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New Zealand Journal of Geology and Geophysics

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The Kerepehi Fault, Hauraki Rift, North Island,New Zealand: active fault characterisation andhazard

M Persaud, P Villamor, KR Berryman, W Ries, J Cousins, N Litchfield & BVAlloway

To cite this article: M Persaud, P Villamor, KR Berryman, W Ries, J Cousins, N Litchfield &BV Alloway (2016) The Kerepehi Fault, Hauraki Rift, North Island, New Zealand: active faultcharacterisation and hazard, New Zealand Journal of Geology and Geophysics, 59:1, 117-135,DOI: 10.1080/00288306.2015.1127826

To link to this article: http://dx.doi.org/10.1080/00288306.2015.1127826

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RESEARCH ARTICLE

The Kerepehi Fault, Hauraki Rift, North Island, New Zealand: active faultcharacterisation and hazardM Persauda, P Villamorb, KR Berrymanb, W Riesb, J Cousinsb, N Litchfieldb and BV Allowayc

aNatural Hazards Division, OMVAktiengesellschaft, Vienna, Austria; bGNS Science, Lower Hutt, New Zealand; cSchool of Geography,Environment and Earth Sciences, Victoria University of Wellington, Wellington, New Zealand

ABSTRACTThe Kerepehi Fault is an active normal fault with a total onshore length of up to 80 kmcomprising six geometric/rupture segments, with four more offshore segments to thenorth. For the last 20 ± 2.5 ka the slip rate has been 0.08–0.4 mm a–1. Average faultrupture recurrence intervals are 5 ka or less on the central segments and 10 ka or more onlow slip rate segments to the north and south. Characteristic earthquakes for a singlesegment rupture range from Mw 5.5 to 7.0, and up to Mw 7.2 or 7.4 in the unlikely event ofrupture of all the onshore fault segments. Fault rupture would result in damage tounreinforced masonry buildings, chimneys and parapets in Auckland (45 km nearestdistant). Very severe damage to buildings in towns within the Hauraki Plains withoutspecific seismic design (those built before 1960) may pose a significant risk to life andlivelihood.

ARTICLE HISTORYReceived 29 June 2015Accepted 11 November 2015

KEYWORDSActive faulting; earthquake-related damage; HaurakiPlains; Hauraki Rift; KerepehiFault; LiDAR; majorearthquake occurrence;paleoseismology

Introduction

The Kerepehi Fault, in the Hauraki Plains region ofnorthern North Island, New Zealand, is a NNW-trend-ing active normal fault (Figure 1). The Kerepehi Faultand its offshore extension in the Hauraki Gulf hasreceived some attention in previous studies related togeological mapping (Houghton & Cuthbertson 1989),active faulting and earthquake hazard (de Lange &Lowe 1990; Stirling et al. 2012; Litchfield et al. 2014),and tsunami hazard (Chick et al. 2001). These studiescharacterised the fault as a series of right-stepping enechelon traces ranging in length from 10 to 43 kmwith step-overs ranging from 3 to 6 km. Recent accessto LiDAR (light detection and ranging) data hasrevealed a far more complex pattern of Holocenefault traces displacing the alluvial terrace surfaces ofthe Hauraki Plains than earlier appreciated. Whereasearlier mapping suggested relatively simple faultstrands, the LiDAR-based interpretation indicates asurface faulting pattern very similar in terms of densityand complexity to that described from the Taupo Rift(e.g. Villamor & Berryman 2001; Berryman et al.2008). Typically there are one or two main strands ineach segment and a myriad of generally shorter sec-ondary strands that splay from, or are parallel to, themain strands. The Kerepehi Fault is the most northerlyactive fault in New Zealand with clear evidence foractivity in the Holocene; it therefore makes a contri-bution to seismic hazard in large northern North Islandcities including Auckland, Hamilton and Tauranga.

The aim of this study is to document the location ofactive traces of the Kerepehi Fault and to derive themagnitude of earthquakes associated with surfacerupture and their recurrence. We assess the rupturehistory of each of the main on-land strands of thefault using tectonic geomorphology and paleoseismol-ogy studies. We have used a high-resolution digitalelevation model (DEM) derived from LiDAR data tomap the faults, and traditional paleoseismic trenchingtechniques combined with radiocarbon dating toassess the earthquake history and derived potentialearthquake magnitudes (see Table S1 for more detailsof methods). From these data we obtain an indicationof ground-shaking effects, and possible damage inmajor population centres of northern North Island,associated with large earthquakes on the KerepehiFault.

Geologic and tectonic setting

The Hauraki Rift

The Kerepehi Fault is located within the Hauraki Rift,a half-graben which extends from the Taupo RiftNNW towards the Hauraki Gulf (Figure 1) separatingthe Coromandel Peninsula and Great Barrier Islandfrom the Auckland and Northland Peninsula. TheHauraki Rift is c. 20–40 km wide and c. 250 kmlong (Hochstein & Nixon 1979; Hochstein et al.1986; de Lange & Lowe 1990; Hochstein & Ballance1993), and comprises three distinct sections from

© 2016 GNS Science

CONTACT P Villamor p.villamor@gns.cri.nzSupplementary material available online at www.tandfonline.com/10.1080/00288306.2015.1127826

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north to south: the Hauraki Gulf, the Firth of Thamesand the onshore Hauraki Depression (Figure 1).

The Hauraki Rift has been active since late Neogenetime (Hochstein & Nixon 1979; Davidge 1982) beforerifting shifted to the Taupo Rift at c. 2–3 Ma (Figure1; Wilson & Rowland 2016). The Hauraki Rift is situ-ated in the back-arc of the Hikurangi subduction mar-gin, parallel to the now-extinct volcanic arc of theCoromandel (Hochstein & Ballance 1993). Our studyarea, the Hauraki Depression, contains two mainNNW-striking normal faults: the now-inactive Haur-aki Fault at the foot of the Kaimai Ranges at the easternmargin of the depression; and the Kerepehi Fault, an

active central-depression normal fault (Figure 2). Thewestern boundary is formed by a minor hinge fault,the Firth of Thames Fault (Hochstein & Nixon 1979;de Lange & Lowe 1990) (Figure 2).

Numerous hot springs occur within the HaurakiDepression. Hochstein and Nixon (1979) calculated,from geothermometry of the springs, that an anoma-lously high heat flow exists below the southern partof the rift. This suggests that there is an upper mantleswell below the Hauraki Depression (Davidge 1982),indicating thinned crust in this region.

Present-day seismic activity in the region is diffuse,but with a concentration in the central and southern

Figure 1. A, Location of the Hauraki Rift on the North Island of New Zealand with respect to the Taupo Rift (dashed grey rectangle)and the Hikurangi Subduction Zone. B, Location of main onshore active fault traces (red lines) and offshore active faults (dashedblack lines) mapped prior to this study. All mapped traces are dipping to southwest. Onshore traces in the Hauraki Depressionbelong to the Kerepehi Fault. WNF is the Wairoa North Fault, the only other active fault known in the region (Wise et al. 2003).Offshore traces were mapped by Chick et al. (2001). Also plotted is the location of earthquakes with Mw >3.0 for the historic perioduntil May 2015. The largest recent event is the 1972 Mw 5.1 Te Aroha earthquake.

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Hauraki Depression (Figure 1). Most earthquakes in theregion have focal depths of ≤12 km. In a microearth-quake survey, Backshall (1982) recorded 21 earthquakes

with ML 0.3–1.2 located between Te Aroha and Mor-rinsville at depths of ≤10 km and concluded that thenature of the macro- and microearthquake activity

Figure 2. Active fault traces of the Hauraki Rift, mapped here using a high-resolution DEM derived from LiDAR on a backgroundgeology map of the Hauraki Rift (simplified from Edbrooke 2001). Note that faults dipping to the southwest and the northeast are inred and grey-blue, respectively. Base map is a low-resolution DEM from LINZ. Shorelines are also mapped with LiDAR-DEM (see fullDEM in Figure S2). Shore line 3 corresponds to that mapped by Schofield (1976). Solid black diamonds represent the location oftrenches. Dark red numbers show the fault scarp heights (m). Black transverse lines are location of topographic profiles presented inFigure 3. Offshore fault segment is from Chick et al. (2001).

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beneath the Hauraki Plains is typical of an active con-tinental rift. The largest recent earthquake was the 1972Te Aroha earthquake of Mw 5.2 (Adams et al. 1972)(Figure 1). In July 2005 a swarm occurred within 5km of Te Aroha (the largest event was Mw 3.7; http://info.geonet.org.nz/display/quake/2005/07/02/Jul+2+2005+-+Te+Aroha+Earthquake+Swarm) and two eventsoccurred in November 2014 close to Waharoa (largestevent was Mw 3.9; http://info.geonet.org.nz/display/quake/2014/11/07/Moderate+earthquakes+rattle+Waikato+region). Geodetic data in the region were insufficientto enable the Kerepehi Fault to be delineated in theNorth Island block model of Wallace et al. (2004).Differences in velocities of widely spaced GPS stationsin the region (Wallace et al. 2004, figure 2) suggest thatany strain across the Kerepehi Fault was below thedetection level. A denser network of stations in theregion of the Hauraki Rift may resolve possible con-temporary strain rates.

