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Influenceoftyphoonsandearthquakesonrainfall

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Influenceoftyphoonsandearthquakesonrainfall-inducedlandslides andsuspendedsedimentsdischargeGuan-WeiLin a,HongeyChen a,⁎,Yi-HuiChen a,Ming-JameHorng baDepartmentofGeosciences,NationalTaiwanUniversity,No.1,Section4,RooseveltRoad,Taip ei,Taiwan,ROCbWaterResourceAgency,MinistryofEconomicAffairs,Taiwan,ROCReceived1May2007;receivedinrevisedform21October2007;accepted3December2007 Availableonline27December2007Abstract Theaimofthispaperistopresentdataontheoccurrenceoflandslidestriggeredbytyphoonandear thquakeevents,andtodiscussthebasicearthquaketriggeringmechanismsinvolved.Foureventsoftyphoonandearthquaketriggeredlandslidesint heChenyoulanRivercatchmentofcentralTaiwanduring1996to2004werestudiedinordertoidentifytheircontrollingfactors.Thelandslidesareas weremeasuredbycomparingaerialphotostakenoverthepast18years,andsedimentdischargewascountedathydrometricstations.Investigationres ultsdemonstratethathillslopegeomaterialsthat hadbeen disturbedbystrongseismicshakinginducedexpansionoflandslideareasduringsubsequentev ents.AnalysisofNormalizedDifferenceVegetationIndex(NDVI)alsoregisteredthechangesofuncoveredconditionsonthehillslopecausedbylandslidin g.Whenflowdischargeislowerthan100m3s−1,post-seismicsedimentconcentrationisroughlyfourtimeshigherthanpre-seismicconcentrationwitht hesameflowdischarge.Thisfateindicatesthatthe amountofsedimentdischargeatnormalflowdischargeisaffectedbysuppliedsedimentvolume, whichtendstoincreasewithseismicactivity.©2007ElsevierB.V.Allrightsreserved.Keywords: Landslide;Sedimentdischarge;Rainstorm;Earthquake1.IntroductionPreviousstudies(Gelietal.,1988;Sitaretal.,1992;ChenandSu,2001;SklarandDietrich,2001;KhazaiandSitar,2004)identifymaterialstrength,discontinuousplanesetting,earth-quake,precipitation,landformandhydrologyastheprincipal naturalcontrolsthataffectlandslidesanddebrisflows. Dadsonetal.(2003) reassesssedimentdischargeinestimatingtheerosionrateinTaiwan.Theyconsiderthatthedecadal-scale erosionratesarecorrelatedwithhistoricalearthquakesandstorm-drivenrunoff,andthatlongscaleerosionratesarecontrolledbystreampower.TheaverageerosionrateintheChoshuiRiverbasin,centralTaiwan,increasedfourfoldfollowingthe1999Chi-Chiearthquake(Mw=7.6).Onaverage,fourtyphoonssweepandinundateTaiwanperyear,andthesubtropicalclimatethereentailsameanprecipita-tionof2500mmyr−1(Shieh,2000).Thesummerprecipitation,occurringfromMaytoOctober,causeslarge-scalewastingfromhillslopeareas.Inmostyears,thisprecipitationinducesseveral floodsandevenvastoverlandflowdrivesofcolluvialsedimentsintotheriverchannels.Duringtheperiodof1996–2004,thefollowingsequenceof eventsoccurredinthecentralTaiwan(Fig.1):(1)typhoonHerb(August1996);(2)Chi-Chiearthquake(September1999);(3)typhoonToraji(July2001);and(4)typhoonMindulle(June2004).IntheChenyoulanRivercatchment,landslidesincreasedby1.5timesand44newdebrisflowsemergedduringtyphoonHerbinJuly1996.Afterthe1999Chi-Chiearthquake,anareaofapproximately12.9×106m2wasaffectedbylandslidingintheChenyoulanRiverbasin.TheChenyoulanRivercatchment isatributaryoftheChoshuiRiverincentralTaiwan,locatedabout12kmeastoftheepicentralareaofthe1999Chi-Chiearthquake.However,bythe2001typhoonTorajiandthe2004typhoonMindulle,thelandslideareaexpandedto24.4×106m2and31.6×106m2,respectively.Thisfactimpliesthatthemain causeoftheexpandinglandslideareawasthedisturbanceof geomaterialbystrongearthquakes.BystudyingthelandslidingAvailable online at EngineeringGeology97(2008)32–41/locate/enggeo⁎ Correspondingauthor.Tel.:+886233662946;fax:+886223636095.E-mailaddress: hchen@.tw (H.Chen).0013-7952/$-seefrontmatter©2007ElsevierB.V.Allrightsreserved.doi:10.1016/j.enggeo.2007.12.001andthechangesofthesedimentdischarge,weattempttoar riveatanunderstandingoftheseismiceffectofthesediment transportationandthestrengthofthehillslopematerialswiththe impactfactoroflandsliding.2.GeologicalsettingofstudyareaTheChenyoulanRiverinNantouCountyis42kmintotal lengthandtheareaoftheChenyoulanRiverbasinis367km2;itsmarginisamountainrangewatershedwithlongitudinalvalleytopography.TheChenyoulanRiveroriginatesinabrookonYushanMountain.TheChenyoulanRiverbasinis,forthemostpart,over2000minelevation,graduallydecreasingfromsouthtonorth.Therivernetworkinthesouthwesternpartofthe ChenyoulanRiverbasinincludesthreetributarieswhich convergenorthwardlyintotheHosheRiveratShenmu,and severalbrooksthatfeedintothemainstreamoftheHosheRiver