A comparison of historical and paleoseismicity in a newly formed fault ...

JOURNALOF GEOPHYSICALRESEARCH,VOL. 101,NO. B3,PAGES6021-6036, MARCH 10,1996

Acomparison of historicalandpaleoseismicity in a newlyformed faultzoneand a maturefault zone,NorthCanterbury,New Zealand

Hugh Cowan 1and Andrew Nicol 2 Department ofGeology, University of Canterbury, Christchurch, NewZealand

PhillipTonkin Department of SoilScience, Lincoln University, Lincoln, NewZealand

Abstract.The timingof largeHoloceneprehistoric earthquakes is determined by dated surface rupturesandlandslides at theedgeof theAustralia-Pacific plateboundary zonein NorthCanterb•, New Zealand.Collectively,thesedataindicatetwo large(M > 7) earthquakes duringthe last circa2500 years,withina newlyformedzoneof hybridstrikeslipandthrustfaultinghereindescribed asthePorter'sPass-to-Amberley FaultZone (PPAFZ).Two earliereventsduringtheHolocenearealsorecognized, butthedatapriorto 2500yearsarepresumedto be incomplete.A returnperiodof 1300-2000yearsbetween largeearthquakes in the PPAFZ is consistent with a late Ho!ocenesliprateof 3-4 mm/yr if eachdisplacement is in therange4-8 m. Historicalseismicityin thePPAFZ ischaracterized byfrequentsmall andmoderatemagnitudeearthquakes anda seismicityratethatis identical to a region surroundingthe stmcturallymatureHope fault of the Marlborough FaultSystemfarthernorth.This is despitean order-of-magnitude differencein sliprate between the respectivefault zonesandconsiderable dffœerences in therecurrence rateof largeearthquakes. The magnitude-frequency distribution in theHopefaultregionis in accord with the characteristic earthquake model,whereastherateof largeearthquakes in thePPAFZis approximated (but overpredicted)by the Gutenberg-Richter model.The comparison of thesetwo œault zonesdemonstrates theimportance of the structural maturity ofthefault zone in relationto seismicityratesinferredfrom recent,historical,and palcoseismic data.

earthquakes in thePorter'sPass-to-Amberley FaultZone(PPAFZ), a regionof newly formed strike-slipfaultingthat definesthe Earthquake hazardassessment traditionally incorporates the deformationfront at the edge of the Australia-Pacific plate analysis of historicandcontemporary seismicity, supplementedboundaryzonein northeastern SouthIsland. where possible bygeological information aboutaverage sliprates, North Canterburystraddlesthe edgeof the Australia-Pacific rupture lengths andslipperevent.Thetimingandmagnitude of plate boundaryzone betweenthe HikurangiTrough and the prehistoric earthquakes resulting in surface rupture aredeterminedSouthernAlps (inset,Figurel a). Dextralstrike-slipfaultsof the Introduction

bydating features offsetbythefaultandbyexamining thegeometry Marlborough FaultSystem(MFS), farthernorth,transfer oblique ofthefaulttrace.However,thesedataaloneoftendo notprovide platemotionbetweentheAlpinefaultandtheHikurangisubduction sufficient informationto characterizelarge palcoseismic events zone. These faults have smaller cumulative offsets toward the [e.g.Ward,1994]. southeastbut higher slip rates that reflecta southwardrelative InNorthCanterbury, NewZealand, wehavesupplemented data migrationof thelocusof deformation across theregionduringthe fromfaulttraceswith datafrom landslides distributed throughout lateQuaternary[e.g.,YeatsandBerryman,1987;H. Andersonet al., theeastern foothillsof the Southern Alps.Thesedataareused 1993].The mostsoutherly elementof theMFS is the Hopefault collective!y to assess the late Holocene chronology of large (Figurel a). Sixty kilometerssouthof the Hope fault, a zoneof disseminated shearis evolvingalonga similartrend(Figurela) [Rynnand Scholz, 1978; Carter and Carter, !982; t!emer and

--l Now atNorwegian Seismic Array, Kjeller, Norway.

