 50 km Unresolved slip segment

PORTOLA VALLEY USGS NEHRP PENINSULA SEGMENT OF THE SAN ANDREAS FAULT Regional Fault Map

WILLIAM LETTIS & ASSOCIATES, INC. Figure 1

12/20/07 1719 Portola Valley

2.0 PENINSULA SEGMENT SEISMOTECTONIC SETTING

The Peninsula segment (SAP) of the northern San Andreas fault extends from the Golden Gate southward to near Los Gatos, where a large-scale restraining bend defines the segment boundary between the Peninsula segment and the Santa Cruz Mountains segment (WGCEP, 2003). The segment boundaries of the SAP are based on geologic, historical, and seismic data from the 1838, 1906, and 1989 earthquakes, variability of geodetically determined and geologically measured slip in the 1906 earthquake, differences in long-term geologic slip rates, and changes in fault orientation relative to the bounding and adjacent fault segments (Figures 1 and 2). For instance, the southern end of the Peninsula segment is coincident with a restraining bend in the fault, the northern extent of the Loma Prieta aftershock zone, and a major geologic contact defined by the northern extent of the Logan Gabbro (WGCEP, 2003).

Previous paleoseismic studies on the Peninsula segment provide evidence of poorly constrained surface- fault rupturing earthquakes occurring in the late Holocene (Wright et al., 1999; Hall et al., 2001), as well as geologic evidence suggestive of a large earthquake that could be the 1838 earthquake known from historical records (Toppozada and Borchardt, 1998). In contrast, paleoseismic investigations on the Santa Cruz Mountains and North Coast segments provide moderately well constrained event chronology information over the last approximately 2,000 years (Fumal et al., 2004; Niemi et al., 2004). WGCEP (2003) hypothesize that much of the seismic moment released by the SAF occurs primarily during large 1906-type events about every 380 years, rather than smaller events confined to the Peninsula segment, such as the 1838 earthquake. Paleoseismic investigations on the North Coast and Santa Cruz Mountains segments have estimated mean recurrence intervals of surface-fault ruptures that vary from of 105 to 250 years (Fumal et al., 2004; Niemi et al., 2003) to 200 to 400 years (Prentice, 1989; Prentice et al., 2000; Prentice et al., 2001). Whether if any of the paleoseismic events documented at these sites correlate with 1906-type multi-segment ruptures, or could be due to more local, smaller, single-segment events remains uncertain. The Peninsula segment lies between the North Coast and Santa Cruz Mountains segments, and thus developing a well constrained event chronology for the Peninsula segment would provide a key to better understanding which paleoseismic events documented on the North Coast segment may have extended through to the Santa Cruz Mountains segment.

At least one investigation at the Filoli estate about 13 km northwest of Portola Valley suggests that coseismic displacement during the 1838 earthquake could have been about 1.6 ± 0.7 m (Hall et al., 1999). Several recent paleoseismic research investigations indicate repeated surface-fault rupture along the Woodside trace over the last 2,000 to 3,000 yrs (Wright et al., 1999; Geomatrix, 2001 and 2002). The

4

recent WGCEP (2003) estimates a mean recurrence value of 378 years for 1906-type events, and a repeat interval of about 230 years for ruptures occurring only on the Peninsula segment of the northern San Andreas fault. Schwartz and others (1998) derived an estimated age for the penultimate 1906 type event, rupturing multiple segments on the Northern San Andreas, between the mid‐ 1600s to mid‐ 1700s based on overlapping evidence from multiple paleoseismic investigations along the fault. More recent paleoseismic investigations along the Santa Cruz Mountains segment (Fumal et al., 2004) and North Coast segment (Niemi et al., 2003; Zhang et al., 2003) suggest the occurrence of surface-fault rupture at these sites as frequently as every 105 to 250 years. It is unknown if these shorter recurrence intervals represent local, smaller events, or if they could have been associated with though-going events that included the Peninsula segment.

The modern slip rate on the SAP is slightly lower than on adjacent segments of the San Andreas fault. Hall et al. (1999) calculated a slip rate range of 17 ± 4 mm/yr from dated offset stream channels at the Filoli estate, north of Woodside, CA (Figure 1). This slip rate is the same within the error of that of the North Coast (SAN) and the Santa Cruz Mountains (SCZ) segments of the SAF. The minimum slip rate on the SAN as determined by Noller et al. (1996) is 16 to 18 mm yr‐ 1. The slip rate on the SAN has been estimated to be 19 ± 4 mm/yr (Prentice et al., 2006; Prentice et al., 2000, 2001; Muhs et al., 2003). On the SCZ segment the slip rate is estimated to be approximately 20.5 ± 1.5 mm/yr (Fumal et al, 2004), also no different within the error of the other studies.

5 Healdsburg M San Andreas aacama

Fort Ross Lake Berryessa 1865 Santa 5.5 Rosa Rodgers

Green Creek

West Napa 1891 5.5

1898 Pacific Ocean 6.2 Point Valley Suisun Reyes Bay San Pablo Bay

1870 1955 5.3 5.4 1889 6 Hayward

San

Francisco Greenville Seismicity San Francisco B 1861 1980 1906 Oakland 5.6 5.8 San Andreas 7.5 to 8.5 7.8

Calaveras 1838 1868 1980 7.2 5.4 6.5 to 7.4 7.6 ay

5.5 to 6.4 1866 Less than 5.4 5.5 Site 1858 6.1

San Jose

0 15 miles San

Scale

Calaveras Modified from "Fault Activity Map of California Gregorio 1911 and Adjacent Areas" by Jennings, dated 1994; seismicity data from EZ-FRISK (Version 5.7). 6.6

PORTOLA VALLEY USGS NEHRP PENINSULA SEGMENT OF THE SAN ANDREAS FAULT

Historical Seismicity Map

WILLIAM LETTIS & ASSOCIATES, INC. Figure 2

12/19/07 1719 Portola Valley

3.0 PORTOLA VALLEY GEOLOGIC SETTINGS

Portola Valley is located in the eastern foothills of the northern Santa Cruz Mountains, west of San Francisco Bay. According to previous geologic mapping by Pampeyan (1993), Brabb and Pampeyan (1983), and the Town Geologic Map (1975), bedrock geology in Portola Valley is structurally complex resulting from numerous episodes of faulting and folding. In Portola Valley, the San Andreas fault is expressed as an approximately 800-to 1,200-ft-wide rift valley that narrows in width to the southeast and broadens to the northwest in the vicinity of the Town Property. Near the Town, the rift valley is characterized by a local topographic and structurally-controlled basin with long-term Holocene subsidence rates of up to 4.4 mm/yr (Hall et al., 2001). The valley edges are linear, bound by the steep slopes of the Santa Cruz Mountains to the west and a much smaller, linear ridgeline to the east. The deep valley has been infilled with fine- to coarse-grained colluvium and alluvial fan material, derived from adjacent hillslopes, underlain by the Pleistocene Santa Clara and Eocene Butano Formations, and the meandering Sausal Creek. Bedrock at the town center is at considerable depth, stepping down to the west near Portola Road. Soil borings drilled directly to the southeast of the Town of Portola Valley site extended to a maximum depth of about 89 feet within the alluvial deposits and did not encounter bedrock (Harlan Tait Associates, 1991).

The San Andreas fault consists of two primary traces in the site area, designated from west to east as the Woodside and Trancos traces (Figure 3). The locations of the Woodside and Trancos traces of the San Andreas fault at the Town Property are based primarily on the mapping of Dickinson (1970), a WLA Alquist-Priolo (A-P) investigation (2003), and historic accounts of the 1906 rupture across Portola Road (Lawson, 1908; Taber, 1907). The Woodside trace is known to be the location of the 1906 surface-fault rupture near the Town Property, and was therefore the focus of this investigation.

The Woodside trace is not a simple, single through-going fault. The Woodside trace is characterized by a complex zone of deformation that includes tilting, warping, folding and brittle surface-fault rupture along northwest- and northeast-striking faults (WLA, 2003)(Figure 4). This zone of deformation occurs locally within the valley as a northwest-trending alignment of primary and secondary deformation features that collectively define the “Woodside” trace. The alignment of zones of interpreted deformation in the northern and central part of the Town Property suggest that the Woodside trace, in places, closely aligns with Dickinson’s (1970) mapped trace of the Woodside fault (Figure 3).

7

The Portola Valley site was ultimately chosen for this event chronology investigation because it (1) contains late Holocene alluvial deposits in a linear valley, (2) has undergone minimal cultural disturbance, and (3) provides excellent constraints on the location of the 1906 rupture. Trench locations for this study were chosen primarily from the fault location map derived from the WLA (2003) A-P study for the Town of Portola Valley. The WLA (2003) A-P study, also located within the Portola Valley Town Center, consisted of extensive paleoseismic trenching and geophysical and geotechnical exploration that characterized superb interbedded 1,200-year-old Holocene alluvial, fluvial, and marsh deposits offset and folded across the Woodside trace. Historical photos of the 1906 surface rupture illustrate en echelon rupture fractures in Portola Road. These photos were also used to locate the trench(es) with respect to the fractures and 1906 rupture trace (Figure 5).