Geological setting and geochronologicalmarkers within the Hauraki Depression

The Hauraki Depression is surrounded by volcanic andsedimentary bedrock units. Jurassic meta-greywackesand Tertiary volcanic rocks bound the depression tothe southwest; to the east and northeast the depressionis bounded by Tertiary and Quaternary volcanic rocksof the Coromandel Volcanic Arc (Healy et al. 1964;Schofield 1967; Hochstein & Nixon 1979; Houghton1987; de Lange & Lowe 1990; Figure 2). The southernend of the Hauraki Depression has been in-filled withthe 240 ka Mamaku ignimbrite that was eruptedfrom the Rotorua caldera of the Taupo VolcanicZone (TVZ) (Hochstein & Ballance 1993; Milneret al. 2003). The Hauraki Depression itself is inferredto be filled with c. 2.5–3 km thick Tertiary and Qua-ternary terrestrial sediments (Kear & Tolley 1957;Healy et al. 1964; Hochstein & Nixon 1979; Hochstein& Ballance 1993), the youngest of which are the alluvial

Figure 3. Topographic transects across the Hauraki Rift showing location of active faults and measurement of fault throw. Thethrow range represents the maximum and minimum values taking into account maximum and minimum surface tilt and uncer-tainties due to erosion of terrace surfaces. Note the eastwards tilting of the Hinuera surfaces (H is Hinuera C of Manville and Wilson,2004) on the footwall of the main strands of fault segments.

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volcaniclastic Hinuera Formation, deltaic and estuarinesediments and locally reworked sediments (Healy et al.1964; Hume et al. 1975; Cuthbertson 1981; Houghton &Cuthbertson 1989; de Lange & Lowe 1990) (Figure 2).

The Hinuera Formation is especially important forthis study because it is an extensive marker horizonfor the assessment of the faulting history of the Kere-pehi Fault. The formation contains reworked pumicealluvium from the Mamaku Plateau, the TVZ and theKaimai Range (Cuthbertson 1981) which was depos-ited through coalescing alluvial fans that graded north-wards into a braided river outwash plain. Deposition ofthe earliest Hinuera sediments began directly after theemplacement of the Mamaku ignimbrite at 0. 24 Ma(Cuthbertson 1981; Gravley et al. 2007). The main sedi-mentation phase took place after the Oruanui eruptionhowever (25.4 ka; Vandergoes et al. 2013), and contin-ued until the deposition of the Rerewhakaaitu tephra atabout 17. 6 ka (Manville &Wilson 2004). The ancestralWaikato River abandoned its route across the HaurakiPlains at about this time and re-established an earlierroute across the Hamilton Basin to the Tasman Sea,as occurs to the present day. Within the HaurakiPlains, two aggradational surfaces and associateddeposits are described as the Hinuera 1 and Hinuera2 surfaces (Cuthbertson 1981). They have been rede-fined recently as Hinuera B and Hinuera C surfacesby Manville and Wilson (2004). Hinuera B is dated at26.5–22.5 ka, based on the distribution of Okarekatephra (c. 21.8 ka; Lowe et al. 2008), and Hinuera Cat 22.5–17.6 ka ( (Manville & Wilson 2004). Otherimportant tephra that assist in bracketing the timingof prior surface rupture events include the Tuhuatephra erupted from Mayor Island (7005 ± 155 cal yrBP; Lowe et al. 2008) and the Taupo tephra eruptedfrom the Taupo caldera (c. 1718 ± 5 cal yr BP; Hogget al. 2012).

The oldest Quaternary terrace surfaces in the HaurakiPlains are the Hinuera B surfaces which are found inthe southern part of the Hauraki Depression. Towardsthe centre and the north, the younger and lower-Late--Pleistocene-aged Hinuera C surfaces or degradedHinuera C terraces of Holocene age cover most of theHauraki Depression. Adjacent to the Firth of Thames,the Hinuera C surface is covered by young estuarinesediments deposited under conditions of rising sealevel during the early Holocene transgression. Theshoreline related to the culmination of sea-level rise fol-lowing the last glaciation at c. 7.5 ka (Clement et al.2010) is a useful marker adjacent to the Firth of Thameswith respect to the timing of the most recent surfacerupture of the Kerephi Fault (Figure 2). Discrete tephralayers are found mainly in old oxbows along Pleisto-cene–Holocene river terraces and in extensive swamps(e.g. the Kopouatai Bog, see de Lange & Lowe 1990).Elsewhere, the Holocene tephra sequence is usuallyobserved as a composite unit.

The Kerepehi Fault

In previous studies, the Kerepehi Fault has been exam-ined mainly by geophysical methods, both onshore andoffshore (e.g. Hochstein & Nixon 1979; Tearney 1980;Davidge 1982; Rawson 1983; Hochstein et al. 1986;Hochstein & Ballance 1993; Chick 1999; Chick et al.2001). In the south of the Hauraki Depression, the Ker-epehi Fault bounds the west side of a buried mid-valleyhorst (Hochstein & Nixon 1979). Its location at thenorthern end of the Hauraki Depression towards theFirth of Thames and in the Hauraki Gulf is less wellestablished. The fault strikes 340°, parallel to theboundaries of the depression (Hochstein et al. 1986).The total throw of the fault in the greywacke basementas determined by geophysical methods is 1.1–3.5 km(Hochstein & Nixon 1979; Tearney 1980; Hochstein& Ballance 1993). de Lange and Lowe (1990) suggestedthat the fault crosses Kopouatai Bog north of Elstowbased on the displacement of several tephra layers,but we provide new data that questions this interpret-ation (see ‘Recurrence interval of faulting on the Elstowsegment’).

Active traces of the on-land Kerepehi Fault

Our study shows that the on-land portion of the Kere-pehi Fault is an active, predominantly west-dipping,normal fault expressed at the ground surface by tracesover a length of at least 71 km (mapped traces) andcould be up to 80 km (inferred traces; Figure 2).From the trace geometry we divide the on-land portionof the fault into six geometric segments. We namethese, from south to north, Okoroire (>5 km long),Te Poi (27 km long), Waitoa (43 km long), Elstow(12 km long), Te Puninga (22 km long) and Awaiti(>27 km long) segments. In the Firth of Thames,Chick et al. (2001) identified four segments fromhigh-resolution seismic reflection profiling that rangein length from 9 to 19 km (Figure 2), although thesouthernmost offshore segment is less confidentlyidentified because of very shallow water and gas mask-ing in the sea-bottom sediments (W de Lange, pers.comm. 2015). In addition, we have identified a prob-able linking structure between the Te Poi and Waitoasegments that is 10 km long (Figure 2). There arealso traces up to 3 km west of the Te Poi segment, 4km east of the Waitoa segment and 6 km alongstrikeof the northern end of the Waitoa segment that donot group very well into our description of segments.

Each segment comprises numerous fault traces. Thefault traces are sinuous but relatively continuous fordistances of 20 km or so; overall, the fault is character-ised as a series of overlapping right-stepping sections.The step-overs in between traces and segments are sep-arated by distances of up to 4.2 km. The principalstrands have scarps up to 8 m high across Hinuera C

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surfaces (Figures 2, 3). Several discrete short scarpsoccur away from the main traces, especially close tothe step-overs. The variability in surface expression ofthe fault may represent irregularities in the faultplane at depth, or in the fault propagation throughthe unconsolidated valley fill to the surface. Faultscarps are commonly irregular and include significantbends and embayments, typical of normal faults (e.g.Schwartz & Coppersmith 1984). Although the geo-morphic scarps are similar in nature to river terracerisers, they cross terrace risers and this criterion is evi-dence of the tectonic origin of the traces. Evidence oftectonic tilt is also observed in topographic cross-sec-tions derived from LiDAR data (Figure 3, Figures S1,S2). There are rare exposures of a fault plane associatedwith the trace, and further evidence of deformation hasbeen revealed in trench excavations. The commonoccurrence of stepped scarps probably representsmovement on multiple fault strands.

All traces comprise simple or stepped scarps that aremostly west-facing (W traces of Figure 2). In someareas the scarps are steep and distinct, while in othersthey are broad and sometimes rather subtle warps(Figure S3). Most traces offset the relatively flat uppersurface of the late Pleistocene Hinuera Formationmapped by Cuthbertson (1981; Figure 2). Scarps acrossthe Hinuera surface range from 1 to 8 m (Figures 2, 3)in height. We characterise the fault as predominantly, ifnot exclusively, normal faults with a probable fault dipat a depth of 60 ± 15° (typical of normal faults in theTaupo Rift; Villamor & Berryman 2001). Some scarpscross Holocene degradational stream terraces (e.g.southeast of Matamata, Figure 3). Scarp heightsdecrease on successively younger, lower terraces, butno scarp cuts the youngest terraces adjacent to themodern river channels. A feature of the deformationalstyle is tilting (generally to the east) of the Hinuera Csurface on the footwall of the fault blocks as seen incross-sections constructed from the LiDAR-derivedDEM (Figures 3, S2). The extension of the Awaiti seg-ment northwards toward Kerepehi (Figure 2) is basedupon this noticeable tilt of the Holocene-aged surfaceillustrated in Figure 3 and the DEM of Figure S2.