1985].Elements of thiszonecomprise anastomosing 2Now atFault Analysis Group, Departmeat ofEarth Sciences, Univer-Bradshaw,

sity ofLiverpool, Liverpool, England.

strike-slipand thrustfaults that extend east-northeast from the

Copyright ! 996bytheAmerican Geophysical Union.

Southern Alpsalongthenorthern marginof theCanterbury Plains andpassinto a fold andthrustbeltnearthe townof AmberIcy

Paper Number 95JB01588. 0148-0227/96/95JB.0! 588$05.00

Fault Zone (PPAFZ)to describethe strikeand approximate

(Figures la and2). We usethenamePorter's Pass-to-Amber!ey 6021

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COWANET AL.:PALEOSEISMICITY IN A NEWLYFORMEDFAULTZONE

! i !

i, •

I I

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1170E Figure la. Major structuralelementsof thenortheastern SouthIsland,andthe locationsandyearsof selectedhistorical earthquakes.The onshorestructureis modifiedfrom Gregg[1964], Cowan[1992] and unpublisheddataof the Active TectonicsResearchGroup,Universityof Canterbury. Offshorestructureis simplifiedfrom Barnes [1994]. The numbered rectangles delineatetwo regionsselectedfor analysisof historicalseismicity.AbbreviationsareMFS, MarlboroughFault System;HPF, Hope fault;HRS, HopeRiver segment;KPF, Kakapofault; PPAFZ, Porter'sPassto-AmberleyFaultZone;PBF, PegasusBay fault;PP,Porter'sPass;and AY, AmberIcy.Inset showslocationof Figures la and lb in relationto the Alpine Fault (Air) and Hikurangi Trough (HT), the main elementsof the New Zealandplateboundary.The arrowindicatesthe directionand magnitude(42 mm/yr) of the plate motaonvector [DeMets et al., 1990].

geographic end-points of thezone,whichhasonlyrecentlybeen

HistoricSeismicity of the North CanterburyRegion

mappedcomprehensively [Cowan,1992]. The region betweenthe Hope fault and PPAFZ is one of Earthquakes withinandadjoiningtheNorthCanterbury region transition fromsubduction tocollision[Reyners andCowan,1993], duringtheperiod1942-1994areshownin Figurelb, withthose

wherethegeological structure of theuppercrustis dominated by greaterthanmagnitude 6.5 since1888indicated onFigurela. The northeast strikingreverse andthrustfaultsthatdipsoutheast andare focaldepths formostevents in thecatalogue areuncertain duetothe closelyassociated withgrowingfolds[Yousif,1987;Nicolet al., widespacing of national networkseismographs (typically >50kin), 1994]. This region has accommodated ~12-15% northwest- butlocalstudies of microseismicity, recorded onportable, smallsoutheast shortening duringthePleistocene, andratesofshorteningapertureseismograph arrays,haveindicatedthatmostearthquakes doseto thePacificcoastareabout1%/100kyr [Nicoletal., 1994]. inthisregionarerestricted totheupper10-15kmofthecrust [Rynn Detailed studies alongtheedgeof theplateboundary zoneinNorth andScholz,1978;Cowan,1992;Reyners andCowan,1993]. Canterbury indicate thatdeformation probably commenced within TheHopeRiversegment of theHopefault(Figurela) andan

thelast0.5-1m.y.,disrupting a volume of crustnotsignificantlyadjacent splay(Kakapo fault)haveeachruptured during historic deformed sincetheCretaceous [Cowan,1992;Nicoletal., 1994]. earthquakes of magnitude M---7.3andM~7.0,respectively [Eiby, Thepurposeof thispaperis to evaluatethelateHolocenerecord 1968;Cowan,1991;Yang,1991, 1992].Two moreevents of of large earthquakesin the PPAFZ, in order to understandthe magnitude 6.5.-6.9haveoccurred in the eastern partof North frequency-magnitude distribution ofearthquakes inanewlyformed Canterbury, in 1901and1922,andanother eventofM•,6.7occurred

faultzone.We compare historical andpalcoseismic datafromthe in June 1994, to the west of the PPAFZ. The 1994 earthquake PPAFZwithdatafromtheadjacent structurally mature Hopefault occurredin the upper crust, and the 1901 and 1922 eventsare zone, and use thesedata to evaluate differencesin fault behavior presumed tohavealsobeenshallow, based onrelatively limited felt betweenstmcturallymatureandnewlyformedfaultzones.