8 House in 1906 Explanation photograph with 450.4x T-2 offset fenceline USGS - NEHRP trench location (Figure 4) (THIS STUDY) 1906 en echelon fractures (approximate) WLA T-1 WLA trench location and designation (2003, 2004) Po rtola WCC T2 Woodward-Clyde Consultants x x x x trench location and designation x x ? x USGS Trench T-2 x ? x (1976a, 1976b, 1977) x WCC T5 x x x 444.3 x R x o Geoprobe location (WLA, 2003) Zone of x Zone of ad west-facing WLA T-1 west-facing Church warping warping SL-2 PV-03-B1 Soil boring location and designation WLA T-2 WLA T-3 (WLA, 2004) Greatest inflection of SL-2 Seismic line location and warping SL-1 designation (WLA, 2003) x WLA T-5 447.8 WCC T6 Photolineament interpreted as TENNIS COURTS N18û to 20ûW striking reverse possible paleochannel fault (flower USGS Trench T-1 structure) Sausal Creek diversion pipe 449.2x Zone of west- Trancos Trace facing warping West-facing monocline x 449.1 Portola Valley WLA T-6 WLA T-7 Town Center x 454.5 PV-03-B5 450.3 x Woodside Trace PV-03-B6

Dense trees 449.3x

454.8 WLA T-9 x x Soccer 452.9 WCC T3 field x (1976a) 453.7 SL-3 and 3A WCC T1 x WCC T4 454.8 Open ditch PV-03-B10 Dense trees WLA T-4 WCC T2 (1976a) PV-03-B11 HTA B-1 (1992) PV-03-B9 Spring Down N PV-03-B8 Equestrian Sausal Creek Center PV-03-B1 WLA T-8 PV-03-B7 0 160 ft. PV-03-B3 PV-03-B2 PV-03-B4 Scale

PORTOLA VALLEY USGS NEHRP PENINSULA SEGMENT OF THE SAN ANDREAS FAULT Site Investigation Map

WILLIAM LETTIS & ASSOCIATES, INC. Figure 3

12/20/07 1719 Portola Valley Explanation 440 450.4 Approximate elevation (MSL feet) House in 1906 x of base of Unit 40 photograph with WLA T-1 offset fenceline 1906 WLA trench location (WLA 2004) en echelon fractures WCC T2 Woodward-Clyde trench location and designation Geoprobe location Po rtol SL-2 Seismic line location and a designation x x x x x x x Sausal Creek diversion pipe USGS Trench T-2 x x x WCC T5 x x R x oa West-facing monocline 444.3 x x d x 444 WLA T-1

438 445

439 SL-2 Church WLA T-3 WLA T-2 443

439 Softball N SL-1 field 0 100 ft. x WLA T-5

T 447.8 E WCC T6 Scale N N 441 IS C O U R 2 T 448.6 S

USGS Trench440 T-1 449.2 x 441 Basketball 443 court 442 x 449.1

Trancos Trace 442 WLA T-6

WLA T-7 x 454.5 Central Projection

450.3 x

Dense trees

x 449.3

x 454.8

PORTOLA VALLEY USGS NEHRP PENINSULA SEGMENT OF THE SAN ANDREAS FAULT

Structure Contour Map

WILLIAM LETTIS & ASSOCIATES, INC. Figure 4

12/20/07 1719 Portola Valley North-trending en echelon fractures across Portola Road following the 1906 earthquake. View is looking south toward present-day softball field at the Town Property.

View to the north-northeast showing fence offset from the 1906 surface-fault rupture. The fence was located directly north of Portola Road (see below) (photograph from Stanford University Archives, J.C. Banner Collection, courtesy of the U.S.G.S.). House modified in 2002, but shown on Figure 3. PORTOLA VALLEY USGS NEHRP PENINSULA SEGMENT OF THE SAN ANDREAS FAULT Historic 1906 Rupture

WILLIAM LETTIS & ASSOCIATES, INC. Figure 5

03/18/07 1719 Portola Valley

3.1 Woodside Trace On the basis of the 1906 surface-fault rupture across Portola Road, coupled with previous investigations within the area (WLA, 2003; WCC 1976a, 1976b and 1977), and the findings of this fault study, the Woodside trace consists of a complex zone of near-surface northwest-trending deformation (tilting, warping, folding and brittle surface-fault rupture). The trench exposures indicate discontinuous, en echelon fault traces. The Woodside trace is characterized by one or more of the following structures: en echelon northwest and/or northeast-striking faults, folds and west-facing monoclines (Figures 3 and 4). Photographs of the 1906 rupture taken near the site (Photo from the Stanford University Archives, J.C. Branner Collection, courtesy of the U.S. Geological Survey), and structural information collected from the trenches, provide limited constraints on the length and spacing of individual en echelon, northeast- striking and northwest-striking faults associated with the Woodside trace. Interpretation of the photograph of Portola Road indicates that the pattern of fracturing consists of several closely-spaced, north-trending en echelon extensional fractures (Figure 5).

Northwest-striking faults exposed in trench T-2 are east-dipping faults that show apparent reverse slip with the east side upthrown consistent with previous trenches. We interpret these east-dipping faults as flower structures linked at depth with a near-vertical strike-slip fault. Along with brittle surface-fault rupture, deformation along the Woodside trace also is accommodated by broad zones of anticlinal folding and west-facing monoclinal warping, that indicates an overall down-to-the-west vertical separation (Figure 4). Warping of late Holocene fluvial deposits into west-facing monoclines appears to be the most widely recognized tectonic signature of the Woodside trace at the Town Property. The sense of displacement across the monoclines is consistent with the sense of vertical separation across the faults encountered in trenches.

12

4.0 PALEOSEISMIC RESULTS

Our field studies in October 2005 focused on the San Andreas fault’s Woodside trace, which is the trace that ruptured in 1906. Of the two trenches, trench T-2 provides stratigraphic evidence suggestive of paleoearthquakes (as described below); where as trench T-1 showed possible evidence of very minor warping of laterally continuous marsh and alluvial sediments. Trench T-2 provides evidence of multiple earthquakes based on several colluvial deposits that we interpret as being shed episodically from a northwest-facing scarp associated with the Woodside trace. Detailed logs of trenches T-1 and T-2 were made for both the south and north walls at a scale of 1 inch = 0.5 meter (see Figures 6 to 8).

4.1 Near Surface Stratigraphy Portola Valley is filled with bedded late Quaternary sediments of variable thickness that includes undifferentiated Quaternary alluvium, as well as Holocene fluvial deposits of Sausal Creek, and lowland marsh deposits. These younger valley-fill sediments locally interfinger with distal alluvial fan and colluvial deposits along the valley margins (WLA, 2003). Geologic units encountered during our subsurface investigation are divided into five main units: (1) Holocene fluvial deposits, (2) Holocene marsh stratigraphy, (3) Holocene colluvial wedges, (4) undifferentiated Quaternary alluvium and (5) artificial fill. These units were further subdivided during our logging of the trenches, and where possible, correlated between the two trenches. General descriptions of the surficial units are provided below.

4.1.1 Holocene Fluvial Deposits Both trenches were excavated within the Sausal Creek meander corridor and encountered stream and overbank deposits associated with the creek. The fluvial deposits consist of clayey silt, silty clay and sandy silts with discontinuous coarse sand and silt or gravel lenses, occasional laminae, and abundant charcoal. The deposits are marked by fining upward sequences of bedded coarse channel deposits with overlying fine-grained silt and clay of overbank deposits. This material was deposited by Sausal Creek during episodes of overbank flooding and channel migration. Thin soils and/or marsh deposits (see below) are developed into the fluvial deposits and are characterized by weak subangular blocky structure, high clay content, and a dark color with traces of peaty material. The soils represent four to five brief periods of soil development and are generally buried by overbank deposits. Based on exposures in trench T-2 and other trenches excavated at the site (WLA, 2003), the eastward lateral extent of Sausal Creek fluvial deposits (in the northern part of the site) are restricted by the development of a west-facing monocline associated with the Woodside trace.

13

4.1.2 Holocene Marsh Deposits Numerous marsh/peaty deposits interpreted as buried soil (surface) horizons were encountered across the entire site. These deposits are characterized by dark grayish brown to black organic-rich silt and clay. The units are generally thinly-bedded and interbedded with the fluvial deposits. The weak to well- developed peaty soils that are developed within the marsh setting generally overlie Holocene overbank deposits.

14 Southeast Northwest

0 0 S89 E N70 E Bend in trench

135 1 1

130

120 135 110 100

2 90 2 Depth (meters) 80 Depth (meters) 70 1 60

50

40 3 40 3 30 20 30 20

4 4

012345678910 Horizontal Distance (meters)

Explanation Soil developed in marsh deposit Notes: 1. Refer to Trench T-2 North Wall (Figure 7) for unit descriptions. 2. Partially drafted log overlying field log. 3. Not fully drafted because of absence of paleoseismic data.

