Specifications Part 3 of 4.pdf

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Earth Mechanics, Inc.

Geotechnical & Earthquake Engineering

14.0 APPENDIX G, SITE EXPLORATION PHOTOS

Figure G-1: Antioch Toll Bridge

Figure G-2: Representative Soil Column at Antioch Toll Bridge

Figure G-3: Representative Soil Column at Antioch Toll Bridge

Figure G-4: Mobile B-80 Drill Rig on the Quin Delta

Figure G-5: Quin Delta

Figure G-6: P- and S-Wave Logging by GEOVision

Figure G-7: Drilling and CPT Operations at Antioch Toll Bridge

Figure G-8: Drilling and CPT Operations at Antioch Toll Bridge

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Earth Mechanics, Inc.

Geotechnical & Earthquake Engineering

15.0 APPENDIX H, SEISMICITY REPORT

Antioch Bridge Potential Seismic Sources Review and Evaluation for:

Earth Mechanics Inc.

17660 Newhope Street, Suite E

Fountain Valley, California, 92708 by:

Douglas H. Hamilton, Ph D., CEG, Consulting Geologist 2 Bassett Lane

Atherton, California 94027

August, 2007

Antioch Bridge Potential Seismic Sources Review and Evaluation

Outline

Section Page

Executive Summary ES-1

I. Introduction 1

II. Findings

III. Regional Setting

A. Regional Structure and Tectonics

B. Stratigraphy and Geologic History

C. Late Cenozoic, Late Quaternary and Contemporary

Tectonics and Deformation

IV. Potential Seismic Sources

A. General Features

B. Descriptions of Selected Potential Seismic Sources

V. References

TABLE

A Parameters of Estimated Potential Seismic Source Structures and Associated Earthquakes in the Region of the Antioch Bridge, Solano and Contra Cost Counties, California

ILLUSTRATIONS

Figure No. Title

1. Regional Late Quaternary Faults and Earthquake Epicenters, 1836-1994

2. Regional Terrain and Principal Late Quaternary Faults i

ILLUSTRATIONS

(continued)

Figure No. Title

3. Conditions Driving Tectonism in the Suisun Embayment Region

4. Motion of GPS Stations Along a Transect Between the Pacific Coast and the Sierra

Nevada Near the Latitude of the Suisun Embayment

5. Terrain and Faults, Suisun Embayment and Antioch Bridge Area

6. Terrain, Faults, and Earthquake Epicenters, 1934-2005, Suisun Embayment and

Antioch Bridge Area

7. Surface Geology of Suisun Embayment and Vicinity

8. Cross Section Profile, Central Valley to Western Los Medanos Hills

9. Cross Section Profile, Montezuma Hills through Los Medanos Hills

10. Cross Section Profiles of Late Quaternary Geology, Lower Mokelumne and Lower

Sacramento/San Joaquin Rivers.

11. Seismic Reflection Images of the Midland-Brentwood fault, and the West Tracy fault

12. BASIX Seismic Reflection Images of the Kirby Hills Fault and Zone of Deformation, Sacramento River Crossing

13. Map and Profiles Showing Seismicity Associated with the Kirby Hills fault

14. Focal Mechanisms of Seismicity Associated with the Kirby Hills Fault

15. Map Showing Seismicity in the Suisun Embayment and Brentwood-Los Medanos

Hills-Northern Diablo Range Region, with Profiles of Hypocenters in the Brentwood-

Los Medanos Hills Region

16. Profiles of Hypocenters in the Potrero Hills-Kirby Hill Area

17. Late Quaternary Tectonics of the Suisun Embayment and Vicinity

18. Isostatic Residual Gravity in the Suisun Embayment Region

19. Exploration and Key Features Map ii

(continued)

APPENDICES

Appendix

A Stratigraphy of the Montezuma Formation in the Montezuma Hills Region iii

Executive Summary

This report presents the results of a review and evaluation of the geologic and neotectonic character of potential seismic source structures present in the region of the Antioch Bridge. The purpose of this review and evaluation is to locate and characterize the potential earthquake sources that define the seismic hazard to the bridge in order to assist development of appropriate design criteria for upgrading its seismic resistance.

The Antioch Bridge is located in the lowland embayment that interrupts the continuity of the regional terrain and tectonic boundary between the California Coast Ranges and Central Valley.

The lowland area is referred to in this report as the Suisun Embayment.

The Suisun Embayment is a region of active tectonic deformation as indicated by both contemporary seismicity and late Quaternary crustal movements including local areas of uplift or subsidence and movements along several faults and fault systems. The tectonic deformation occurs in a transpressional environment that is affected by plate boundary San Andreas-type dextral shear on the west and by Coast Ranges-Central Valley convergence plate boundary-normal compression in the east. N-S to NNE-SSW contraction that may occur in response to N-

S compression probably related to an abrupt "kink bend" change in Coast Ranges and San

Andreas system faults orientation, has resulted in development of basal detachment thrusts that underlie the lowland area and contribute to deformation and earthquake hazard along its south margin where the Antioch Bridge is located.

The geology of the Suisun Embayment interior is largely concealed beneath marsh, delta, and river channel lowlands, but is generally well exposed in the uplands and hills around the embayment margins.

The subsurface geology of the embayment region is known in considerable detail from 70 years of oil and gas exploration and development and 30 plus years of US Geological Survey and other agency sponsored surveys and research. Using the data set that is available from this work, we have attempted to develop an understanding of the mechanics of late Cenozoic deformation and

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to identify and characterize seismic source structures potentially significant to an assessment of the earthquake hazard to the Antioch Bridge.

