introducing the cajon pass earthquake gate area

IntroducingtheCajonPassEarthquakeGateAreaJulianLozosCaliforniaStateUniversity,Northridge

SCECAnnualMee=ng•PalmSprings,California•12September2017

AModernChaconne:AStructuralandHarmonicAnalysisof“HotelCalifornia”JulianLozosCaliforniaStateUniversity,Northridge

SCECAnnualMee=ng•PalmSprings,California•12September2017

TheEarthquakeofDecember8th,1812:ACasefortheCajonPassEarthquakeGateAreaJulianLozosCaliforniaStateUniversity,Northridge

SCECAnnualMee=ng•PalmSprings,California•12September2017

EarthquakeGates

• Cri=calpointinafaultsystemthatmay(ormaynot)stopruptureormaycontrolwhichwayarupturegoes.• Dogatesswingthesamewayevery=me?

CajonPassAjunc=onbetweenprimaryplateboundaryfaults,inadensely

populatedregion.

SomeKeyQues=ons• Howdofaultinterac=onsaffecttheprobabilityofthrough-goingearthquakes?

• Howmuchdoesthegeometryofthecomponentfaultsmodulatethis?

• Howdopreviousrupturesaffectfuturerupturepathsandprobabili=es?

• Doesthecurrentstressfieldrepresentrecentrupturehistory?

TheEarthquakeofDecember8th,1812• HeavydamageatMissionSanGabriel,40deathsatMissionSanJuanCapistrano.

•  Ini=allyconsideredaNewport-Inglewoodevent.

• ReassignedtotheSanAndreasbasedontreeringevidence.

•  Sinceiden=fiedinmanySanAndreaspaleoseismictrenches.

Icannotdefini=velyproveanythinghere.

HistoricandPaleoseismicRecords

ModelingApproach

• 3Dfullydynamicrupturemodeling.•  Finiteelementmethod(FaultMod,Barall,2009).

• Realis=cini=alcondi=ons:•  Faultmeshwithcomplexgeometry.•  Complexregionalvelocitystructure(SCECCVM).•  Regionalstressfieldfromseismicitystudies.

• Varyini=alstressamplitudesun=lmodelsproduceslipconsistentwithpaleoseismicrecords.

ModelGeometry

SAF

SJF

ModelGeometry

RegionalStresses

Maximumhorizontalcompressivestressorienta=oninsouthernCaliforniaYangandHauksson,2013

On-FaultStresses

•  Addi=onalheterogeneityfromfaultgeometry.

PhysicalandComputa=onalParameters

Vp FromSCECCVM

Vs FromSCECCVM

Density FromSCECCVM

µstaMc0.6

µdynamic0.2

d0 0.4m

Principalstresses Variable(Bestfit:σV=30MPa,σNS=36MPa,σEW=12MPa)

StressorientaMon SAF:N5Enorthofjunc=on,N15Wsouthofjunc=on;SJF:N12E

Elementsize 200minnearfield,400minfarfield

NucleaMonradius 3000m

SlipintheEarly1800s

Site Early1800sSlipPallett Creek 2 - 6 m

Wrightwood 2.5 - 4.5 m

Cajon Creek ~4 m

Pitman Canyon 3 - 4 m

Plunge Creek No surface rupture

Burro Flats Several cm on secondary structures

Colton Large liquefaction features

Quincy 1.8 - 3 m

Mystic Lake 1.8 - 3 m

ModelSlipDistribu=ons

GroundMo=on:SAFnorth

GroundMo=on:SAFatjunc=on

GroundMo=on:SJFatjunc=on

GroundMo=on:SJFsouth

CoulombStress:SJFsouth

PreferredModel

•Groundmo=onsconsistentbothwithMissionrecordsandwithPBRs.•Stresschangesconsistentwithnorthward-propaga=ngEQsequence.

Implica=onsfor1812

Implica=onsforSAF-SJFInterac=ons•  TheSanAndreasandSanJacintocanrupturetogether.•  Eveniftheydidn’tin1812,it’sphysicallyplausibleundercurrentcondi=ons.

•  CorroboratedbypaleoseismoloyandPBRs.

•  HowloadedistheSanAndreasFaultsouthofitsjunc=onwiththeSanJacintoFault?

•  DoesthischangethemostlikelyReallyBigOneforsouthernCalifornia?

•  HowoRendoesthishappen?IsthisausualpaTern?

Now,wewanttoaddmoredetail,clarity,andlengthtotheCajon

Passstory.

SomeKeyQues=ons• Howdofaultinterac=onsaffecttheprobabilityofthrough-goingearthquakes?

• Howmuchdoesthegeometryofthecomponentfaultsmodulatethis?

• Howdopreviousrupturesaffectfuturerupturepathsandprobabili=es?

• Doesthecurrentstressfieldrepresentrecentrupturehistory?

SomeKeyQues=ons• Howdofaultinterac=onsaffecttheprobabilityofthrough-goingearthquakes?

• Howmuchdoesthegeometryofthecomponentfaultsmodulatethis?

• Howdopreviousrupturesaffectfuturerupturepathsandprobabili=es?

• Doesthecurrentstressfieldrepresentrecentrupturehistory?

CajonPassFaultGeometryCajon Pass Earthquake Gate Area

New CFM-5.2 faults

inferred 1812 rupture

Nicholsonetal.,2017

DipontheSouthernSanAndreas

Fuis

et a

l., 2

012

SomeKeyQues=ons• Howdofaultinterac=onsaffecttheprobabilityofthrough-goingearthquakes?

• Howmuchdoesthegeometryofthecomponentfaultsmodulatethis?

• Howdopreviousrupturesaffectfuturerupturepathsandprobabili=es?

• Doesthecurrentstressfieldrepresentrecentrupturehistory?

RecentRuptureHistory

SomeKeyQues=ons• Howdofaultinterac=onsaffecttheprobabilityofthrough-goingearthquakes?

• Howmuchdoesthegeometryofthecomponentfaultsmodulatethis?

• Howdopreviousrupturesaffectfuturerupturepathsandprobabili=es?

• Doesthecurrentstressfieldrepresentrecentrupturehistory?

Maximumhorizontalcompressivestressorienta=oninsouthernCaliforniaYangandHauksson,2013

CajonPassPanelDiscussion

3-4PMtoday,inthisroom.