fromPerilousEarth:UnderstandingProcessesBehindNaturalDisasters,ver.1.0,June,2009byG.H.Girty,DepartmentofGeologicalSciences,SanDiegoStateUniversity Page1Chapter8Landslides fromPerilousEarth:UnderstandingProcessesBehindNaturalDisasters,ver.1.0,June,2009byG.H.Girty,DepartmentofGeologicalSciences,SanDiegoStateUniversity Page2IntroductionLandslidesoccurinawidevarietyofsettings,includingsubductioncomplexes,volcanicarcs,transformfaults,andintraplatesettingslikeHawaii.Theyalsoarecommonanywhereonthe Earths surface where steep topographic slopes exist or where weak water saturatedmaterial lies on slopes ranging from a fraction to many degrees of inclination. Under suchconditions,thedownslopemovementofmaterialiscommonlyreferredtoasmasswasting.Inthefollowingsections,Ifirstdefinethisphenomenoninmoredetail,andthenreviewtherolethatgravityplaysinmovingmaterialdownslope.Ithencoverthevarioustypesoflandslides,andthenendthischapterbylookingattwoimportantcasestudies.MassWastingandLandslidesMasswastingreferstothedownslopemovementofEarthmaterialssuchasregolithorsolidrockundertheinfluenceofgravity.Regolithisatermusedtorefertoallofthematerialslying between unweathered rock below and the Earths surface above. It therefore includesweathered rock, soils, and unconsolidated deposits derived from flowing water, ice (glaciers),and wind. When such material rests on horizontal surfaces, then it is relatively stable.However, if it rests on an inclined or sloped surface, then the degree to which the inclined orsloped surface varies from the horizontal, determines its stability. In such settings theresistance of the regolith to down slope motion is dependent upon its cohesiveness and itsfrictionalresistancetomotion.Inaddition,plantrootstendtobindtheregolith,andthereforeactasastabilizingagent.GravityShowninFigure1Aisalargeboulderrestingonahorizontalsurface.Insuchasetting,theonlyforceactingontheboulderisthatduetogravity.Asyoumightrecallfromhighschoolphysics, a force is a vector and therefore has both direction and magnitude. In Figure 1A, thegravitational force is depicted by the vertical black arrow pointing downward. The length ofthisarrowisthemagnitudeofthegravitationalforce.Ifthesurfaceonwhichtheboulderrestsslopes at some angle other than zero, then the gravitational force will consist of twocomponents, one acting parallel to the sloping surface (see blue arrow in Figures 1B, 1C, and1D),andtheotheractingorthogonallyornormaltoit(seeredarrowinFigures1B,1C,and1D).Theformeristhetangentialwhilethelatteristhenormalcomponentofgravity.Thetangentialcomponent is that part of the gravitational force that tends to pull the boulder down theslantedsurface.Incontrast,thenormalcomponentisthatpartofthegravitationalforcethattendstoholdtheboulderontotheinclinedsurface.If the surface on which the boulder rests makes an angle of say 40orelative to thehorizontal, then the normal component of gravity will be greater than the tangentialcomponent, and the boulder likely will not move down the slope (Figure 1B). However, if theangle of inclination is higher, say 50o, then the magnitude of the tangential component willexceed the normal component, and thus the likelihood that the boulder will slide down theinclinedsurfacewillbegreatlyincreased(Figure1Cand1D).fromPerilousEarth:UnderstandingProcessesBehindNaturalDisasters,ver.1.0,June,2009byG.H.Girty,DepartmentofGeologicalSciences,SanDiegoStateUniversity Page3Figure1.Theeffectsofgravity.Seetextforexplanation.As mentioned in the introductory comments, the physical properties of material thatrests on, or directly underlies, a segment of the Earths surface that is steeply inclined play animportant role in whether or not they will slide down such slopes under the influence of thetangential component of gravity. For example, if the Earths surface is underlain