Seal Beach Wetlands: A Potential Paleoseismic and/or Tsunami Record for Southern California.

Report for SCEC Award # 14041 Submitted September 11, 2016

Investigators: Brady P. Rhodes, Matthew E. Kirby

I. Project Overview ................................................................................................................................... 1A. Abstract ........................................................................................................................................... 1B. SCEC Annual Science Highlights .................................................................................................... 1C.Exemplary Figure ............................................................................................................................ 1D.SCEC Science Priorities .................................................................................................................. 1E. Intellectual Merit .............................................................................................................................. 2F. Broader Impacts .............................................................................................................................. 2G.Project Publications ......................................................................................................................... 2

II. Technical Report ................................................................................................................................... 4A.Introduction ........................................................................................................................................ 4 B. Newport-Inglewood fault zone .......................................................................................................... 4 C SealBeachwetlands.........................................................................................................................................5D. Stratigraphicevidenceforcoseismicsubsidence............................................................................................5 E. NIFZ structure in the Sunset Gap ..................................................................................................... 7 F. Earthquake magnitude and sequencing ........................................................................................... 8 G. A new understanding of SBW evolution, coseismic deformation, and earthquake hazards for

southern California .......................................................................................................................... 9 B. References .................................................................................................................................... 11

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I. Project Overview

A. Abstract In the box below, describe the project objectives, methodology, and results obtained and their signifi-cance. If this work is a continuation of a multi-year SCEC-funded project, please include major research findings for all previous years in the abstract. (Maximum 250 words.) TheSealBeachwetlands straddle theNewport-Inglewood fault zonewithin theperimeterof theU.S.NavalWeaponsStationSealBeach.Anewpaleoenvironmentalstudyon55sedimentcorescollectedthroughoutthewetlands reveals evidence for late Holocene coseismic subsidence events, representing previously unrecog-nizedecologicalandsocioeconomicalhazardsalongthesouthernCaliforniacoastline.Wepresentevidenceforthe coseismic subsidence events based on detailed sedimentological analyses in two vibracores with well-constrainedradiocarbonchronostratigraphy.Thecoresrevealdistinctandconsistentchangesinstratigraphy,percent total organicmatter, grain size,magnetic susceptibility, and diatom and foraminiferal assemblagesindicatingrepeated,rapidsubsidenceandfillingofasmallbasin.Together, thecoredata indicatethatthreesubsidenceeventsoccurredinthewetlandsduringthepast2000calendaryears.Fromthesedatawesuggestareleasingstep-overalongtheNewport-Inglewoodfaultzoneresultsintheverticaldisplacementofanapprox-imately5-km2area,consistentwiththefootprintofanestuaryidentifiedinpre-developmentmaps.Thisstudyidentifiesanewseismichazard in coastal southernCaliforniaandprovides insight to coseismicdeformationandearthquakecontrolsontheevolutionofthewetlandsduringtheHolocene.

B. SCEC Annual Science Highlights Each year, the Science Planning Committee reviews and summarizes SCEC research accomplishments, and presents the results to the SCEC community and funding agencies. Rank (in order of preference) the sections in which you would like your project results to appear. Choose up to 3 working groups from be-low and re-order them according to your preference ranking. 1. Earthquake Geology 2. Earthquake Forecasting and Predictability (EFP) 3. Tectonic Geodesy

C. Exemplary Figure Select one figure from your project report that best exemplifies the significance of the results. The figure may be used in the SCEC Annual Science Highlights and chosen for the cover of the Annual Meeting Proceedings Volume. In the box below, enter the figure number from the project report, figure caption and figure credits.

Figure 3. Schematic diagram showing the chronological cycle in the Seal Beach wetlands of marine embayment followed by intertidal saltmarsh accretion, earthquake occurrence (coseismic subsidence) and basin formation, in-filling with allochthonous sediment, and intertidal saltmarsh accretion.