PORTOLA VALLEY USGS NEHRP PENINSULA SEGMENT OF THE SAN ANDREAS FAULT USGS Trench T-1 South Wall

WILLIAMLETTIS& ASSOCIATES, INC. Figure 6

12/26/07 1719 Portola Valley Zone of tilting Fault zone (brittle rupture) of Woodside trace ? Southwest Northeast

N60°E 0 Wood parking block 0 Christ Church parking lot Wood parking block Wood parking block Asphalt Wood parking block Wood parking block Asphalt Weathered iron 135 Wire FILL and MIXED ALLUVIUM 34N 5 Nail Pottery chip COL4 (1906) 132 130 11 COL3 12N 33N 9 1 Historic ground surface (1906) 21 38N 1 1 3 120 2N 36N 110 COL 30N COL1 110 2 115 22N 5 2 7 8 10 3 1 4 SOIL 4 5 MIXING Root 105 3 6 Area not logged 2 6 cast Depth (meters) Depth (meters)Depth (meters) 100 23 3a 2 7 90 16 SOIL 5a 11 88 MIXING 2 17 18N 3 2 2 2 85 80 ? 25N 70 18 10 1 1a 22 4a 60 12 4 9 20 19 13 30 25 8 50 19N35 14 40 15 Water level (October 20, 2005) Inferred northeast-dipping faults Not logged 3 3

0 1 2 3 4 5 6 7 8 9 10 11 12 13 Horizontal Distance (meters)

Explanation Bedding Orientation Data Unit Descriptions Unit Descriptions (continued) Unit Descriptions (continued) Bedding Orientation Data West of Fault East of Fault Fracture Orientation Fault Orientation

1 N36°W 9°SW 13 N46°W, 11°SW 22N26°W, 33°SW 1 N65°W, 90° 1a N16°W, 80°NE 135 Brown to light yellow-brown sandy gravel to gravely silt, 88 Dark yellow-brown clayey silt to silty clay, massive, 25 Gray-brown silty sand, fine-grained sand, massive bedded with clasts <0.5 cm, rounded with granular zones of oxidation, trace sand (Holocene alluvium - 2 N22°W, 5°SW 14 N32°W, 9°SW 23N24°W, 72°SW 2 N75°W, 88°SW 2a N18°W, 74°NE

matrix, slightly silty sand overbank, massive, clearly and Light brown gravelly fine to medium-grained sand with 15 3 3a overbank deposit) 20 3 N19°W, 14°SE N30°W, 18°SW N6°E, 90° N30°W, 80°NE abruptly cuts into underlying unit (Artificial fill) silt and clay, massive, saturated (Holocene alluvium 4 N28°W, 11°SW 16 N35°W, 9°SW 4 N31°W, 90° 4a N25°W, 75°NE Yellow clayey silt with sand, strongly oxidized, 85 5 N37°W, 12°SW 17 N37°W, 13°SW 5 N61°W, 80°SW 5a N15°W, 78°NE Dark brown to light brown clayey sandy gravel, weakly massive, reduced zones, clear to gradual basal Light brown silty clay to clayey silt, strongly oxidized, fine 132 5 6 N10°W, 7°SW 18 N47°W, 9°W 6 N65°W, 85°SW bedded to massive (Holocene alluvium) contact (Holocene alluvium) CaCO3 nodules (Quaternary alluvium) 7 N23°W, 0 - 3°SW 19 N30°W, 6°W 7 N8°W, 88°NE 20 8 N10°W, 80°NE Pale brown sandy silt, occasional light orange clasts Yellow to olive-brown clayey silt to silty clay, Brown silty clay, massive, unclear contacts,abundant 8 N37°W, 3°SW N55°W, 4°SW 130 4 21 9 <0.5 cm bioturbated with rootlets; plastic found in this 80 massive, abundant charcoal, very diffuse basal rootlets (Quaternary alluvium) 9 N30°W, 7°SW N21°W N18°W, 82°NE unit (Artificial fill) contact (Paleosol developed in alluvium) 10 N27°W, 11°SW 10 N16°W, 80°NE N42°W, 15°SW 11 N40°W, 90° 3 Tan to light brown silty sand, hard, mottled with oxidized 11 Dark brown to gray-brown clayey silt to silty clay, large to Yellow-brown clayey silt with sand, faint orange 120 blotches, fine CaCO3 nodules, white siltstone clasts 12 N39°W, 13°SW medium blocky peds, massive, gradual basal contact, 70 oxidation mottles, massive,abundant charcoal at (Holocene marsh deposit) with soil development (Quaternary alluvium) base, sharp basal contact (Holocene alluvium - Symbols Pale brown to light brown silty sand to sandy silt, overbank deposit) Dark brown clay, trace sand and gravel, few CaCO3 2 Soil developed in marsh deposit 2 Fracture orientation measurement 115 massive, friable, faint FeO2 mottles, rootlets (Holocene nodules (Quaternary alluvium with paleosol) alluvium - overbank deposit) Black to dark gray silty clay, massive, brown silty 3S K 60 mottles, abundant charcoal, clear basal contact Light olive-brown clay with sand, contains large (1 - 2 cm Charcoal sample and designation Krotovina 1 Brown to dark brown sandy gravel to gravelly sand (Holocene marsh deposit) diameter) CaCO3 nodule (see Table 1) imbricated pebbles, massive to bedded layers undulating s (Quaternary alluvium with Fault 110 paleosol) and interfingering with tan silty sand (Holocene alluvium) Dark brown to dark yellow-brown silty clay, massive 23 Bedding orientation measurement 50 with trace fine sand, orange-brown oxidation Brown to dark yellow-brown sandy silt, faint horizontal mottles, diffuse basal contact (Holocene alluvium) COL4 Mottled brown sandy silt with small angular pods of silt and clay, massive contains a fragment of milled wood 105 bedding, strongly mottled (Holocene alluvium) Dark gray-brown to gray-black silty clay, massive, (Holocene colluvium)

Dark to olive-brown clayey silt, zones of brown silty occasional gray reduced zones, diffuse to gradual 2 mottles, abundant charcoal, gradual basal contact 40 Dark yellowish brown clayey sandy silt, massive, friable, contact (Holocene alluvium) COL3 100 (Holocene marsh deposit) with small angular silty fragments, weak pedogenic Dark brown silty clay, massive with trace fine sand, development, gradational and diffuse upper contact and Dark brown sandy clay, massive abundant charcoal, trace faint dark orange-brown mottles, reduced clear basal contact (Holocene colluvium) PORTOLA VALLEY USGS NEHRP 35 PENINSULA SEGMENT OF THE SAN ANDREAS FAULT 95 clear basal contact (Holocene marsh deposit) zones (Holocene alluvium) Dark brown clayey sandy silt, massive, with small COL2 Tan to light yellow-brown silt to fine sand, firm to soft, Dark gray clayey sand, fine sand with coarse sand angular silty fragments (Holocene colluvium) USGS Trench T-2 North Wall mottled, clear basal contact with some undulations clasts, massive, wavy to smooth lower boundary 90 30 (Holocene alluvium) Tan to light brown silty clay to clayey silt, massive, COL1 unclear contacts, abundant rootlets WILLIAMLETTIS& ASSOCIATES, INC. Figure 7

03/06/08 1719 Portola Valley Northeast Southwest

N39°E

0 0 135

Milled wood Plastic 0.5 COL1 0.5 132 COLCOL4 130 3S 9S COLCOL3-2 120 Root cast 5 7S 6S 1.0 115 1.0 110 100 COL1 35 95 4 90 1.5 88 90 1.5 3 60 Root cast 85 2 Depth (meters) Depth (meters) 50 70 2.0 2.0 40 60 1 30 / 35 25? 2.5 Water level (October 20, 2005) 2.5

3.0 3.0

0 0.51.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0 8.5 9.0 9.5 10.0 10.5 11.0 11.5 12.0 12.5 13.0 13.5

Horizontal Distance (meters)

Explanation

Soil developed in marsh deposit Notes: 1. Refer to Trench T-2 North Wall (Figure 7) for unit descriptions. 3S Charcoal sample (see Table 1- ages 2. Drafted log modified from Wetenkamp (2007) thesis. are 2 ) 3. Colors are from Wetenkamp (2007) which do not match Figure 7 of this NEHRP report.

PORTOLA VALLEY USGS NEHRP PENINSULA SEGMENT OF THE SAN ANDREAS FAULT USGS Trench T-2 South Wall

WILLIAMLETTIS& ASSOCIATES, INC. Figure 8

03/18/07 1719 Portola Valley

4.1.3 Undifferentiated Quaternary Alluvium Undifferentiated Quaternary alluvial fan deposits are present in the east end of trench T-2. These deposits are characterized by tan to light-brown, poorly bedded sandy clay, silty sand with minor gravel lenses, and represent the oldest deposits exposed in the site trenches. The older alluvium contains carbonate nodules and vertical filaments filled with gray clay, and are strongly mottled to a rust orange and light brown color. Several of the alluvial units are interrupted by pedogenic horizons noted by an increase in dark brown color and clay content.

4.1.4 Scarp-Derived Colluvium Three to four colluvial deposits (“wedges”) derived from the undifferentiated Quaternary alluvium, and pedogenic horizons developed on an elevated portion of the west-facing monocline of the Woodside trace overlie and interfinger with the Holocene fluvial and marsh deposits to the west (Figure 7). The colluvial wedges (designated from oldest to youngest based on relative stratigraphic position include COL 1 to 4) are derived from a northwest-facing scarp caused by repeated tectonic uplift along the active Woodside trace. COL 1 is a massive colluvial deposit with gradational contact boundaries. Directly overlying COL 1 are colluvial units designated as COL 2 and 3. Interpretation of stratigraphic relations in the north wall of trench T-2 is permissive of two distinct colluvial packages (COL 2 and 3), however, this distinction is less obvious on the south wall, suggesting that COL 2 and COL 3 may be a single colluvial unit. Their stratigraphic relationship and ages, discussed below, provide evidence for either a single wedge (COL 2- 3) or two wedges (COL 2 and 3). Unit COL 4 is the youngest colluvial deposit, and is associated with an elongated shape-possibly from cultural modification-and likely represents a single earthquake. All of the wedges consist of mottled brown sandy silt and contain fragments of siltstone derived from the undifferentiated Quaternary alluvium.

4.1.5 Artificial Fill Varying thicknesses of artificial fill overlie the Holocene marsh and fluvial deposits. The artificial fill encountered in the trenches is between 1.5- to 3.0-feet in thickness, with a large accumulation of artificial fill along the existing buried Sausal Creek corridor. Historical artifacts and modern garbage, including wire, pottery and plastic, were encountered within the fill.