directly bysolid fracturefree rock, then it can form very steep slopes without any material breaking freeandslidingdownward.Attheotherendofthespectrum,ifthesolidfracturefreerocksurfaceis overlain by loose granular materials (e.g., the regolith), then such material will slide downslopes with even small inclinations. Lying between these two extremes is a wide variety ofmaterials with different physical properties that will determine if they will move under thetangential component of gravity or not. Some of this material will break into coherent blocksthatwillfallfreelythroughtheairpriortobouncingorrollingdownhill,whereasothermaterialwhen saturated with water will flow with the consistency of wet cement down slope. All ofthese processes and their resulting deposits are often lumped under the general heading oflandslideswhichD.J.Varnes(1958)definedasthedownwardandoutwardmovementofslopeforming materials composed of natural rock, soils, artificial fills, or combinations of thesematerials. The moving mass proceeds down slope by falling, sliding, spreading, flowing, orsomecombinationofthesethreeprocesses.Themovementoffalls,slides,spreads,andmostflows are perceptible to the human eye, while creep and solifluction occur so slowly that thehumaneyecantdetectthedownslopemotioncharacteristicofthesetwoprocesses.Alandslideisclassifiedonthebasisof(1)thetypeofmaterialthatexistedpriortothelandslide and (2) the type of movement that dominates during the landslide. The types ofmaterialthatmightexistpriortoalandslidearerock,soil,earth,mud,anddebris.Rockisdefinedasanyintact,hard,andfirmmassthatexistedinitsnaturalplacepriorto the landslide. Examples include igneous, metamorphic, and lithified sedimentary rocks. AtfromPerilousEarth:UnderstandingProcessesBehindNaturalDisasters,ver.1.0,June,2009byG.H.Girty,DepartmentofGeologicalSciences,SanDiegoStateUniversity Page4the other end of the spectrum is soil, an aggregate of minerals and rock fragments plus orminusorganicmaterialthatformedfromtheinsituweatheringofrockorsediments.Inasoil,pores or open voids between the minerals and rock fragments are often filled with gases orwater.Material defined as earth is composed of ~80% or more particles smaller than 2 mm(the upper size limit of sand) while mud is composed of 80% or more particles smaller thanabout0.06mm(theupperlimitofsilt).Finally,debriscontains20%to80%particleslargerthan2mm,whiletheremainderisgenerallylessthan2mm.The four generalclasses of movement during any given landslide are fall, slide, spread,and flow. Hence, to classify a landslide you first determine the type of material that existedprior to the landslide and then attach to that name the class of movement. Using such ascheme, a rock fall is a landslide that involved intact, hard, and firm material that fell downslopewhileanearthflowisalandslidethatinvolvedtheflowageofearthmaterialdownslope.Acomplexlandslidecommonlyinvolvestwoormoreoftheclassesfall,slide,spread,orflow.Forexample,initslowerparts,arotationalslidecommonlytransformsintoanearthflow.Hence,suchalandslidewouldbeclassifiedasacomplexrotationalslideearthflow.FallsRockfallsareproducedwhensolidmaterialorsoilbecomedetachedfromasteepslopeandthenfallfreelyforsomedistanceorbounceandrolldowntheslope(Figures2and3).Figure2.Timesequencefordevelopmentofairfall.Attime2blockisdislodgedfromoverhang.Itthenfallsfreelythroughtheatmosphereduringtimeintervals3through6.Betweentimeintervals6and7itimpactsgroundandshatters.fromPerilousEarth:UnderstandingProcessesBehindNaturalDisasters,ver.1.0,June,2009byG.H.Girty,DepartmentofGeologicalSciences,SanDiegoStateUniversity Page5Detachment of solid blocks or soils along such steep slopes occurs as a result of freezethaw,ground shaking during earthquakes, saturation by water during torrential rainstorms, orweakening by the growth of plant