Fault rupture history of Kerepehi Fault

To investigate the rupture behaviour of the fault wehave excavated trenches across the main strands ofeach of the geometric segments of the fault, with theexception of the Te Puninga segment on the centralwestern side of the Hauraki Depression (identified inthis study for the first time) and the Okoroire segment(Figure 2). We selected sites for trenching where thescarp was simple and sharp, hoping that stratigraphicrelationships would serve as a basis for determiningboth the timing and size of past surface rupture events.The Westlake and Madill trenches are on the Te Poi

segment, the Simonsen trenches are on the Waitoa seg-ment, the Buchanan trench is on the Elstow segmentand the Davey trench on the Awaiti segment (Figure2). An additional trench adjacent to the Davey trenchdid not provide evidence for faulting on an adjacent,parallel lineament which is probably a terrace riser(Figure 2). We additionally trenched a scarp in thenorthern part of the depression near the Piako Riverwhich we thought could have been the northern con-tinuation of the Awaiti segment (the Tye trench), butthis trench did not expose a fault. Detailed logs of thetrenches and stratigraphic unit descriptions can befound in Datasets S1–S2 (Figures S4–S9; Tables S2–S9), and are presented in summary form in Figure 5.

Okoroire segment

Fault scarps were mapped in the area between Okoroireand Hinuera (Figures 1, 2) in 1986 (Foster & Berryman1991). Scarps measure 2–3 m in height, both east- andwest-facing, across surfaces of Hinuera age, and up to10 m high facing west across older Pleistocene surfaces.The fault trace is mapped for at least 5 km. No exca-vations have been undertaken on this segment and it isoutside the area covered by our recently obtainedLiDARcoverage. It is possible that this fault segment con-tinues to the SSE towards State Highway 5 about 6 kmeastward from Tirau, which would extend the segmentlength to at least 10 km. The traces are located 3–4 kmwest of the southern endof theTePoi segment (Figure 2).

Te Poi segment

The Te Poi segment is 27 km long and extends from TePoi, 8 km southeast of Matamata, to Hungahunga, 14km north of Matamata (Figure 2). It consists of severalfault traces with vertical surface displacements thatrange from 1 to 8 m (Figure 2). The scarp reaches itsmaximum height (8 m) around Matamata and fromthere decreases in height towards the northern andsouthern ends of the segment. In the south, some ofthe smallest displacements on this fault segmentoccur on young river terraces near the Waihou Riverand Mangawhero Stream (Figures 2, 4). Here, displace-ments progressively decrease in height on lower sur-faces, indicating that there has been repeatedmovement on this segment of the fault. The lowestand therefore youngest terraces are not displaced bythe fault, and the highest terrace level that is displacedby the fault is a slightly degraded Hinuera C surfacewith 8 m of vertical offset (Figure 4).

Except for the 1.2 m high scarp just to the south ofthe Waihou River (Figure 4), 1.5–1.8 m high scarpsare the smallest found along this segment and may rep-resent a single-event displacement (SED). To confirmthis we plotted the scarp height versus river terrace levelsfrom a detailed topographic map constructed with a

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differential GPS system and LiDAR around the WaihouRiver and Mangawhero Stream area (Figure 4). The sur-vey extended for about 3 km along the fault, coveringapproximately 500–1000 m on each side of the faultscarp. Terrace-level elevations are plotted against thescarp heights in Figure 4 and it suggests that the displa-cement increases in steps of about 1.5 m with eachhigher terrace level, indicating that the single-event dis-placement is indeed about 1.5 m.

Based on this progressive displacement, we estimatethe number of events recorded on the Hinuera C

surface as probably four to five for a scarp height of 8m (Figure 2). Manville and Wilson (2004) estimatedthe culmination of the accumulation phase of HinueraC aggradation was 22.5–17.6 ka, so we estimate that themaximum displacements of 8 m occurred at c. 20 ± 2.5ka (Figure 2).

Trenches on the Te Poi segmentTwo trenches, Westlake and Madill, have been exca-vated on the southern section of the main strand ofthe Te Poi segment.

Figure 4. A, Topographic map compiled from detailed RTK-GPS study and LiDAR around the Westlake and Madill trenches on theTe Poi segment of the Kerepehi Fault, demonstrating the progressive displacement of terraces cut by the Mangawhero Stream andthe Waihou River. The colour coding of the DEM is given in metres above mean sea level (asl). Black numbers are fault scarp heights(m). Coordinates are New Zealand transverse Mercator projection. B, Plot of terrace height (asl) versus fault throw (scarp height;blue points) on the hanging wall of the Kerepehi Fault. Younger (lower) terraces show less displacement (scarp height) because theyhave been ruptured by fewer paleoearthquakes, whereas older terraces show larger cumulative scarp heights. The displacement onsuccessively higher and older terraces increases by about 1.5 m which we assume to be the single-event displacement ofpaleoearthquakes on this segment of the fault. Plot also shows a terrace age (from literature and field data, see text) v. terraceheight curve (red line). The curve shows there was very rapid down-cutting after the formation of the aggradation surface atabout 20 ± 2.5 ka , but down-cutting slowed down dramatically after c. 16 ka . Note that the two x-axes of the plot do not correlatewith each other. C, Topographic profiles along the fault on the footwall and hanging wall with measured throw (scarp heights; rednumbers in metres) along the fault. Note the increase of fault throw for similar height terraces towards the NW.

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Westlake trenchWestlake trench southeast of Matamata (Figures 2, 4,Figure S4) was selected because the trace consists of asimple scarp on a low-elevation degradational terraceat c. 49 m elevation and is probably representative ofonly a single surface displacement. The excavationrevealed a sequence of Hinuera deposits overlain by a0.5 m thick Holocene-aged accretionary (up-building)soil sequence containing multiple ash and lapilli inter-beds of mostly rhyolitic composition derived from thevolcanic centres of the central North Island (Figures 5,S4; Table S2).

Deformation in this trench is manifest as small fis-sures (infilled with unit 2) nested within largermetre-scale wedge-shaped fissures infilled with unit 3,similar to the deformation mapped along the 1987 rup-ture of the Edgecumbe Fault (Beanland et al. 1989).The large fissures are associated with displacement ofunit 5 and older. Deformed horizons are overlain byunit 1 (Figure 5). A radiocarbon sample with an ageof 540–320 cal yr BP from unit 2 (WE 1/1; Tables 1,S1) provides a useful maximum estimate for theyounger episode of fissuring that we interpret as sec-ondary displacement related to rupture of an adjacentsegment of the fault based on the size of fissure 2(see more discussion of interpreting rupture eventsfrom fissuring below). The older displacement hasc. 1.5 m of vertical displacement on the base of unit

5, and fissuring in this event truncates the Holocenetephra unit.

Based on the preliminary information of ages ofdegradational terraces below the Hinuera C surfacenear the Waihou River, we estimate the age of the ter-race at the Westlake trench to be about 16 ka based onthe stratigraphy in auger hole 2, located in a cut-offmeander of the Waihou River at the same elevation(Figure 4; Table S3). From a peat overlying degrada-tional Waihou River gravels (Table S3), Raupo(Typha orientalis) leaves gave an age of 16,947–16,381 cal yr BP (sample 77511/3; Tables 1, S1). A pri-mary fault rupture on the Te Poi segment thereforeoccurred about the same time as the formation of thecut-off meander, that is, at c. 16 ka (Table 2).

Madill trenchThe Madill trench is situated 200 m northwest of theWestlake trench (Figures 2, 4, S5). At this locationthe scarp is c. 5.5 m high. The stratigraphy exposedin the trench (Table S4) comprises Hinuera depositsat the base and a succession of younger river gravel,overbank sand units, tephra, paleosols and a youngerfluvial channel sequence of gravel, channel fill andyoung tephra that contains lapilli from the 1718 calyr BP Taupo eruption (Figure 5; Table 1).

Based on the 5.5 m scarp height (see Figure 4), theresults of the Westlake trench 200 m away along the

Figure 5. Trench logs: A,Westlake trench (see full log in Figure S4). B,Madill trench (see full log in Figure S5). C, Simonsen trench 1(see full log and log of Simonsen trench 2 in Figure S6). D, Buchanan trench (see full log in Figure S8). E, Davey trench (see full log inFigure S9). Note that radiocarbon samples in red on Davey trench were collected on the north wall and displayed at the correlatinglevel on the south wall. Full unit descriptions are in Tables S2 and S4–S9. Note that the generalised legend uses similar colour tonesfor units that are approximately the same age but not necessarily the same environment. Unit numbers do not correlate acrosstrenches.