18

4.2 Trench T-1 Trench T-1 is located along the Woodside trace of the SAF as interpreted from previous trenching and drilling studies performed at the site (Woodward-Clyde Consultants, 1976; WLA, 2003; Figure 3). Previous trenching in the direct vicinity of the site exposed a discontinuous northeast-striking fault and/or subtle to clear west-facing monocline associated with the Woodside trace (WLA, 2003). Trench T-1 is located approximately 75 m southwest of trench T-2 and 80 m south of Portola Road. The trench was excavated along the southern margin of the Town Center softball field where it intersects a black-topped basketball court. The eastern extent of trenching was limited due to the presence of an active leach line and septic tank. The trench was excavated to a maximum depth of about 2.7 m where groundwater was encountered.

Trench T-1 exposed interbedded, laterally continuous, stratified Holocene fluvial deposits and weak soils developed in marsh deposits (Figure 6). Based on previous radiocarbon age-dating of detrital charcoal, pedologic development, and correlation of stratigraphy across the site, these units are late Holocene in age (WLA, 2003). The deposits generally dip gently (1 to 4 degrees) to the northwest, consistent with the regional gradient of Sausal Creek. Sandy gravel channel deposits exposed in the upper western part of the trench (unit 110) represent former distributary channels of Sausal Creek. In the northern part of the site, these coarse-grained sandy gravel units (i.e. basal unit 20) of Sausal Creek typically are constrained west of the Woodside trace and its associated west-facing monocline (topographic escarpment)(WLA, 2003). Several incipient and moderately developed soils developed in the fine-grained marsh deposits provide evidence for intermittent periods of stability in the valley that allowed time for pedogenic development.

No offset stratigraphy, faulting or shearing was observed in trench T-1, however, a weak, broad zone of west-facing monoclinal warping is inferred based on the trench exposure and bedding data. The basal deposits may record a weak overall down-to-the-west vertical separation within the well-bedded, laterally continuous units at depth but no brittle offset. Figure 5 illustrates the suggestion of a broad westward tilt of basal units 30 and 60. In the general region of trench T-1, the brittle rupture previously observed in trench WLA T-5 (2003) located 60 feet to the northwest should project through the center of the trench, however, either the fault: (1) dies out to the south as a very subtle monocline, or (2) faulting steps easterly beyond the east end of trench T-1. Both the fault termination and left-stepping, en echelon pattern of deformation are consistent with the observed faulting and folding directly north of trench T-1 (WLA, 2003). Regardless, the trench T-1 does not provide paleoseismic data useful for event chronology evaluation, and thus is not considered further in this study.

19

4.3 Trench T-2 Trench T-2 is located across the Woodside trace and extends along the southern edge of the Christ Church parking lot near Portola Road (Figure 3). The trench is positioned along a southeast projection to a series of 1906-event en echelon fractures in Portola Road that were observed and photographed shortly after the 1906 earthquake (Photo from the Stanford University Archives, J.C. Branner Collection, courtesy of the U.S. Geological Survey) (Figure 4). Fences bounding both sides of the road at this location were damaged during the 1906 event (Figure 5). The trench overlaps with a zone of monoclinal warping exposed in previous trenches WCC-T5 and WLA-T-1 to the southeast (Figure 3). The trench was located within a narrow undeveloped strip of land between the Christ Church parking lot (paved) and the northern fenceline of the Town of Portola Valley which apparently was an orchard at the time of the 1906 earthquake. The trench location was constrained in length and orientation due to existing cultural constraints (paved parking lot and a recently refurbished soccer field). The trench was excavated to a maximum depth of about 3 meters. Groundwater was encountered at a depth of approximately 2.5 meters in the western end of the trench and limited the depth of the trench.

The eastern end of trench T-2 exposed sheared and folded undifferentiated Quaternary alluvium and pedogenic units whereas the western part of the trench exposed bedded late Holocene alluvial, fluvial and marsh stratigraphy (Figures 7 and 8). A well-developed zone of upward branching and anastomosing faults and fractures is present at the east end of the trench between stations 9 and 12 m on the north wall and stations 0.5 to 13.5 m on the south wall. An uplifted block of undifferentiated Quaternary alluvium depicting clear east-side up vertical separation is juxtaposed at or near the Woodside trace against late Holocene interbedded, vertically stratified units to the west. Units 1 through 4 of the undifferentiated Quaternary alluvium appear to be offset along pervasive, northeast-dipping faults, with up to 16 cm of west-side down vertical offset (Figures 7 and 8). A northeast-dipping fault at station 11 m, defined by a zone of up to 11 cm of fault gouge, appears to vertically displace unit 4 about 60 cm along the fault to within about 1.5 m of the ground surface (Figure 7). Because of the absence of Holocene stratigraphy directly overlying the undifferentiated Quaternary alluvium east of the fault zone, and the pervasive fracturing present in unit 5, none of the faults can be confidently traced to the ground surface. The undifferentiated Quaternary alluvium and weak pedogenic units form a distinct buried west-facing topographic escarpment. The buried escarpment strikes northwest and contains units dipping 33° (unit 1) and up to 72° (unit 3) to the southwest. These deposits and weak pedogenic soil horizons faulted by the Woodside trace contain no dateable material; however, the presence of calcium carbonate filaments

20

suggests antiquity to the deposits. For these reasons, the observed unit offsets provide no direct evidence on the timing of surface rupture events, and the 1906 ground rupture event was not identified. We note, however, that the shears observed in the trench align to the northwest with the en echelon fractures observed in Portola Road shortly after the 1906 earthquake.

On the western side of the west-facing scarp, laterally continuous, stratified Holocene marsh and fluvial units deposited by Sausal Creek, are deposited against the uplifted undifferentiated Quaternary alluvium fault block (Figures 7 and 8). Basal units composed of interbedded marsh and fluvial deposits, up through at least unit 60 (and possibly through unit 85), appear gently tilted to the southwest in relation to units 88 and above. Above unit 88 the strata consist of relatively flat-lying well-bedded marsh deposits with weak incipient pedogenic horizons interbedded with coarse-grained fluvial deposits. Units 25 to 40 may be undifferentiated Quaternary alluvium or Holocene alluvium draped over the older alluvial deposits. The alluvium includes the oddly shaped unit 30 that has likely undergone mixing with the alluvial unit above (unit 40) from a root throw, fissure fill or bioturbation. Minor bioturbation was observed in the upper units. The Sausal Creek and marsh deposits contain abundant peat and detrital charcoal, and were amply sampled for age dating purposes. The results of the age dating are discussed below. Lastly, the entire trench is capped with up to 1 m of multiple artificial fill units.

4.3.1 Event Stratigraphy Although dateable offset features were not encountered in the trenches, we interpreted colluvial units to be derived from an episodically uplifted west-facing fault scarp. In addition, we interpret west-tilted stratigraphy to indicate the occurrence of a prehistoric earthquake. Using these features we have developed a preliminary paleoearthquake record for the Town of Portola Valley site.

4.3.1.1 Scarp-Derived Colluvial Wedges Three and possibly four colluvial wedges were interpreted in trench T-2 (units COL 1 through 4). Based on the textural compositions of the wedges and their stratigraphic relations, units COL 1 to 4 are all derived from the older undifferentiated Quaternary alluvium and Holocene marsh strata (Figures 7 and 8). The colluvial deposits are briefly described below from oldest to youngest.

Unit COL 1: The oldest colluvial wedge interpreted at the site is comprised by unit COL 1. Based on the relatively large size and morphology of unit COL 1, this massive colluvial deposit may result from multiple, indistinguishable surface-rupturing events. Recognizing discrete colluvial bodies within unit

21

COL 1 was not possible due to mixing from bioturbation, possible tree and root throw, and soil forming processes. Unit COL 1 directly overlies the oldest alluvial deposits exposed in the trench and drapes the uplifted block of undifferentiated bedded Quaternary alluvium. Unit COL 1 is composed of light brown clayey silt to silty clay with fragments of siltstone.

Units COL 2 and 3: The colluvial units COL 2 and COL 3 are differentiated only on the north wall of trench T-2, and appear to form a single wedge on the south wall (Figures 7 and 8). The colluvium consists of a dark yellow brown to dark brown clayey sandy silt with small angular silty fragments likely derived from the uplifted older undifferentiated Quaternary alluvium. Despite the color and textural variations delineating the two colluvial units on the north wall, there is no evidence of two separate colluvial bodies associated with units COL 2 and 3 on the south wall. This absence of correlation between the two walls may be due to the fact that part of the south wall was inadvertently saturated by sprinklers during the first couple of days the trench was open. Subsequently, the south trench wall remained moist and never fully cured similar to the drier north wall. The persistent moisture may have obscured the color change observed between colluvial units COL 2 and 3 exposed on the north wall. Stratigraphic and structural relations on the south wall were more difficult to decipher, thus we entertain the possibility of the existence of two separate colluvial wedges (units COL 2 and 3), as well as a combination of the two units (i.e., unit COL 2-3) forming a single colluvial wedge.

Unit COL 4: The youngest colluvial wedge, unit COL 4, overlies unit 120 (historic), units COL 3 and COL 1 (Figure 7). The colluvium of unit COL 4 is truncated by unit 132, a weakly bedded historic paleochannel deposit. Unit 132 also truncates another historic stream channel deposit (unit 130) with remnants of plastic and wood directly west of the buried escarpment. Unit COL 4 is composed of massive, mottled brown sandy silt with small angular pods of silt and clay, and contains a fragment of milled wood. Stratigraphic and structural relations suggest that unit COL4 was formed by degradation of a 1906-fault scarp. There is no surface expression of the scarp in present-day topography at the site because of cultural modification of the escarpment through grading and farming across the valley; however, we infer the presence of a former scarp based on the presence of the underlying tilted and folded Holocene stratigraphy in trench T-2 and previous trenches excavated across the 1906 rupture trace (WLA, 2003). The available historic photos capturing the 1906 rupture across Portola Road, however, do not show a fault scarp as interpreted in the Portola Valley trenches. The absence of a fault scarp in Portola Road following the 1906 earthquake may be because the Woodside trace has exited the structurally

22

controlled basin of the Portola Valley site.