D. SCEC Science Priorities In the box below, please list (in rank order) the SCEC priorities this project has achieved. See https://www.scec.org/research/priorities for list of SCEC research priorities. For example: 6a, 6b, 6c

1a, 4a

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E. Intellectual Merit How does the project contribute to the overall intellectual merit of SCEC? For example: How does the research contribute to advancing knowledge and understanding in the field and, more specifically, SCEC research objectives? To what extent has the activity developed creative and original concepts?

This project is one of the first in southern California to evaluate the potential of coast wetland to record an earth-quake chronology. We have foundevidencethattheSealBeachWetlandsareawasamarineembayment~4,000calyrsBP.Afterformationofintertidalmarsh,thewetlandsabruptlysubsided3timesinthelast2000years.Aftereachsubsidenceeventrapid infillingoccurredandintertidalmarshformed.WehaveproposedthatthewetlandssubsidedduringlargemagnitudeearthquakesontheNIFZ.ThisisthefirstconcretegeologicrecordofearthquakeactivityontheNewportInglewoodFaultZone,andprovidesadditionalconstraintsontheseismichazardsofsouth-ernCalifornia.

F. Broader Impacts How does the project contribute to the broader impacts of SCEC as a whole? For example: How well has the activity promoted or supported teaching, training, and learning at your institution or across SCEC? If your project included a SCEC intern, what was his/her contribution? How has your project broadened the participation of underrepresented groups? To what extent has the project enhanced the infrastructure for research and education (e.g., facilities, instrumentation, networks, and partnerships)? What are some possible benefits of the activity to society?

1. Our data suggest that a significant hazard of sudden subsidence may exist for the Seal Beach Wetlands, and the Seal Beach Naval Weapons Station, which stores large numbers of naval weapons including nuclear warheads. Our data suggest that planning for sudden subsidence may be indicated for this installation. 2. Out project has a large educational component. SCEC helped fund 2 completed MS theses and 4 undergraduate theses. Additionally, over 10 additional undergraduate and graduate students assisted with field work and laboratory analyses. Most of these student were woman and/or minorities. Many are currently pursuing advanced degrees.

G. Project Publications All publications and presentations of the work funded must be entered in the SCEC Publications data-base. Log in at http://www.scec.org/user/login and select the Publications button to enter the SCEC Pubi-cations System. Please either (a) update a publication record you previously submitted or (b) add new publication record(s) as needed. If you have any problems, please email web@scec.org for assistance. Leeper, R.J., Rhodes, B.R., Kirby, M.E., Scharer, K.M., Starratt, S., Hemphill-Haley, E., Bonuso, N., Bal-

maki, B., Garcia, D., and Creager, D., 2014, Evidence of Coseismic Subsidence Along the Newport-Inglewood Fault Zone During the Late Holocene, presented Southern California Earthquake Center Annual Meeting, Palm Springs, California, September 7-10.

Leeper, R.J., Rhodes, B.R., Kirby, M.E., Scharer, K.M., Starratt, S., Hemphill-Haley, E., Bonuso, N., Bal-

maki, B., Garcia, D., and Creager, D., 2014, Evidence of Coseismic Subsidence Along the Newport-Inglewood Fault Zone During the Late Holocene, presented at 2014 Annual Meet-ing, GSA, Vancouver, BC, October 19-22.

Leeper, R.J., Rhodes, B.R., Kirby, M.E., Scharer, K.M., Starratt, S., Hemphill-Haley, E., Bonuso, N., Bal-

maki, B., Garcia, D., and Creager, D., 2014, Evidence of Coseismic Subsidence Along the Newport-Inglewood Fault Zone During the Late Holocene, presented at 2014 Annual Meet-ing, AGU, San Francisco, CA., December 15-19.

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Leeper, Robert J., Rhodes, Brady P., Kirby, Matthew E., Scharer, Katherine M., Creager, Dlissa O., And Garcia, Dylan J., 2013, Does Evidence of Abrupt Coseismic Subsidence and Tsunami During the Late Holocene Existing Seal Beach Marsh Stratigraphy? Geological Society of America 2013 Annual Meeting, Denver, Co.