roots. Topples are rock falls that involve the forwardrotation of a detached block above a pivotal point located in the lower part of the detachedmaterial (Figure 4). Common everyday evidence of a rock fall or topple is a road littered withrockdebrisderivedfromanadjacentsteepslope(Figure5).Figure3.PhotographtakenfromRoyalArchesRouteclimbfromacrossthecanyonbyrockclimberLloydDeForrestwhiledanglingontherope2000feetabovethevalleyfloor.Source:USGSLandslidePhotoCollection.Figure4.Atoppleisarockfallinwhichthedislodgedblockrotatesaboutanaxisinitslowerpart(time2)asitpitchesforward(times2and3)priortofalling(time4).fromPerilousEarth:UnderstandingProcessesBehindNaturalDisasters,ver.1.0,June,2009byG.H.Girty,DepartmentofGeologicalSciences,SanDiegoStateUniversity Page6Figure5.Resultsofrockfall,TruckeeRiver,California.LargebouldersanddebrisalongroadweredislodgedfromsteepslopeinforegroundduringanearthquakeonSeptember12,1966.Theyfellandbounceddowntheslopeshatteringonimpactwiththeroad.PhotographbyR.Kachadoorian,UnitedStatesGeologicalSurvey.Slides Slides,asynonymoustermisslumps,formwhenacoherentmassofregolithorbedrockbreaksfreeandthenslidesdownslopealongeitheraplanarorcurvedsurface.Thegeometryof the detachment or rupture surface and the degree to which the sliding material remainscoherentdeterminesthetypeofslide. Translational:Ifthedetachedlandmassslidesalongarelativelyplanarsurface,thentheresulting slump is called translational (Figure 6). Relatively common planar surfaces of failurearejointsorbeddingplanes.Ifthematerialinthetoeregionliquefies,thenthedownslopeFigure6.Notethattheslidemassbrokefreeandthensliddownslopealongabeddingplaneandthatthetoeregionisanearthflow.Cracksthatformperpendiculartothelongdimensionofthetranslationalslideareextensional.fromPerilousEarth:UnderstandingProcessesBehindNaturalDisasters,ver.1.0,June,2009byG.H.Girty,DepartmentofGeologicalSciences,SanDiegoStateUniversity Page7endofatranslationalslidemayturnintoanearthflow.A block slump is a translational slump in which the detached landmass consists of asingleorafewcloselyrelatedunitsthatmovedownslopeasarelativelycoherentmass(Figure7).Figure7.Blockslumpimpactinganddestroyinghouse. Rotational: When a coherent mass of regolith breaks free from a slope along a curvedslip surface, the resulting slump is called rotational (Figures 8 and 9). As the coherent massmoves down slope along the curved slip surface it rotates downward leaving at its head acrescentshapedscarporcrown.Attheoppositeendoftheslump,inthetoeregion,materialmay lose coherence and flow slowly down slope as an earthflow. In such cases, the slide is acomplexrotationalslumpearthflow.Figure8.Acomplexrotationalslumpearthflowconsistsofarotationalslumpwithanearthflowinitslowertoeregion.fromPerilousEarth:UnderstandingProcessesBehindNaturalDisasters,ver.1.0,June,2009byG.H.Girty,DepartmentofGeologicalSciences,SanDiegoStateUniversity Page8Figure9.PhotographfromUnitedStatesGeologicalSurvey.DuringtheMarch27,1964Alaskanearthquake,alargeslumpoccurredintheTurnagainHeightsareaofAnchorage.Notethevarioushousesforscale.Thevariousslumpblocksslidealongthevariousscarpsfromupperrighttolowerleftandrotateddownwardinaclockwisesense.SpreadsLateralspreadscommonlyoccuronverygentleorflatslopes.Failureoccursasaresultofliquefaction,theprocessbywhichsaturated,loose,cohesionlesssedimentsaretransformedfromasolidintoaliquidstate.Suchchangesinstatecanbeinducedbygroundshakingduringearthquakes.Ifliquefactionoccurswithinalayeroverlainbymorecoherentmaterials,thentheupperlayersmayfractureandthensubside,translate,rotate,disintegrate,orliquefyandflow(Figure10).Figure10.Cutawayviewofalateralspread.Greenanddarkbrownlayersinupperpartofillustrationarerigidfirmmudstones.Underlyingthesetwouppermostlayersisanintervalofwatersaturatedsiltstonethatrestsonimpermeablebedrock.Duringanearthquakegroundshakingliquefiesaportionofthewatersaturatedsiltstoneanditspreadslaterallydownslopesandtowardclifffaces.fromPerilousEarth:UnderstandingProcessesBehindNaturalDisasters,ver.1.0,June,2009byG.H.Girty,DepartmentofGeologicalSciences,SanDiegoStateUniversity