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same scarp and indications of single-event vertical dis-placement observed at the Westlake trench, weexpected to find evidence for perhaps three earthquakeruptures in this trench.

The age of the terrace at the Madill site, at 62 mabove sea level, is estimated to be 19 ± 1 ka. The alti-tude of the terrace at Madill site is lower than theHinuera C surface (69 m) but higher than the terracewith auger hole 2 mentioned above (49 m) (Figure4). The terrace at the Madill site must therefore beolder than the terrace with the auger hole 2(i.e. c. 16 ka), but less than the 20 ± 2.5 ka ageassigned to the Hinuera C surface by Manville andWilson (2004).

We have plotted the ages of terrace surfaces near theWaihou River against their height above sea level inFigure 4. This plot suggests there was very rapiddown-cutting after the formation of the aggradationsurface (Hinuera C) at about 20 ± 2.5 ka. By about19 ± 1 ka (Madill trench surface), the river haddown-cut by about 7 m from c. 69 m to 62 m, andwas at 49 m (12 m of down-cut) by c. 16 ka (augerhole 2 terrace). Between 16 ka and the present withthe river at c. 36 m there are no further controls onthe down-cutting (Figure 4). The general form of thedown-cutting is very similar to that described by Berry-man et al. (2010) from the Waipaoa River in the Gis-borne district. With many more terraces dated there,

Table 1. Radiocarbon ages.

Sample number (lab number) δ13Ca 14C age (yr BP)b95% HPDF calendarage range (cal yr BP)c Observations (location, unit: fraction dated)

WE 1/1 (NZ 7894) −26.1 453 ± 67 540–320 Westlake trench, buried soil unit: soilSI 1/3 (NZA 2143) −30.4 4790 ± 110 5736–5068 Simonsen trench #1, channel fill deposit: leaves and small twigsSI 1/1 (NZ 7892) −284.8 6716 ± 64 7655–7437 Simonsen trench #1, organic rich horizon: woodSI 2/1 (NZ 7893) −26.9 651 ± 86 719–500 Simonsen trench #2, fissure fill: soilC4 (NZA 19732) −35 5863 ± 45 6740–6495 Buchanan trench, fissure fill: soilC5 (NZA 19733) −31 12051 ± 60 14,057–13,728 Davey trench, peat layer: soilC6 (NZA 19734) −35.5 15853 ± 80 19,314–18,864 Davey trench, fissure fill: soilC7 (NZA 19735) −28.5 2787 ± 35 2944–2761 Davey trench, overbank silts with tephras: soilC10 (NZA 19736) −28 5157 ± 40 5982–5741 Davey trench, organic rich soil: soilC9 (NZA 23135) −26.4 6898 ± 35 7788–7611 Davey trench peat: small pieces of woodTye 1 (NZA 18657) −0.1 6226 ± 40 6783–6551 Tye trench, unit at 2.4 m depth: bivalve shellTye 6 (NZA 18762) −27.3 5672 ± 35 6490–6312 Tye trench, unit at 1.9 m depth; fragments of wood77511/3 (NZA 19419) −27.1 13823 ± 65 16,947–16,381 Auger hole 2, organic soil at 82–97 cm depth: Raupo leavesaδ 13C normalisation is always performed using δ13C measured by AMS, thus accounting for AMS fractionation.bConventional radiocarbon ages in years BP or before present (1950) determined by Rafter Laboratory.cCalibrated age ranges as determined using Oxcal v4.2.4 (Bronk Ramsey 2013) and Southern Hemisphere Atmospheric data from Hogg et al. (2013). Forsample Tye 1, the 2013 Marine calibration curve (Reimer et al. 2013) and a Delta-R correction of 0 ± 13 was used (McSaveney et al. 2006). Ranges representtwo standard deviation statistics or 95% probability. HPDF, probability density function.

Table 2. Earthquake events, single-event displacements (vertical), recurrence intervals and slip rates (vertical). All SEDs and sliprates are vertical values. The dip of Kerepehi Fault is unknown. If we assume similar fault dips as those described for the TaupoRift (Villamor & Berryman 2001), a dip value of 60 ± 15 can be used.Segment SED (m) Events* Slip rate RI**Surface or trench (scarp height) geomorph or trench trench or Hinuera/SED (mm/yr) (ka)

OkoroireHinuera (2–3 m) nd nd 0.08–0.17 nd

Te PoiHinuera (6–8 m) nd 4–5 (1Y, 20 ± 2.5 ka) 0.27–0.46 ndWestlake Tr nd 1 (2Y, <0.5–0.3 ka) nd ndWestlake Tr 1.5–1.8 1 (1Y, 16 ka), MRE nd ndMadill Tr (5.5m) 1.8 3 (1Y, 16 ka to 19 ± 2 ka) 0.26–0.32 6.0–6.6

WaitoaHinuera (6 ± 1 m, 2 scarps) nd nd 0.22–0.4 ndSimonsen 1 Tr (0.8 m) 0.8–1.6 1 (1Y, 5.3 ± 0.5 ka), MRE nd ndSimonsen 2 Tr (1.5 m) 0.8–1.6 1 (1Y, > 7.4 ka), PE nd ndSimonsen 2 Tr (1.5 m) nd 1 (2Y, 0.7–0.5 ka) nd ndSimonsen 2 Tr (1.5 m) 0.8–1.6 ?3 (1Y, 20 ± 2.5 ka) nd 6.2–7.3

ElstowHinuera (4 m) nd nd 0.17–0.23 ndBuchanan Tr 1.6 1 (1Y, 6.6 ± 0.2 ka), MRE nd ndBuchanan Tr 1.6 2–3 (1Y, 20 ± 2.5 ka) nd 5.8–11.2

Te PuningaHinuera (2.0–3.7 m) nd nd 0.08–0.21 nd

AwaitiHinuera (1 m) nd nd nd ndDavey Tr 1 1 (1Y, 5.9–1.8ka), MRE nd ndDavey Tr 1 1( 1Y , 19.2–13.7ka), PE nd 8.1–17.5

*2–3 (1Y, 20 ± 2.5 ka) = number of events (primary 1Y or secondary 2Y, time interval). For this example, there are two or three events that are primary andoccurred in the time period 20 ± 2.5 ka.

**RI calculated from information within the segment.MRE, most recent event; nd, no data or not determined; PE, penultimate event.

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the conclusion drawn was that when sediment supplydeclined the river switched to rapid down-cuttingbecause it was no longer choked with sediment andhad the power to erode to achieve a new grade. Thedown-cutting pulse was progressive up the Waipaoacatchment with earlier down-cutting downstream andlater down-cutting upstream. In the Hauraki Plains asimilar down-cutting process to that observed in theWaipaoa catchment is likely, implying that the periodof degradational terrace formation may have been lar-gely completed by 12–10 ka.

Within the Madill trench most of the faulting iswithin the Hinuera units (units 16 and 17), with thelargest displacements found in the lowermost exposedHinuera gravels (Figures 5, S5). The main fault of thetrench displaces the Hinuera gravels (unit 17) at thebottom of the trench vertically by about 1 m (Figure5). Faults in the lowermost gravel splay upwards intoseveral faults with much smaller displacements in theHinuera sands (unit 16). The majority of faults termi-nate within the Hinuera sands, and do not penetrateupwards into the overlying Waihou River gravels(unit 15). Only eight to nine of the faults seen in theHinuera sands also displace the base of the WaihouRiver gravels.

Folding as well as faulting is a prominent feature ofthe deformation. Further up in the sequence in theHinuera sands (unit 16), the main fault plane becomesshallower and rolls over towards the west, similar tomost of the faults in the upper Hinuera sands. Due tothe bending of the top of the faults, they show anapparent reverse displacement (Figure 5). The faultingextending through the Waihou River gravel (unit 15)extends into unit 9.

The occurrence of at least two separate ruptureevents is confirmed by cross-cutting relationships onsmall-scale faults in the Hinuera sands and folding.The small-scale faults appear to have formed as verticalfaults but rotated at their upper ends. After rotation, anew set of vertical faults was formed that cross-cut theolder rotated set. The timing and size of each of theseevents is not possible to determine from the trench.However, if we divide the total displacement at theMadill trench of 5.5 m by the average SED of 1.5 mfrom progressive displacement of the Waiohou Riverterraces, at least three surface rupture events of 1.5–1.8 m probably occurred since 20–18 ka (Table 2).