Cordova, J., Rhodes, B. P. Kirby, M. E., Bonuso, N, and Leeper, R. J., 2013, A Potential Paleotsunami

Shell-Hash layer from the Los Penasquitos Marsh, San Diego County, California. Southern California Earthquake Center 2013 Annual Meeting.

Creager, D’lisa O., Rhodes, Brady P., Kirby, Matthew E., and Robert J. Leeper, Robert J., 2013, Pale-

otsunami Research at the Seal Beach Wetlands, Seal Beach, California, Southern California Earthquake Center 2013 Annual Meeting.

Leeper, Robert J., Rhodes, Brady P., Kirby, Matthew E., Scharer, Katherine M., Creager, D'lisa O., and

Garcia, Dylan J., 2013, Does Evidence of Abrupt Coseismic Subsidence and Tsunami During the Late Holocene Exist in Seal Beach Marsh Stratigraphy?, 2013, Southern California Earthquake Center 2013 Annual Meeting.

Rhodes, Brady P., Kirby, Matthew E., and Leeper, Robert J., 2013, The Search for a Paleotsunami Rec-

ord in the Coastal Wetlands of Southern California: A Progress Report, Southern California Earthquake Center 2013 Annual Meeting.

Leeper, R. R., Rhodes, B. P., Kirby, M. E., Scharer, K. M., Hemphill-Haley, E., Starratt, S. W.,and Carlin, J., Abrupt Subsidence in The Seal Beach Wetlands, Southern California, Geological Society of America Cordilleran Section 112th Annual Meeting- 2016.

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II. Technical Report A. IntroductionTheeffectsofcoseismicsubsidenceincoastalwetlandsalongsubductionzonesarewellknown(e.g.,At-water,1987;Kelseyetal.,2002;Cisternasetal.,2005),andevidenceofsimilarchangesintheelevationofcoastalwetlandsisdocumentedalongstrikeslipfaults innorthernCalifornia(e.g.,Knudsenetal.,2002;Koehleret al., 2005). In coastalwetlands, abrupt changes inelevationdue to coseismic subsidenceareshown todrastically alter depositional environments andecological communities. For example, deposi-tionalenvironmentsmayshiftfrommuddyorganic-richintertidalsedimentstosilty-to-sandylagoonal,ormarinesediments;ecologicaltransitionsareoftenreflectedbyabruptchangesinmicrofossilassemblagessuchasdiatomsandforaminifera(e.g.,Hemphill-Haley,1995;AtwaterandHemphill-Haley,1997;Nelsonetal.,1996). Ifthesedepositionalchangesarefollowedbysustainedsubmergenceorrapidaggradationandshowntobelaterallyextensiveandoccurinzonesofactivestrike-slipfaulting,coseismicsubsidenceis considered the likelymechanism for thesubsidence (e.g.,Knudsenetal.,2002;Koehleretal.,2005).Here,we evaluate a coastal saltmarsh in southern California that containsmultiple buried organic-richlayers,which are common inwetlands that subside coseismically, however, uniquewhen compared toother southern California coastal wetlands (e.g., Scott et al., 2011; Rhodes et al., 2013;Wilson et al.,2014).