Page9FlowsEarthflows: Watersaturated finegrained slope material that liquefies and then runsout,leavingabowlshapeddepressionontheslopinglandsurfacearecalledearthflows(Figure11).Astheearthflowmovesdownslopeitremainscoveredwithvegetation.Attheheadofthebowlshaped depression, where the material in the earthflow pulled away from the slope, ascarp is commonly present. Lying between the bowlshaped depression and the area ofdepositionisthemaintrack,theareawheretheearthflowtraveledfromthesourceareatothedepositional site. Dependent upon how much water they contain, earthflows travel at variousspeeds ranging from 0.17 to 20 kilometers perhour (0.11 to 12.4 miles per hour). The greaterthe water content, the faster they travel. However, earthflows are generally slower thanmudflows(seebelow).Figure11.Earthflowsoccurwhenwatersaturatedfinegrainedmaterial(~80%oftheparticlesarelessthanabout2.0mminsize)lyingonaslopeliquefies.Theliquefiedmaterialthenflowsdownslopeleavingbehindabowlshapeddepression.The deposit resulting from an earthflow is elongated and composed of more than 80%finegrained sediment and/or soil particles finer grained than ~2.0 mm. In plan view (i.e.,lookingfromtheskyverticallydownontheearthflowsystem),thebowlshapeddepressionandsiteofdepositioncommonlyformtheoutlineofanhourglass(Figure11).Debrisflows:Followingintensetorrentialrainfall,orduringmeltingoflargeamountsofsnoworice,loseregolithonsteepslopesmaybecomewatersaturatedandunstable,andasaresult, give way flowing down slope accumulating in and moving down ravines and othernaturalchannelsasadebrisflow(Figure12).Suchflowsaresaidtohavetheconsistencyofwetcement. In debris flows grain size varies considerably, ranging from less than ~0.004 mm toover ~256 mm (Figure 13). In such a watersaturated mixture, grains larger than sand (~2.0mm) make up between 20% 80% of the solid material. Debris flows can reach speeds up to~56kilometersperhour(~35milesperhour).fromPerilousEarth:UnderstandingProcessesBehindNaturalDisasters,ver.1.0,June,2009byG.H.Girty,DepartmentofGeologicalSciences,SanDiegoStateUniversity Page10Figure12.Debrisflowscommonlyoccurfollowingheavyrainfall.Figure13.Snoutofdebrisflow,SanBernardinoMountains,California.Notethemanyblockslargerthan256mm,theapproximatediameterofabasketball,andthewiderangeinsizeofmaterialincorporatedintotheflow.PhotographbyD.Morton,UnitedStatesGeologicalSurvey.Mudflows: If slopes are covered by watersaturated soils or finegrained sedimentarydeposits, then following an intense rainfall or the melting of large amounts of snow and ice,suchmaterialmaybecomeliquefiedresultinginamudflow(Figures14).Ofthesolidmaterialincorporated into such flows more than 80% are less than silt size (~0.06 mm). Mudflows aregenerallyfasterthanearthflowsandarefinergrainedthandebrisflows(Figure14).fromPerilousEarth:UnderstandingProcessesBehindNaturalDisasters,ver.1.0,June,2009byG.H.Girty,DepartmentofGeologicalSciences,SanDiegoStateUniversity Page11Figure14.Themajordifferencebetweenamudflowandadebrisflowisthesizeofthematerialcarriedbytheflow.Asshowninthisillustration,thegeneralsizeofparticlesinadebrisflowissignificantlylargerthanthesizeofparticlesinamudflow.Bothtypesofflowscommonlyfollowandplugupstreamchannels.Creep The imperceptible slow and steady down slope movement of the regolith is defined ascreep.Thoughyoucantperceivesuchmotionwiththenakedeye,curvedorbentsegmentsoftreetrunks(Figure15),restrainingwalls,fences,roads,orrailroadtracks(Figure16)alongwiththe down slope tilting of layered rocks (Figure 17) and tilted telephone poles, all attest