On the basis of this data set, we have identified 5 primary potential seismic source structures located within 20 km of the bridge, 2 systems of nearby secondary potential seismic source structures, and several more distant potential seismic source structures. Together, these structures define the overall seismic hazard to the Antioch Bridge. The structures are listed below, noting the estimated maximum potential earthquake on each and its mechanism, estimated slip rate, and distance from the closest potential seismic rupture surface to the Antioch

Bridge.

Potential Seismic Source Structure

Potential Maximum Earthquake (Mo) and Mechanism

Estimated Slip Rate (mm/yr)

Distance of Seismic Rupture to Bridge (km)

Primary Structures Midland-Brentwood M7.0, Reverse 2±1 8 Montezuma Hills-Antioch M6.5-6.7, Thrust/Reverse <1 7-8 Kirby Hills M7.0, Right Reverse <1 13 Los Medanos Hills M6.3-6.5, Thrust <1 12 Marsh Creek- Greenville - Clayton

M7.0, Strike- Slip/Reverse

4 17-20

Secondary Structures Faults in the upper plate of the Montezuma Hills – Antioch thrust where affected by late Cenozoic movement

M6.0, Reverse <1 1-10

Faults of the fold and thrust belt in the upper plate of the Los Medanos Hills thrust

M6.0, Reverse <1 14-28

Distant Structures Concord-Green Valley M6.9, Strike-Slip 5± 3 24-30 Cordelia-Wragg Canyon M6.7, Strike-Slip 0.5-5 38 Tassajara Hills fold and thrust belt

M 6.4, Thrust Unknown 25

Calaveras M6.9, Strike-Slip 10±4 30 Hayward M7.3, Strike-Slip 9±2 46 San Andreas M8.0, Strike-Slip 24±3 76

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The possibility of surface deformation along the alignment of the Antioch Bridge resulting from tectonic faulting is extremely remote but not inconceivable, since the 60+ km-long, west-vergent

Sherman Island fault projects up-dip more or less directly toward the bridge site. The Sherman

Island fault is the largest, after the Midland fault, of the pre-mid Tertiary "growth" faults in the

Antioch-Montezuma Hills area. It is not known to have been overprinted by late Cenozoic reverse movement but, to our knowledge, has not been investigated for possible late Cenozoic activity.

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I. Introduction

This report presents the results of a review and evaluation of geotectonic conditions relevant to establishing appropriate seismic criteria for analysis and possible upgrading of the seismic design of the Antioch Bridge. The bridge carries State Highway 160 across the San Joaquin

River between the city of Antioch and Sherman Island, and crosses the boundary between Contra

Costa and Solano counties. The structure is owned by the California Department of

Transportation (Caltrans) and operated by the Bay Area Transportation Authority (BATA). The report presented herein is part of a series of studies for a seismic upgrade of the Antioch Bridge being performed by Earth Mechanics Incorporated (EMI) under a contract with BATA.

This review and evaluation of geotectonic conditions in the region of the Antioch Bridge has been directed at identification and characterization as to their earthquake generation potential, of potential seismic source structures located close enough to the Antioch Bridge to possibly affect its seismic design criteria. The study has therefore focused primarily on the lower Delta and bordering uplands region, which is referred to herein as the Suisun Embayment (Figure 1). The subsurface geology of this area is known from many decades of drilling and geophysical surveying for gas and oil resources, but only in about the last 35 years has it begun to be studied for active tectonics and earthquake potential. Despite previous neglect of this aspect of geotectonic conditions in the Suisun Embayment region, several studies during the last decade have brought together existing available data and provided both important new data and new insights that have greatly benefited the present study. Recent studies by Weber-Band (1998), WLA (1998), WLA/Unruh (2000), the USGS (Parsons et. al. 2001) and DRMS (2007) have been especially important and helpful.

The report includes a series of illustrations that depict terrain, geologic, and geotectonic conditions in the region and vicinity of the Antioch Bridge. Most of the illustrations are either reproduced directly or adapted from available sources, but several were prepared to depict interpretations developed in connection with this review and evaluation. Parameters of estimated potential earthquake source structures and associated maximum earthquakes in the region of the Antioch Bridge are summarized in Table. A.

This review and evaluation has been performed within the scope of the overall investigation by

EMI by Douglas H. Hamilton, Ph D, C.E.G. Significant contributions have included discussions with Dr. Janine Weber, C.E.G., plotting of epicenter and hypocenter data by Martha Merriam, C.E.G. of Caltrans, and assistance in recovery of data from power plant investigations in the

Montezuma Hills by Dr. Andre Sarna-Wojciciki of the USGS, and George Saucedo of the

California Geological Survey. Dr. Roy Shlemon provided a reprint of his article on the

Holocene evolution of the Sacramento-San Joaquin Delta. Both Dr. Janine Weber and the

Pacific Section of the American Association of Petroleum Geologists kindly permitted reproduction of illustrations from copyrighted materials.

This review and evaluation reported herein, as noted above, is based on the results of many previous investigations, studies, and analyses by others, and many of the findings and conclusions presented herein reflect or directly mirror others in various reports and the published literature. Several important interpretations, however, are presented herein that we believe are not seen in any of the materials available for our review. We assume full responsibility for both the overall evaluation and any new interpretations, and for any errors or omissions in the report presented herewith.

II. Findings

1. The Antioch Bridge is located in a region of active tectonic deformation as indicated by both contemporary seismicity and late Quaternary crustal movements, including local areas of uplift or subsidence and movements along several faults and fault systems.