Recurrence interval of rupture on the Te PoisegmentIf we accept that interpretations from the Westlake andMadill trenches apply to the whole Te Poi segmentthen we have at least a secondary faulting event andthree primary events. The secondary event is close inage to 540–320 cal yr BP (recorded in Westlaketrench), and the most recent primary faulting event,with 1.5 m vertical displacement, occurred at c. 16 ka

(recorded at Westlake trench and one of the threeevents interpreted from the Madill trench). The twoolder primary faulting events, averaging 1.5–1.8 m ver-tical displacement, occurred during the intervalbetween 20–18 ka (Madill trench) and 16 ka. Scarpsthat are 7–8 m high on the Hinuera C surface (Figures2, 4) suggest one or two additional earlier events wereprobable at c. 19 ± 1 ka and 20 ± 2.5 ka to account forthe additional 1.5–2.5 m difference in scarp heightbetween the Madill trench site and fault scarp heightsacross the Hinuera C surface (Figure 4). From theseobservations a very clustered history of fault ruptureon this segment can be deduced, with four to fiveevents in the period 22.5–16 ka (at maximum), andnothing more in the last 16 ka other than a veryyoung secondary rupture at c. 0.5–0.3 ka (Table 2).The average recurrence intervals of 6.0–6.6 ka shownin Table 2 are averaged over the whole timeframeand are therefore misleading. It is possible that notall surface ruptures will necessarily involve the mainstrand of the segment, and so the interpretation ofthe rupture history at a single site along the mainstrand may be modified when data from more siteson this segment are investigated.

Waitoa segment

The Waitoa segment may be up to 43 km long andstretches from Wardville/Hungahunga to about 8 kmNNW of Otway within the Kopouatai Bog (Figure 2).It is separated from the Te Poi segment by a rightstep of about 3 km, although a 10 km minor trace con-nects the two segments at the southern end suggestingthe Waitoa and southern Te Poi segment may rupturetogether, at least on some occasions. The segment con-sists of a relatively continuous main strand but splaysnear its southern end with a divergent trace extendingthrough Okauia. Fault scarps range in height from 1–2m in the south, 3–5 m in the centre (near Waitoa) toabout 8 m locally near Springdale before the splays inthe north (Figure 2). These larger scarp heights are gen-erally all on the Hinuera C surface, so offsets of 2 m onthe southern section of the segment are probablyequivalent to the 5 m scarps near Waitoa. Scarpsalong this segment are mostly single trace distinctscarps, with the exception of one location in the centreof the segment that shows a double scarp with 3 m ofsurface displacement on each scarp.

Trenches on the Waitoa segmentTwo trenches (Simonsen 1 and 2; Figures 5, S6, S7)have been excavated at the southern end of this faultsegment to the east of Hungahunga (Figure 2).

Simonsen trenchesThe Simonsen site was selected for trenching becausethe floor of a small stream channel is displaced by

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only 0.8 m and the adjacent terrace surface is displacedby 1.5 m, suggesting information on the size and thetiming of the last two faulting events on this strandof the Kerepehi Fault could be obtained.

Simonsen 1 trench was excavated across the 0.8 mhigh scarp and exposed Hinuera pumice alluvium(unit 5) overlain by a younger Holocene channel-filldeposit (unit 3), Taupo ash (unit 2) and topsoil (unit1) (Figures 5, S6; Table S5). A distinct fault was seendisplacing the Hinuera alluvium (unit 5) and the baseof the channel-fill deposits (unit 3) near the middleof the trench. The channel-fill deposit (unit 3) is thickeron the downthrown side of the fault, suggesting it filledacross a pre-existing scarp. The channel fill unit itself isdisplaced, so at least two events have occurred in theperiod since deposition of the Hinuera deposits ( onepre- and one post-channel fill; unit 3). The defor-mation of the channel-fill unit suggests at least c. 0.35m and up to 1.5 m of displacement (if the base of thechannel was horizontal before the deformation). Thetiming of the most recent event (MRE) can be esti-mated from radiocarbon ages of samples obtainedfrom unit 3. The faulting is certainly younger thanthe basal age of 7655–7437 cal yr BP (S1/1 in Table1; Figure 5) and may be a little younger or similar tothe upper sample age of 5736–5068 cal yr BP (S1/3 inTable 1; Figure 5), depending on whether the horizonfrom which the upper sample was collected goes acrossthe top of the fault tip or was displaced. The most prob-able timing of this faulting event is a little earlier thanthe upper sample (i.e. c. 5.3 ± 0.5 ka).

Simonsen 2 trench exposed no actual fault planesbut showed that deformation had been accommodatedby warping and ground cracking (Figure S6). A peatunit within the Hinuera deposits (unit 5; Table S6) iswarped down by about 1.5 m, the same as the groundsurface (Figure S6). The larger scarp height at Simon-sen 2 site suggests that there is an additional eventolder than that at Simonsen 1 trench. Cracks are filledwith an old soil that has an age of 719–500 cal yr BP (SI2/1; Table 1) and is different in character from the pre-sent topsoil that occurs on the upthrown side of thefault (Figures 5, S6), suggesting a secondary faultingevent at this time (Table 2).

Recurrence interval of rupture on the WaitoasegmentObservations from the two trenches show that at leasttwo primary faulting events have occurred at this sitesince the formation of the Hinuera surface: one inthe period after terrace formation on the Hinueradeposits at c. 20 ka (Simonsen 2 trench) and olderthan the base of unit 3 at 7655–7437 cal yr BP (oldestage from Simonsen 1 trench); and the second event atc. 5.3 ± 0.5 ka (Simonsen 1 trench; Table 2). Inaddition, a secondary faulting event occurred close to719–500 cal yr BP. These data suggest two or perhaps

three (if the latter were primary rather than secondary)primary rupture events in the past c. 20 ka. For the twoevents that are clearly primary, an average recurrenceinterval of 6.2–7.3 ka can be estimated if the MRE iscloser in age to 5.3 ka (i.e. two events in 14.7 yearsduring 20–5.3 ka ) or to 7.5 ka (two events per 12.5ka), respectively (Table 2).

About 200 m west of the Simonsen trench sites thescarp is about 4 m in height compared with the 1.5 mobserved at the trench sites, even though the terraceapparently remains the same; at one locality alongthis segment of the fault shows a double scarp with 3m of surface displacement on each scarp. Furthernorth along this segment scarp heights reach 8 mnear Springdale on a surface estimated to be 20 ± 2.5ka in age (Figure 2). The smallest scarps measured inthe same vicinity are about 3 m (field measurementsfrom Foster & Berryman, 1991), suggesting two tothree ruptures in the past 20 ± 2.5 ka in this area.Together these data suggest an average recurrenceinterval of primary rupture on the Waitoa segment of6.2–7.3 ka (Table 2).

Elstow segment

Scarps of the Elstow segment extend for 12 km fromnear Waihou northwards into the eastern part of theKopouatai Bog (Figure 2). This short segment couldalso be a northern splay of the Waitoa segment. How-ever, we choose to interpret it as a separate segmentbecause trench data indicate a different rupture eventand there is a 3–4 km step between the main strands.Scarps of the Elstow segment south of Kopouatai Bogcomprise broad single and double traces which are1.5–4 m high on the Hinuera C surface.

De Lange and Lowe (1990) trace the Elstow segmentthrough the Kopouatai Bog, inferred from apparentdisplacements of tephra layers from drillhole data(Figure S7). However, our aerial photo, LiDAR-derived DEM and paleoseismic trenching shows thatthe Elstow segment of the Kerepehi Fault extendsinto the bog by only 2.5 km and then steps to theright by 3 km, paralleling the Kopouatai Bog just out-side its eastern margin (Awaiti segment, Figures 2, 3,S7). Several c. 2 km long traces have been mapped inthe bog as potentially active based on DEM analysis,but these do not coincide with the fault locationmapped by de Lange and Lowe (1990).

Trenches on the Elstow segmentBuchanan trench. The only trench on the Elstow seg-ment, the Buchanan trench site, was chosen at thelocation of a relatively narrow distinct scarp alongthis relatively short strand of the Kerepehi Fault(about 1 km south of Mellon Road near Elstow Hall,Figure 2). At this site, the trace consists of a 1.75 mhigh scarp, but a few metres northwest and southeast

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of the trench site the trace has two stepped scarps eachwith c. 0.75–2 m vertical offset.

The trench exposes Hinuera pumice alluvium (units5 and 6) overlain by a clay unit (unit 4), a paleosol(units 2 and 3) on the clay and the modern topsoil(unit 1) (Figures 5, S6; Table S7). The Hinuera pumicealluvium contains liquefaction features that reach upinto the clay unit. In addition, some larger cracks inthe Hinuera pumice alluvium are filled by material ofthe clay unit. The paleosol units 2 and 3 are truncatedby the topsoil in the upper part of the scarp.

The deformation in the trench manifests itself bywarping and ground cracking similar to the defor-mation caused by the Edgecumbe earthquake in 1987(Beanland et al. 1989). All units are down-warped by1–1.6 m. The cracks are filled by yellow-brown soilmaterial different from the organic-rich topsoil. Thesoil in the cracks was radiocarbon dated to 6740–6495 cal yr BP (C4 in Tables 1, 2; Figure 5). We there-fore assign a 6.6 ± 0.2 ka age to the earthquake thatcaused ground fissuring and warping.

