The San Francisco Bay Area DOCUMENTS DHDT . DEC 2 I Ground Shaky 1995 April ASSOCIATION OF BAY AREA GOVERNMENTS SAN FRANCISCO PUBLIC LIBRARY REFERENCE BOOK Not to be taken fromthe Library — The San Francisco Bay Area On Shaky Ground APRIL 1995 ASSOCIATION OF BAY AREA GOVERNMENTS Mailing Address: P.O. Box 2050 - Oakland, CA 94604-2050 Location: Joseph P. Bort MetroCenter — Eighth and Oak Streets — Oakland - Phone: (510) 464-7900 Fax: (510) 464-7970 InternetAccess through abagOnline at http://www.abag.ca.gov Publication Number: P950011 : . D REF 551.22 As78s 1995 Association of Bay Area credits Governments ... The San Francisco Bay Area -- on shaky ground ReportAuthors: 1995 . Jeanne B. Perkins ~ EarthquakeProgramManager, Assoc. ofBay Area Governments — JohnBoatwright Geophysicist, U.S. Geological Survey Technical Support: Paul Wilson — President, MapFrame Corp. ~ GIS software and support BryanMagers and MichaelDumer, ABAG~ Internetpublicationassistance ABAG Management: Eugene Y. Leong -- ExecutiveDirector Gary Binger ~ PlanningDirector Acknowledgments ABAG would liketo acknowledgethe efforts ofseveral reviewers, including David M. Boore and Carl Mortensen who provided the internal review for the U.S. Geological Survey, and the following ABAG members oftwo Review Committees: ABAGEarthquake Mapping forLocal Governments Review Committee: CatherineBauman-PlanningDept., CityandCountyofSanFrancisco EdBortugno—Geologist, CaliforniaOfficeofEmergencyServices DanBerman-GISManager, CityofSanRamon LeslieCarmichael- SeniorPlanner, CityofFosterCity JohnLane-BuildingRehabilitationSupervisor, OfficeofEmerg. Services, CityofSanJose DaleMarcum~Geologist/GeotechnicalEngineer, CityofSaratoga(Wm. CottonandAssoc.) CarlMortensen-Geophysicist,U.S. Geological Survey — CarolPrentice Geologist, U.S. Geological Survey RichardScott~Prin. EngineeringTechnician, CityofNovato GrantWilson-PlanningConsultant, CityofSanBruno ABAG Earthquakes and Housing Losses Review Committee: ThelmaRubin~CommitteeChair-Councilmember, CityofAlbany ChristopherArnold-President,BuildingSystemsDevelopment(BSD), Inc. RogerBorcherdt-Geophysicist,U.S. GeologicalSurvey JohnHarrald—Dir. andProf,InstituteforDisasterandCrisisManagement,TheGeorgeWashingtonUniv. LaurenceKornfield-ChiefBuildingInspector, CityandCountyofSanFrancisco FrankMcClure—ConsultingStructuralEngineer PatrickMcClellan-EarthquakePreparednessCoordinator, CityofSanLeandroFireDept. GreggO'Ryon~Director,DisasterServices, AmericanRedCross-BayArea PaulaSchulz- StateHazardMtigationOfficer, CaliforniaOfficeofEmergencyServices RoySchweyer—Mgr.,Housing&Neigh. Preservation, Off. ofComm. Dev.,Oakland MarthaBlairTyler—SpangleAssociates,UrbanPlanningandResearch FrannieWinslow~Director,OfficeofEmergencyServices, CityofSanJose PHOTOCREDITS-Page 11 -DaveRogers; Page 12-DegenkolbAssoc.; Page 14(top)and 19 -JeannePerkins, Pageivand 14 (bottom)-Anonymous CARTOGRAPHY-EurekaCartography(Berkeley, CA)usingABAG'sgeographicinformationsystem(BASIS) maps COVERGRAPHICS -PeterBeelerusingABAG'sgeographicinformationsystem(BASIS)mapofgeologicmaterials The writing and production ofthis report were financed in part with Grant No. 1434-94-G-2394 from the U.S. Geological Survey (USGS). Additional funds have been provided by ABAG. The research is based upon work by the authors supported by National Science Foundation (NSF) Grant No. BCS-9302612 and USGS Project No. 9930-12343. Any opinions, findings, conclusions or recommendationsexpressedinthismaterialarethoseoftheauthorsanddonotnecessarilyreflecttheviewsofNSForUSGS. 