to itsexistence.Figure15.Benttreetrunksasaresultofdownslopecreep.PhotographbyG.K.Gilbert1969,UnitedStatesGeologicalSurvey.fromPerilousEarth:UnderstandingProcessesBehindNaturalDisasters,ver.1.0,June,2009byG.H.Girty,DepartmentofGeologicalSciences,SanDiegoStateUniversity Page12Figure16.Effectsofhillsidecreeponrailroadtracks.PhotographbyW.W.Atwood,UnitedStatesGeologicalSurvey.Figure17.Creepinshale.PhotobyG.W.Stose,UnitedStatesGeologicalSurvey.Solifluction Permafrost is soil that remains frozen for at least two consecutive years. During thesummer, the surface layer melts creating a watersaturated mobile layer that then movesslowlydownslopeasitslidesoverthestillfrozenandimpermeableunderlyingregolith(Figure18). In areas of permafrost, this slow imperceptible motion down slope is referred to assolifluction.Figure18.SolifluctionlobesSewardPeninsula,Alaska.PhotographbyP.S.Smith,UnitedStatesGeologicalSurvey.fromPerilousEarth:UnderstandingProcessesBehindNaturalDisasters,ver.1.0,June,2009byG.H.Girty,DepartmentofGeologicalSciences,SanDiegoStateUniversity Page13Summary Landslides are the result of the mass wasting of the Earths surface. Down slopemovement is the result of gravity and/or its tangential component often acting in conjunctionwith a change in the physical state of the acted upon material. Lets take a look at tworelatively recent examples that exemplify well why we should know something about thisimportantandubiquitousprocess.TwoCaseStudiesTheMountSoledadLandsideOn Wednesday morning, just before 9:00 am, October 3rd, 2007 a large mass of theslope lying between Soledad Mountain Road and Desert View Drive, La Jolla, Californiadetached and began sliding down slope (Figures 19). As a result, in the area of the slide, gasmains,waterlines,andsewerlinesbroke.Becauseofextensivedamage,ninehouseswereredtagged. The costs were estimated at $26 million for public works such as broken sewer andwater mains and a sunken section of Soledad Mountain Road, and $22 million for privatepropertylosses.ShouldtheresidenceinvolvedintheMountSoledadslidehavebeensurprised?Figure19.AerialviewofMountSoledadlandslide.PhotographcourtesyM.Hart.Before answering this question lets review some of the geology and history of theMountSoledadarea.(1)MuchoftheMountSoledadareaisunderlainbythemiddleEoceneArdathShale,aformationthatisknowntobesusceptibletolandslides(Figure20).fromPerilousEarth:UnderstandingProcessesBehindNaturalDisasters,ver.1.0,June,2009byG.H.Girty,DepartmentofGeologicalSciences,SanDiegoStateUniversity Page14(2)ThegeologicmapforSanDiegoshowsmanyancientlandslidedepositsintheMountSoledad area. In fact, the October 2007 landslide occurred in an area where geologists hadmappedseveralancientlandslides(Figure20).(3) The Mount Soledad area is transected by the Rose Canyon Country Club faultsystem (Figure 20), first recognized in the 1970s. This fault system was not recognized in the1950s and 1960s when many of the houses were built in the Mount Soledad area.Nevertheless,theOctober2007landslideoccurredalongthetraceoftheCountryClubfault,asdid several other ancient landslides shown on the geologic map of the Mount Soledad area(Figure20).(4) A landslide destroyed 7 homes under construction in December 1961, whileadditionallandslidesoccurredin1990and1994.Inshort,thereisalonghistoryoflandslideactivityintheMountSoledadarea,andthefact that another one occurred is not at all surprising. Do you think that still others will occursometimeinthefuture?Figure20.GeologicmapofMountSoledadarea,SanDiego,California.TheLaConchitaLandslide La Conchita is located along the Pacific Coast of California about midwaybetweenVenturaandSantaBarbara(Figure21).OnMarch4,1995at2:03pm,asectionofthehills in back of La Conchita dropped downward and flowed slowly toward the Pacific Ocean(Figure 22). This particular