untilthislaterconvergesintothemainstreamoftheChenyoulanRiverintheHoshearea(Fig.2).Thetributaries convergeatthecentervalleyofthebasin,andthenaredelivered northwardintotheChoshuiRiver. TheChenyoulanRiverbasinliesatthejunctionwherethe metamorphicShuehshanRangekeepsabreastthesedimentaryWesternFoothills(Fig.2).TheChenyoulanRiver,closelyruns alongtheChenyoulanfaultline,themajorboundaryseparating thesetwogeologicalregions.Theterraintotheeastofthe ChenyoulanfaultisclassifiedasPaleogenemetamorphicrockoftheShuehshanRange,composedofslateandmeta-sandstone.ThemajorformationsofeastChenyoulanfaultareLushanFormation,ChiayangFormation,TachienSandstone ShihpachuangchiFormation,ShuichangliuFormation,andPailengFormation.LushanFormation,locatedintheeasternedgeofthe ChenyoulanRiverbasinandcomposedmainlyofgrayslateandalittlefinesandstone,hasstrikesofN5°E–N20°Eandsouthwarddipsof40°–66°.ChiayangFormation,composedofgrayslate,conformstotheDaganSandstone.TachienSandstone,Fig.1.TracksoftyphoonandthelocationofChi-Chiearthquakeepicenter.33G.-W.Linetal./EngineeringGeology97(2008)32–41composedofmeta-sandstoneandslate,h asstrikesofN30°W–N20°Eandsouthwarddipsof30°–60°.ShihpachuangchiFormation,composedofblackandgrayslate,hasstrikesofN29°E–N57°Wandsouthwarddipsof51°.Shuichangliu FormationandPailengFormation,composedofgrayhardslateandmeta-sandstone,havestrikesofN49°E–N9°Eandsouthwarddipsof50°–65°.Therearemorethantwosetsofdiscontinuityin therockmassofShuichangliuFormationandPailengFormation.Thediscontinuitydensityaverages16.9m−3forthesesixformations,indicatingthattherockmassonthehillslopeis fracturingconsiderablyintheChenyoulanRivercatchment. TheterraintothewestoftheChenyoulanfaultisclassifiedas NeogenesedimentaryrockoftheWesternFoothills,composed oftheshaleandsandstone.Themajorformationswestof ChenyoulanfaultareNanchuangFormation,HosheFormationandAlluvium.Shaleandcementedsandstonearethedominantlithologiesinthearea.NanchuangFormation,locatedinthe westernmostpartoftheChenyoulanRiverbasin,hasstrikesofN17°E–N39°Eandnorthwarddipsof48°–71°.Themajor lithologyofNanchuangFormationiscomposedofthicksandstoneandshale.ThediscontinuitydensityofNanchuangFormationaverages10.4m− 3.HosheFormation,which outcropsintheupperpartoftheChenyoulanRivercatchment,hasstrikesofN50°W–N21°Eandnorthwarddipsof30°–80°. ThelithologyinHosheFormationisthesameasthe compositioninNanchuangFormation.Thediscontinuity densityofHosheFormationaverages8.2m−3.AlluviumisdistributedalongthemainstreamoftheChenyoulanRiverand theconfluencesoftributaries.ndslideanalysisLandslidesweremappedfrom10and20mresolutionSPOT satelliteimageswithdetailedfieldandair-photocheckingintheChenyoulanRiverbasin(Linetal.,2004).The20mmulti-spectraland10mpanchromaticspatialresolutionofSPOTcan onlybeusedtomaplandslidesgreaterthan1600m2withaccuracy.Omissionoflandslidessmallerthan1600m2canresultinunderestimationoftheareaandvolumeofmaterialdisturbed.Fig.2.GeologicalmapoftheChenyoulanRiverbasinandlocationofhydrometricstation.34G.-W.Linetal./EngineeringGeology97(2008)32–41Weusedthelandslideratio,thenewgenera tionratioandthereactivatedratiooflandslidetodiscusslandslidecondition.The landslideratioisdefinedastheratioofthetotallandslideareatothecatchmentarea.Wealsodefinedthenewgenerationratioas theratioofnewlandslideareatothetotallandslidearea.Additionally,wedefinedthereactivatedratioastheratioofreactivatedlandslideareatotheoldlandslidearea.Theformula canbewrittenasfollows:Newgenerationratio ¼ Newlandslideareaaftertheevent Totallandslideareaafterevent100kð1ÞReactivatedratio ¼ Reactivatedlandslideareaaftertheevent Landslideareabeforetheevent100kð2Þ“Newlandslideareaaftertheevent” meansthatthelandslide appearedonlyafterthatevent; “reactivatedlandslideareaafter theevent” meansthatthelandslideareawasreactivatedafter thatoneevent; “oldlandslideareabeforeevent” meansthat landslidealreadyexistedbeforethatevent. Calculationandstatisticsoflandslidenumber,area,landslideratio,reactivatedratioandnewgenerationratioindicatethat landslidescontinuetoexpandintheChenyoulanRiverbasin. Thelandslideareahasincreased2.8timessinceSeptember1999.Furthermore,aftertyphoonsTorajiandMindulle,the landslideareaincreasedby1.9timesand1.3times,respec-tively.AftertyphoonHerb,thereactivatedratiooflandslidereached93.5%and,followingtheChi-Chiearthquake,theratio ofnewgenerationlandslidedecreasedprogressively.The reactivatedratiodecreasedto40.0%aftertyphoonMindulle(Fig.3).Moreover,theratioofnewgenerationlandslidesgrewfrom41.0%to70.0%aftertheearthquake(Fig.4).Following