Recurrence interval of faulting on the ElstowsegmentFrom paleoseismic evidence, we show that rupture onthe Elstow segment last occurred at 6.6 ± 0.2 ka. Thetotal displacement in the Buchanan trench is 1.6 m(by warping), but we cannot conclusively say whetherthis displacement is entirely due to just one earthquakeor to several events. From the difference in scarp heights(up to 4.5 m) along the Elstow segment we assume twoto three events in the past 20 ± 2.5 ka if the 1.6 m foundin the Buchanan trench is indicative of a single-eventdisplacement (Table 2). The recurrence interval is there-fore likely to be in the range of 5.8–11.2 ka (Table 2).

On the Elstow segment of the fault, de Lange andLowe (1990) have inferred small (0.2–0.7 m) ruptureevents at c. 1400, c. 5600, c. 6800 and c. 9000 yearsago based on tephra horizons at different elevationsin the Kopouatai Bog about 20 km northwest of TeAroha. The Kopouatai Bog has had a complex history,evolving from a forest swamp at c. 12 ka to the currentraised bog. There was also a large environmentalchange during the marine transgression up to c. 6 kawith spatial and temporal changes in peat accumu-lation (Newnham et al. 1995).

The displacements inferred by de Lange and Lowe(1990) are much smaller and the average recurrenceinterval rather shorter than those we have identifiedby fault mapping and trenching on several segmentsof the fault. We identify a likely event at 6.6 ± 0.2 kaat the Buchanan trench which coincides with anevent inferred by de Lange and Lowe (1990), but wesee no evidence of younger events. At the Davey trench(see also ‘Awaiti segment’ below) which lies just east ofKopouatai Bog we show that displacements are not

achieved from many small displacements but ratherseveral larger (1.0–1.5 m) displacements per event.

It is possible that fault traces have been obscured bypeat growth and are not visible in the LiDAR DEM.However, without locating the fault planes inferredby de Lange and Lowe (1990) with respect to the drill-holes and understanding the depositional geometry ofthe peat layers (dependent on the type of mire, i.e.raised bog v. forest swamp), it is difficult to interpretthe elevation differences of tephra horizons in theKopouatai Bog peats as displacement along the Kere-pehi Fault. The drillholes referred to by de Lange andLowe (1990) are widely spaced (e.g. southern mostpair is at least 1 km apart) and the differences inelevation of the tephra are small. We therefore suggestthat differences in the ground surface level at the timeof tephra deposition, or post-deposition sediment com-paction in the actively developing peat, are the mostlikely origin of the elevation differences in the tephrahorizons. However, additional fault strands throughthe Kopouatai Bog cannot be definitively discounteduntil further detailed investigations to establish thepresence of faulting and displacement of the tephrahorizons across the fault planes have been undertaken.

Te puninga segment

Recent acquisition of LiDAR coverage of the HaurakiPlains has revealed a zone of active faults along the wes-tern margin of the plains in the vicinity of the re-entrantin the range front along the Firth of Thames Fault(Figures 1, 2). This raises the possibility of reclassificationof the western boundary fault of the Hauraki Depressionas an active fault. The traces are distributed over a lengthof 22 km and a width of 5 km. A feature of this segment isthe occurrence of some east-facing scarps along the wes-tern side of the zone, although there is a predominance ofwest-facing scarps overall. No trenches have been exca-vated on this segment. Interpretation of the LiDAR datasuggests scarps 2.0–3.7 m high across probable HinueraC surfaces (22.5–17.6 ka). The preliminary data suggesta surface slip rate of 0.08–0.21 mm a–1 (Table 2).

Awaiti segment

The Awaiti segment is the northernmost onshore seg-ment of the Kerepehi Fault, separated from the Elstowsegment by a 3–4 km right-step (Figure 2). The Awaitisegment extends from Tirohia (2 km east of KopouataiBog) at least 27 km to 3 km northeast of Ngatea (Figure2). The extension of the fault north from Kerepehi isbased entirely on the eastwards tilt of the terrace sur-face (Figure 3) as have observed further south in associ-ation with main strand fault scarps (Figure 3). Thesurface scarp is removed by fluvial erosion fromabout 2 km north of Awaiti, although we infer thefault location is not very far west of the fluvial terrace

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riser (Figure 2). Traces along this segment between Tir-ohia and Awaiti are 1–3 m in height.

Trenches on the Awaiti segmentThree trenches – Davey, Lucy and Tye (Figure 2) –have been excavated on this segment. We show thetrench log for the Davey trench in Figures 5 and S9(unit descriptions in Table S8), but the Lucy (2 kmnorth of Davey trench) and Tye (about 2 km northwestof Kerepehi) trenches were, with hindsight, excavatedacross the terrace riser discussed above (Figure 2)and contained no faults. The Lucy and Tye trenchlogs are therefore not shown. We used some infor-mation from the Tye trench (Table S9) to assess agesof relevant geomorphic surfaces.

Davey trenchAt the Davey trench site, the scarp is a double scarpparallel to the edge of Kopouatai Bog on AwaitiRoad, about 1.5 km west of Tirohia (Figure 2). TheDavey trench was excavated across the lower scarpwith 1.2 m of vertical offset. Two parallel normalfault planes were exposed at the lower end of the trench(Figure 5). These faults offset all but the uppermost twosoil layers. On the upthrown side of these faults,Hinuera pumice alluvium (unit 9; Table S8) is warpeddown by 0.9 m. This is overlain by a paleosol (unit 8)that shows ground fissuring which occurred whenthis paleosol was the ground surface. The fissures arefilled with material morphologically and texturallydifferent from the overlying peat unit. The Hinuerapaleosol (unit 8) also shows some fractures that reachup into the overlying peat unit (unit 7) and its paleosol(unit 6), probably as a result of the younger event.

On top of the peat sequence on the upthrown side ofthe fault (units 6 and 7) there are a soil unit (unit 4)rich in organic material and a tephra (TuT) consistingof coarse rounded lapilli. This tephra was identified asthe 7005 ± 155 cal yr BP Tuhua Tephra from MayorIsland based on its aegirine > clinopyroxene ferromagne-sian mineralogy and two bracketing radiocarbon agesobtained from immediately above and below this unitin the southern wall of the trench (Figure 5; Table 1).

On the downthrown side of the faults, the lowestexposed unit is a peat unit (unit 5) which is almostentirely composed of twigs and pieces of wood.Above this unit lies the tephra interbed identified asTuhua tephra (TuT), which is in turn overlain by anorganic-rich clay unit (unit 3) which is missing fromthe upthrown side of the fault.

Unit 2 extends across the whole trench and containstwo other closely spaced tephras that extend as discon-tinuous lenses within the unit. From the chronostrati-graphic position and the tephra sequence within thebog, the upper discontinuous tephra is correlatedwith Kaharoa Tephra (636 ± 12 cal yr BP; Lowe et al.2008) (Figure 5) from Okataina Volcanic Centre,

while the lower is correlated with Taupo Tephra(1718 ± 5 cal yr BP; Figure 5) from Taupo VolcanicCentre on the basis of its similarity in grain size andappearance to known Taupo Tephra occurrences inthe area. A single radiocarbon age of 2944–2761 calyr BP (C7 in Table 1) was obtained from a mixed soilhorizon that includes Kaharoa and Taupo tephralenses.

In the Davey trench we find evidence for two rup-ture events. Evidence of the first (oldest) earthquakeevent in this trench consists of ground fissuring onthe paleosol of the Hinuera pumice alluvium (unit 8),undated here but younger than c. 22.5 ka . The cracksare filled with soil material with an age of 19,251–18,898 cal yr BP, and the overlying peat unit has aradiocarbon age of 14,047–13,767 cal yr BP (Table 1).The fissuring is likely to have occurred shortly afterthe underlying soil horizon was formed c. 19,252 yrBP. The earthquake event is therefore bracketedbetween 19,252 and 13,767 yr BP. A second (mostrecent) event formed two parallel faults which termi-nate just below the middle tephra layer which is ident-ified based on its morphological expression andthickness as the Taupo ash (c. 1.7 ka). Minor faultingon the south wall displacing up to the base of unit 2is also associated with this event. A radiocarbon ageof faulted soil material below this horizon (at the topof unit 3) is 5982–5741 cal yr BP (C10 in Table 1;Figure 5). The most recent rupture event is thereforeconstrained to between 5982 and 1718 yr BP (Table 2).

Tye trenchThe Tye trench is located about 2 km north of Kerepehi(Figure 2) on what we now, with the benefit of LiDARdata after trenching was undertaken, recognise as a ter-race riser. The trench did not expose faults or warping,but in situ shallow estuarine bivalve shells that werefound at 2.4 m below the ground surface were dated at6783–6582 cal yr BP (Tye 1 in Table 1) and a piece ofwood at 1.9 m depth was dated at 6490–6312 cal yr BP(Tye 6 in Table 1; Table S8). These sediments and thecoastal plain in this area are related to the highest Holo-cene sea-level transgression that culminated at c. 7.5 ka inthis region (Clement et al. 2010). The shoreline retreatedto its present position due to alluvial sedimentation, seenas an overbank of the Piako River above the shelly depos-its. The age of the land surface is unclear butmust be con-siderably younger than 7.5 ka because three paleosolshave developed within the overbank alluvial stratigraphyabove the dated shelly layer (Table S8).