4.3.2 West-Tilted Stratigraphy Additional event stratigraphy at the site is interpreted based on an increasing degree of tilted fluvial and marsh deposits with depth. On the basis of bedding measurements collected at the base of stratigraphic contacts, units 40 to 60 are warped across the buried west-facing monocline associated with the Woodside trace. Collectively, the units represent a vertical separation that is down-to-the-southwest with respect to the ground surface and the overlying stratigraphy (Figure 7). The change in dip above and below unit 60 indicates a possible deformation event large enough to tilt these older strata. The stratigraphic units 60 and older are tilted westerly up to 18° compared to the younger overlying deposits that typically contain shallow westerly dips. The total vertical separation accommodated by the tilt, as measured along the clear basal contact between units 60 and 70, is 0.58 m. With the exception of unit 88, the warped units maintain a relatively constant thickness away from the uplifted fault block, consistent with our model of an uplifted fault block. On the basis of stratigraphic relations and bedding dip, the tilting event occurred after the deposition of unit 60, and prior to deposition of unit 88, which maintains a horizontal upper contact and thickens away from the Woodside trace.

4.4 Age of Deposits Age estimates of the surficial deposits at the Portola Valley site are derived from cross-cutting stratigraphic relationships and radiocarbon analysis of fourteen detrital charcoal fragments. The marsh, fluvial and colluvial deposits contained ample material for sampling and analysis. The samples collected from trench T-2 were analyzed by accelerator mass spectrometry (AMS) at Lawrence Livermore National Laboratory or the NSF-Arizona Center for Mass Spectrometry. Several of the samples were pre-treated and processed for analysis at the U.S. Geological Survey C14 Laboratory in Reston, Virginia, and subsequently analyzed at Livermore or Arizona. The radiometric dates were dendrochronologically calibrated to calendar years (2 sigma) according to the procedure of Stuiver and Reimer (2004) and compared to several previous samples collected and analyzed from trenches excavated at the site (WLA, 2003). The results of the radiometric analysis are presented in Table 1, and collectively define a relatively consistent stratigraphic sequence with depth.

The silty clay to clayey silt and weak soil of unit 120 is interpreted as the likely ground surface at the time of the 1906 event. Historical objects collected from units above and inset into unit 120 strongly suggest that the minimum age for this unit is very close to historical. For instance unit 130, a historical fluvial deposit, contains several small pieces of thin plastic, nails and debris near the basal contact with unit 120.

23

In addition, milled wood was collected from unit COL 4 on the south wall, which overlies unit 120. Lastly, the absence of an overlying soil above unit 120 suggests that this deposit was at or near the former ground surface during the 1906 earthquake. A single charcoal sample (RC-34N) collected from unit 130 yielded a calibrated age of A.D. 1650 to 1950, consistent with the cultural material collected from the unit. Two charcoal samples collected from unit 120 (RC-33N and RC-3S) provided a consistent suite of calibrated ages of A.D. 1440 to 1640 and A.D. 1440 to 1630, respectively. These older ages of unit 120 represent a maximum age, and suggests that unit 120 may have been present near the ground surface for at least several hundred years prior to cultural modification and burial by historical deposits (units 130, 134, and COL 4).

Unit COL 4 is a well defined, massive colluvium that originated from the uplift and erosion of older undifferentiated Quaternary alluvium and marsh deposits. Colluvial unit COL 4 overlies unit 120 (A.D. 1440-1640). The colluvial unit also is truncated to the west by unit 132, which overlies unit 130, a fluvial deposit with historical debris and a calibrated age of A.D. 1650 to 1950. On the basis of stratigraphic position and geometric shape of the deposit, as well as the presence of silt and clay fragments similar to the undifferentiated Quaternary alluvium, unit COL 4 is interpreted as scarp-derived colluvium related to the 1906 earthquake.

Colluvial unit COL 3 is defined by friable dark yellowish brown sandy silt with weak pedogenic development. It has a gradational and diffuse upper contact and a relatively clear basal contact noted by a lighter brown color compared to adjacent units COL 2 and 1 (with respect to the north wall of trench T- 2). The estimated age of unit COL 3 is constrained by the overlying unit 120 (A.D. 1440 to 1640) and the underlying colluvium of unit COL 2. Unit COL 2 has an estimated calibrated age of A.D. 1410-1490 (see below). An anomalously old age yielded by sample RC-12N from unit COL 3 provides a calibrated age of B.C. 370 to 120, and is not considered further in the analysis. Therefore, stratigraphic relations and radiocarbon data provide an age for unit COL 3 that ranges between A.D. 1410 and 1640.

The age of colluvial unit COL 2 is constrained by its interfingering with unit 100, as well as the presence of older underlying units COL 1 and 80, and the younger unit 105 overlying COL 2. The base of COL 2 is interfingered with unit 100 directly west of the scarp, meaning unit 100 is coeval with the deposition of COL 2. A detrital charcoal sample (RC-22N) collected from unit 100, a thinly-bedded marsh deposit that interfingers with unit COL 2, provides a calibrated age of A.D. 1410 to 1490 (99% probability). This age represents an upper age limit for unit COL 2. Sample 25N from unit 80 provides the youngest calibrated age of A.D. 1260 to 1390 that provides a lower age limit of A.D. 1260 for COL 2. This lower bound is in

24

agreement with the maximum age provided by charcoal sample (RC-2N), extracted from the colluvium of unit COL 2, yielding a calibrated age of A.D. 1270 to 1390, and age data yielded by samples from COL 1. Four of five calibrated C14 ages derived from unit COL 1 closely overlap in age with sample RC-2N collected from the overlying COL 2 colluvium. The stratigraphically inconsistent ages between units COL 1, 2 and 80 strongly suggest that sample RC-2N was derived from an uplifted and reworked unit COL 1, and thus represents a maximum age for unit COL 2. In summary, the stratigraphic and age relations indicate that the age of unit COL 2 ranges from A.D. 1260 to 1490.

Alternatively, units COL 3 and 2 may represent one colluvial deposit (i.e., unit COL 2-3) derived from a single event, presumably the penultimate event. If we assume that the two colluvial units represent one colluvial wedge, the age constraints for this earthquake are the same as COL 2, described above.

Four detrital charcoal samples collected and analyzed from unit COL 1 yield ages ranging from A.D. 1260 to 1420. A fifth sample, RC-6S, yielded an anomalous age of A.D. 260 to 530 that appears to be too old with respect to the four samples collected from the same unit. This discrepancy likely indicates that sample RC-6S represents reworked older charcoal, and thus is not included in our analyses. Based on stratigraphic relations exposed in the south wall, unit 60 directly underlies unit COL 1 and has a maximum age of A.D. 1030 to 1210 (charcoal sample RC-18N from unit 60). The four charcoal samples extracted from the massive colluvial wedge of COL 1 and sample RC-18N, indicate that the age of unit COL 1 is in the range of A.D. 1030 to 1390.