typhoonsTorajiandMindulle,theratiosofnewgenerationlandslidewere66.0%and68.2%(Table1),respectively. Thedecreaseinreactivatedratiomayindicatethatthe activityofoldlandslidesbecamemoderatedorhadstopped.The landslideratiointheChenyoulanRiverbasinisontherise,and thenewgenerationratiomaintainedathighratioabout70%. ThelandslideareasmappedfollowingtheChi-Chiearthquake andtyphoonsHerb,TorajiandMindulledisplayamagnitude–probabilitydistributionthatcanbedescribedbyapowerlawover theapproximatelythreeordersofareamagnitudeforwhich reliablemeasurementsareavailable(Fig.5).ThepowerlawFig.3.RatiosofreactivatedlandslideintheChenyoulanRiverbasin.Dashed lineisregressionofallobserveddata.Fig.4.RatiosofnewgenerationlandslideintheChenyoulanRiverbasin. Dashedlineisregressionofallobserveddata.Table1Landslidenumber,landslidearea,landslideratio,reactivatedratio,andnew generationratiooffourtyphoon-inducedandearthquake-inducedeventsEventsPre-HerbtyphoonHerbtyphoonChi-ChiearthquakeTorajityphoonMindulletyphoonLandslidenumber7161168232137352120Landslidearea(106m2)2.994.5912.8524.4331.58Landslideratio(%)0.821.263.536.718.67Reactivatedratio(%) – 93.4983.1664.6140.55Newgenerationratio(%)–39.1770.3166.0368.62Exponentoflandslideareaprobability, γ1.881.991.831.861.88Fig.5.ProbabilitydistributionoflandslideareasduringtyphoonHerb,Chi-Chiearthquake,typhoonToraji,andtyphoonMindulle. γ istheregressionexponents ofpowerlaw.35G.-W.Linetal./EngineeringGeology97(2008)32–41distributionisvalidoverawiderangeofland slideareas(Guzzettietal.,2002),andisusedtodiscussthedominantareaoflandslide. Thisprobabilitydistributionmaybewritteninacumulativeform(Hoviusetal.,1997),PAzAc ðÞ¼ jA ð3Þwhere P(A≥Ac)istheprobabilityofthenumberoflandslidesof magnitudegreaterthanorequalto Ac, κ isthefrequen cyoflandslideperunitarea,and γ isadimensionlessscalingexponent.Wedefine Ac =2000m2,andobtainthebestfittingpowerlawmodelbylinearregression.ForlandslidesfromtheChi-Chiearthquakeandthetyphoons,thescalingexponents, γ,wereestimatedusingthemaximumlikelihoodmethod(Rileyetal.,2002)ashigherthan1.5overarangebetween2000m2and1×106m2(Table1). Theseexponentsindicatethatsmallandfrequentlandslidesare reallyandvolumetricallyimportantintheChenyoulanRiver basin.4.Uniaxialcompressivestrength Thefieldinvestigationindicatesthatthedepthoflandsliding intheChenyoulanRivercatchmentisdeeperthanthethickness ofsurfacesoilandcolluviumafterthe2004typhoonMindulle. Therefore,weperformedtheuniaxialcompressivestrength (UCS)teston128setsofrocksamplesobtainedfromevery formationinChenyoulancatchmentsoastofindacorrelation betweenmechanicalstrengthofthebatholithineachformation andlandslide.Eachsetofrocksamplesinclude3to5rockcores whichhavealengthof30cmandaradiusof15cm.For37setsofsandstonesamplesfromNanchuangForma- tion,theaverageUCSis70MPa,whilefor12setsofshale samples,itislessthan10MPa.Theratioofsandstonetoshale inNanchuangFormationisabout1:1,thereforetheaverage UCSfortheentireNanchuangFormationiscalculatedtobe42MPa.For32setsofsandstonesamplesfromHosheFormation,the averageUCSis96MPa,whilefor10setsofshalesamples,it variedbetween37and49MPa.Theratioofsandstonetoshale inHosheFormationisalso1:1,hencetheaverageUCSof HosheFormationiscalculatedtobe70MPa.For37setsof metamorphicrocksamplesobtainedfromeastregionof Chenyoulancatchment,theaverageUCSis102MPa. ThecomparativelylowerUCS(b10MPa)ofshalein NanchuangFormationisamajorfactorcausingtheaverage UCSoftheentireNanchuangFormationtobelowerthanthatof theotherformations.AlthoughtheaverageUCSofmeta- morphicrockisthehighestinChenyoulancatchment,the variationofUCSofmetamorphicrockthereisthegreatest becausethemetamorphicrockincludeshighUCSmetamorphic sandstoneandlowUCSslate(Table2).5.AnalysisofNormalizedDifferenceVegetationIndex Onthehillslopedisturbedbylandsliding,thevegetation wouldbewasted.Thedensityofvegetationcanbeanother targettounderstandthedegreeoflandslidingeffect.Green leavescommonlyhavelargerreflectioninthenear-infrared lightthanintheinfraredlight.Asthegreenplantscomeover heavyrainstorm,becomediseasedordie,theywouldreflectsignificantlylessinthenear-infraredlight.Arangeofsatellite derivedvegetationindiceshasbeenwidelyusedtoclassifylandcover.WeusedSPOTsatelliteimaginestocountthedensityofvegetation,whichcouldbeindirectlycalculatedfromthevisibleandnear-infraredlightreflectedbyvegetationasaNormalized DifferenceVegetationIndex(NDVI).TheNDVIresultscan assistindeterminingthedensityofgreenplantpresenceusing thewavelengthsdifference.Theformulausedtocountcanbewrittenasfollows(Deering,1978).NDVI ¼ nearQinfraredreflectionNIR ðÞinfraredreflectionIR