B. Newport-Inglewood fault zone TheNewport-Inglewood fault zone (NIFZ) isa seismicallyactive structurally complex right-lateral strike-slipfaultcomprisedofenechelonfaultstrands,oil-bearinganticlines,andsubsidiaryfaultsegmentsex-hibiting normal and reverse displacement (e.g., Poland and Piper, 1956; Barrows, 1974; Bryant, 1985;Wright,1991).TheNIFZislocatedalongthewesternmarginoftheLosAngelesbasinandextendsonshorefor~75kmfromtheCityofBeverlyHillsinthenorthtotheCityofNewportBeachinthesouth(Figure1).SouthofNewportBeach,thefaultzonecontinuesoffshoreroughlyparalleltothecoastlinefor~90kmintotheSanDiegoregionwhereitisknownastheRoseCanyonFault(e.g.,Barrows,1974;Treiman,1999)(Figure1).TheNIFZ/RoseCanyonfaultsystemisoneofaseriesofmajorstrike-slipfaultslikelykinemati-

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callylinkedalongthe300-kmlongactiveCoastalFaultZone,forwhichearthquakecontemporaneityandmagnitudeispoorlyknown(GrantandRockwell,2002). Limiteddataexist regarding theextent andmagnitudeof lateHolocene seismicityon theNIFZbe-causedenseurbanizationandahighwater tableprohibit conventionalpaleoseismicmethods (Grantetal.,1997).Thus,Grantetal.(1997)foundevidenceforthreetofiveearlytomiddleHoloceneearthquakesinHuntingtonBeachusingconepenetrometertesting. Inthisstudy,weexamineadensearrayofcorescollectedfromtheSealBeachwetlands(SBW)anddetailpaleoenvironmentaldataontwovibracores.

C.SealBeachwetlandsThe SBW are the only remaining undeveloped part of an estuary known as the Anaheim Bay estuary(ABE),whichoncecoveredalargeareaintheSunsetGapbetweenLandingHillandBolsaMesaPriortodevelopment,theABEhadlargefringingfreshwaterwetlands,saltflats,andalkalimeadows(Chase,1873;Steinetal.,2007;Grossingeretal.,2011).ReclaimedareasoftheABEproximaltotheNIFZarenowoccu-pied bymilitary,municipal and industrial infrastructure including the U.S. NavalWeapons Station SealBeach,HuntingtonHarbor,the965-acreSealBeachNationalWildlifeRefuge,andanactiveoil-extractionoperation established in 1954. Consequently, understanding the earthquake history of the NIFZ in theSBWiscritical fora largeareaofcoastalsouthernCaliforniawherecoseismicsubsidencecouldproducesignificantecologicalandsocioeconomicalhazards.D.StratigraphicevidenceforcoseismicsubsidenceInmostsouthernCaliforniacoastalsaltmarshes,thetypicaldepositsconsistofthickbasalsandoverlainbyinterlayeredfinelagoonalsand,silt,andmud,toppedbyasingle,uppermostlayeroforganic-richmarshsediment(e.g.,Scottetal.,2011;Rhodesetal.,2013;Wilsonetal.,2014).Thistypeofstratigraphicse-quence