2. Contemporary tectonic deformation of the area referred to herein as the Suisan embayment, where the Antioch Bridge is located, is transpressional and occurs in response to a combination of plate boundary-parallel right lateral shear, plate boundary-normal compression and approximately north-south compression.

3. The plate boundary-parallel shear is concentrated locally along a trend of right lateral and right-reverse faulting, referred to herein as the Greenville-Green Valley trend, which forms the west boundary of the Suisun Embayment. This fault trend is part of the zone forming the easternmost branch of the San Andreas fault system in the Central Coast

Ranges. The closest segments of this fault trend to the Antioch Bridge are the Clayton and Marsh Creek faults located within approximately 17 to 20 km distance from the bridge. These are high angle faults that do not extend significantly closer to the bridge in the subsurface.

4. The plate boundary-normal faulting in the region of the Antioch Bridge occurs within an approximately 40 km wide zone of east-west contraction referred to herein as the Coast

Ranges-Central Valley convergence zone (CR-CV c.z). East-west compression within this zone has overprinted several generally north-south aligned pre-late Tertiary normal faults with late Cenozoic to late Quaternary reverse movement. The bridge site is bracketed between two such faults, the Midland-Brentwood on the east and the Kirby

Hills on the west. The Midland-Brentwood fault dips toward and beneath the bridge site.

The down-dip plane of this present day reverse fault has a minimum distance to the bridge of about 8 km. The nearly vertical Kirby Hills fault is about 13 km distant from the bridge.

5. N-S contraction across the north-south extent of the Suisun Embayment may be driven by compression resulting from the sharp bend or "kink" in the orientation of the Coast

Ranges at the latitude of the embayment. The contraction within the embayment area is accommodated by relative southward movement and south margin upwarping and uplift of the upper plates of two parallel detachment thrusts that underlie most of the embayment area. The two thrust plates are separated by the NNW-SSE to N-S aligned near-vertical Kirby Hills fault and zone of deformation. The upper plate of the thrust on the west, known as the Los Medanos Hills thrust, has been deformed internally by subsidiary thrusting and folding, probably in response to back thrusting at depth prior to development of the main detachment thrust sole. This subsidiary deformation is referred to herein as the "Los Medanos Hills detachment upper plate fold and thrust belt." The upper plate of the thrust on the east, referred to herein as the Montezuma Hills – Antioch thrust, appears to be riding up a shallowly north dipping ramp which results in uplift of the Montezuma Hills without internal deformation of the thrust plate.

The northeast-southwest contraction within the embayment is disconnected structurally from the central part of the Coast Ranges on the west by the Greenville-Green Valley fault trend.

The uplift and upwarping along the southern margin of both thrusts, which is expressed as the Los Medanos Hills topographic range and structural homocline, apparently results from movement of the thrust upper plates up a ramp formed by the upper leading surface of an underlying tectonic wedge, that is believed to consist of thrust-emplaced Franciscan

Formation rock (Weber Band, 1998). This concealed ramp structure along with possible backthrusts along the sole of the tectonic wedge form the most likely sources of thrust mechanism earthquakes associated with the Los Medanos Hills and Montezuma Hills –

Antioch thrusts. The sole of the steeper part of the latter thrust is here interpreted as directly underlying the Antioch Bridge at a depth of 7-8 km.

Local "kink-bend" – related compression may also be responsible for development of the

Thornton Arch anticlinal structures in the southern Sacramento Valley opposite the

Suisun Embayment.

6. The Kirby Hills fault and zone of deformation is a NNW-SSE to N-S near vertical deep seated structural zone that extends through the crust beneath the Los Medanos Hills on the south and, probably, faults in the Lagoon Valley area on the north. The fault is notable for extending down-dip entirely through the crust to the Mohorovicic ("Moho")

Discontinuity at the crust-mantle boundary and for providing a structural separation between the large detachment thrust faults located adjacent to it on the west and east, as described in finding No. 5 above. Recent high-resolution seismic reflection profiles across the Kirby Hills zone beneath the Sacramento River, obtained by the USGS, suggest significant late Quaternary deformation along it. Also, a zone of persistent seismicity in the crystalline basement crust at depths between 15 and 25 km directly underlies the surface trace of the Kirby Hills zone and, as noted above, continues southward beneath the Los Medanos Hills. Movement along the Kirby Hills fault could occur either as primary strike-slip or right-reverse displacement, or as secondary tear displacement accommodating lateral movement along the boundary between the upper plates of the adjacent detachment thrusts. At point of closest approach the Kirby Hills fault surface trace is about 13 km distant from the Antioch Bridge.

7. In addition to the primary potential seismic source structures noted above, other nearby potential source structures in the area of the Antioch Bridge include the contractional fold and thrust structures in the upper plate of the Los Medanos Hills thrust located west and northwest of the Antioch Bridge, and several of the larger pre-mid Tertiary faults that may have been overprinted by late Cenozoic reverse faulting, in the upper plate of the underlying Montezuma Hills-Antioch thrust. Movements along either of these systems of faults would probably be secondary adjustments occurring in response either to large movements or to accumulated strain along the underlying or bordering thrust and reverse faults.