ðÞnearQinfraredreflectionNIR ðÞþ infraredreflectionIR ðÞð4ÞNIRandIRcanbeinvestigatedintheSPOTsatelliteimages byobservingthedistinctcolorsinwavelengthsreflectedbytheplants.ThecalculationofNDVIforagivenpixelintheSPOTsatelliteimagesalwa ysresultsinanumberthatrangesfrom −1to+1.BecausethevariationsofphotosyntheticactivityinTable2Mean,minimum,maximumrockstrength,andISRMclassificationofeachlitho-formation FormationNanchuangFormationsandstone(MPa)NanchuangFormationshale(MPa)HosheFormationsandstone(MPa)HosheFormationshale(MPa)Metamorphicterrain(MPa)Mean69.886.3796.4743.14101.77Maximum116.319.80156.5248.42178.41Minimum29.404.0050.4637.9145.18StrengthclassificationofISRMR3 R1 R4 R3 R5Fig.6.NDVIvalueandlandslideratioofeachsub-catchmentinChenyoulancatchment.Dashedlineisregressionofallobserveddata.36G.-W.Linetal./EngineeringGeology97(2008)32–41differentseasonsimpactthecalculationof NDVI,weselectedSPOTimagestakeninthemonthsfromJunetoDecember.Priorto1996typhoonHerb,theresultofNDVIanalysisin ChanyoulanRivercatchmentrangedbetween −0.25and0.77,withanaverageof0.55.AftertyphoonHerb,theaveragevalueofNDVIbecame0.47;moreover,theaveragedroppedto0.43,0.32and0.23afterthe1999Chi-Chiearthquake,2001typhoon Torajiand2004TyphoonMindulle,respectively(Fig.6).These resultsshowthataverageNDVIvalueafteranyrainstormor earthquakeeventdecreasedcontinuouslyduringtheperiod from1996to2004,signifyingthatthevegetationcover conditionwasimpactedandreduced.ThevariationofNDVI valueindifferentsub-catchmentsmayresultfromthediversity ofvegetation,geomaterials,gradient,andotherfactors. IntheChenyoulancatchment,asthelandslideratioincreases withthesuccessiverainstormsandheavyearthquake,theNDVI valuesdecrease.Weattributethistomasserosionresultingfrom landsliding;theresultingwastageofvegetationcausesthedrop ofNDVIvalue.Further,wefindthatthevariationofNDVI valuesinsub-catchmentsafterrainstormsarehigherthanthe variationafterheavyearthquake.Hence,subsequentrainstorms causemoredivergentdestructionofvegetationineachsub- catchment,andthismaybeinfluencedbyprecipitation distributionandpathoftyphoon.6.EstimationofsedimentdischargeHydrometricdata,includingsedimentconcentrationand flowdischarge,arerequiredinordertoapplytheMonthly WeightedAverageMethod(MWA)(Cohn,1995)toestimate sedimentdischarge.ThesamplingfrequencyofWaterResource Agency(WRA)increasesinthesummermonths(fromMayto October).Employinganaverageofmonthlyaveragecan eliminatebiasassociatedwithincreasedsamplingfrequency. Thedatausedinourstudywereobtainedfromthehydrometric stationsoftheWaterResourcesAgencyintheChenyoulan Riverwatershed.TheobserversofWRAgetthesuspended sedimentsampleaverage30±2timesperyearandtheyalso measuretheflowdischargeastheygetthesuspendedsediment sample(WRA,2007).Theformulausedtocalculatethe sedimentdischargecanbewrittenasfollows,EMWA ¼ 112X12i1miXmij¼1Qsijð5Þwhere EMWA isthecalculatedamountofsedimentdischarge(tday−1), mi isthemeasuredtimesnumberfrequencypermonth,and Qsij isthemeasuredvalueofsedimentdischarge(tday−1).Lackingsufficientmeasurementdatafromthetyphoonperiodstocalculatethetyphoon-inducedsedimentdischarge, weusedtheRatingCurveMethod(Walling,1977)toestimate sedimentdischargeduringthetyphoonperiods.TheRating CurveMethodisusefulwhenanempiricallycalibratedpower lawrelationbetweentheflowdischargeandthesedimentconcentrationcanbedefined.ThestandardformulaisQs ¼ aQcð6Þwhere Qs issuspendedsedimentconcentrationand Q isflowdischarge.Theformulaislinearizedbytakingthelogarithmofeachcoordinateaxis.Theparameters a and c aredeterminedbyusingleast-squareregression.Then,the Qs wouldbeobtainedbyback-transformationofthemeasurementsofflowdischargeTable3 Statisticsofsedimentdischargeandflowdischargeoffourtyphoonswhichinducehazardouslan dslidesEventsDate(days)Totalsedimentdischarge(t)Calculateddate(days)Averagesedimentdischargeperday(tday−1)Averageflowdischarge(m3s−1)Unitsedimentdischarge(tm−3s−1)Herbtyphoon7/29–8/1(4)28,743,0587/31–8/2(3)9,581,019913.7031,457Torajityphoon7/28–7/31(4)11,555,9227/30–8/2(4)2,888,980254.8845,338 Mindulletyphoon6/28–7/3(6)9,918,4946/28–7/5(8)1,239,811203.2848,791Aeretyphoon8/23–8/26(4)34,146,5348/23–9/3(12)2,845,544897.5738,043 Table4 Averageflowdischargeanddailysedimentdischargeintheperiodoftyphoon TyphoonDateSedimentdischarge(t)Averageflowdischarge(m3s−1)Herb1996/7/31405,716186.261996/8/125,727,1771875.831996/8/22,610,166698.33Totalsedimentdischarge28,743,058Toraji2001/7/28276820.072001/7/29191317.082001/7/3010,426,733730.382001/7/31910,888251.992001/8/1164,860119.492001/8/253,44273.07Totalsedimentdischarge11,555,922Mindulle2004/6/28455,123146.122004/6/29443,531144.562004/6/30433,875143.372004/7/1426,486142.422004/7/2738,302172.002004/7/31,964,797252.502004/7/44,862,211356.192004/7/5594,170221.98Totalsedimentdischarge9,918,494Aere2004/8/23100,30899.772004/8/24596,613222.392004/8/254,268,531874.642004/8/263,003,188637.112004/8/274,884,710987.662004/8/284,930,563996.012004/8/294,884,710987.662004/8/305,116,4491,029.792004/8/313,584,263747.222004/9/11,543,763349.752004/9/21,202,453304.762004/9/330,98458.83Totalsedimentdischarge34,146,53437G.