is consistent with saltmarsh accretion duringmarine transgressions (e.g., Van Straaten, 1961,Bouma,1963,Cole andWahl, 2000).However, in the SBW,Wilsonet al. (2014) and Leeper (2016)ob-servedadifferent stratigraphic sequence;multipleburiedorganic-rich layers,whicharenotexpected ifsealevelchangeweregradual.Therefore,weexamined55coresthroughouttheSBWtoinvestigatetheoriginortheburiedorganic-richlayersandreconstructthepaleoenvironmentalhistoryoftheestuary.In45coresalongthetrendoftheNIFZweobservedsharpsedimentologicalchangesfrombrown-to-grayorganic-richsilttooverlyingbrownorganic-poormica-richsiltysand.Tounderstandtheecologicalchangesassociatedwiththedistinctsedi-mentological transitions observed at similar depths, we paired detailedmicrofossil analyses with sedi-mentologicaldataintwovibracores,02VCand18VC,whichareseparatedinthewetlandsby~650m.In 02VC, ecological and sedimentological changes occur at 320 cm, 228-222 cm, and 40 cmdepth. Forexample,achangeinthediatomandforaminiferalassemblagesisrecordedat228cmdepthwhereasub-tle sedimentological change occurs (Figure 2A). Below 228 cm, intertidal diatoms and foraminifera arepreservedinhighconcentration(Figure2A).At227cm,intertidaldiatomsdecreasebyanorderofmagni-tude, intertidal foraminifera decrease in concentration by nearly 45%, and sparse, mostly poorly pre-servedfresh-to-freshbrackish(f-fb)diatomsappearsuggestingfreshwatermixingatthesite(Figure2A).Weinterpretthatat229cm,therelativelyhighorganiccontentdeterminedthroughlossonignition(LOI),lowmagnetic susceptibility (MS), and dominance of intertidal diatoms and foraminifera indicate a saltmarshenvironmentwherethesedimentationrateisrelativelylowandautochthonousintertidalmaterialisaccumulating.At227cm,thedecreaseinLOI,increaseinsiltandMS,andfirstappearanceoff-fbdia-toms commensuratewith a reduction in intertidal diatomsand foraminifera suggest a shift to anenvi-ronmentwhereautochthonousandfreshwaterallochthonoussedimentisaccumulating(Figure2A).This change in depositional environment persists up the core. By 225 cm, f-fb diatoms becomemoreabundantthanintertidaldiatomsandnointertidalforaminiferaarepresent,LOIandMSareintermediate,andsiltconcentrationcontinuestoincrease(Figure2A).At222.5cmthereisadistinctdepositionalcon-tactwhere theunderlying organic-rich silt is buriedby a 122.5 cm thick deposit ofmica-rich silty sand(Figure2A).At222cm,mostly limited intertidalandf-fbdiatomsarepresent.Furtherupthecore,onlysparsef-fbdiatomsoccuruntildisappearingfromthesiltysandcompletely.Althoughthethicknessesof