3 1223 03902 5581 ii S.F PUBLIC LIBRARY 28111 TABLE OF CONTENTS ... page Background iv The Importance ofShakingHazard Maps 1 TheBay AreaIsEarthquake Country 2 — Plate Fault Traces 3 HowBig IsBIG-- MeasuringEarthquake Size 4 How Strong Is STRONG~ Measuring Shaking 5 How Distance and DirectivityAffect ShakingIntensity 6 How GeologicMaterials Affect Shaking Intensity 8 Plate — GeologicMaterials 9 ForMoreDetailed Information 10 A Callto Action 1 WhatYOUCanDoNext 12 As aResident orHomeowner 1 As a School orBusiness Owner orWorker 14 As aLocal GovernmentElected Official or StaffMember 16 - OUR What'sNext FuturePlans 1 ShakingIntensityMap Plates 20 ModifiedMercalliIntensity Scale 20 Plate —LomaPrietaModeledEarthquake 2 Plate ~ SanAndreasEarthquake -Peninsula Segment 22 - Plate SanGregorio Earthquake 23 Plate —NorthernHaywardEarthquake 24 — Plate SouthernHaywardEarthquake 25 Plate —HaywardEarthquake — EntireLength 26 Plate —Healdsburg-Rodgers CreekEarthquake 27 Plate ~MaacamaEarthquake 28 Plate -WestNapaEarthquake 29 ~ Plate Concord-GreenValleyEarthquake 3 Plate --NorthernCalaverasEarthquake 3 Plate ~ GreenvilleEarthquake 32 iii page References 33 A Technical Appendix - Source Modeling forMapping Intensity 36 Introduction 36 Background 36 General Model Approach 38 A Composite SourceModel 38 A Trilateral RuptureModel 40 Calibrating IntensitiesNormal to theFault Trace 45 Testingthe IntensityModelby Comparing Actual Versus 46 Predicted Red-Tagged HousingUnits inPastBayArea Earthquakes Conclusion 48 Technical AppendixB - Occurrence ofand Average Predicted 49 Intensity forthe Geologic Units inthe SanFrancisco Bay Area BACKGROUND ... During the last twenty years, ABAG, with funding from both the U.S. Geological Survey and the National Science Foundation, has developed a number of earthquake hazard maps for the nine-county San Francisco Bay Area. Those maps were last revised inthe mid-1980s and resulted in the publication in 1987 ofthe first On Shaky Ground report. The Loma Prieta earthquake in 1989 and the Northridge earthquake in 1994 were devastating in their effects on northern and southern California. However, they have also provided us with valuable information to test the hypotheses forming a basis for those earlier maps and to develop a better understanding ofthe physical processes that occur in earthquakes. The maps described and shown on the following pages are the result ofthis research and better understanding. They are an updated version ofthe maps documented inthe 1987 OnShaky Groundreport. This report documents ABAG's shaking hazard maps to encourage appropriateplanningfor and mitigation ofearthquakehazards. THE IMPORTANCE OF SHAKING HAZARD MAPS ... Maps includedin this The San Francisco Bay Area is in "earthquake country." reportareforthe entireSan Francisco In some earthquakes, the surface ofthe ground can rupture BayArea Mapsfor - - along a fault or a landslide can be triggered or individualcities areavailablefrom underground sand layers may flow (liquefy) - or a tsunami ABAG's officesand ("tidal" wave) may be generated in water . But in ALL on theInternetat earthquakes, the ground shakes. In large magnitude abagOnlineat earthquakes, more ground shakes, and it shakes longer, http:/Avww.abag.ca.gov. than in small magnitude earthquakes. Ground shaking City maps aredetailed causes damage tens ofmiles away from the fault source. enough to showstreet patterns. (Seepage 10.) When the ground shakes, damage occurs to buildings, facilities and their contents. People can be injured or killed. People find that they may no longer be able to sleep in their homes, or even have access to their belongings. Businesses can't function and segments of the economy suffer. Hazardous materials are released which can be damaging to people andthe environment. Various options are available to avoid, reduce or otherwise YOU mitigate these results. What do to prepare for shaking can minimize or eliminate these effects. Most earthquake damage is caused by the shaking of the ground itself Yet, at the same time, many existing local and Stategovernment hazardreductionprograms andregulations focus on other earthquake hazards. Our purposes in preparing this booklet are to expose ground shaking as a significant hazard, to show (using maps) the areas with the strongest expected shaking, and to suggest ways to mitigate shakingdamage. THE BAY AREA IS EARTHQUAKE COUNTRY... The fact that a devastating earthquake ~ occurred in 1906 the San Francisco earthquake — is common knowledge. Larger earthquakes generally affect larger areas; the San Francisco earthquake caused extensive damage in Oakland, San