areas[McKay,1902;Skey,1925].Manymorefaultsin North

COWAN ETAL.-PALEOSEISMI••IN A NEWLYFORMED FAULT ZONE

6023

o ß

O0 o

Figurelb. ShaLlow seismicity (7 earthquakesin the PPAFZ.

Temporal variationsin seismicityrate may profoundlyaffectthe ---2500years is only sparselypreserved.Moreover, while the frequency-magnitude distributionand apparentrelationshipscombinedevidenceof faultingandlandslides between2000 and between seismic productivityand fault slip rate, structural 2500yearsB.P.is quitecompelling, thiscannotbesaidof the500complexity, or maturity.Forexample,hadit beenpossible tosample 700 yearB.P.event,for whichourinterpretation restslargelyonthe seismicity in theHopefaultregionfor the50 yearsstraddling the landslide dataalone.

1888 earthquake, theproductivity couldhavebeenhigherandthe If ourrecordoflateHolocene rupture of thePPAFZiscomplete, frequency-magnitude relationships impliedbysuchdatamightnot theimplied interval of 1300-2000 yearsbetween thelasttwoevents have underpredicted therecurrence rateforthelargest events onthe wouldrequke 4-8 m displacement pereventtobeconsistent with

Hope faultto thesamedegree asthecurrent dataset(Table 3). theinferred sliprateof 3-4 mm/yron thePorter's Passfault. Conversely, lowseismicity ratesmaytypifythebehavior of the Displacements perevent of thisorderhavebeen documented for Hope fault, since it isclearthattheM--,7.3event ofonecentury ago numerous Holocene faults throughout NewZealand [Bcrr3,'man and

has not been followed byongoing high rates ofactivity.

Beanland, 1991] and fi,rnad6ttir etal.[1995] have modeled upto5

Return periods forlarge rareevents areclearly sensitive toslight m of displacement forthe1994Arthur's Pass earthquake (Figure changes inrecurrence parameters whenregressions areperformed la). Thustheinferred return period, displacement perevent, and ondatadominated by smaller events, or whenthecatalogue is sliprateneed notbeinconsistent. Moreover, itseems reasonable to heterogeneous with respect to magnitude [e.g.,Okal and conjecture thatlarger displacements perevent couldbeexpected

l•omanowicz, 1994; Kagan andJackson, 1995]. It isperhaps ironicwithin anewly formed faultzone composed ofnumerous short (7.0)earthquakes withinthePPAFZduringthelast 669, 1994. ~2500 years.Two earliereventsduringthe Holoceneare also Berryman, K., ActivefaultingandderivedPHS directions in theSouth Island, NewZealand, inTheOriginoftheSouthern Alpsedited byR.[ recognized, but the dataprior to 2500 yearsarepresumed to be WalcottandM.M. Cresswell, Bull.R. $oc.N.Z., 18, 29-34,1979. incomplete. A returnperiodof 1300-2000yearsfor ruptureof the Berryman, K.R., andS. Beanland, Variationin faultbehavior in different PPAFZwouldbeconsistent with theinferredsliprate(3-4 rnm/yr) tectonicprovinces of New Zealand,J. Struct.Geol.,13, 177•189,1991. ofitsprincipalelement, thePorter'sPassfault,provided thattheslip Brown, LJ., Geological mapofNewZealand, sheet S76"Kaiapoi", 1st ed. per eventis in the range4-8 m.

scale1:63,360,Dep.of Sci.Ind.Res.,Wellington, NewZealand, 1973.

