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Recent earthquakes involving multiple fault ruptures highlight the need to evaluate complex coastal deformation mechanisms, which are important for understanding plate boundary kinematics and seismic and tsunami hazards. We compare ages and uplift of the youngest Holocene marine terraces at Puatai Beach and Pakarae River mouth (∼10 km apart) in the northern Hikurangi subduction margin to examine whether uplift is the result of subduction earthquakes or upper-plate fault earthquakes. From stepped platform-cliff morphology, we infer uplift during 2–3 earthquakes and calculate an average uplift-per-event of 2.9±0.5 m at Puatai Beach and 2.0±0.5 m at Pakarae River mouth. Radiocarbon ages from the youngest beach deposit shells on each terrace and a tephra coverbed on one terrace constrain the timing of earthquakes to 1770–1710, 1100–910, and 420–250 cal. B.P. at Puatai Beach, and 1490–1290 and 660–530 cal. B.P. at Pakarae River mouth. The ages differ at each site indicating uplift is neither the result of subduction earthquakes nor single upper-plate fault earthquakes. A reinterpretation of new and existing bathymetry and seismic reflection data, combined with dislocation modeling, indicates that near-shore fault segmentation is more complex than previously thought and ruptures likely involve multiple upper-plate faults. Future updates of the New Zealand National Seismic Hazard Model should revise the northern Hikurangi subduction seismic sources so that rupture does not uplift Puatai Beach and Pakarae River mouth and include new near-shore upper-plate faults as multifault sources.

The Hikurangi subduction margin, New Zealand, has not produced large subduction earthquakes within the short written historic period (~180 years) and the potential of the plate interface to host large (M > 7) to great (M > 8) earthquakes and tsunamis is poorly constrained. The geological record of past subduction earthquakes offers a method for assessing the location, frequency and approximate magnitude of subduction earthquakes to underpin seismic and tsunami hazard assessments. We review evidence of Holocene coseismic coastal deformation and tsunamis at 22 locations along the margin. A consistent approach to radiocarbon age modelling is used and earthquake and tsunami evidence is ranked using a systematic assessment of the quality of age control and the certainty that the event in question is an earthquake. To identify possible subduction earthquakes, we use temporal correlation of earthquakes, combined with the type of earthquake evidence, likely primary fault source and the earthquake certainty ranking. We identify 10 past possible subduction earthquakes over the past 7000 years along the Hikurangi margin. The last subduction earthquake occurred at 520–470 years BP in the southern Hikurangi margin and the strongest evidence for a full margin rupture is at 870–815 years BP. There are no apparent persistent rupture patches, suggesting segmentation of the margin is not strong. In the southern margin, the type of geological deformation preserved generally matches that expected due to rupture of the interseismically locked portion of the subduction interface but the southern termination of past subduction ruptures remains unresolved. The pattern of geological deformation on the central margin suggests that the region of the interface that currently hosts slow slip events also undergoes rupture in large earthquakes, demonstrating different modes of slip behaviour occur on the central Hikurangi margin. Evidence for subduction earthquakes on the northern margin has not been identified because deformation signals from upper plate faults dominate the geological record. Large uncertainties remain in regard to evidence of past subduction earthquakes on the Hikurangi margin, with the greatest challenges presented by temporal correlation of earthquake evidence when working within the uncertainties of radiocarbon ages, and the presence of upper plate faults capable of producing deformation and tsunamis similar to that expected for subduction earthquakes. However, areas of priority research such as improving the paleotsunami record and integration of submarine turbidite records should produce significant advances in the future.