25 Table 1. Summary of Radiocarbon Analyses, Trench 2, Portola Valley Site

95.4% (2σ) cal age Stratigraphic Sample 14C Age, 95.4% (2σ) cal age ranges# for relative area under Sample Name Lab No. @ +/- Unit Description yr B.P. total range# discontinuous total distribution distributions 1650 AD to 1680 AD 0.32 [305 BP TO 266 BP] 1650 AD to 1950 AD 1730 AD to 1810 AD PV-T2-RC-34N CAMS 123577 130 Charcoal 210 30 0.53 [305 BP TO -1 BP] [216 BP TO 144 BP] 1930 AD to 1950 AD [21 0.15 BP TO -1 BP] 1440 AD to 1530 AD 0.54 1440 AD to 1640 AD [507 BP TO 418 BP] PV-T2-RC-33N CAMS 5637 120 Charcoal 375 45 [507 BP TO 315 BP] 1540 AD to 1640 AD 0.46 [413 BP TO 315 BP] 1440 AD to 1520 AD 0.77 [512 BP TO 427 BP] 1440 AD to 1630 AD 1560 AD to 1560 AD PV-T2-RC-3S AA 5635 120 Charcoal 397 31 0.001 [512 BP TO 321 BP] [390 BP TO 390 BP] 1570 AD to 1630 AD 0.23 [378 BP TO 321 BP] 370 BC to 155 BC [2319 0.97 Reworked 370 BC to 120 BC BP TO 2105 BP] PV-T2-RC-12N CAMS 123575 COL 3 2175 35 Charcoal [2319 BP TO 2065 BP] 140 BC to 120 BC [2087 0.03 BP TO 2065 BP] 1270 AD to 1330 AD 0.52 Reworked? 1270 AD to 1390 AD [677 BP TO 625 BP] PV-T2-RC-2N CAMS5634 COL 2 665 35 Charcoal [677 BP TO 556 BP] 1340 AD to 1390 AD 0.48 [606 BP TO 556 BP] 1410 AD to 1490 AD 0.99 1410 AD to 1610 AD [540 BP TO 459 BP] PV-T2-RC-22N CAMS 123581 100 Charcoal 450 35 [540 BP TO 340 BP] 1600 AD to 1610 AD 0.01 [347 BP TO 340 BP] 1260 AD to 1320 AD 0.62 1260 AD to 1390 AD [689 BP TO 626 BP] PV-T2-RC-36N CAMS 123579 COL 1 Charcoal 685 40 [689 BP TO 557 BP] 1350 AD to 1390 AD 0.38 [605 BP TO 557 BP] 1270 AD to 1370 AD 0.71 1270 AD to 1410 AD [682 BP TO 580 BP] PV-T2-RC-30N CAMS 123578 COL 1 Charcoal 590 30 [682 BP TO 537 BP] 1380 AD to 1410 AD 0.29 [571 BP TO 537 BP] 1300 AD to 1370 AD 0.63 1300 AD to 1420 AD [651 BP TO 581 BP] PV-T2-RC-9S CAMS 123576 COL 1 Charcoal 575 35 [651 BP TO 527 BP] 1380 AD to 1420 AD 0.37 [569 BP TO 527 BP] 1260 AD to 1320 AD 0.68 1260 AD to 1390 AD [688 BP TO 632 BP] PV-T2-RC-7S CAMS 5636 COL 1 Charcoal 690 35 [688 BP TO 560 BP] 1350 AD to 1390 AD 0.32 [598 BP TO 560 BP] 260 AD to 290 AD [1691 0.06 BP TO 1656 BP] 320 AD to 440 AD [1628 0.9 260 AD to 530 AD BP TO 1514 BP] PV-T2-RC-6S CAMS 123580 COL 1 Charcoal 1660 30 [1691 BP TO 1422 BP] 490 AD to 510 AD [1460 0.02 BP TO 1441 BP] 520 AD to 530 AD [1432 0.01 BP TO 1422 BP] 1260 AD to 1310 AD 0.81 1260 AD to 1390 AD [688 BP TO 641 BP] PV-T2-RC-25N CAMS 123582 80 Charcoal 700 30 [688 BP TO 564 BP] 1360 AD to 1390 AD 0.19 [589 BP TO 564 BP] 1030 AD to 1210 AD 1030 AD to 1210 AD PV-T2-RC-18N CAMS 123584 60 Charcoal 900 40 1 [916 BP TO 736 BP] [916 BP TO 736 BP] 40 BC to 90 AD [1993 0.96 40 BC to 120 AD [1993 BP TO 1862 BP] PV-T2-RC-19N CAMS 123583 40 Charcoal 1965 35 BP TO 1828 BP] 100 AD to 120 AD [1848 0.04 BP TO 1828 BP]

Samples are listed in stratigraphic order, with stratigraphically highest samp Lab No. @: CAMS = Center for Accelerator Mass Spectrometry (CAMS), Lawrence Livermore National Laboratory AA = University of Arizona (AMS Technique) # Calibrated based on Stuiver and Reimer (2004) using CALIB v. 5.0.1 Sample Name indicates: N = North wall, S = South wall

5.0 DISCUSSION

The Portola Valley trenches provide evidence that we interpret to represent at least three earthquake events, including the 1906 earthquake, all of which appear to be large magnitude, scarp-forming events. The event chronology developed form Portola Valley is preliminary and is based on the exposures provided from a single trench (trench T-2). The trench provides evidence for the formation of at least three scarp-derived colluvial wedges (designated from oldest to youngest as units COL 1, 2-3 and 4), as well as possibly an additional colluvial wedge (unit COL 3), in addition to tilting of Holocene fluvial deposits. The event chronology information is discussed below in relation to other investigations performed on the Peninsula segment and the northern segment of the San Andreas fault.

5.1 Event 1 Evidence for the oldest event is the tilting of laterally-continuous and stratified marsh and fluvial deposits exposed in trench T-2. The tilting event includes all basal units of trench T-2 up through unit 60 and possibly including units 70, 80 and 85. The event is based on the distinct change in dip between strata above and below unit 60 (or 85). A large earthquake likely caused the greater westward tilt observed in units 40 through 60. Strata younger than unit 85 typically are not tilted. It is impossible to discern whether units 70 through 85, which do tilt slightly to the west, were deposited prior to the tilting and were truncated by erosion prior to the deposition of unit 88, or deposited post-tilt and pinch out against the tilted units (units 40 to 60). Unit 80 appears to thin slightly toward the monocline, and exhibits less westerly tilting, but the relationship is not definitive. Unit 60 has a C14 calibrated age of A.D. 1030 to 1210 (sample RC-18N) which is stratigraphically consistent with the overlying dates. The upper bound for the tilting event is provided by sample RC-22N (unit 100), thus the full age range for Event 1 is A.D. 1030 to 1490. Note that four out of five charcoal samples analyzed from unit COL 1 (calibrated ages range from A.D. 1260 to 1420) overlap with the timing of the tilting event, suggesting unit COL 1 may have formed in response to the event that warped units 40 through 60 or 85. Lastly, we note that the inclination of unit 40 increases near the base of the trench perhaps indicating greater tilt with depth and possibly earlier events. The calibrated age of unit 40 is 40 B.C. to 120 A.D. (sample RC-19N), and any possible earlier event would postdate the deposition of this sample.

Event 1 overlaps with at least two pre-historic earthquakes on the North Coast segment of the San Andreas fault (Figure 9). Although no distinct stratigraphic boundaries were identified within unit COL 1, the overall size of the colluvial wedge deposit could account for multiple events consistent with this interpretation. Paleoseismic investigations (Kelson et al., 2001; Noller et al.,1993; Simpson et al.,1996;

27

Knudsen et al., 2002; Kelson et al., 2006) on the North Coast segment have reported two large events (i.e., Events 3 and 4 of Figure 9) that overlap with Event 1 of the Town of Portola Valley. The timing of these two previously interpreted events include: (a) Event 4 estimated to have occurred between A.D. 1040 and 1190, and (b) Event 3 dated between A.D. 1220 and 1380. Event 1 of the Portola Valley site encompasses both of these moderately constrained events (Figure 9).

5.2 Event 2 The simplest interpretation for the origin of colluvial units COL 3 and 2 is that these two colluvial deposits represent a single event that post dates the tilting described above and the formation of unit COL 1. Under this scenario, we consider event 2 as consisting of a single colluvial wedge (designated unit COL 2-3) derived from a west-facing fault scarp produced by the Woodside trace. Based on stratigraphic and structural relations, unit COL 2-3 was deposited after the penultimate event for the Portola Valley site. The upper bound of Event 2 is constrained by the fact that the base of the colluvium interfingers with unit 100 (calibrated age of A.D. 1410 to 1490). The lower bound of the event is constrained by the age of unit 80 (A.D. 1260 to 1390). Therefore, Event 2 occurred between A.D. 1260 and 1490 (shown as a vertical black bar on Figure 9). Event 2 may correlate with a large earthquake interpreted to have occurred after A.D. 1450 at Filoli (Hall et al., 1999) located 13 km northwest of Portola Valley on the Peninsula segment of the San Andreas fault (Figure 9). Event 2 also overlaps with the age of a large event on the Santa Cruz Mountains segment that occurred before A.D. 1665 (Schwartz et al., 1996) and A.D. 1670 (Heingartner, 1998) at the Grizzly Flat and Arano Flat sites, respectively (Figure 9).

5.2.1 Alternative Interpretation of Event 2 Horizon The combined colluvial wedge of COL 2-3 can be interpreted as the product of two separate events, as delineated only on the north wall of trench T-2. If the combined wedge of unit COL 2-3 is assumed to represent two different events, the colluvial wedges noted by units COL 2 and COL 3 are significantly smaller than unit COL 4 that represents the 1906 earthquake (see Event 3 below). The apparent smaller size of the colluvial bodies may be the result of smaller magnitude events, or the 1906 event strata have been modified by cultural processes and the size of the wedge is artificially large from grading and plowing. Alternative interpretations for event horizon COL 2-3 are discussed below.

28 N 0 50 100 km

1 12 11 San Juan Cape 5 6 7 8 9 10 Fault Bautista 3 4 Mendocino San Andreas San Gregorio Pt. Arena Fault PACIFIC OCEAN

2 Santa Cruz Mts. segment Offshore segment North Coast segment Peninsula segment

Explanation 2000 Shelter Cove (Prentice et al., 1999 ) Noyo Canyon (Goldfinger et al., 2003a) Alder Creek (Baldwin, 1996) Scaramella Ranch (Prentice, 1989) Prentice et al., 2000) Fort Ross Orchard (Kelson et al., 2003; Fort Ross Archae Camp (Noller et al., 1993; Simpson et al., 1996 Bodega Harbor (Knudsen et al., 2002) Vedanta Marsh (Niemi et al., 2004; Zhang et al., 2004) Bolinas Lagoon Knudsen et al., 2002) Filoli (Hall et al., 1999) Portola Valley (THIS STUDY ) Grizzly Flat (Schwartz e al., 1998) Mill Canyon/Arano Flat (Heingartner, 1998; Fumal et al., 2004) 8 7 1 2 3 4 5 5 6 9 12 1906 10 11 Multiple event interpretation (2A and 2B 1900 1838 this study) ? ? 1800 1770 No Event 2 1700 record? 1665 1670 1700 1670 Notes: 1. Open bars show 1-sigma confidence 1640 intervals; solid bars show 2-sigma 1600 confidence limits. Colored horizontal 1490 1500 bands show interpreted time 1450 1490 1430 windows for ruptures on the North 1400 Event 3 Coast segment based on data from 1300 the Fort Ross Orchard site, from 1260 1260 Kelson et al., 2006. 1200 2. Modified from Kelson et al., 2006. Event 4 1100 2 events?