8. In addition to vibratory ground motion from earthquakes occurring along the faults noted above, the site of the Antioch Bridge is subject to ground motion from more distant faults located in the Coast Ranges and the Coast Ranges – Central Valley convergence zone to the west, south, and north of the Suisun Embayment area. The cumulative probability of the occurrence of moderate to large magnitude earthquakes on distant source structures is much greater than it is for local potential source structures. However, even the closest

"intermediate-distance" potential source structures are more than 30 km distant from the bridge site. Maximum vibratory ground motion from such potential source structures will therefore be significantly reduced by distance attenuation compared with motion from earthquakes along any of the principal faults within and directly bordering the

Suisun Embayment.

9. The possibility of surface deformation along the alignment of the Antioch Bridge resulting from tectonic faulting is extremely remote but not inconceivable, since the 60 plus km long west-vergent Sherman Island fault projects up-dip more or less directly toward the bridge site. The Sherman Island fault is the largest, after the Midland fault, of the pre-mid Tertiary "growth" faults in the Antioch-Montezuma Hills area. It is not known to have been overprinted by late Cenozoic reverse movement but, to our knowledge, has not been investigated for possible late Cenozoic activity.

III. Regional Setting

A. Regional Structure and Tectonics

The Antioch Bridge is located in the southeasterly sector of a large topographic and structural embayment in the east margin of the central/northern Coast Ranges of California. The major terrain and geologic features of this region are, from west to east, the Coast Ranges, the Central

Valley, and the Sierra Nevada Range. In terms of crustal structure however, the crystalline basement rocks that are exposed in the Sierran Foothills continue west beneath the Central

Valley and the eastern part of the Coast Ranges. Tectonic interaction between the underthrusting

Sierran basement plate and the overriding Coast Ranges margin has resulted in uplift and compressional deformation along the Coast Ranges margin, along with downwarping of the adjacent west margin of the Central Valley. The modern Coast Ranges are geologically very recent, having been mostly formed during latest Tertiary and Quaternary time in response to transpressional deformation associated with the San Andreas fault system. Welt-like ranges, narrow linear fault rift valleys and local intervening basins define most of the terrain in the central and western part of the central and northern Coast Ranges while uplifted blocks and wedges flanked by homoclinal upwarps of strata from the Central Valley characterize much of the east part of the range system (Figure 2). The aforementioned topographic and structural embayment where the Antioch Bridge is located, referred to herein as the Suisun Embayment, is a unique feature along the Coast Ranges – Central Valley margin.

In addition to an obvious interruption in the continuity of the eastern front of the Coast Ranges, the Suisun Embayment also coincides with a marked change in the orientation of both the range front and the major San Andreas system faults along inside the east margin of the Coast Ranges

(Figure 3).

South of the embayment, the overall range front is oriented about N34oW and the two major segments of the easternmost branch of the San Andreas fault system in the northern Diablo

Range, the Greenville and Concord faults, strike N33oW and N32oW, respectively.

North of the embayment the range front is oriented about N17oW and the northerly continuation of the easternmost branch of the San Andreas system, the Green Valley fault, strikes N22oW.

The possibly related Cordelia-Wragg Canyon fault zone follows an even more northerly alignment, striking N04oW. The Suisun Embayment is therefore the locale of a sharp bend or kink in the regional structural orientation of as much as 17 degrees. The visible kink bend may be related to the northward widening of the northern Coast Ranges, and it clearly defines a structure where local N-S compression must occur on the inside of the bend. This compression appears to be the driving force for the SSW-directed thrust faulting that occurs deep within the crust beneath the Suisun Embayment and, probably, for the local westward bend in the range front (the Los Medanos Hills) along the south margin of the embayment. The local zone of kink bend-related compression may also be responsible for development of the ENE-WSW striking, anticlinal Thornton Arch structure that extends across the western half of the southern

Sacramento Valley east of the Montezuma Hills.

The proximate cause of the topographic depression of the Suisun Embayment appears to be a compressional down-bowing of the crust between the south end of the Vaca Mountain on the north and the Los Medanos Hills on the south. This may be related to the local N-S compression described above. This compression is probably also responsible for the Montezuma Hills uplift in the outer part of the Suisun Embayment. No terrain feature like this low north-sloping ramp formed by a local uplift of the late Quaternary age Montezuma Formation, exists anywhere else along the west margin of the Central Valley although other local uplifts that are clearly anticlinal arches, (e.g. the Dunnigan Hills) are present farther north along the valley margin. The interpretation adopted for this report is that the uplift of the Montezuma Hills is part of the same tectonic process that has formed the Los Medanos Hills uplift and homocline. This process is interpreted to involve relative southward movement of the upper plates of a system of detachment thrusts, up a ramp formed by the leading edge of a concealed tectonic wedge, possibly consisting of a body of Franciscan Formation rock that was tectonically emplaced over the Sierran crystalline basement (e.g.: Weber Band, 1998). The driving force for this southward thrust movement, as noted above, may be the local "kink bend" – related N-S compression in the inside angle of the local bend in the orientation of the Coast Ranges, as illustrated on Figure 3.

The principal late Cenozoic structural features that form the margins of the Suisan embayment terrain lowland are the faults of the Greenville – Marsh Creek – Clayton – Concord – Green

Valley system (referred to herein as the Greenville – Green Valley fault trend) on the west and the Midland – Brentwood fault zone on the east. The Greenville – Green Valley fault trend is part of the easternmost major branch of the San Andreas fault system in the Coast Ranges of central California. This fault trend is characterized by right lateral strike-slip and local associated compressional reverse fault movements.