-W.Linetal./EngineeringGeology97(2008)32–41Q.Therelationbetweenflowdischargean dsedimentconcentrationwasconstructedbythemeasureddataofoneyear.ForthecaseoftyphoonMindulle,thethirtymeasureddataof2004areusedtoconstructtheratingcurveof2004,andthen thesedimentdischargeduringtyphoonMindulleisestimatedbytheratingcurveof2004andthehourlyflowdischargeduringtyphoonMindulle.Weestimatedthesedimentdischargeof4typhoons,includingtyphoonsHerb,Toraji,MindulleandAerefrom1996–2004.Estimationofdataatthehydrometricstationshowsthat,duringtheperiodof1972–2004,theaverageannualsediment dischargeatthedownstreamNeimaopustationwas6.2Mtyr−1,whiletheannualsedimentdischargewas107Mtin2004.Ifweexcludedthe2004datafromourestimation,theannualsedimentdischargewouldbe2.9Mtyr−1.DataestimationattheupstreamHoshestationshowsthat,duringtheperiodfrom1972to2000,theaverageannualsedimentdischargewas0.5Mtyr−1,thatis,one-tenthoftheaverageannualsediment dischargeatthedownstreamNeimaopustation.Nevertheless, theannualsedimentdischargeatHoshestationwas2.7Mtyr−1in1994,butlessthan100tyr−1in1987.Theabove-mentioned conditionshowsthatthevariationoftheannualsediment dischargeattheHoshestationisgreat.Wealsoestimated sedimentdischargesduringmajorstorms,including1996typhoonHerb,2001typhoonTorji,2004typhoonMindulleand2004typhoonAere(Table3).During1996typhoonHerb,thecatchmentofupstreamNeimaopustationsupplied28.7Mtofsedimentdischargeoveraperiodof3days(Table4).Specificallyon1August,25.7Mt ofsedimentwasdelivereddownstream,withtheaverageFig.7.Therecordofwaterdischargeandsedimentdischargeintheperiodfrom1972to2004. Fig.8.(a)SuspendedsedimentratingcurveoftheChenyoulanRiver.Dashedlineispowerlawfun ctionfittedtopost-earthquakedata;solidlineispowerlawfunctionfittedtopre-earthquakedata.(b)Theconcentrationratioofpre-earthquakeandpost-earthquake.Solidlineispowerlawregression.38G.-W.Linetal./EngineeringGeology97(2008)32–41sedimentconcentrationforthisdayreachi ng158.7×103ppm,muchhigherthantheaverageannualsedimentconcentrationof2.3×103ppm.During2001typhoonToraji,thecatchment upstreamofNeimaopustationsupplied11.6Mtofsediment,with10Mtofsedimentdelivereddownstreamon30July.The totalsedimentduringtyphoonTorajiwaslessthanthatduringtyphoonHerb,buttheaverageflowdischargeduringtyphoonTorajiwas254.9m3s−1,onlyaquarteroftheaverageflowdischargeduringtyphoonHerb.Theunitconcentrationof sedimentduringtyphoonTorajiwas4.5Mtm−3s−1greaterthantheunitconcentrationduringthetyphoonHerb.During2004typhoonMindullefrom28Juneto3July,4.5Mt ofsedimentweredeliveredtothedownstreamarea.However, stormsfollowingtyphoonMindulleinduced4.9Mtofsedimenton4July.DuringtheperiodoftyphoonAere,theupstream catchmentofNeimaopustationyielded34Mtofsediment,with anaveragedailysedimentdischargeof2.8Mtday−1.TheunitsedimentconcentrationsduringtyphoonsMindulleandAerewere4.8Mtm−3s−1and3.8Mtm−3s−1,respectively.7.Thechangeofsedimentdischarge Fortherelationshipbetweenaverageflowdischargeand measuredsedimentdischargeduringtheperiodof1972–2004,wefoundthatpost-earthquakemeasuredsedimentdischargesweregreaterthan1tday−1(Fig.7),whichdiffereddiscerniblyfromthepre-earthquakemeasuredsedimentdischarges,whichwerefrequentlylessthan1tday−1.IntheChenyoulanRiverbasin,thesedimentdischarges increasedaftertheChi-Chiearthquake(Fig.8a).Theaverage annualsedimentdischargewas2.9Mtyr−1(7.9×103tyr−1km−2)intheperiodof1972to1999,anditwas37.8Mtyr−1(103×103tyr−1km−2)intheperiodfrom2000to2004.The changeindicatesthatsedimentdischargesincreasedby13times aftertheearthquake.Theaverageannualsedimentdischargeof theChoshuiRiverincentralTaiwanwas5.4Mtyr−1(1.8×103tyr−1km−2)intheperiodfrom1986to1999,anditincreasedby2.6timesintheperiodbetweentheChi-Chiearthquakeandthe endof2001(Dadsonetal.,2004).TheareaoftheChoshui Riverbasinis2989km2,8timesgreaterthantheChenyoulanRivercatchment.Inadditiontotherelationshipbetweenthe measuredflowdischargeandthesedimentconcentration,the post-seismicconcentrationincreased,andtheratioofpre- seismicandpost-seismicconcentration(Cpost/Cpre)canbe describedbyapowerlawformulawithmeasuredflowdischarge(Fig.8b).Thisformulacanbewrittenasfollows,logCpostCpre¼ 0:352 log Q þ 1:27 ð6Þ where Cpost isthepost-seismicsedimentconcentration, Cpre is thepre-seismicsedimentconcentration,and Q isthemeasuredflowdischarge.Theexponentialdecayrevealsthattheratio,Cpost/Cpre,islowerwhentheflowdischargeishigher. Whentheflowdischargeislessthan100m3s−1,post-seismicsedimentconcentrationis4timeshigherthanpre-seismic concentrationwiththesameflowdischarge.Dadsonetal.