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thedepositsobserved in02VCdiffer, this sequence from intertidal sediment to sediment that containsmostlypoorlypreservedf-fbdiatomsisalsofoundin02VCat320cmand40cmdepth.

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Similarecologicalandsedimentologicaltransitionsoccur invibracore18VCat253.5cm,190cm,and45cmdepth(Figure2B).Forexample,at258cmdepth inorganic-richsilt, intertidaldiatomsoccur inhighconcentrationandnoforaminiferaarepresent(Figure2B).Justabove,at253cmdepthinmica-richsiltysand, no intertidal diatomsor foraminifera are present (Figure 2B). Similarly, from50.5 cm to 45.5 cmthereisamixofintertidaldiatomsandforaminiferapreservedinorganic-richsilt.Thischangesat45cmtomica-richsiltysandinwhichnodiatomsorforaminiferaoccur.Inboth02VCand18VC,weidentifythreeseparateevents(E1,E2,andE3)whereecologicalandsedimen-tologicalchangesoccurfromorganic-richintertidalsilttooverlyingallochthonousorganic-poormica-richsiltysand.Weinterpretthatthethreeeventsobserved inthevibracoresarethesamebasedonsimilardepthsofoccurrence,andcorrelatethemtosimilarsharpsedimentologicalchangesobservedbyLeeper(2016)in43othersedimentcorescollectedacrosstheSBW.Thethreeeventsareobservedatcomparabledepthsandcorrelateoverhorizontaldistancesthatexceed1kmTodeterminethetimingof theevents,weobtainedradiocarbonagesonmacrophytes,charcoal,andshellsfromdepositswithsimilarcharacter-isticsatcomparabledepthsinvibracores02VCand18VC.Calibratedradiocarbondatesindicatethattheeventsoccurred2045-1947calBP(E3),1554-1355calBP(E2),and590-477calBP(E1).Inordertounderstandthetransitionfromintertidalsilttothesiltysandcontainingsparseallochthonousf-fbdiatomsweconsider indetail02VC,whereweconducted thehighest resolutionanalyses.Onbothsidesofthesharpdepositionalcontactat222.5cm,thediatomassemblageconsistsofamixtureofinter-tidalandf-fbtaxa,andforaminiferanolongeroccur.Upthecoreintertidaldiatomsdisappearandonlyf-fbdiatomsarepresent.Afewofthef-fbdiatomvalvesarewellpreserved,butmostarefracturedorbro-kensuggestingthatthediatomsweretransportedintothewetlandsafteroriginatingnearby.Theabsenceof foraminifera suggestsa freshwater sourceof the sediment. The f-fbdiatomtaxa thatoccurare con-sistentwithaerophileandshallowfreshwaterandalkalienvironments,possiblysimilartothosehistorical-lymappedfringingtheABE.Thus,themicrofossilresultssuggestthatsiltysanddepositscouldbealloch-thonousfreshwatermaterialtransportedintothesite.Weconsideredifthethicksiltysandunitscouldberiverdepositsfrommajorfloodingeventspreservedabovetherelicwetlandsurfaceasthereishistoricalevidenceofmajorfloodingthatresultedin>1-mthickdeposits of silty sand across the lowgradient coastal plains of southernCalifornia (e.g., CADWR, 1968;Steinetal.,2007;Grossingeretal.,2011).However, radiocarbondating indicatesthatthesedimentac-cumulatedin~300-670yearsatanaveragerateof2.5mm/yr,notasingleevent.ThisrateismuchfasterthanthelateHolocenerelativesealevelrisedeterminedforsouthernCalifornia(0.8+/-0.3mm/yr,Reyn-olds andSimms,2015), and requires that subsidenceoccurred inorder topreserve the122 cmof siltysandandeventuallyreturntointertidalconditionsby100cmdepth.Onemechanismforpreservingthesedimentistodeposititbelowsealevel,aswouldoccurifthewetlandsurfacerapidlysubsided,providingaccommodation space and producing a subaqueous environment where the sediment would be pre-served(e.g.,Knudsenetal.,2002,Koehleretal.,2005).E.NIFZstructureintheSunsetGapThegeometryoftheNIFZwithintheSunsetGapprovidesamechanismthatwouldproducetheproposedsubsidenceevents(Figure1).ExtensiveoilandwaterwelldataintheregionshowthatwithinSunsetGapprimaryaquifers100-200mdeeparefoldedintoabroadstructurallowonthenortheastsideofthefaultbetweenLandingHillandBolsaMesa(CADWR,1968).UpperMioceneandearlyPlioceneunitsalsostepdownon thenortheast sideof the faultat theSBW(Wright,1991),providingevidence thatatabroadscale, both older andmore recent deposits showpersistent verticalmotion along theNIFZ thatwouldisolatethe landwardsideof thefault fromsea levelduringtectonicactivity.Thefault-zonestructureoftheNIFZ itselfwouldalso tend toaccentuatesubsidence incenterof theSBWdue toa small (5°) rightstepinthefaultbetweenBolsaMesaandLandingHill.Thisstepwouldberesolvedasasmallpull-apartbasinat least1-kmwide in theSBW, similar in scale toagrabennorthwestofHuntingtonBeachMesawherepaleoearthquakesaredocumented(Grantetal.,1997)andasmallpush-upridgeatSignalHill lo-catedtotheNW(Figure1).