Multiple (>20 \ud >20\ud ) crustal faults ruptured to the ground surface and seafloor in the 14 November 2016 M w \ud Mw\ud 7.8 Kaikōura earthquake, and many have been documented in detail, providing an opportunity to understand the factors controlling multifault ruptures, including the role of the subduction interface. We present a summary of the surface ruptures, as well as previous knowledge including paleoseismic data, and use these data and a 3D geological model to calculate cumulative geological moment magnitudes (M G w \ud MwG\ud ) and seismic moments for comparison with those from geophysical datasets. The earthquake ruptured faults with a wide range of orientations, sense of movement, slip rates, and recurrence intervals, and crossed a tectonic domain boundary, the Hope fault. The maximum net surface displacement was ∼12 m \ud ∼12 m\ud on the Kekerengu and the Papatea faults, and average displacements for the major faults were 0.7–1.5 m south of the Hope fault, and 5.5–6.4 m to the north. M G w \ud MwG\ud using two different methods are M G w \ud MwG\ud 7.7 +0.3 −0.2 \ud 7.7−0.2+0.3\ud and the seismic moment is 33%–67% of geophysical datasets. However, these are minimum values and a best estimate M G w \ud MwG\ud incorporating probable larger slip at depth, a 20 km seismogenic depth, and likely listric geometry is M G w \ud MwG\ud 7.8±0.2 \ud 7.8±0.2\ud , suggests ≤32% \ud ≤32%\ud of the moment may be attributed to slip on the subduction interface and/or a midcrustal detachment. Likely factors contributing to multifault rupture in the Kaikōura earthquake include (1) the presence of the subduction interface, (2) physical linkages between faults, (3) rupture of geologically immature faults in the south, and (4) inherited geological structure. The estimated recurrence interval for the Kaikōura earthquake is ≥5,000–10,000 yrs \ud ≥5,000–10,000 yrs\ud , and so it is a relatively rare event. Nevertheless, these findings support the need for continued advances in seismic hazard modeling to ensure that they incorporate multifault ruptures that cross tectonic domain boundaries.

The evolution of the continental intra-arc Taupo Rift in the North Island, New Zealand is rapid, significantly faster than comparative intra-continental rifts such as the African Rifts. Based on our faulting data and published geological, geophysical and borehole data, we show that activity in the ~2 Ma Taupo Rift has rapidly and asymmetrically narrowed via inward and eastward migration of faulting (at rates of ca. 30 km My-1 and 15 km My-1, respectively) and has propagated southwards along its axis ~70 km in 350 kyr. The loci of voluminous volcanic eruptions and active faulting are correlated in time and space, suggesting that a controlling factor in the rapid rift narrowing is the presence of large shallow heterogeneities in the crust, such as large rhyolitic magma bodies generated by subduction processes, which weaken the crust and localize deformation. Eastward migration of faulting also follows the eastward migration of the volcanic arc which may be related to rollback of the Pacific crust slab at the Hikurangi subduction zone. Southward propagation of the rift is linked with southward migration of the Hikurangi plateau/Chatham Rise subduction point and occurs episodically aided by stress changes associated with voluminous local volcanism. The large magma supply during early continental intra-arc rift stages explains faster evolution (from tectonic to magmatic) than intracontinental rifts. However, the fast changes in magma supply from the subduction zone can also lead to evolution reversals (more evolved magmatic stages reverting to less evolved tectonic stages), rift cessation, and thus failed continental break-up.

Coseismic coastal deformation is often used to understand slip on offshore faults in large earthquakes but in the 2016 M W 7.8 Kaikōura earthquake multiple faults ruptured across and sub-parallel to the coastline. Along ∼110 km of coastline, a rich dataset of coastal deformation comprising airborne lidar differencing, field surveying and satellite geodesy reveals highly variable vertical displacements, ranging from −2.5 to 6.5 m. These inform a refined slip model for the Kaikōura earthquake which incorporates changes to the slip on offshore faults and inclusion of an offshore reverse crustal fault that accounts for broad, low-amplitude uplift centered on Kaikōura Peninsula. The exceptional detail afforded by differential lidar and the high variability in coastal deformation combine to form the highest-resolution and most complex record of coseismic coastal deformation yet documented. This should prompt reassessment of coastal paleoseismic records that may not have considered multi-fault ruptures and high complexity deformation fields.