Calendar Age (AD) Calendar 1030 1000 1020 Re-analysis in progress 900 Event 5 800

700 PORTOLA VALLEY NEHRP Point Arena Point 2 events Golden Gate

600 Los Gatos Re-analysis in progress No well constrained timing data

500 San Juan Bautista Preliminary Earthquake Chronology 400 WILLIAM LETTIS & ASSOCIATES, INC. Figure 9

03.19.08 1719 Portola Valley

5.2.1.1 Event 2A (Unit COL 2) If unit COL 2 represents a separate event predating unit COL 3, then this event horizon demarcates the pre-penultimate earthquake at the site. Unit COL 2 interfingers and is coeval with unit 100, dated at cal A.D. 1410 to 1490, providing an upper bound for event 2A. There is little constraint on the lower bound of Event 2A because all available age data for strata directly underlying unit COL 2 are either from the colluvium of unit COL 1 or unit 80 (Table 1). The ages derived from these two underlying deposits are similar (Table 1). Based on a single charcoal fragment collected from Unit 80 (sample RC-25N) the age of unit 80 is cal A.D. 1260 to 1390. Therefore, the timing of Event 2A is the same as the age for Event 2, and ranges from cal A.D. 1260 to 1490 (shown a vertical blue bar on Figure 9).

5.2.1.2 Event 2B (Unit COL 3) In this interpretation, the event horizon below unit COL 3 represents the penultimate earthquake for the site. The upper age of this event is constrained by unit 120 (cal 1440 to 1640 AD), which overlies unit COL 3. The lower age of this event is constrained by the age of unit 100 (1410 to 1490 AD). Therefore, if unit COL 3 represents a separate event horizon, Event 2B occurred between 1410 and 1640 AD (shown as vertical blue bar on Figure 9).

5.3 Event 3 -1906 Earthquake The 1906-event is represented in trench T-2 as a colluvial wedge (COL 4) overlying unit 120, the interpreted 1906 ground surface. As discussed above, the elongated geometry and size may be the result of cultural modification, or alternatively, the colluvial deposit may represent the 1906 event overprinting the smaller 1838 earthquake of Toppozada and Borchardt (1998) and Hall et al. (1999). If colluvial units COL 2 and 3 indeed represent separate event horizons, then it is plausible that the colluvium deposited in response to the 1838 event could be contained within unit COL 4 based on size alone. However, we interpret unit COL 4 as a single event associated with the 1906 earthquake. We did not observe faults rupturing through the 1906 surface in the exposed fault block. In summary, available stratigraphic and radiocarbon age data, as well as cultural debris, do not permit the interpretation of any events between the deposition of unit 120 and 1906, such as the 1838 event.

30

6.0 CONCLUSIONS

Paleoseismic trenching within the Portola Valley Town Center site provides preliminary late Holocene event chronology data for the Peninsula segment of the San Andreas fault. Trench T-2 exposed colluvial units, fault gouge, and dipping fluvial and marsh stratigraphy from which we interpret multiple paleoearthquakes. Several distinct packages of fine-to medium-grained, colluvium derived from the uplifted undifferentiated Quaternary alluvium provide evidence for three, and maybe four, events within the last approximately 1,000 years. Based on stratigraphic and structural relations, radiocarbon dating, and the presence of historical artifacts, at least three surface-deforming events occurred: (1) A.D. 1030 to 1490, (2) A.D. 1260 to 1490, and (3) A.D. 1906. An possible alternate interpretation of Event 2 is that a fourth event (Event 2B) may have occurred between A.D. 1410 and 1640.

The southwestward dip of older units exposed in trench T-2 provides evidence for the earliest event (Event 1) exposed in this trench, and is perhaps consistent with the formation of colluvial wedge unit COL 1. The timing of Event 1, A.D. 1030 to 1490, overlaps with the event chronology for two prehistoric earthquakes (Events 3 and 4) interpreted from other paleoseismic sites on the northern San Andreas fault (Figure 9). The stratigraphic and structural relations exposed in trench T-2 do not provide data to further constrain the timing of Event 1 with respect to these two prehistoric earthquakes.

The age of the interpreted penultimate event (Event 2) for the Portola Valley site is inconsistent with the findings of previous paleoseismic studies performed along the northern San Andreas fault north of the Golden Gate, and it slightly overlaps with sparse timing data on the penultimate event on the Peninsula and Santa Cruz Mountains segments. The timing of the penultimate event from multiple trench studies in the North Coast, Peninsula, and Santa Cruz Mountains segments of the SAF has been compiled for the northern San Andreas fault (Figure 9). The age range of combined Event 2 from Portola Valley overlaps with ages provided for Event 3 on the North Coast segment. Lastly, Schwartz et al. (1998) estimated a minimum age of A.D. 1665 for the penultimate event on the Santa Cruz Mountains segment, which also is consistent with the Portola valley site.

The alternate interpretation that COL 2-3 is the product of two separate events is more consistent with North Coast, Peninsula and Santa Cruz segment event chronologies (Figure 9). Event 2A dated between A.D. 1260 and 1490 is consistent with Event 3 on the North Coast segment. Event 2B dated between A.D. 1490 and 1640 is consistent with an event from the Filoli Estate. The trench exposures at the Filoli Estate, 13-km to the northwest (Hall et al., 1999) is the only other investigation on the Peninsula segment

31

that provides event chronology data on the northern San Andreas fault. Hall et al. (1999) provide a maximum age of A.D. 1450 for a penultimate “1906-type” event. The timing of Event 2B also correlates with the age of an event at Coward Creek (i.e., Arano Flat) located on the Santa Cruz Mountains segment (Figures 1 and 9). The Coward Creek site suggests an event occurred between A.D. 1430 and A.D. 1670 (Heingartner, 1998).

More recent studies by Fumal et al. (2004) at the Arano Flat (Coward Creek) and Mill Canyon sites on the Santa Cruz Mountains segment provide evidence for three ground-rupturing earthquakes since about 1500 A.D., with the penultimate event dated at A.D. 1700 to 1770 (Figure 9). This age is much younger than the age of the penultimate event at the Portola Valley site; however, recent investigations on other segments north of the Golden Gate also make the penultimate earthquake significantly younger. For instance, fault studies at Fort Ross on the North Coast segment provide an age of A.D. 1660 to 1812 for the penultimate event (Kelson et al., 2006) (Figure 9). Regardless, Event 2, as well as Events 2A and 2B, does not appear to correlate with the age of the penultimate event on the northern San Andreas fault, north of the Golden Gate. The various interpretations of Event 2 at the Town of Portola Valley indicate that the Peninsula segment may behave differently than segments directly to the north and south. In addition, if Event 2B occurred, it may have only ruptured the Santa Cruz and Peninsula segments of the San Andreas fault.

In summary, some of the age ranges for events interpreted at the Portola Valley site are consistent with results from several other sites along the Peninsula segment and northern San Andreas fault. However, the age constraints allow broad ranges in event timing that may influence these apparent consistencies. Additional age dating is ongoing in an attempt to further constrain Events 1, 2 and the potential Event 2B. We note that event 2B does not appear to correlate with the earthquake chronology for the northern San Andreas fault, north of the Golden Gate, but is consistent with events from the Peninsula and Santa Cruz segments. The occurrence of the 1906 earthquake along the entire northern San Andreas fault, and the possibility that the 1838 earthquake ruptured along only the Peninsula fault segment of the San Andreas fault (Toppozada and Borchardt, 1998) suggests that the northern San Andreas fault may rupture along all or just some of its 435-km length. This model is further supported by evidence for another event (event 2B) on the Peninsula and Santa Cruz segments that is not consistent with North Coast event chronology. While this investigation provides some preliminary data about the Peninsula segment of the SAF, the current state-of-knowledge is inadequate to define the rupture behavior. Additional data is needed to constrain the fault characteristics of the San Andreas fault, and in particular the Peninsula Segment.

32

7.0 ACKNOWLEDGEMENTS

This research was supported by the U.S. Geological Survey (USGS), Department of the Interior, under USGS award number 05HQGR0073. The views and conclusions contained in this document are those of the authors and should not be interpreted as necessarily representing the official policies, either expressed or implied, of the U.S. Government. The town of Portola Valley graciously provided permission to conduct this investigation within the Portola Valley Town Center. A special thanks to the Town Board and maintenance crew of Portola Valley. Eurotech Construction ably excavated and backfilled the trenches. Mr. Sean Sundermann (WLA) provided invaluable assistance both in the trenches and in completing this report. Dr. Stephen Thompson (WLA) assisted with some of the trench documentation efforts. Thanks also to Dr. David Schwartz of the U.S. Geological Survey (Menlo Park) for providing insightful field and office discussions. Our appreciation also goes to Jack McGeehin (U.S. Geological Survey, Eastern Earth Surface Processes Team) for completing radiometric analyses.

33

8.0 REFERENCES

Brabb, E.E., and Pampeyan, E.H., 1983, Geologic map of San Mateo County, California, U.S. Geological Survey Miscellaneous Field Investigations Map-1257A; 1:62,500-scale. Bakun, W.H., 1999, Seismic activity of the San Francisco Bay Region: Bulletin Seismological Society of America, v. 89, no. 3, p. 764-784.

Baldwin, J.N., 1996, Paleoseismic investigation of the San Andreas fault on the north coast segment, near Manchester, California: M.S. thesis San Jose State University, 127 p. plus plates.

