REGULATORY EARTHQUAKE HAZARD ZONES

Publications and Information Office801 K Street, MS 14-34Sacramento, CA 95814-3532(916) 445-5716

Bay Area Regional Office345 Middlefield Road, MS 520Menlo Park, CA 94025(650) 688-6327

Southern California Regional Office888 South Figueroa Street, Suite 475Los Angeles, CA 90017(213) 239-0878

PAGE 4CALIFORNIA GEOLOGICAL SURVEY

SAN FRANCISCO BAY AREAEARTHQUAKE-INDUCED GROUND FAILURE HAZARDS

Surface rupture—occurs whenmovement on a fault deep within theearth breaks through to the surface.Fault rupture almost always followspre-existing faults, which are zonesof weakness. Rupture may occursuddenly during an earthquake orslowly in the form of fault creep.Sudden ruptures are more damagingto structures because they areaccompanied by shaking. There wasover 20 feet of surface rupture nearOlema, Marin County, during the“Great San Francisco Earthquake” of1906 (photo by G.K. Gilbert).

Liquefaction—asa geologic term,refers to the loss ofstrength of saturatedsoils during shaking.An earthquake cancause soil particlesto shift and becomebuoyant, as porespaces filled withwater, whichweakens the abilityof a soil to support structures on the surface. Liquefaction causedsome of the damage to buildings in San Francisco’s Marina Districtduring the 1989 Loma Prieta earthquake (photo by U.S. GeologicalSurvey).

Offices of the California Geological Survey:

Landslides—are the downhill movement of ground caused primarily by gravity acting on weakened rock orsoil. Slopes are weakened by weathering, erosion, saturation, and the addition of weight in the form of artificialfill, structures, snow, or rock. Landslides that occur during earthquakes typically originate from these steep andweakened slopes. A large number of landslides occurred in the Santa Cruz Mountains and along coastal bluffsduring the 1989 Loma Prieta earthquake (photo by U.S. Geological Survey).

THE RESOURCES AGENCYMIKE CHRISMAN

SECRETARY FOR RESOURCES

STATE OF CALIFORNIAARNOLD SCHWARZENEGGER

GOVERNOR

DEPARTMENT OF CONSERVATIONBRIDGETT LUTHER

DIRECTOR

Seismic Hazards Resources and Earthquake Education Information:Regulatory Seismic Zonation Maps - www.consrv.ca.gov/cgs/geologic_hazards/regulatory_hazard_zones/index.htm

Earthquake Education Information - www.consrv.ca.gov/cgs/information/EdResCenter.htm

California Geological Survey (CGS) designates zonesthat delineate areas where ground failure is more likelyto increase earthquake damage by liquefaction, land-slides, or surface fault rupture (the hazard of strongearthquake shaking is addressed separately by theCalifornia Building Code). These areas are commonlyreferred to as “zones of required investigation.” Citiesand counties are required by state law to withholdbuilding permits within these zones until geotechnicalinvestigations are conducted to assess seismic hazards ona site-specific basis. If liquefaction or earthquake-induced landslide hazard is identified, appropriate designand/or ground improvement measures must be appliedin order to reduce potential for structural failure. Morerestrictive measures are applied within earthquake faultzones, where proposed structures must be set back fromthe trace of active faults. In all cases, real property sellersare required to check seismic zonation maps producedby CGS to determine whether property being sold fallswithin a seismic hazard zone. The seller is required toprovide a “Natural Hazard Disclosure Statement” to thebuyer.

Detail map of the seismic hazard zones for potentialliquefaction in the Marina District of San Francisco.

CGS geologists delineate liquefaction zones by assessingthe engineering behavior of soils based on surfacegeology and geomorphology, measuring soil propertiesfrom subsurface borings, assessing the potential degreeof soil saturation, and evaluating the potential for suchsoils to liquefy under estimated levels of ground shakingfrom future earthquakes. Earthquake-induced landslidezones are based on a similar analysis that also includesmapping locations of existing landslides, and an analysisof rock strength, geologic structure and surface topogra-phy in order to assess the stability of slopes under futureearthquake shaking. As of April 2006, CGS has re-leased 112 official maps covering about 7000 squaremiles.

These maps show zones of liquefaction and earth-quake-induced landslides. Twenty-two of the completedmaps cover parts of San Francisco, Santa Clara, SanMateo, and Alameda counties; these maps are compiledinto one map inside this CGS Note. CGS will continueproducing seismic zonation maps for liquefaction andearthquake-induced landslides in the Bay region.

Detail map of the seismic hazard zones for potentiallandslides in the Lincoln Park area of San Francisco.