The Midland – Brentwood fault is part of a system of east-vergent reverse faults and west-vergent back thrusts that extends along the west margin of the Central Valley. This fault system forms the principal structure within the local reach of the regional Coast Ranges – Central Valley convergence zone (CR-CV c.z.). The zone is represented in the terrain by the generally abrupt east slope of the Coast Ranges (Figure 2A) and by less obvious features resulting from deformation of late Quaternary deposits along the adjacent valley margin.

The Midland – Brentwood fault is the name used herein for the compressional late Cenozoic over printed faulting that mostly follows the plane of the pre-mid Tertiary Midland "growth" fault, as described by Weber Band (1998).

The principal structural feature recognized in the region of the Southern Sacramento Valley that lies east of the Midland-Brentwood fault zone is known as the Thornton Arch. This feature consists of a NE-SW aligned series of broad anticlines that extends approximately 27 km NE from the Midland-Brentwood fault opposite the Montezuma Hills. Knowledge of the geology of the Thornton Arch is based almost entirely on gas exploration and development data, since the feature has no obvious surface expression. DRMS (2007) suggests, however, that the arcuate course of the Mokelumne River across the valley floor may result from deflection around a zone of subtle up-arching and that this may indicate continuing late Quaternary activity of faulting related to the development of the arch.

We note that the Thornton Arch lies within the expected zone of compression in the Suisun

Embayment kink bend in the alignment of the Coast Ranges (Figure 3), and may therefore result from on-going deformation related to that structure.

As noted above, the late Cenozoic internal structure of the Suisun Embayment is dominated by the near vertical, N-S to NNW-SSE aligned Kirby Hills fault zone and, apparently, by near horizontal detachment thrusts at the base of the sedimentary section on the west and east sides of the Kirby Hills zone. The thrust fault on the west is called the Los Medanos Hills thrust (e.g., Hoffman, 1992, Weber Band, 1998, WLA, 1998). The thrust fault on the east, which underlies the Montezuma Hills and the Antioch Bridge, is referred to herein as the Montezuma Hills –

Antioch thrust. The two thrust planes apparently mostly coalesce southward as the Kirby Hills fault dies out in the upper sedimentary section, however some separation evidently continues through the Los Medanos Hills homocline, since there is an abrupt discontinuity in the north flank of the hills across the apparently related Kirker fault. As noted above this homocline is interpreted herein as an upwarp developed in response to relative southward movement of the upper plates of detachment thrusts riding up an underlying ramp formed by the leading surface of an underlying tectonic wedge.

The contemporary kinematics and tectonic regime in the Suisun Embayment region is defined by two data sets. These are the GPS-based geodetic data analyzed by Prescott et. al., (2001) (Figure

4) and the earthquake focal mechanism analyses reported in Parsons et. al. (2001) (Figure 14).

The geodetic data show the expected plate boundary – parallel motion of the Coast Ranges west of the Greenville – Green Valley fault trend, but also show a zone of plate boundary-normal

(right angle) convergence or contraction along the Coast Ranges – Central Valley margin, including where the Suisan embayment is located. The data appear to show contemporary kinematics that correspond generally to motion expected from right slip along the NW-SE aligned Greenville-Green Valley trend faults, and reverse dislocation along the N-S aligned

Midland-Brentwood fault.

The earthquake focal mechanisms study by Parsons et.al. (2001) was based on analysis of numerous small earthquakes that were recorded from the basement root of the Kirby Hills fault.

The focal mechanisms showed a predominance of right lateral strike-slip events together with a scattering of ENE-WSW oriented thrust faulting events. The ENE-WSW strike thrust faulting would appear to be driven by the local approximately N-S compression described above.

All of the available data sets (paleoseismic geology, geodetic, and seismicity) indicate that the

Suisun Embayment region is undergoing ongoing tectonic deformation.

B. Stratigraphy and Geologic History

General Features

The stratigraphy of the Suisun Embayment region that is of interest for evaluating the seismic hazard to the Antioch Bridge encompasses the entire crustal section from the base of the crust at about 25 km depth to the surface. This crustal section consists of three major elements: 1) crystalline basement in the form of a slab about 15 km thick and probably consisting of ophiolite, oceanic, meta-volcanic and sedimentary rock that is coextensive with the Sierran basement exposed in the Sierran Foothills to the east, 2) a wedge of probable Franciscan Formation, locally thrust over the Sierran basement, and 3) a column of clastic sedimentary and minor volcaniclastic strata about 10 km thick that extends from a basement contact to the present surface.

Neither the basement nor the Franciscan rock are exposed anywhere within the Suisun

Embayment, but Franciscan Formation is exposed in the core of the Mt. Diablo uplift directly across the Clayton fault southwest of the Los Medanos Hills. The entire overlying sedimentary rock section is exposed in the homoclinal upwarp of the Los Medanos Hills and contiguous northern Diablo Range on the south. This section ranges in age from early Cretaceous or possibly late Jurassic to late Quaternary. Overlying deposits include older fan alluvium, older channel alluvium, deltaic sediments, and peat. The peat extends from around 7 Ka to contemporary in age.