Baldwin, J.N., Knudsen, K.L., Lee, A., Prentice, C.S., and Gross, R., 2000, Preliminary estimate of coseismic displacement of the penultimate earthquake on the northern San Andreas fault, Pt. Arena, California, Proceedings of the 3rd Conference on Tectonic problems of the San Andreas fault system, eds. Bokelmann, G. and Kovach, R., Stanford University Publications, Geological Sciences, XXI, p. 355-369.

Bay Area Paleoseismologic Experiment, 1999, Preliminary results: Paleoseismic study of the peninsula segment of the San Andreas fault at the Sausal Creek site, Portola Valley, San Mateo County, California; prepared by Geomatrix Consultants Inc. for David Schwartz, U.S. Geological Survey, pp. 11.

Cotton, W.R., Hall, N.T., and Hay, E.A., 1980, Holocene behavior of the San Andreas fault at Dogtown, Point Reyes National Seashore, California: U.S. Geological Survey Final Technical Report, NEHRP contract 14-08-0001-18229, Open-File Report 80-1142, 33p.

Dickinson, W.R., 1970, Commentary and reconnaissance photogeologic map, San Andreas rift belt, Portola Valley, California: report to Town of Portola Valley Planning Commission, dated July 6, 1970.

Fumal, T.E., Heingartner, G.F., and Schwartz, D.P., 1999, Timing and slip of large earthquakes on the San Andreas fault, Santa Cruz Mountains, California: Geological Society of America Abstracts with Programs, v. 31, no. 6, p. A-56.

Fumal, T.E., Dawson, T.E., Flowers, R., Hamilton, J.C., Kessler, J., Samrad, L., 2004, A 100-yr average recurrence interval for the San Andreas fault, southern San Francisco Bay area, California, in Proceedings of the First Annual Northern California Hazards Workshop, January 13-14, 2004, ed. Zoback, Mary Lou, p 42.

Geomatrix Consultants, Inc., 2001, Fault investigation-Lands of White, 689 Portola Road, Portola Valley, California, Geomatrix Consultants, Inc., Oakland, dated April 16, 2001, pp. 13. Geomatrix Consultants, Inc., 2002, Paleoseismic Investigation of the San Andreas Fault Zone in Portola Valley, California: U.S. Geological Survey Final Technical Report, 12 p. + 7 figures, 2 plates.

Goldfinger, C., Nelson, C., and Johnson, J., and the Shipboard Scientific Party, 2003a, Holocene earthquake records from the Cascadia subduction zone and northern San Andreas fault based on precise dating of offshore turbidites: Annual Rev. Earth Planet. Sci., 31, p. 555-577.

34

Goldfinger, C., Nelson, C., and Johnson, J., and the Shipboard Scientific, Party 2003b, Deep-water turbidites as Holocene earthquake proxies: The Cascadia subduction zone and northern San Andreas fault systems: Ann. Geophysics, v. 46, no. 5, p. 1169-1194.

Hall, N.T., 1984, Holocene history of the San Andreas fault between Crystal Springs Reservoir and San Andreas Dam, San Mateo County: California, Bulletin of the Seismological Society of America, v. 74, no. 1, p. 281-299.

Hall, N.T., Wright, R.H., and Clahan, K.B., 1999, Paleoseismic studies of the San Francisco peninsular segment of the San Andreas fault zone near Woodside, California: Journal of Geophysical Research, v. 104, no. B10, p. 23,215-23,236.

Hall, N.T., Wright, R.H., and, C.S. Prentice, 2001, Studies along the peninsula segment of the San Andreas fault, San Mateo and Santa Clara Counties, California in Ferriz, H., and Anderson, R., eds., Engineering Geology Practice in Northern California: California Division of Mines and Geology, Bulletin 210, Special Publication 12, p. 193 to 210.

Harlan Tait Associates, 1991, Geologic and seismic hazards study, Spring Down Property, Portola Valley, California: prepared for Mr. Stan Goodstein, dated December 19, 1991, pp. 27.

Kelson, K.I., Streig, A., Koehler, R., 2006, Timing of late Holocene paleoearthquakes on the northern San Andreas fault at the Fort Ross Orchard site, Sonoma County, California: Bulletin of the Seismological Society of America, v. 96, no. 3, pp. 1012-1028.

Knudsen, K.L. Witter, R.C., Garrison-Laney, C.E., Baldwin, J.N., and Carver, G.A., 2002, Past Earthquake-Induced Rapid Subsidence Along the Northern San Andreas Fault: A Paleoseismological Method for Strike-Slip Faults: Bulletin of Seismological Society of America. v. 92, no. 7, p. 2,612-2,636.

Lawson, A.C., 1908, The earthquake of 1868; in A.C. Lawson, ed., the California earthquake of April 18, 1906: Report of the State Earthquake Investigation Commission (vol. 1): Carnegie Institute of Washington Publication 87, p. 105-107, 434-448.

Niemi, T.M., and Hall, N.T., 1992, Late Holocene slip rate and recurrence of great earthquakes on the San Andreas fault in northern California: Geology, v. 20, p. 195-198.

Niemi, T., 2003, Zhang, H., and Generaux, S., 2003, Determination of a high resolution paleoearthquake chronology for the northern San Andreas fault at the Vedanta marsh site, Marin County, CA: U.S. Geological Survey Annual Project Summary for NEHRP Award No. 03HQGR0015.

Niemi, T.M., Zhang, Hongwei, and Generaux, S.L., 2004, A 2,500-year record along the Northern San Andreas fault at Vedanta Marsh, Olema, CA, in Proceedings of the First Annual Northern California Hazards Workshop, January 13-14, 2004, ed. Zoback, Mary Lou, p 74.

Noller, J.S., and Lightfoot, K., 1997, An archaeoseismic approach and method for the study of active strike-slip faults: Geoarchaeology, An international Journal, v. 12, p. 117-135.

Prentice, C.S., 1989, Earthquake geology of the northern San Andreas fault near Point Arena, California: Pasadena, California, California Institute of Technology, Ph.D. Dissertation, 252 p.

35

Prentice, C.S. Langridge, R., and Merritts, D J., 2000, Paleoseismic and Quaternary tectonic studies of the San Andreas fault from Shelter Cove to Fort Ross [abs.], in Bokelmann, G. and Kovach, R.L., eds., Proceedings of the Third Conference on Tectonic Problems of the San Andreas System, Stanford University Publications, p.349-350.

Prentice, C.S., Langridge, R., Baldwin, J.N., Dawson, T., Merritts, D.J., and Crosby, C.J., 2001, Paleoseismic and Quaternary tectonic studies of the San Andreas Fault between Shelter Cove and Fort Ross, Northern California, USA [abs]: Ten Years of Paleoseismology in the ILP: Progress and Prospects, 17-21 December, 2001, Kaikoura, New Zealand, Program and Abstracts. Institute of Geological and Nuclear Sciences Information Series 50, p. 52-53.

Schwartz, D.P., Pantosti, D., Okumura, K., Powers, T.J., and Hamilton, J., 1998, Paleoseismic investigations in the Santa Cruz mountains, California: Implications for recurrence of large- magnitude earthquakes on the San Andreas fault: Journal of Geophysical Research, v. 103, no. B8, p. 17,985-18001.

Simpson, G.D., Noller, J.S., Kelson, K.I., and Lettis, W.R., 1996, Logs of trenches across the San Andreas fault, Archae camp, Fort Ross State Historic Park, Northern California: U.S. Geological Survey NEHRP Final Technical Report.

Tabor, S., 1907, Some local effects of the San Francisco earthquake: in D.S. Jordan, ed., the California earthquake of 1906, A.M. Robertson, San Francisco, pp. 303-315.

Toppozada, T.R., and Borchardt, G., 1998, Re-evaluation of the 1836 “Hayward Fault” and the 1838 San Andreas fault earthquakes: Bulletin of the Seismological Society of America, v. 88, p. 140-159.

William Lettis & Associates, Inc., 2003, Seismic hazard evaluation, Portola Valley Town Center, Portola Valley, California, dated February 28, 2003, p. 94.

Woodward-Clyde Consultants, 1976a, Results of fault study conducted for the Town of Portola Valley, Portola Valley Elementary School site: Woodward-Clyde Consultants, Oakland, letter report dated August 7, 1976.

Woodward-Clyde Consultants, 1976b, Results of Phase II fault study, Portola Valley Elementary School site: Woodward-Clyde Consultants, Oakland, letter report dated September 29, 1976.

Woodward-Clyde Consultants, 1977, Results of Phase III fault study, Portola Valley Elementary School site: Woodward-Clyde Consultants, Oakland, letter report dated January 4, 1977.

Working Group on California Earthquake Probabilities (WGCEP), 1999, Earthquake probabilities in the San Francisco Bay Region: 2000 to 2030 – A summary of findings: U.S. Geological Survey Open File Report 99-517, version 1.0, 36 p.

Working Group on California Earthquake Probabilities (WGCEP), 2003, Earthquake probabilities in the San Francisco Bay Region: 2002 to 2031 – A summary of findings: U.S. Geological Survey Open File Report 03-214, 235 p.

Wright, R.H., Clahan, K.B., and Hall, N.T., 1999, Preliminary results: Paleoseismic study of the Peninsula segment of the San Andreas fault at the Sausal Creek site, Portola Valley, California: Harlan Tait Associates, San Francisco, California, unpublished BAPEX report to the U.S. Geological Survey, 11p.

36

Zhang, H., Niemi, T.M., Allison, A., and Fumal, T.E., 2004, Noncharacteristic slip on the northern San Andreas fault at the Vedanta marsh, Marin County, California [abs]: EOS Trans. American Geophysical Union 85 (47), Fall Meeting, Supplemental Abstract T13C-1395, San Francisco, CA.

37