Recurrence interval of faulting on the AwaitisegmentRupture of the Awaiti segment is best constrained of allof the segments of the Kerepehi Fault because of clearevidence exposed in the Davey trench. These eventsoccurred during the intervals 5982–1718 yr BP (most

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recent event) and 19,251–14,047 yr BP (penultimateevent) with displacements of about 1 m during eachevent (Table 2). The recurrence interval indicated bythese events is in the range of 8.1–17.5 ka (Table 2).

The recurrence interval can also be interpreted fromsingle-event displacement and scarp heights along thisfault strand. Scarp heights range between 1.5 and 2 m(Figure 2). From the trench, we can assume the single-event displacement to be about 1 m. We can thereforeconfirm that there were only two events during the last20 ± 2.5 ka and that the recurrence interval is c. 10 ka,which lies within the range estimated from the paleo-seismic data (Table 2).

Earthquake history of the Kerepehi Fault

Data from the six geometric segments of the fault, sum-marising the interpretation of incremental fault displa-cement archived in the geomorphic record and eventspecific timing of past events from trenches, are pre-sented in Table 2. The information available is primar-ily on the main strands (largest scarps) of each of thecomplex geometric segments, and the interpretationhere is necessarily cautious. We have few data fromthe northern- and southernmost on-land segments,but the indications are of only modest total displace-ment of 2–3 m of the Hinuera C surface with an ageof 20 ± 2.5 ka. The average slip rate on these segmentstherefore appears to be appreciably lower than on seg-ments in the middle parts of the Hauraki Depression(Table 2). Data on single-event displacement (SED)are available from a number of the segments indicatingthat, except at the very end of fault strands, SEDmeasurements are in the range of 0.8 m (on thesouthern end of the Waitoa segment, but probablydouble that in the middle of the segment) to 1.8 m(Te Poi segment). Not particularly large SED measure-ments of 3 m or more were documented and the impli-cations of this are discussed in the following.

If we assume the scarps of the main strands on eachsegment have grown by increments indicated by avail-able measurements of SED, then we can infer the num-ber of displacement events required to build the throwobserved since the formation of the Hinuera C aggra-dation surface and also derive the average recurrenceinterval (RI) for surface displacement along each seg-ment of the fault (Table 2). This simple calculationsuggests recurrence intervals on individual segmentsof 4.0–6. 0 ka are common on the central segmentsbut may be less frequent on the Okoroire, Elstow andTe Puninga segments where total throw on the mainstrands of the segments is smaller.

The third suite of observations pertains to the tim-ing of surface ruptures on each of the segments fromthe limited number of trenches excavated on the Kere-pehi Fault to date (Table 2). We clarify first why wehave interpreted fissures as evidence for secondary

faulting in some occasions, while on other occasionswe have interpreted fissures as evidence for primaryfault rupture. The occurrence of fissures is often a con-sequence of folding deformation in near-surface weaksedimentary layers, but they also occur in associationwith surface scarp formation.

When deformation is expressed as faulting, such asat the Davey trench, the sediments have mechanicalproperties that favour fracture and fissures aroundthe fault may occur. Fissures occur if there is drag fold-ing in the footwall or there is some gravitationalinstability created in the footwall as a consequence ofcreation of topography (scarp) by fault displacement.In this type of setting, if there is an isolated fissure inclose proximity to the fault but the age of the fissuredoes not coincide with a large displacement on thefault, then it is likely that the fissure is associatedwith shaking resulting from rupture of a differentfault or fault segment. This was observed in the Simon-sen trench 2. This secondary fissuring will be a conse-quence of gravitational instability of the upper part of asteep fault scarp or of differential compaction acrossthe fault plane if sedimentary units have differentstrengths or different thicknesses on either side of thefault.

When the mechanical properties of the near-surfacesediments across the fault are weaker (e.g. water satu-rated, loose sands), folding rather than fracture scarpsform such as at the Buchanan trench site. In this set-ting, fissures are commonly associated with bendingof the surface and near-surface horizons. The 1987Edgecumbe surface rupture exemplified this style ofsurface deformation at many localities (Beanlandet al. 1989). At some locations there were displace-ments across large fissures illustrating a mixed modesurface rupture comprising both fracturing and fold-ing, as we observe in the Westlake trench. Fissuresthat develop across the monoclinal flexure of a foldare often evidence for primary deformation at thesite. However, it is often difficult to associate the ageof fissure development with the folding, and the possi-bility therefore exists that the fissures could haveformed as a consequence of shaking associated withrupture of a different fault. Repeated exposures of thesame relationships of fissures and folding, ideallywith age control, provide higher levels of confidencein the interpretation of primary versus secondaryfaulting.

Small-scale fissuring, interpreted to be secondaryshaking-induced deformation related to a large earth-quake nearby, was observed in trenches on the TePoi and Waitoa segments within the last 700 yr, butthey may not have been related to the same event asthe most likely interpretation of the timing of eachevent does not overlap. We do not know whether thelarge nearby earthquake or earthquakes were onanother segment of the Kerepehi Fault or further

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away. We do not have data on the most recent event(MRE) from the Okoroire, Te Puninga or any of theoffshore fault segments. We do have quite good con-straints on the MRE from the Te Poi (16 ka ), Waitoa(5.3 ± 0.5 ka), Elstow (6.6 ± 0.2 ka) and Awaiti (in theinterval 5.9–1.8 ka) segments. These constraints allowfor the possibility that the Waitoa and Awaiti segmentsruptured at c . 5.9 ka , but not the strand of the inter-vening Elstow segment we have characterised. It istherefore more likely that a cluster of three large earth-quakes occurred on the Kerepehi Fault during theperiod 7–5 ka (Table 2).

Earthquake hazard characterisation of theKerepehi Fault

Earthquake magnitudes associated withsegment rupture

Using empirical regressions of fault length or fault areaagainst earthquake magnitude (e.g. Wells & Copper-smith 1994; Stirling et al. 2013) we can use the availableinformation to estimate the earthquake magnitudesconsistent with the surface fault parameters we haveobserved. Stirling et al. (2013) reviewed the applicationof the empirical regressions for different tectonicregions of the world and concluded that the regressionfrom Villamor et al. (2001), used for Taupo Rift faultsby Villamor et al. (2007) and Berryman et al. (2008), isthe most appropriate for the Hauraki Rift as well as thenearby Taupo Rift. Regressions of both length on mag-nitude and average displacement on magnitude areavailable (Equations 1 and 2):

Mw = 4.80+ 1.33 log L, (1)

where L is subsurface fault length (km), taken as 1.164times the surface rupture length (Villamor et al. 2001,2007).

Earthquake magnitude is related to subsurface dis-placement as follows:

Mw = 3.78+ 1.33 logD, (2)

where D is average subsurface displacement (cm) andsurface displacement is about 80% of subsurface displa-cement (Villamor et al. 2001).

In Table 3 we calculate possible earthquake magni-tudes from observed segment lengths and from the

SED measurements, assuming these are average values.Considering the sparseness of the data there is areasonable level of agreement in comparing magni-tudes calculated from length or displacement. Notableexceptions are for the Waitoa segment that is very longbut appears to have a small SED (but this might be upto 1.6 m for the centre of the segment; see Table 3 forimplications for earthquake magnitude). If the SEDvalues are reasonable, then this may suggest that thesegment may rupture in events that do not break thewhole segment and, as a consequence, there may bemore frequent earthquakes than estimated from anysingle location. The Elstow segment appears to be tooshort for the SED observed, suggesting that it mightusually rupture with one of the adjacent segments.The Okoroire segment is very much a minimum lengthbecause the segment has not been mapped south of theLiDAR coverage.

The prevalence of estimates ofMw 6.3–7.0 earthquakesfor rupture of various segments of the Kerepehi Fault(Table 3) from both segment length and single-event dis-placement is compatible with the only historic normalfault surface rupture in the region, that is, the 1987 Mw

6.6 Edgecumbe earthquake (Beanland et al. 1989).

Hazards posed by the rupture of theKerepehi Fault

Modified Mercalli shaking intensities due torupture

Three major population centres of New Zealand arerelatively close to the Kerepehi Fault, includingAuckland (45 km from the northernmost offshoresegment), Hamilton (40 km from the central on-land segments of the fault) and Tauranga (35 kmfrom the central segments of the fault). Shakingintensities for ruptures of the defined individual seg-ments or the entire Kerepehi Fault can be estimatedusing the earthquake loss models for New Zealand(Cousins 2004; Cousins et al. 2014), which arebased on the MMI (Modified Mercalli Intensity)attenuation model of Dowrick and Rhoades (2005).Table 4 lists the expected MM intensities likely tobe experienced in the larger population centres andlocal towns following rupture of various segmentsof the Kerepehi Fault. Auckland, Hamilton and

Table 3. Kerepehi Fault segment lengths, single-event displacement (SED) and assorted Mw.