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Thus, thethreepaleo-environmental transitionsweobserve from intertidalorganic-richsilt tooverlyingallochthonousorganic-poorsiltysandareconsistentwiththehypothesisthatearthquakesalongtheNIFZresultedincoseismicsubsidenceofthewetlands(e.g.,Knudsenetal.,2002;Koehleretal,2005).EvidenceofapastmarineembaymentandthreecoseismicsubsidenceeventsThedeepestsedimentarydepositwecoredintheSBW(4.26mdepth)iscomprisedofcleangrayorganic-poor silty sand, sparsemica, and scatteredand clustered shell fragments, and thediatomassemblagesconsistoftaxathatsuggestamarineorigin(SupFile1).Mostofthediatomsarebrokenandshowerosionor abrasion at the edges, and shells that occur are fragmented and abraded. A bivalve, Chione cali-forniensis, preserved in core02VC, is commonly recognized living inmud flats, estuarineenvironmentsandshallowmarinesettings(e.g.,Fitch,1953).Thefragmentedandabradeddiatomsandshellsindicatethat theorganismswere living in anenvironmentexposed towaveenergy.Thegrain size, presenceofmarineorganisms,andradiocarbondatingindicatethataround3500calyrsBPtheSBWareawasashal-lowmarineembayment;as sedimentationexceeded sea level rise, thebaybecamean intertidalmarshapproximately2700calyrsBP.Followingformationoftheintertidalmarshoverthemarineembayment,wesuggestthatcoseismicsub-sidenceepisodically lowered thewetlandsalongandNEof the fault zone (Figures1and3).Thiswouldinstantly provide accommodation space, creating a subaqueous depositional environment with an in-creasedtidalprismandfetchinanewlyformed,largeranddeeperlow-energybasin(e.g.,DavisandFitz-Gerald,2004).Theareawouldbe immediately filledbyseawater,andthefringingperennial freshwaterwetlandsandalkalimeadowswouldbeexposedtotidalandwaveenergyanderosionoftheinlandareawould occur.Mostly intertidal diatoms and foraminifera, but some f-fb diatoms, would accumulate inorganic-rich silt preserved above the subsidedwetlands surface. Subsequent infillingwith organic-poormica-richsiltysandwouldoccurbasinwidethrougherosionofthefringingfreshwaterwetlandhabitats.Foraminiferawouldbeabsent,intertidaldiatomswouldberare,andmostlybrokenandpoorlypreservedf-fb diatoms would becomemore prevalent as non-marine sediments become dominant. During localfluvial flooding events the basin would accumulate sediment rapidly and no diatoms or foraminiferawouldbepreserved.Astheaccommodationspacecreatedbythecoseismicsubsidenceisfilled,thetidalprism is reduced, the subaqueous environment disappears and the system returns to a similar pre-earthquakesaltmarsh,andintertidaldiatomsandforaminiferawouldreturntoprominence.F.EarthquakemagnitudeandsequencingThelargesthistoricearthquakeontheNIFZisthe1933(ML=6.3)LongBeachEarthquake,whichruptureda13-16kmlongsectionofthefaultzonebetweenthecitiesofNewportBeachandLongBeachandre-sultedinextensivestructuraldamageintheLosAngelesregionand120deaths(Wood,1933;HaukssonandGross,1991) (Figure1). Therewasno surface rupturemappedduring field investigations followingthe1933earthquake,althoughliquefactionandlateralspreadingwereobserved(Wood,1933;Barrows,1974; Bryant, 1988). Cracking was reported “along and near the structural [fault] zone” in the SBW(Wright,1991)butweobservenoevidenceofthe1933earthquakeinthecores,consistentwithminimalland-levelchangesduringthismoderatesizedearthquake.HaukssonandGross(1991)estimatethathori-zontaldisplacementsforthiseventwere85-120cmatseismogenicdepths.Thethicknessesofthesiltysanddepositsabovetheeventhorizons in02VCand18VCsuggeststhattheaveragesubsidenceduringthesepaleoearthquakesis~62cm(Figures2AandB)andisdeepestinthecen-terof thewetlands (Leeper,2016).Wedonothaveanestimate for thehorizontaldisplacementof thepaleoearthquakes,but canuse theML6.3historicearthquakeasaminimumestimate, since thatearth-quakewasnotassociatedwithanysubsidence.Sahakian(2015)improvedcharacterizationofstepoversalongtheRoseCanyonFault,showingthatthefaultsystemcouldrupture insingleeventsbetweentheregionsofSanDiegoandNewportBeach,yieldinglargemagnitude(M7.5)earthquakes.Themostrecent(E1) and penultimate (E2) earthquakes identified in the SBW are contemporaneous with two surface-rupturing earthquakes documented along the Rose Canyon Fault (e.g., Rockwell and Murbach, 1999;Grant andRockwell, 2002; SCE, 2012). Rockwell andMurbach (1999) determine that about 3mof slipoccurred during the earthquake along theRoseCanyon Fault dated to 491-241 cal BP.Using empiricalscaling relationships ofWesnousky (2008) and assuming that 3 m was the average displacement, thisearthquakecouldhaveproduceda225-kmlongrupturethatcouldhaveextendednorthtotheSBWex-