Discovery and investigation of millennial-scale geological records of past large earthquakes improve understanding of earthquake frequency, recurrence behaviour, and likelihood of future rupture of major active faults. Here we present a ∼2000 year-long, seven-event earthquake record from John O'Groats wetland adjacent to the Alpine fault in New Zealand, one of the most active strike-slip faults in the world. We linked this record with the 7000 year-long, 22-event earthquake record from Hokuri Creek (20 km along strike to the north) to refine estimates of earthquake frequency and recurrence behaviour for the South Westland section of the plate boundary fault. Eight cores from John O'Groats wetland revealed a sequence that alternated between organic-dominated and clastic-dominated sediment packages. Transitions from a thick organic unit to a thick clastic unit that were sharp, involved a significant change in depositional environment, and were basin-wide, were interpreted as evidence of past surface-rupturing earthquakes. Radiocarbon dates of short-lived organic fractions either side of these transitions were modelled to provide estimates for earthquake ages. Of the seven events recognised at the John O'Groats site, three post-date the most recent event at Hokuri Creek, two match events at Hokuri Creek, and two events at John O'Groats occurred in a long interval during which the Hokuri Creek site may not have been recording earthquakes clearly. The preferred John O'Groats–Hokuri Creek earthquake record consists of 27 events since ∼6000 BC for which we calculate a mean recurrence interval of 291 ± 23 years , shorter than previously estimated for the South Westland section of the fault and shorter than the current interseismic period. The revised 50-year conditional probability of a surface-rupturing earthquake on this fault section is 29%. The coefficient of variation is estimated at 0.41. We suggest the low recurrence variability is likely to be a feature of other strike-slip plate boundary faults similar to the Alpine fault.

The Alpine fault in south Westland, New Zealand, releases strains of Pacific–Australian relative plate motion in large earthquakes with an average interevent spacing of ∼330 years. A new record of earthquake recurrence has been developed at Hokuri Creek, with evidence for 22 events. The youngest Hokuri Creek earthquake overlaps in time and is believed to be the same as the oldest of another site about 100 km to the northwest near Haast. The combined record spans the last 7900 years and includes 24 events. We study the recurrence rate and conditional probability of ground ruptures from this record using a new likelihood‐based approach for estimation of recurrence model parameters. Paleoseismic parameter estimation includes both dating and natural recurrence uncertainties. Lognormal and Brownian passage time (BPT) models are considered. The likelihood surface has distribution location and width parameters as axes, the mean and standard deviation of the log recurrence for the lognormal, and the mean and coefficient of variation for the BPT. The maximum‐likelihood (ML) point gives the parameters most likely to have given rise to the data. The ML point, 50‐year conditional probabilities of a ground‐rupturing earthquake are 26.8% and 26.1% for the lognormal and BPT models, respectively. Contours of equal likelihood track the parameter pairs that are equally probable to have given rise to the observed data. Conditional probabilities on the lognormal 95% boundary around the ML point range from 18.2% to 35.8%. An empirical distribution model completely based on past recurrence times gives a similar conditional probability of 27.1% (9.6%–50.2%). In contrast, the time‐independent conditional probability estimate of 13.6% (8.8%–19.1%) is about half that of the time‐dependent models. A nonparametric test of earthquake recurrence at Hokuri Creek indicates that time‐dependent recurrence models best represent the southern Alpine fault of the South Island, New Zealand.

A fundamentally important but typically abbreviated component of seismic‐hazard analysis is the selection of earthquake scaling relationships. These are typically regressions of historical earthquake datasets, in which magnitude is estimated from parameters such as fault rupture length and area. The mix of historical data from different tectonic environments and the different forms of the regression equations can result in large differences in magnitude estimates for a given fault rupture length or area. We compile a worldwide set of regressions and make a first‐order shortlisting of regressions according to their relevance to a range of tectonic regimes (plate tectonic setting and fault slip type) in existence around the world. Regression relevance is based largely on the geographical distribution, age, and quantity/quality of earthquake data used to develop them. Our compilation is limited to regressions of magnitude (or seismic moment) on fault rupture area or length, and our shortlisted regressions show a large magnitude range (up to a full magnitude unit) for a given rupture length or area across the various tectonic regimes. These large differences in magnitude estimates underline the importance of choosing regressions carefully for seismic‐hazard application in different tectonic environments.

A team of earthquake geologists, seismologists, and engineering seis- mologists has collectively produced an update of the national probabilistic seismic hazard (PSH) model for New Zealand (National Seismic Hazard Model, or NSHM). The new NSHM supersedes the earlier NSHM published in 2002 and used as the hazard basis for the New Zealand Loadings Standard and numerous other end-user applica- tions. The new NSHM incorporates a fault source model that has been updated with over 200 new onshore and offshore fault sources and utilizes new New Zealand-based and international scaling relationships for the parameterization of the faults. The dis- tributed seismicity model has also been updated to include post-1997 seismicity data, a new seismicity regionalization, and improved methodology for calculation of the seismicity parameters. Probabilistic seismic hazard maps produced from the new NSHM show a similar pattern of hazard to the earlier model at the national scale, but there are some significant reductions and increases in hazard at the regional scale. The national-scale differences between the new and earlier NSHM appear less than those seen between much earlier national models, indicating that some degree of consis- tency has been achieved in the national-scale pattern of hazard estimates, at least for return periods of 475 years and greater. Online Material: Table of fault source parameters for the 2010 national seismic- hazard model.