(2004) discussedtheaverageovera6-monthperiodofsedimentconcentrationfrom1986to2002intheChoshuiRiver,and foundthatthedataforwinterseasonsdisplayedanexponential decayinsedimentconcentrationfromtheelevatedpost-seismicvaluetothepre-1999wintermean.AlthoughtheChenyoulan RiverisatributaryoftheChoshuiRiver,itsconditionof sedimentdischargeduringwinterseasonsisnotthesameasthatoftheChoshuiRiver.Moreover,theaveragesedimentdischargeforpost-earth-quakewinterseasons(1999–2004)waslessthantheamountfor allavailablerecordedwinterseasons(1972–2004).Thatmay havebeenbecausetheaverageannualprecipitationfor1999to2004wasalsolessthantheamountfor1972to2004.ThisalsoFig.9.Timeseriesofsedimentdischargeovera6-monthperiodbetween1972and2004intheChe nyoulanRiver.Dottedlineshowstyphoonseasonmean;dashedline showswinterseasonmean.39G.-W.Linetal./EngineeringGeology97(2008)32–41indicatesthatpatternofsedimentdischarg eduringwinterseasonsisdeterminedbytheprecipitation.Additionally,fromthecomparisonoftheaverageovera6- monthperiodofsedimentdischarge,wefoundthatin1999the sedimentdischargeoftyphoonseason(fromMaytoOctober)was15263timesthesedimentdischargeofwinterseason(from DecembertofollowingApril),andtheaveragesediment dischargeoftyphoonseasonswas38timestheaverage sedimentdischargeofwinterseasonsfrom1972to2004.This revealsthatsedimentdischargesoftyphoonseasonsandwinter seasonsdisplayenormousdisparityintheChenyoulanRiverbasin(Fig.9).8.DiscussionInNanchuangFormation,thetyphoonMindullelandslideratiowas17timesthetyphoonHerblandslideratio(Fig.10).But,inHosheFormation,thelandslideratiofromtyphoonHerb totyphoonMindulleincreasedlessthan5times.Theincreaseof landslideratiofortheNanchuangFormationisdiscerniblygreaterthanthatoftheHosheFormationintheperiodfrom1996typhoonHerbto2004typhoonMindulle.Thisobserva- tionshowsthatthelandslideratiodifferencebetween NanchuangFormationandHosheFormationmayberelated tothefactthattheaverageuniaxialcompressivestrengthof NanchuangFormationisthelowestamongthreeformations.InAlluvium,withsedimentstrengthlessthan10MPa(CGS,2003),thelandslideratioalsoincreasedbylessthan5times fromtyphoonHerbtotyphoonMindulle.Keefer(2000) suggestedthattheproportionofsurfacearea disturbedbylandslidingincreaseswithproximitytotheearth- quakeepicenterandearthquakefaultandthedecreasebecomes moreobviouswithproximitytotheepicenter. Dadsonetal.(2004) studiedthelandslidedataofChi-Chiearthquake,and foundthatthedecreaseinareaaffectedbylandslidingawayfrom theChelungpufaultwasrapidatdistancesinexcessof20kmfromthefault.IntheChenyoulanRiverbasin,afterChi-Chiearthquake,thelandslideratiodecreasedatincreaseddistances fromtheChelungpufaultandalsodecreasedwiththedecayin peakverticalgroundacceleration(PGA)(Leeetal.,1999). Oncethedistanceawayfromfaultexceeded20kmandPGAwaslowerthan0.2g,thelandslideratiodecreasedtolessthan1%quickly(Fig.11).ThiswasparticularlyevidentaftertyphoonToraji.Moreover,oncethedistanceawayfromfaultexceeded20kmandPGAwaslowerthan0.2g,thelandslide ratiooftyphoonTorajidecreased7.2%morequicklythanthe decreaseofthelandslideratiooftheearthquake.Nevertheless, aftertyphoonMindulle,thisappearanceofdecayoflandslide ratiowasmoderated.Theobservationofastrongerearthquake- relatedsignalinthetyphoonToraji-inducedlandslidedistribu-tionsuggeststhat,eveninareasthatexperiencednolandsliding duringanearthquake,thesubstratewaspreconditionedtofail.ndslideratiosinresponsetoearthquakewithdistancefromChelungpufault. ndslideratiosoffourformationsinfourtyphoon-inducedandearthquake-inducedlandslideandUCSoffourformations.40 G.-W.Linetal./EngineeringGeology97(2008)32–419.Conclusion Theprobabilitydistributionoflandslideareaforfourevents,includingthe1996typhoonHerb,1999Chi-Chiearthquake,2001typhoonToraji,and2004typhoonMindulle,correlating wellwithapowerlawrelation,regressionexponentslessthan1.5,indicatesthatthelandslideconditionintheChenyoulan Riverbasinisdominatedbysmallandfrequentlandslides. DuringtheperiodfromChi-ChiearthquaketotyphoonMindulle,theratioofnewgenerationlandslidesinthe。