Segment

Length

Mw

SED (cm)

MwSurface Subsurface Surface Subsurface

Okoroire >5 >5.75 5.8Te Poi 27 31 6.8 180 230 6.9Waitoa 43 49 7.1 80 100 6.4Elstow 12 14 6.3 160 200 6.8Te Puninga 22 25 6.7Awaiti >27 >31 6.8 100 130 6.6Offshore 9–19 10–22 6.2–6.6

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Tauranga may be expected to experience MMI 6–7 asthe maximum impact associated with rupture of thenearest segments of the fault. The likely damage atMMI 7 shaking is moderate with the onset ofstructural damage in older, unreinforced masonryconstruction, but with extensive contents damageand potential collapse of poorly secured parapetsand chimneys. Liquefaction can be expected at sus-ceptible sites, but is unlikely to damage structuresto a significant extent except at the most susceptiblesites where damage more akin to MMI 8 shakingcould be expected. At this level, unreinforcedmasonry buildings can be expected to be heavilydamaged with some collapse. Structural damagemay also occur in reinforced masonry and concretebuildings with no specific seismic design. Residentialbuildings that are not tied to foundations couldmove, most unreinforced chimneys are likely to bedamaged and unsecured contents are likely to fall.

In the towns of the Hauraki Plains in close proximityto segments of the Kerepehi Fault, the earthquake inten-sities are expected to be in the range of MMI 8–9 whenthe nearest segment ruptures, andmay even reachMMI10 in the extreme case involving combined rupture ofthe Te Poi, Waitoa and Awaiti segments (Table 4).The damage expected at this level of shaking is greatand poses significant life safety issues for all structureswithout specific seismic design (Table S10). Structuraldamage to buildings with seismic design, even thosebuilt after 1980, can be expected in some instances.Landsliding and liquefaction impacts will be severe.Major earthquakes on theKerepehi Fault when combin-ing recurrence intervals for all segments (six segmentswith an average recurrent interval of 6 ka) occur onaverage every 1000 yr or 1% likelihood per decade.

The risk posed by the Kerepehi Fault throughout theHauraki Plains is therefore at a level where checksshould be made of the strength of buildings withhigh occupancy or which house critical facilities. Inplaces where there is significant pedestrian foot traffic,any unreinforced masonry buildings, parapets andchimneys should be retrofitted as soon as possible.

Similarly, the location of important facilities withrespect to potential liquefaction-driven ground defor-mation and landslide risk should be mapped withurgency.

Conclusions

The Kerepehi Fault is an active normal fault orientedapproximately NNW–SSE along the centre of theHauraki Plains and extending northwards into theFirth of Thames and Hauraki Gulf. The on-land partof the fault consists of six overlapping right-steppingsegments that are 10–43 km long and have a totallength of 71–80 km. Four offshore segments vary inlength over the range 9–19 km.

Fault scarps along the various traces range from 1to 8 m in height with the largest scarps displacing the20 ± 2.5 ka Hinuera C surface. A series of trenchesthat have targeted the main strand scarps of manyof the segments, coupled with other dating control,has identified the timing and size of several past sur-face-rupturing earthquakes. Single-event displace-ments vary between 1 and 1.8 m. The fault has anaverage vertical slip rate that varies among segmentsfrom c. 0.08 mm a–1 to 0.4 mm a–1, but trench datasuggest strongly clustered events on at least some ofthe fault segments. The Te Poi segment appears tohave ruptured four or five times in the period 22.5–16 ka, with no more recent rupture except for second-ary fracturing within the past 500 yr. On other faultsegments the evidence for clustered event timing isnot as clear but it is possible that the Waitoa andAwaiti segments ruptured at the same time at c. 5.9ka, but not the intervening Elstow segment. It ismore likely that a cluster of at least three large earth-quakes occurred on the Kerepehi Fault during theperiod 7–5 ka (Table 2).

Estimates of the earthquake magnitudes associatedwith surface rupture of the Kerepehi Fault have beenobtained from fault segment length and single-eventdisplacement (Table 3). Both methods suggest thatearthquakes in the range of Mw 6.3–7.0 have occurred

Table 4. Estimated MMI values in cities resulting from the rupture of Kerepehi Fault segments. MMI scale descriptions from NZSEE(1992) and Dowrick et al. (2008). See Table S10 for description of damage associated with each value of MM intensity.Segment name Mw Auckland Hamilton Tauranga Matamata Te Aroha Thames

Okoroire 5.8 4 5 5 7 6 4Te Poi 6.8 5 7 7 9 7 6Waitoa 7.1 6 7 7 8 9 7Elstow 6.3 5 6 6 6 8 6Te Puninga 6.7 5 7 6 7 8 6Awaiti 6.8 6 6 6 6 8 7Offshore south* 6.5 6 6 5 5 6 8Offshore south-central* 6.2 5 5 5 5 5 7Offshore north-central* 6.4 6 5 5 5 5 7Offshore north* 6.6 6 5 5 5 5 7Te Poi + Waitoa* 7.2 6 7 7 9 9 7Te Poi + Waitoa + Awaiti* 7.4 6 7 7 8 10 7

*Mw calculated using Equation (1) in text from fault lengths defined by Chick et al. (2001) for offshore segments and fault length from this study for potentialmultiple-segment ruptures.

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in the past and should be considered with respect tofuture hazard and risk.

The three large population centres of northern NorthIsland–Auckland, Hamilton and Tauranga – mayexperience MMI 6–7 shaking as the maximum impactassociated with rupture of the nearest segments of thefault. In the towns of the Hauraki Plains, in close proxi-mity to segments of the Kerepehi Fault, the earthquakeintensities are expected to reach MMI 9 on someoccasions and perhaps evenMMI 10. Major earthquakeson the Kerepehi Fault occur with a recurrence of perhaps1000 yr or 1% likelihood per decade when combiningevents from all segments. The risk is therefore at a levelwhere buildings with high occupancy rates or whichhouse critical facilities should undergo strength checks,and in places where there is significant pedestrian foottraffic any unreinforced masonry buildings, parapetsand chimneys should be retrofitted as soon as possible.

Supplementary data

Dataset S1. Figures S1–S9 including DEMs, fault trenches,photographs of fault excavations and location of drillholesin the de Lange & Lowe (1990) study.

Figure S1. Active fault mapping.

Figure S2. LIDAR digital elevation model with active faulttraces.

Figure S3. Photos of fault scarps and paleoseismic trenches.

Figure S4. Westlake trench log.

Figure S5. Madill trench log.

Figure S6. Simonsen trenches 1 and 2 logs.

Figure S7. Location of de Lange & Lowe (2001) study.

Figure S8. Buchanan trench log.

Figure S9. Davey trench log.

Dataset S2. Tables S1–S10 including a description of researchmethods, unit descriptions from fault trenches and augerhole 2, and a simplified description of the Modified MercalliIntensity scale.

Table S1. Methods.

Table S2. Unit descriptions Westlake trench.

Table S3. Unit descriptions Augerhole 2.

Table S4. Unit descriptions Madill.

Table S5. Unit descriptions Simonsen 1.

Table S6. Unit descriptions Simonsen 2.

Table S7. Unit descriptions Buchanan.

Table S8. Unit descriptions Davey.

Table S9. Unit descriptions Tye.

Table S10. Simplified MMI scale.

Acknowledgements

The authors wish to dedicate this research to John Beavan.While John did not work directly on paleoseismologicaltopics he did take a keen interest in all data pertaining toNew Zealand tectonics. He always sought to integrate all evi-dence for present and past activity into our understanding ofseismic hazard and the risks that future activity pose onsociety. John’s contribution to this paper is contained in dis-cussion and debate that sits in the thinking behind the wordsin the manuscript. In addition, we would also like to notethat Sarah Beanland made some of the earliest observationson the Kerepehi Fault which are incorporated into thispaper. We also thank landowners Walter Tye, Ian Madill,Stuart Davey, Wayne Lucy, James Buchanan and the Simon-sen family for giving permission for trenching, and WaikatoRegional Council for providing the LiDAR dataset. We alsothank: Kate Clark, Sandra Staller and Bennie Thiebes forassistance with field work; Neville Palmer and Dion Mathe-son for technical support during the GPS surveys; and Bil-jana Luković, Tim Browne and Shivaun Hogan fordigitising trench logs. Radiocarbon ages were provided bythe GNS Science Radiocarbon Laboratory, Lower Hutt.This paper also includes data from a 1986 unpublishedreport by Sarah Beanland and Kelvin Berryman (Beanland& Berryman 1986). Steve Edbrooke reviewed an earlier ver-sion of this manuscript. Vasiliki Mouslopoulou and twoanonymous journal referees provided useful comments.

Guest Editor: Dr Ian Hamling.

Disclosure statement

No potential conflict of interest was reported by the authors.

Funding

Mira Persaud was supported by Swiss National ScienceFoundation grant No. 81BE-69172. GNS Science contri-butions to this study were funded from GNS Science DirectCrown Funding.

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