Strata and fault relationships revealed in five trenches excavated across the recent trace of the Alpine fault at the Haast, Okuru, and Turnbull Rivers, South Westland, New Zealand, record the three most recent surface‐faulting events. Using back‐stripping techniques to remove the three faulting events and the sedimentary units associated with the faulting restores the cross‐sections to gravel‐bed floodplains at the Haast and Okuru Rivers, at about A.D. 750. Horizontal and vertical offsets of stream channels and terrace risers reveal characteristic displacements of about 8–9 m dextral and up to 1 m vertical per event. Cumulative dextral displacement is 25±3 m in the past three events. The most recent surface‐rupture event was probably in A.D. 1717, and the next prior events were about A.D. 1230±50 and about A.D.750±50. The timing of these events is consistent with past large‐great earthquakes on the southern section of the Alpine fault inferred from off‐fault data, but there are fewer events identified in trenches. Our three‐event dataset indicates the average surface‐rupture recurrence interval for the South Westland section of the fault is about 480 years, much longer than the current elapsed time of 295 years. Therefore, the Alpine fault in South Westland may not be close to rupture as is often speculated.

The formation and uplift of Holocene marine terraces along a 160-km-long stretch of the Wairarapa coast in the southern Hikurangi subduction margin, North Island, New Zealand, are interpreted in terms of causative faults and associated large earthquakes during the past circa 7000 cal yr B.P. Distinctive stepped terrace morphology, the clustering of radiocarbon ages on each uplifted terrace, and the historic occurrence of coseismic uplift in other parts of eastern and southern North Island, support the contention that coastal uplift occurred suddenly and repeatedly during large-magnitude earthquakes. The rupture of five west-dipping reverse or reverse-oblique faults within the Australian plate, whose surface traces lie a short distance seaward of the present shoreline, are inferred to be responsible for coastal uplift. Individual structures range in length from 25 to 60 km. These lengths, together with uplift events in the range of 1–4 m, suggest earthquakes in the range of M w 7–7.5. Approximately 20% of the radiocarbon ages are anomalously young for their elevations, but many have ages indistinguishable from accepted ages for terrace uplift. We infer that many of the anomalous ages are from samples deposited by tsunami that occurred in association with offshore fault rupture on adjacent sections of the coast.

The El Salvador earthquake of 13 February 2001 (Mw 6.6) caused tectonic rupture on the El Salvador fault zone (ESFZ). Right-lateral strike-slip surface rupture of the east–west trending fault zone had a maximum surface displacement of 0.60 m. No vertical component was observed. The earthquake resulted in widespread landslides in the epicentral area, where bedrock is composed of volcanic sediments, tephra, and weak ignimbrites. In the aftermath of the earthquake, widespread damage to houses and roads and the hazards posed by landslides captured the attention of responding agencies and scientists, and the presence of surface-fault rupture was overlooked. Additionally, the tectonic context in which the earthquake took place had not been clear until mapping of the ESFZ was completed for the present study.We identified several fault segments, the distribution of surface ruptures, the aftershock pattern, and fault-rupture scaling considerations that indicate the 21-km-long San Vicente segment ruptured in the 2001 event. Static Coulomb stress transfer models for the San Vicente rupture are consistent with both aftershock activity of the 2001 sequence and ongoing background seismicity in the region. At Mw 6.6, the 2001 earthquake was of only moderate magnitude, yet there was significant damage to the country’s infrastructure, including buildings and roads, and numerous deaths and injuries. Thus, earthquake hazard and risk in the vicinity of the ESFZ, which straddles the city of San Salvador with a population of >2 million, is high because even moderate-magnitude events can result in major damage, deaths, and injuries in the region.