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ceedingMw7.6.Alternatively, consideringdating limitationsandCoulombmodelingbySahakian (2015),E1andE2couldrepresent thenorthernmost rupturesofasequenceof intermediatesizedearthquakesthatsequentiallyrupturedupthecoast(GrantandRockwell.2002).The only long-term estimate of paleoearthquake recurrence for the NIFZ is from near the HuntingtonBeachoilfield,whereGrantetal.(1997)usedcontinuousconepenetrometerboringstolocateoffsetsinstratigraphicunits inasmallgrabenontheNIFZ(Figure1).TheyinterpretfiveearlytomiddleHoloceneearthquakesandweakevidenceofasixth, lateHoloceneearthquake. Ofthisrecord,onlytheyoungestearthquakeneartheHuntingtonBeachoilfieldcouldbecorrelativewiththeeventsinSBW.TheabsenceoftheSBWpaleoearthquakes inthestudyneartheHuntingtonBeachoil field is likelyexplainedby lowsedimentation rates in the late Holocene,whichwould limit preservation of any offsets and/or lateralvariationsintheyoungestunits,preventingdetailedcorrelationrequiredtoidentifyfaultingevents(Grantetal.,1997). Incombination,theSBWandHuntingtonBeachoilfieldeventssuggestthatabout8largeearthquakeshaveoccurredontheNIFZsince11.7ka,foranaverageintervalof1600years.G.AnewunderstandingofSBWevolution,coseismicdeformation,andearthquakehazardsforsouth-ernCalifornia Wepresentpaleoenvironmentalevidencethat theSBWareawasamarineembayment~4,000calyrsBP.After formationof intertidalmarsh, thewetlandsabruptlysubsidedthreetimes inthe last2000years.Aftereachsubsidenceeventrapidinfillingoccurredandintertidalmarshformed.WeproposethatthewetlandssubsidedduringlargemagnitudeearthquakesontheNIFZ.TheNIFZhasasmallextensionalstepintheareaoftheSBWandhaslong-term,downonthenortheastdisplacement,consistentwithre-curringcoseismicsubsidenceof thewetlands. It ispossible that theentire reclaimedareaof theABE isrelatedtocoseismicdeformationalongtheNIFZ.Coseismicsubsidenceof13cmanduptoasmuchas122cmmayonlyoccurduringlarge-magnitude(>M7)earthquakes. TwoofthreelargemagnitudeearthquakesrecordedintheSBWarecontemporaneouswithtwolargemagnitudeearthquakesthatoccurredalongtheRoseCanyonFaultinSanDiego(e.g.,RockwellandMur-bach,1999;GrantandRockwell,2002;SCE,2012).Thus, it ispossible that theearthquakes recorded intheSBWmaybethenorthernextentofaM7.6+ruptureinvolvingtheRoseCanyonFault.Ifthetwopale-oearthquakesrecorded intheSBWandalongtheRoseCanyonFaultarethesame,theaverage intervalforlargemagnitudesurface-rupturingearthquakesalongtheNewport-Inglewood/RoseCanyonFaultsys-temis~750years.Futureearthquakesthatresult inabruptsubsidenceoftheSBWmaypresentserioushazardstotheU.S.NavalWeaponsStationSealBeach,SealBeachNationalWildlifeRefuge,HuntingtonHarbor,andsouthernCaliforniacoastalcommunities.

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