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预防地震的英语建议作文

预防地震的英语建议作文

预防地震的英语建议作文Title: Suggestions for Earthquake Prevention。

Introduction:Earthquakes are natural disasters that can cause immense destruction to life and property. Therefore, it is crucial to take proactive measures to prevent or minimize their impact. In this essay, I will explore various suggestions for earthquake prevention based on widely downloaded materials from the internet, while also providing additional insights and recommendations.1. Public Awareness and Education:Educating the public about earthquake preparedness is paramount. Government agencies, schools, and community organizations should conduct awareness campaigns to inform people about the risks associated with earthquakes and the appropriate actions to take before, during, and after anearthquake. This includes creating emergency kits,identifying safe shelters, and practicing evacuation drills.2. Strengthening Building Codes:One of the most effective ways to mitigate earthquake damage is to enforce strict building codes that ensure structures are resilient to seismic activity. This involves using earthquake-resistant materials, reinforcing foundations, and implementing engineering designs that can withstand ground motion. Governments should invest in retrofitting older buildings to bring them up to modern safety standards.3. Early Warning Systems:The development and implementation of early warning systems can provide valuable seconds to minutes of advance notice before an earthquake strikes. These systems use a network of sensors to detect seismic waves and send alertsto individuals and authorities, allowing them to take immediate action to protect lives and infrastructure.Investing in the expansion of early warning networks is crucial for enhancing earthquake preparedness.4. Land-Use Planning:Proper land-use planning plays a significant role in reducing earthquake risk. Governments should enforce zoning regulations that restrict construction in high-risk areas, such as fault lines or liquefaction zones. Additionally, infrastructure projects should consider seismic hazards during site selection and design to minimize exposure to potential earthquake damage.5. Community Resilience:Building community resilience is essential for effectively responding to earthquakes and recovering from their aftermath. This involves fostering strong social networks, establishing community emergency response teams, and promoting self-sufficiency through initiatives such as community gardens and local food storage. By empowering communities to support each other during crises, theoverall impact of earthquakes can be mitigated.6. International Cooperation:Earthquakes can transcend national borders, making international cooperation vital for effective disaster management. Countries should collaborate on sharing seismic data, best practices in earthquake preparedness, and mutual assistance agreements for disaster response. Through joint efforts, the global community can work towards minimizing the loss of life and property caused by earthquakes.Conclusion:In conclusion, preventing earthquakes entirely may not be feasible, but implementing proactive measures can significantly reduce their impact. By raising public awareness, strengthening building codes, deploying early warning systems, planning land use effectively, building community resilience, and fostering international cooperation, we can enhance our preparedness and resilience to earthquakes. It is essential for governments,communities, and individuals to work together to prioritize earthquake prevention and create safer, more resilient societies for future generations.。

地震前兆的英语作文

地震前兆的英语作文

地震前兆的英语作文Earthquakes, one of the most powerful and destructive natural phenomena, have long been a subject of fascination and fearfor humanity. While they are often sudden and without warning, scientists have been studying various phenomena that may precede an earthquake, offering a glimpse into the complex world of seismology. In this essay, we will explore some ofthe potential precursor signs of earthquakes and discusstheir significance in earthquake prediction.One of the most well-known precursors is foreshock activity. Foreshocks are smaller earthquakes that occur before a larger mainshock. They can provide valuable information about the stress build-up within the Earth's crust. However, not all earthquakes are preceded by foreshocks, and their occurrence does not guarantee a subsequent major earthquake.Another sign to consider is the behavior of animals. There have been numerous anecdotal reports of unusual animal behavior prior to significant earthquakes. Dogs barking excessively, birds flying in erratic patterns, and fishleaping from the water are just a few examples. While these observations are not scientifically proven to be directly linked to earthquakes, they do suggest that animals might be sensitive to subtle changes in their environment that humans cannot perceive.Seismic activity is also closely monitored through the use ofseismographs. A sudden increase in the frequency of smaller tremors, known as an earthquake swarm, can sometimes indicate that larger seismic events are imminent. However,interpreting these data requires expert analysis and cannot be relied upon as a standalone predictor.Changes in ground deformation can also be a sign of an impending earthquake. GPS technology and satellite imagery can detect minute shifts in the Earth's surface that may be indicative of tectonic stress. While these measurements are more scientific and reliable, they are still not foolproof in predicting the exact timing or location of an earthquake.In addition to physical signs, there are also chemical changes that can occur before an earthquake. Some studies suggest that gases such as radon may leak from the ground in areas where tectonic plates are about to move. The increase in radon levels can be detected by specialized equipment, but again, this is not a definitive precursor.Lastly, the possibility of using infrasound to predict earthquakes is an area of ongoing research. Infrasound are low-frequency sound waves that are below the range of human hearing. They can be generated by various natural events, including earthquakes. The detection of these waves before an earthquake could potentially serve as a warning, but the technology and understanding of this phenomenon are still in the early stages.In conclusion, while there are several potential precursor signs of earthquakes, predicting them with accuracy remains acomplex challenge. The study of these signs is crucial for advancing our understanding of earthquake science and for developing better early warning systems. As technology and research continue to evolve, the hope is that one day we may be able to provide more reliable warnings to protect lives and property from the devastating effects of earthquakes.。