Determining the spatial and temporal deliveries of sediment from the Waipaoa River catchment in the post-glacial period is a major goal of integrated source-to-sink landscape evolution studies in the region. In a 2.2 km long section of the Waihuka tributary (217 km 2 ) of the Waipaoa River (2150 km 2 ) in eastern North Island, New Zealand, a sequence of at least ten fluvial terraces and abandoned meanders up to 45 m above the present river records rates and times of post-glacial downcutting. Dateable organic and tephra horizons in the terrace and meander infill stratigraphy indicate that downcutting at this location on the Waihuka tributary was dominated by a short-lived event in the early Holocene from 10–8 ka BP, when as much as half of the downcutting was accomplished in only 10–15% of post-glacial time. The large downcutting event is interpreted to record the passing of a succession of major knickpoints or a knickzone through this tributary. The initiation of knickpoint retreat in the catchment was primarily a consequence of reduction in sediment supply at the end of MIS 2. Based on the timing of rapid downcutting in the Waipaoa River mainstem, we estimate that the rate of knickpoint retreat along the Waihuka tributary was about 2 km/kyr. In the period before and after rapid downcutting in the Waihuka tributary, modest incision rates, averaging 1–2 mm/yr, probably represent the regional tectonic uplift rate.

One of the major environmental controls on sedimentary systems is tectonics, and thus in order to make quantitative explanations and predictions of the behaviour of sedimentary systems, it is important to quantify tectonic rates and mechanisms. In this study we show how a combined Holocene fluvial and marine terrace dataset can be used to place constraints on coastal uplift mechanisms at Pakarae River mouth, to the east of the Waipaoa sedimentary system. Mapping and surveying of fluvial terraces shows that the terraces can be divided into three sets, (i) PgT, a fill terrace formed during post-glacial sea level rise (~ 10000–7000 cal. yr BP), (ii) a sequence of post-7000 cal. yr BP degradation terraces, and (iii) historical (post ~ AD 1880) flood terraces. A conceptual model of terrace evolution proposes that coseismic uplift has resulted in the observed back-tilting, downstream divergence, and successive abandonment of tectonic terraces. Coastal uplift mechanisms are then examined by comparing the fluvial and marine terrace uplift distribution and the uplift-per-event from the marine terraces with those predicted by forward elastic dislocation models. The models agree with the interpretation that the terrace uplift can be explained by coseismic slip on a steeply west-dipping reverse fault located close to the shore, possibly in combination with rupture of the Hikurangi Margin subduction interface 10–24 km below. This study also provides insights into the tectonic drivers and impacts on the adjacent Waipaoa sedimentary system including: the possible role of subduction earthquakes, growth of structures on the shelf and upper slope, and the role that the interplay between sea level and tectonic controls has on sediment dispersal.

SUMMARY The landmass of New Zealand exists as a consequence of transpressional collision between the Australian and Pacific plates, providing an excellent opportunity to quantify the kinematics of deformation at this type of tectonic boundary. We interpret GPS, geological and seismological data describing the active deformation in the South Island, New Zealand by using an elastic, rotating block approach that automatically balances the Pacific/Australia relative plate motion budget. The data in New Zealand are fit to within uncertainty when inverted simultaneously for angular velocities of rotating tectonic blocks and the degree of coupling on faults bounding the blocks. We find that most of the plate motion budget has been accounted for in previous geological studies, although we suggest that the Porter’s Pass/Amberley fault zone in North Canterbury, and a zone of faults in the foothills of the Southern Alps may have slip rates about twice that of the geological estimates. Up to 5 mm yr −1 of active deformation on faults distributed within the Southern Alps

A series of large hydrothermal eruptions occurred across the Waiotapu geothermal field at about the same (prehistoric) time as the ~AD1315 “Kaharoa” rhyolite magmatic eruptions from Tarawera volcano vents, 10–20 km distant. Triggering of the Waiotapu hydrothermal eruptions was previously attributed to displacement of the adjacent Ngapouri Fault. The Kaharoa rhyolite eruptions are now recognised as primed and triggered by multiple basalt intrusions beneath the Tarawera volcano. A ~1000 t/day pulse of CO2 gas is recorded by alteration mineralogy and fluid inclusions in drill core samples from Waiotapu geothermal wells. This CO2 pulse is most readily sourced from basalt intruded at depth, and although not precisely dated, it appears to be associated with the Waiotapu hydrothermal eruptions. We infer that the hydrothermal eruptions at Waiotapu were primed by intrusion of the same arrested basalt dike system that drove the rhyolite eruptions at Tarawera. This dike system was likely similar at depth to the dike that generated basalt eruptions from a 17 km-long fissure that formed across the Tarawera region in AD1886. Fault ruptures that occurred in the Waiotapu area in association with both the AD1886 and ~AD1315 eruptions are considered to be a result, rather than a cause, of the dike intrusion processes.