T Moutoux, andW.M.Phillips, Lichen dating Palcoseismic datafrom the adjacentstmcturally matureHope Bull,W.B.,J.King,E Kong, of coseismic landslide hazards in alpine mountains, Geomorphology, fault zone farthernorthindicatesignificantdifferences in fault 10, 253-264, 1994. b•haviorwith respectto the PPAF'Z,with recurrence intervalsfor Burrows, CJ.,A 500-year oldlandslide in theAcheron Rivervalley, Caw terbury,N.Z. J. Geol.Geophys., 18, 357-360,1975. largeearthquakes apparently differingby a factorof 5 or more. at Paradoxically, thedifferences in historical seismicity ratesfor the Carter,R.M., and L. Carter,The MotunauFault and otherstructures southern edgeof theAustralian-Pacific plateboundary, offshore Marl-

Hope fault and PPAFZ ( zones i and 2,respectively, Figure lb)are borough, New Zealand, Tectonophysics, 88,133-159, 1982.

negligible despite anorder ofmagnitude difference insliprateand Chinn, T.J.H., Useofrock weathering-rind thickness forHolocene absolu!e in NewZealand,Arct.Alp.Res.,13, 33-45,1981. cumulative offsetassociated withthemajorfaults.Themagnitude- age-dating K.I.,Temporal andspatial clustering ofearthquake activity frequency distribution fortheHopefaultregionisin accord withthe Coppersmith, centralandeasternUnitedStates,Seismol.Res.Lett.,59, 299-304, characteristic earthquake model,whereas themagnitude-frequency the 1988.

distributionin the PPAPZ is approximated, but slightly Cornell,C.A.,Engineering seismic riskanalysis, Bull.Seismol. Soc. overpredicted, by theGutenberg-Richter relationship (Table3 and 58, 1583-1606, 1968.

Figure7). The comparisonof thesetwo fault zonesdemonstratesCowan,H.A., LateQuaternary displacements ontheHopefaultatGly•m Wye,NorthCanterbury, N.Z.J. Geol.Geophys., 33,285-293,1990. theimportance of thestructural maturityof thefaultzonein relation H.A.,TheNorthCanterbury earthquake of September 1, 1888, J. toseismicity ratesinferredfromrecent, historical, andpalcoseismicCowan, R. Soc.N.Z.,21, !-12, 199!.

Cowan,H.A., Structure, seismicityand tectonics of the Porter's AmberleyFaultZone, North Canterbury, New Zealand.Ph.D.th•s,

Acknowledgments.This study was fundedby the New Zealand Univ.of Canterbury, Christchurch, New Zealand,1992. UniversityGrantsCommittee andNewZealand'Earthquake Commission,Cowan,H.A., (Compilor),Fieldguideto New Zealandactivetecto•esandformedpartof a Ph.D.programmewithintheActiveTectonicsResearch

Groupat the Universityof Canterbury. BrianMolloy andNeville Moar providedradiocarbon agesand pollendatafrom sitesat Porter'sPass.We

IASPEI94,27thGeneralAssembly of IASPEI,January 1994,Welllag. ton, R. Soc.N.Z. Misc. Sen, 27, 1994.

H.A., andM.S.MeGlone, LateHolocene displacements and thankWarwickSmithandtheNewZealandSeismological Observatory for Cowan, acteristicearthquakes on the HopeRiver segment of theHopefault, seismicity dataandConradLindholmfor helpwithdatareprocessing at New Zealand,J.R. Soc.N.Z., 21,373-384, 1991. NORSAR.Reviewsby JohnAdams,KelvinBerryman, Bill Bull,Hilmar

Coyle, S., The Porter'sPassFault, M.Sc. thesis,Univ. of Canterbury,

Bungum, Jarg Pettinga, Maud McSaveney, Mark Stirling, and Bob Yeats Christchurch, New Zealand, 1988. improved thepaper and provided impetus toextend some aspects ofthe Davis, T.L.,andS.S.Namson, A balanced cross-section oftheI994 work, which was completed while the first author worked atthe Norwegian Northridge earthquake, southern California, Nature, $72,167-169, GeotechnicalInstituteand NorwegianSeismicArray (NORSAR), 1994.

supported by a fellowship fromtheResearch Councilof Norway.

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