masswasting event has been classified as a complex slumpearthflow. The scarp at the head of the 1995 slump is clearly visible in Figure 22. The lowerpartofthelandslidethattunedintoanearthflowisthatpartencroachingonthehomesinthelower half of the photograph. No one was killed during the March 4 event, but the earthflowdestroyedorseverelydamaged9houses.fromPerilousEarth:UnderstandingProcessesBehindNaturalDisasters,ver.1.0,June,2009byG.H.Girty,DepartmentofGeologicalSciences,SanDiegoStateUniversity Page15Figure21.LocationofLaConchitamidwaybetweenVenturaandSantaBarbara,California.Figure22.LaConchitalandslide,March4,1995.PhotographfromUnitedStatesGeologicalSurvey.SeetextfordiscussionofRinconslide.OnJanuary10,2005at12:30pmanothermassivelandslideoccurred.Thiseventdidnotinvolve new material, but remobilized the southeastern portion of the old 1995 landslide(Figure23).ThishorrifyingexperiencewascaughtonvideobyaTVcameracrew.DuringthefromPerilousEarth:UnderstandingProcessesBehindNaturalDisasters,ver.1.0,June,2009byG.H.Girty,DepartmentofGeologicalSciences,SanDiegoStateUniversity Page16Figure23.LaConchitafollowingthe2005landslide.The2005landslidereactivatedtheSEportionofthe1995landslide.PhotographcourtesyUnitedStatesGeologicalSurvey.January 10 event, the slide material was mobilized nearly instantaneously into a highly fluid,and rapidly moving debris flow that ultimately buried four blocks of the town in over ~9.1meters(~30feet)ofdebris.Tenpeoplewerekilledand14wereinjured.Ofthe166homesinLaConchita,15weredestroyedand16weretaggedasuninhabitable.Both the March 4 and January 10 landslides occurred following intense long periods ofrainfalls that elevated ground water tables and weakened the cliffs above La Conchita. Forexample, the 2005 landslide occurred at theend of a 15day period that producedrecord andnearrecordamountsofrainfallinsouthernCaliforniawhilethe1995eventoccurredduringanextraordinarilywetyear.InvestigationsbyLarryGurrolaandotherscientistsatUCSantaBarbaraindicatethattheMarch 4 and January 10 events are small parts of a much larger ancient landslide called theRincon Mountain slide. The main scarp or this larger ancient landslide is clearly visible in thephotographshowninFigure22.According to Gurrola, landslides similar or larger than the 1995 and 2005 events mayoccur next year or in coming decades, during or shortly after intense rain falls. If theinhabitants had recognized the geologic setting of La Conchita would they still have built theirhomesonanarrowcoastalstripadjacenttoasteepslopecomposedofsoftweaksedimentandclearevidenceofanancientlandslidescarp?IsitsafetocontinuelivinginLaConchita?ReferencesUsedintheDevelopmentofthisChapterBooksandArticlesVarnes, D.J., 1958, Landslide types and processes in Eckel E.B., ed., Landslides andEngineering Practice, Highway Research Board Special Report 29, NASNRC Publication 544,Washington,D.C.,p.2047. fromPerilousEarth:UnderstandingProcessesBehindNaturalDisasters,ver.1.0,June,2009byG.H.Girty,DepartmentofGeologicalSciences,SanDiegoStateUniversity Page17MapsKennedy, M.P., and Tan, S.S., 2005, Geologic map of the San Diego 30' x 60' Quadrangle,California;CaliforniaDepartmentofConservationWebhttp://cfx.signonsandiego.com/uniontrib/20071004/images/slide.pdfhttp://nationalatlas.gov/articles/geology/a_geohazards.html#threehttp://www.nationalatlas.gov/articles/geology/a_landslide.html#onehttp://pbs.usgs.gov/fs/2004/3072/http://sports.uniontrib.com/uniontrib/20071007/news_1n7geology.htmlhttp://wikipedia.orghttp://www.signonsandiego.com/uniontrib/20071004/news_1n4slide.htmlhttp://www.absoluteastronomy.com/topics/landslidehttp://www.ukgeohazards.info/pages/eng_geol/landslide_geohazard/eng_geol_landslides_classification.htmhttp://www.ia.ucsb.edu/pa/display.aspx?pkey=1356http://www.sciencedaily.com/releases/2005/10/051023123104.htm