Sedimentary rocks

The sedimentary stratigraphic column can be divided broadly into three intervals:

1. A basal section consisting of strata deposited in a progressively downwarping trough starting in early Cretaceous or possibly late Jurassic time and continuing into Eocene time. This section comprises roughly 80% of the approximately 10 km thickness of the entire sedimentary column. The sediments accumulated in a north-south aligned trough depocenter as described in numerous references (e.g., Pepper and Johnson, 1992, Krug et.

al., 1992, Weber Band, 1998). Deposition was interrupted intermittently by erosion of submarine canyons, but was not affected locally by significant tectonic deformation or by any noteworthy interruption at the Mesozoic – Cenozoic time boundary at c.55 Ma. The principal structural feature present within the pre-late Tertiary stratigraphic section is a mostly N-S trough–parallel system of "growth" faults. These are normal faults that developed by progressive gravity collapse of elongate wedges of the accumulating sedimentary section toward the downwarping basin axis. The principal faults of this system in the Suisun Embayment region are the pre late-Cenozoic Kirby Hills fault on the west and the Midland fault on the east. These faults, as noted above, later provided preferred planes of weakness that were overprinted by late Cenozoic reverse and strike-slip faulting.

2. Following erosion of the Markley submarine canyon, sedimentation resumed, first by the backfilling and overtopping of the canyon with the sand now known as the Markley

Formation, and then continuing with additional, more or less conformable, layers. Near the top of this section, volcanic-derived layers came to predominate during and following late Miocene time (since approximately 10 Ma). The lower of these strata, called the

Neroly Formation, is composed largely of sand and gravel derived from erosion of the andesitic Mehrten Formation on the western slope of the Sierra Nevada. This sequence of volcanic-derived shallow marine strata is the last marine deposit to accumulate in the

Suisun Embayment area depocenter. The next layer, known as the Tehama Formation, consists mostly of rhyolitic ash derived from explosive volcanism in the northern Coast

Ranges and deposited under continental conditions. This formation includes the 4.8 Ma

Lawlor Tuff, which provides a widespread reliable age-dating and correlation horizon

(Sarna-Wojcicki et. al., 1999). The Lawlor tuff crops out along the lower north flank of the Los Medanos Hills and was sampled in the ESA-1 test boring in the Montezuma Hills

(ESA, 1973).

Following deposition of the Tehama Formation, the entire sedimentary section underlying the northern part of the present Suisun Embayment was deformed by upwarping and local folding and faulting, and the area was then beveled by erosion.

3. After deformation of the interior of the Suisun Embayment had largely ceased and the erosional beveling noted above had occurred, probably between 2 and 1 Ma, the embayment area was downwarped enough to permit accumulation of the Montezuma

Formation, which Weber Band (1998) considers to be 1.0±0.5 Ma in age. This formation consists of predominantly fine-grained clastic sediment of Coast Ranges provenance that accumulated under deltaic to lacustrine reducing conditions. The base of the formation as determined in test boring ESA-1 along the south margin of the Montezuma Hills is at elevation –133 m and the now uplifted remnant upper surface is at elevation approximately 76 m. The formation therefore accumulated to a thickness of at least 210

m. in a progressively down warping basin, which was underlain by the erosion surface developed over the beveled upwarped beds of Tehama and older strata. Accumulation of the Montezuma deposits terminated some time prior to the breaching of the Carquinez

Strait to allow drainage from the Sierra Nevada and Central Valley to reach the Pacific via San Francisco Bay.

4. The latest phases of deposition in the Suisun Embayment region have included partial alluvial backfilling of the Sacramento – San Joaquin River channel from its maximum depth of about 44 m through and downstream from the Montezuma Hills - Antioch gap, and accumulation of shallow deltaic sediment and peat deposits in adjacent lowlands.

C. Late Cenozoic, Late Quaternary, and Contemporary Tectonics and Deformation

The Suisun Embayment, as noted in Section IIIA above, forms an interruption in the pattern of uplift and deformation along the Coast Ranges-Central Valley margin. The four visible features that represent this interruption are 1) the eastward-opening lowland Suisun Embayment in the east front of the Coast Ranges, 2) the westward bend in the range front that forms the Los

Medanos Hills, 3) the abrupt southward termination of the Vaca Mountains on the north, and 4) the low north-sloping ramp uplift of the Montezuma Hills. This local area of anomalous terrain and structure is abruptly terminated on the west by the Concord and Green Valley strike-slip faults of the San Andreas fault system eastern branch referred to herein as the Greenville-Green

Valley trend. These features are illustrated on Figures 1, 2, and 5.

The features noted above are all indicators of the late Quaternary and contemporary tectonics of the Suisun Embayment area, but the actual tectonic structures along which the area is being deformed are largely subtle or invisible. These structures are the Midland-Brentwood and Kirby

Hills fault zones that form the structural east margin and central axis, respectively, of the embayment area, the concealed Montezuma Hills-Antioch and Los Medanos Hills detachment thrusts, and a concealed tectonic wedge that forms a ramp beneath the Los Medanos Hills. In the lowland west part of the embayment, the visible Potrero Hills anticline and a series of other similar structures, which are concealed beneath bay and marsh terrain, apparently represent secondary contractional deformation that has developed in the upper plate of the Los Medanos

Hills detachment thrust. Late Quaternary deformation of this thrust plate, however, appears to consist mostly or possibly entirely of broad down-bowing of the plate, forming the present lowland, and uplift and deformation along the south margin of the plate forming the western end of the Los Medanos Hills and in the thrust zone footwall, the Concord basin (Hoffman, 1992).

As noted in both Weber-Band (1998) and WLA (1998), the fold and thrust belt that includes the

Potrero Hills has accounted for significant crustal contraction and shortening. However, it is our interpretation, as noted above, that this structural effect is confined to the upper plate of an underlying thrust where contemporary movement is concentrated. This upper plate NNE-SSW contraction is probably responsible for the distinct offset to the north across the Kirker fault, of the western Los Medanos Hills relative to their eastward continuation. As noted below there is no indication of north-south contraction of the upper plate of the adjacent Montezuma Hills-

Antioch detachment thrust located on the east side of the Kirby Hills and Kirker fault zone.