Jose and Santa Rosa. More recently, the 1989 Loma Prieta earthquake caused extensive damage in the Santa Cruz Mountains, as well as in Oakland and San Francisco tens of miles away. But many moderate to great earthquakes (over Estimated magnitudeandyearofoccurrenceare shown. magnitude 6.0) have affected theBay Area; 22 Notethatthelevel ofearthquakeactivityinthelast 15 such events have occurred in the last 160 years isclosertotheperiodpriortothe 1906 San Francisco — years for an average of one every seven earthquake,whilethe 1911 to 1979 period,when mostof theBayAreadeveloped,isexceptionally quiet. Courtesyof years. PeterWard,USGS. Earthquakes occur in the Bay Area when forces underground cause the faults beneath us to rupture and suddenly slip. If the rupture extends to the surface, we see movement on a fault (surface rupture). But strong earthquakes can occur when the fault rupture does not extend to the surface. The fault rupture ofthe ground generates vibrations or waves in the rock which we feel as ground shaking. Because faults are weaknesses in the rock, earthquakes tend to occur over and over on these Strike-SlipFaultDiagram same faults. Almost all ofthe major faults in the Bay Area arestrike-slipfaultswhere the rupture extends almost vertically into the ground and the ground on one side movespast the ground on the other side of the fault. Thrustfaults, where ground moves over adjacent ground, are much more common in the Los Angeles area than the Bay Area because the San Andreas fault makes a large bend to the west there before heading northwest. Thrust faults in southern ThrustFaultDiagram California are caused by this bending. 2 HOW BIG IS BIG - MEASURING EARTHQUAKE SIZE Magnitude a Larger magnitude earthquakes generally cause a larger area of is measure ofoverall ground to shake hard, and to shake longer. This relationship is generally well understood. Thus, one principal factor in determining earthquake size. shaking hazard isthe magnitude ofthe earthquake. Seismologists now have several measures ofearthquake magnitude in addition to the familiar Richter (or "local") magnitude. The Richter magnitude has difficulty differentiating the size oflarge and great (7-1/2+) magnitude earthquakes. To overcome this difficulty, modern seismologists use moment magnitude because it best reflects the energy released by the earthquake. The moment magnitude is proportional to the area ofthe fault surface that has slipped. Thus, it is directly related to the fault length. Because the models used to generate the shaking hazard maps in this report are based on fault length, they, in effect, bypass magnitude. (See AppendixA.) Fault segments generate "characteristic" earthquakes. Some faults are weak and tend to generate earthquakes withmoment magnitudes of5 and 6. However, at leasttenfault segments inthe Bay Area are relatively strong and can store up enough energy to generate earthquakes ofmagnitude 7 or so. These stronger faults will generate these large earthquakes, not magnitude 5 and 6 events. The concept of"characteristic" earthquakes means that we can anticipate, with reasonable certainty, the actual damaging earthquakes that will occur on these fault segments. These anticipated events are the scenario earthquakes depicted inthe color maps in the center section ofthis report. The probability of one of these scenario earthquakes occurring varies from fault segment to fault segment. The two Hayward fault segments and the peninsula segment of the San Andreas are felt to have, roughly, a probability ofone in four of occurring in the next 30 years (Ref. 3). Other fault segments are less well understood; equivalent probabilities are being developed. Note: Noprobabilitydataisprovidedin Ref. 3 fortheentireHaywardfault rupturingatonce. However, experts at the U.S. Geological Surveyworkingonamaptobeincludedinthe Uniform Building Code are now assuming that one in every four Hayward earthquakes will involve both segments of the Hayward rupturing at once, yielding a probability of5- 6% in 30 years (personal communication, Art Frankel). 4