在地震中的英语作文

在地震中的英语作文

Earthquakes are natural disasters that can cause significant destruction and loss of life.Writing an essay about an earthquake can be a way to explore the impact of such events,as well as the response and recovery processes.Here are some key points to consider when writing an English essay on an earthquake:Title:The Impact of Earthquakes on SocietyIntroduction:Begin by defining what an earthquake is and its geological causes.Mention the unpredictability of earthquakes and their potential to cause widespread damage.The Science Behind Earthquakes:Explain the tectonic plate theory and how it relates to earthquakes.Discuss the Richter scale and how it measures the magnitude of earthquakes.Case Study:A Specific Earthquake Event:Choose a notable earthquake,such as the2004Indian Ocean earthquake or the2010 Haiti earthquake.Describe the location,date,and time of the earthquake.Provide statistics on the death toll,number of people affected,and the extent of the damage.Immediate Effects:Detail the immediate physical impacts,such as building collapses,landslides,and tsunamis.Discuss the psychological effects on survivors,including fear,anxiety,and trauma.Response and Rescue Efforts:Describe the immediate response from local authorities,international aid organizations, and volunteers.Highlight the challenges faced during the rescue and relief operations,such as accessibility,communication,and resource allocation.LongTerm Impacts:Discuss the longterm effects on the economy,infrastructure,and the environment. Mention the displacement of populations and the challenges of rebuilding communities.Prevention and Preparedness:Explore the measures that can be taken to minimize the impact of earthquakes,such asbuilding codes,early warning systems,and public education.Discuss the role of technology in predicting and preparing for earthquakes.Recovery and Reconstruction:Describe the process of rebuilding after an earthquake,including the physical reconstruction of infrastructure and the emotional recovery of the affected population. Highlight the importance of community resilience and the role of international cooperation in the recovery process.Conclusion:Summarize the key points discussed in the essay.Reflect on the importance of understanding and preparing for earthquakes to mitigate their devastating effects.Recommendations:Suggest ways in which individuals,communities,and governments can work together to improve earthquake preparedness and response.References:List any sources used in the research for the essay,ensuring they are properly cited to maintain academic integrity.Remember to use clear and concise language,and to structure your essay in a logical manner.Providing specific examples and data can help to illustrate the points you are making and make your essay more engaging and informative.。

关于地震的影响英语作文

关于地震的影响英语作文

关于地震的影响英语作文The Impact of Earthquakes。

Introduction:Earthquakes are natural disasters that occur when there is a sudden release of energy in the Earth's crust, causing the ground to shake. They can have devastating effects on both human life and the environment. In this essay, we will explore the impact of earthquakes and how they can be mitigated.Body:1. Loss of human life:Earthquakes can result in the loss of thousands of lives. When buildings collapse or landslides occur, people can be trapped or buried under debris. In densely populated areas, the death toll can be particularly high. For example,the 2010 earthquake in Haiti claimed the lives of over 230,000 people.2. Destruction of infrastructure:Earthquakes can cause significant damage to infrastructure such as roads, bridges, and buildings. This can disrupt transportation systems, making it difficult for emergency services to reach affected areas. The destruction of buildings also leads to the displacement of people, leaving them homeless and in need of shelter.3. Economic impact:The economic impact of earthquakes can be immense. The cost of rebuilding damaged infrastructure and providing aid to affected areas can run into billions of dollars. The loss of businesses and disruption to trade can also have long-term effects on the economy. For example, the 2011 earthquake and tsunami in Japan caused an estimated $360 billion in damages.4. Environmental impact:Earthquakes can have a significant impact on the environment. They can trigger landslides and avalanches, leading to the destruction of habitats and the loss of biodiversity. Earthquakes can also cause tsunamis, whichcan devastate coastal ecosystems. Additionally, the release of harmful substances during earthquakes, such as oilspills or nuclear leaks, can further damage the environment. Mitigation measures:1. Building codes and regulations:Implementing and enforcing strict building codes and regulations can help reduce the impact of earthquakes. Buildings constructed to withstand seismic forces are less likely to collapse during an earthquake. This can savelives and minimize the destruction of infrastructure.2. Early warning systems:Developing and implementing early warning systems can provide people with valuable time to seek shelter and take necessary precautions. These systems use seismic monitoring technology to detect earthquakes and send out alerts before the shaking reaches populated areas.3. Public awareness and preparedness:Educating the public about earthquake safety and preparedness is crucial. This includes teaching people how to respond during an earthquake, such as taking cover under sturdy furniture and staying away from windows. It is also essential to have emergency kits and evacuation plans in place.4. International cooperation:Earthquakes are not confined to national borders, and their impact can be felt globally. International cooperation is vital in sharing knowledge, resources, and expertise to better understand and mitigate the effects of earthquakes. This can involve collaboration in research,technology development, and disaster response.Conclusion:Earthquakes have a profound impact on human life and the environment. The loss of life, destruction of infrastructure, economic consequences, and environmental damage are all significant challenges. However, through the implementation of mitigation measures and international cooperation, we can work towards minimizing the impact of earthquakes and protecting vulnerable communities.。