The evidence for the existence of a Montezuma Hills-Antioch fault is indirect, but we think compelling. The three lines of evidence are 1) a need to account for the late Quaternary ramp-like uplift of the Montezuma Hills in a structurally reasonable way, 2) a need to allow for N-S shortening of the upper part of the crust on the east side of the Kirby Hills fault similar to the amount of contraction that has occurred within the Los Medanos thrust upper plate along its west side, and 3) a need for a structure similar to the Los Medanos Hills thrust to account for the up warping and uplift of the main, eastern part of the Los Medanos Hills.

A detachment thrust moving parallel to, but separated across the deep-seated Kirby Hills fault from the known Los Medanos Hills thrust, appears to fulfill all of these needs. The indicated structure would respond to the same tectonic forces that drive the Los Medanos Hills. However, although the thrust surface forms a ramp as it is deflected upward by a tectonic wedge along the south margin of the Suisun Embayment, it apparently interacts with the more active, high angle strike-slip Clayton-Marsh Creek- faults, and probably, the Brushy Creek cross fault. The continued relative southward movement of the Montezuma Hills-Antioch thrust upper plate is therefore accommodated largely by uplift and inter- and intra- bed shearing within the upwarped section of the eastern Los Medanos Hills.

The Kirby Hills fault and zone of deformation is an important, but enigmatic, structure within the

Suisun Embayment. The surface trace of this fault is almost entirely concealed beneath lowland delta and marsh terrain, so that its main surface expression occurs as the three compressional hills that are known as Bradmoor Island, Kirby Hill, and (informally) south Kirby Hill, and the structurally disturbed east end of the Potrero Hills. However, the horst-like core of the hills around Lagoon Valley (through which Highway 80 passes) to the north is probably also part of the Kirby Hills zone.

The subsurface aspect of the fault, as indicated both by profiles based on deep gas well logs and on seismic reflection imaging, is that of a sharp compressional anticline, at least from north of the Potrero Hills to the Sacramento River. South of the river, in the vicinity of the town of

Pittsburg, however, the aspect of the fault is that of a simple east dipping reverse fault as shown in seismic Line B-1 presented in Weber Band (1998). This reverse fault aspect, however, may be a local condition resulting from a local bend to a NW-SE strike in the orientation of the fault in the upper part of the section. Note that the seismicity in the underlying basement continues directly beneath the surface trace of the Kirby Hills fault south of the San Joaquin River as shown on Figure 13. All of the above discussion relates to the near surface (upper 5 km) form of the Kirby Hills fault, which is clearly a late Cenozoic fault superimposed on the pre-late Tertiary

"growth" fault that existed along the west margin of the main late Mesozoic – early Cenozoic depocenter trough in the area. The more remarkable characteristic of the Kirby Hills fault is that, as defined by seismicity, it extends through the entire 25 km thick crust, into the Mohorovicic

Discontinuity ("Moho") at the crust-mantle boundary. Furthermore, the seismic reflection-defined surface trace directly overlies the deep seismicity-defined trace in the lower part of the basement crust, which exists at depths of 15 to 25 km. A very detailed analysis of the deep seismicity along the Kirby Hill fault by the USGS (Parsons et. al., 2001) (Figure 13 of this report) shows that both NNW-SSE right lateral strike-slip and thrusting along ENE-WSW oriented planes is occurring within this zone.

The north end of the Kirby Hill fault zone is considered by some researchers to be somewhat north of Lagoon Valley (ESA 1980, Mackevitt, 1992, this report). The south end is traceable in the subsurface only as far south as Pittsburg, but the deep seismicity along the zone turns eastward and continues at least a further 8 km beneath the Los Medanos Hills (Figure 13) and may possibly continue a further 10 km to an intersection with the Northern Greenville fault.

Several researchers have suggested that in the near surface section the Kirby Hills fault was once co-extensive with the Kirker fault (Mackevitt 1992, Weber Band, 1998). The northward step in the Los Medanos Hills across the Kirker fault, discussed above, suggests that such a connection may have existed during Quaternary time.

So far as we have been able to determine, no reliable evidence of late Quaternary surface displacement along a trace of the Kirby Hill fault has been reported north of the Sacramento

River. However, as noted above, high resolution over-water seismic imaging by the USGS

BASIX project (Parsons et. al., 2001, Figure 12 of this report) seems to show clear evidence of late Quaternary (possibly Holocene) deformation along the zone beneath the Sacramento River.

IV. Potential Seismic Source Structures

A. General Features

We consider there to be five principal geologically recognizable potential seismic source structures capable of generating earthquakes in the magnitude of 6.5–7.0 range and located within 20 km distance from the Antioch Bridge within and near the Suisun Embayment.

These are, in order of increasing minimum distance from the bridge, 1) the Midland-Brentwood fault and 2) the Montezuma Hills-Antioch thrust, each passing beneath the bridge at depth and within approximately 7 to 8 km minimum distance from the bridge to the fault plane; 3) the

Kirby Hills fault and zone of deformation, located within about 13 km of the bridge; 4) the Los

Medanos Hills thrust, located about 12 km from the bridge, and 5) the Greenville-Green Valley trend of faulting, the closest segments of which (Clayton and Marsh Creek faults) are about 17 to

20 km distance from the bridge.