Earthquake fault zones designated by CGS are delin-eated on a separate series of maps. CGS geologistsplace earthquake fault zones along traces of faultswhere mapping demonstrates surface fault rupture hasoccurred within the past 11,000 years (Holocene time).Construction within these zones cannot be permitteduntil a geologic investigation has been conducted toprove that a building planned for human occupancy willnot be constructed across an active fault. These typesof site evaluations address the precise location andrecency of rupture along traces of the faults andtypically are based on observations made in trenchesexcavated across fault traces. As of April 2006, CGShas released 547 official maps statewide.

Detail map of the earthquake fault zone for a portion ofthe Hayward fault in the City of Fremont, AlamedaCounty.

©California Department of Conservation, 2006. Reproduction of this CGS Note for classroom or public education purposes is encouraged and does notrequire written permission. However, please cite California Geological Survey as source.The Department of Conservation makes no warranties as to the suitability of this product for any particular purpose.

www.conservation.ca.gov/cgs

REGULATORY EARTHQUAKE HAZARD ZONESNOTE

53NOTE

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Diablo

Dublin

Mt. Day

Newark

Milpitas

MidwayHayward

Palo Alto

Altamont

La Honda

Mt. SizerBig Basin

Tassajara

Cupertino

Woodside

Livermore

Los Gatos

San Mateo

Eylar Mtn.

Montara Mtn.

Morgan Hill

Cedar Mtn.

Mindego Hill

San Leandro

Pigeon Point

Point Bonita

San Gregorio

Oakland East

Isabel Valley

Hunters Point

Franklin Point

Oakland West

San Jose East

Half Moon Bay Mountain View

San Jose West

Redwood PointLa Costa Valley

Lick Observatory

Santa Teresa HillsCastle Rock Ridge

Byron Hot SpringsLas Trampas Ridge

San Francisco North

San Francisco South

Mendenhall Springs

Calaveras Reservoir

Clifton Court Forebay

GilroyLaurelPelton Loma Prieta Mt. MadonnaDavenportAno Nuevo

San Rafael ClaytonRichmondSan Quentin Briones Valley Walnut Creek BrentwoodBolinas

Antioch South Woodward Island

San Jose

Fremont

Oakland

Hayward

San Francisco

Alamo

Palo Alto

Danville

Sunnyvale

Livermore

Newark

Milpitas

Union City

Santa Clara

Pacifica

Pleasanton

San Jose

Saratoga

Dublin

San Mateo

Woodside

Los Gatos

Orinda

San LeandroCastro Valley

Berkeley

Alameda

Blackhawk

Menlo Park

Lafayette

San Bruno

Campbell

Fairview

Montara

Byron

Redwood City

Cupertino

San Ramon

Daly City

Morgan Hill

Mountain View

Boulder Creek

Moraga Town

Los AltosPortola Valley Los Altos Hills

Atherton

Belmont

San Carlos

Hillsborough

Ben Lomond

Lexington Hills

Half Moon Bay

El Granada

South San Francisco

Walnut Creek

Millbrae

Brisbane

Felton

Burlingame

Foster City

Stanford

Colma

Alameda

East Foothills

Ashland

Loyola

Sausalito

San Lorenzo

Moss Beach

Redwood City

East Palo Alto

Piedmont

Tiburon

Tiburon

Highlands

Monte Sereno

Emeryville

Redwood City

Cherryland

Menlo Park

Burbank

North Fair Oaks

Emerald Lake Hills

Sausalito BelvedereTamalpais-Homestead Valley

Cambrian Park

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West Menlo Park

San Martin

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DEPARTMENT OF CONSERVATONCALIFORNIA GEOLOGICAL SURVEYSEISMIC HAZARDS ZONATION801 K STREET, MS 12-31SACRAMENTO, CA 95814916 324-7299http://conservation.ca.gov/cgs/shmp

5 0 5 10 15 202.5

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5 0 5 10 15 202.5

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Scale

Seismic Hazard Zonation of Northern California

April 2006

San Francisco

Areas where historic occurrence of liquefaction, or local geological, geotechnical and groundwater condtions indicate a potential for permanent ground displacements such that mitigation as defined in Public Resources Code Section 2693(c) would be required.

Regulatory zones encompassing active faults so as to define those areas within which fault-rupture hazard investigations are required prior to building structures for human occupancy.

Surface Fault Rupture

Earthquake-Induced Landslides

Liquefaction

MAP EXPLANATION

Zones of Required Investigation

Areas where previous occurrence of landslide movement, or local topographic, geological, geotechnical and sub-surface water conditions indicate a potential for permanent ground displacements such that mitigation as defined in Public Resources Code Section 2693(c) would be required.

C A L I F O R N I A

CONSERVATION

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