In addition to the principal seismic sources listed above three additional systems of potential seismic sources are present in the region. These are 1) two local systems of shallow-rooted faults which exist in the upper plates of the Montezuma Hills – Antioch and Los Medanos Hills thrust faults, respectively, 2) faults inferred to exist in association within the anticlinal structures that make up the Thornton Arch, and 3) faults capable of generating earthquakes in the magnitude of the 6.5-8.0 plus range but located more than 20 km distant from the Antioch

Bridge. The local fault systems consist of, first, pre-late Tertiary "growth" faults present in the section between the Midland-Brentwood and Kirby Hills faults that forms the upper plate of the

Montezuma Hills - Antioch thrust, second, late Cenozoic faults of the Los Medanos thrust upper plate fold and thrust belt, and third, the inferred underlying faults of the Thornton Arch. Faults of these local systems probably have a maximum earthquake generation capability of no more than about magnitude 6.0.

The "growth" faults of interest in the Montezuma Hills-Antioch thrust upper plate section include the Montezuma Hills, Denverton Creek, Sherman Island, and Kroutch faults. These faults are all considered to have been inactive since Eocene time by petroleum geologists who have mapped them using correlations between stratigraphic sections identified in gas well logs and seismic reflection data. There is some evidence, however, that late Cenozoic (post

Montezuma Formation) movement may have occurred along several of them (e.g., ESA 1973, 1980; USBR, 1949, Reiche, 1950). The plane of the Sherman Island fault, which has been traced over a strike length of about 60 km, projects up-dip directly toward the Antioch Bridge at the surface.

The fold and thrust belt faults in the upper plate of the Los Medanos Hills detachment thrust are located at distances ranging from 14 to 28 km from the Antioch Bridge. Deformation associated with these faults strongly affects strata as young as the late Pliocene Tehama Formation, but has not, to our knowledge, been shown to affect the late Quaternary Montezuma Formation except in the Concord area where the Los Medanos Hills thrust reaches the surface.

Faults capable of generating large earthquakes but located at greater than 20 km distance from the Antioch Bridge include blind or poorly exposed thrust faults of the Coast Ranges-Central

Valley convergence zone (CR-CV c.z.) present along the Vaca Mountains to the north and the northern Diablo Range to the south, and strike-slip faults in the Coast Ranges to the northwest, west, and southwest of the bridge.

Faults of the CR-CV c.z. located beneath the Vaca Mountains north of the Suisun Embayment region include the Gordon Valley and Trout Creek thrusts, which have been modeled as the sources of the two main shocks of the 1892 Vacaville-Winter earthquakes (O'Connell et. al., 2002). The closest of these thrusts is about 35 km NNW of the Antioch Bridge.

To the south the West Tracy and Vernalis reverse faults extend to within about 25 km of the bridge. The West Tracy fault probably intersects the south end of the Midland-Brentwood fault, and it has a similar aspect in its seismic reflection image, which shows it forming a monoclinal warp that probably extends up-dip to the surface (Figure 12). The north end of the Vernalis fault starts in the footwall of the West Tracy fault and has been mapped extending a further 34 km to the south. This fault is classified by Jennings (1994) as of Quaternary age.

The Thornton Arch structure has an overall axial length of approximately 27 km, and at its point of truncation by the Midland-Brentwood fault, is about 13 km NE of the Antioch Bridge. The overall structure consists of a series of several apparently separate anticlines that DRMS (2007) suggests may be undergoing continuing deformation at an extremely slow rate.

Among the large San Andreas system strike-slip fault zones of the central Coast Ranges the one closest to the Antioch Bridge includes the Greenville-Green Valley trend as referred to herein, which we include among the principal potential seismic sources capable of generating large earthquakes that could influence the seismic design of the bridge. The Clayton fault, which is the segment of this fault trend that is closest to the bridge, is coextensive with the Marsh Creek and Greenville faults to the south. The Clayton, Marsh Creek and Greenville faults together have a strike length of more than 70 km. This zone should be considered capable of generating an earthquake of approximately M7.0.

Going west from the Greenville-Green Valley trend, the next closest large strike-slip faults are in turn, the Calaveras at 30 km distance, the Hayward at 46 km distance, and the San Andreas at 76 km distance.

B. Descriptions of Selected Potential Source Structures

Midland-Brentwood fault zone

The late Cenozoic Midland-Brentwood fault zone extends over a strike length of approximately

50 km across the width of the Suisun Embayment. The northern end of this zone is not well constrained since the pre-late Tertiary "growth" fault system along which the late Cenozoic reverse fault has developed divides into several major branches northward from the Montezuma

Hills. Data available to this review does not indicate how far north the late Cenozoic overprinting extends. The southern end of the late Cenozoic reverse faulting is considered to terminate at an intersection with the late Cenozoic West Tracy reverse fault in the vicinity of the

Clifton Court Forebay impoundment. The down-dip geometry of the Midland-Brentwood fault is clearly imaged in the multi channel seismic reflection line shown on the cover of AAPG

Pacific Section Miscellaneous Publication 41 (Cherven and Edmonson, 1992) and is reproduced as Figure 11A of this report and also as line A-6 of Weber Band (1998). This image forms the basis for part of the cross section shown on Figure 8 of this report. The line shows the fault as a planar dislocation extending to the maximum depth penetrated around 4 km depth.

As Weber Band (1998) points out, the A-6 line seismic image of the Midland-Brentwood fault shows distinct monoclinal warping of strata in the near surface part of the section, thereby clearly indicating late Cenozoic overprinting by west up reverse movement along the original west down normal "growth" fault.