We present a new probabilistic seismic hazard analysis (PSHA) for New Zealand. An important feature of the analysis is the application of a new method for the treatment of historical (distributed) seismicity data in PSHA. The PSHA uses the seismicity recorded across and beneath the country to define a three-dimensional grid of a -values (i.e., parameter a of a Gutenberg-Richter distribution log N/yr = a - bM , in which N /yr is the number of earthquakes per year recorded inside each grid cell equal to or greater than magnitude M ); parameter b and the limiting maximum cutoff magnitude of the Gutenberg-Richter distribution are defined from the surrounding region (14 crustal and 23 subcrustal seismotectonic zones are defined for the country) and then smoothed across the boundaries of the zones. The methodology therefore combines the modern method of defining continuous distributions of seismicity parameters (Frankel, 1995; Frankel et al. , 1996) with the traditional method of defining large area sources and the associated seismicity parameters (e.g., Algermissen et al. , 1990). The methodology provides a means of including deep (subduction zone) seismicity in a PSHA, preserves the finer-scale spatial variations of seismicity rates across a region, avoids the undesirable edge effects produced in the traditional method when adjacent area sources enclose areas of significantly different seismicity rates, and also enables parameters most reliably defined at a regional scale (parameter b and maximum cutoff magnitude of a Gutenberg-Richter distribution, and slip type) to be incorporated into the PSHA. The PSHA combines the modeled seismicity data with geological data describing the location and earthquake recurrence behavior of 305 active faults and new attenuation relationships for peak ground acceleration and spectral acceleration developed specifically for New Zealand. Different attenuation expressions are used for crustal and subduction zone earthquakes. The resulting PSH maps for a 150-year return period show the highest hazard to occur in the center and southwest of the country, in the areas of highest historical crustal and deep subduction zone seismicity. In contrast, the longer return-period maps (475 and 1000 year return period) show the highest hazard to occur from the southwest to northeast ends of the country, along the faults that accommodate the majority of the motion between the Pacific and Australian plates. The maps are currently being used to revise New Zealand's building code, which has previously been based on PSHAs that did not explicitly include individual faults as earthquake sources. Manuscript received 10 April 2001.

Estimates of surface rupture displacement and magnitude for crustal earthquakes from the preinstrumental era (pre-1900) tend to be greater than the corresponding estimates derived from modern scaling relations. We investigate this tendency using an expanded and updated version of the earthquake dataset of Wells and Coppersmith (1994) to fit regression relations of moment magnitude on surface rupture length and rupture area and average surface displacement on surface rupture length. Separate relations are fitted to preinstrumental and instrumental data and the results compared to the equivalent relations of Wells and Coppersmith. We find that our relations for instrumental data remove some, but not all, of the differences between the preinstrumental data and the relations of Wells and Coppersmith. We attribute the remaining differences largely to natural censoring of surface displacements less than about 1 m and surface rupture lengths less than about 5 km from the dataset for the preinstrumental era because regressions constructed from similarly censored instrumental data are indistinguishable from the preinstrumental regressions. Since the regressions for our censored instrumental data (i.e., restricted to moderate to large earthquakes) are different from regressions for our complete dataset of instrumental earthquakes and from the regressions of Wells and Coppersmith (both with a larger proportion of small-to-moderate earthquakes), the results may indicate that large earthquakes have different scaling relationships from those of smaller earthquakes.

The Wellington region is cut by five active right-lateral strike-slip faults : Wairarapa, Wellington, Ohariu, Shepherds Gully/Pukerua, and Wairau faults that have average recurrence intervals of meter-scale surface rupture that range from ∼500 years to 5000 years, and lateral slip rates that range from 1 to 10 mm/yr. Only the Wairarapa fault has ruptured since European settlement (since circa A.D. 1840). Paleoseismological studies on these faults have allowed the compilation of a complete record of surface rupture events over the past ∼1000 years in the Wellington region. Within this time period, there does not appear to be any temporal clustering of surface rupture events on adjacent faults. The M 8 A.D. 1855 Wairarapa earthquake did not trigger rupture on any other fault in the region. The most recent surface-faulting event on the Wellington fault (290-440 cal years B.P.) (cal years are calendar years before A.D. 1950) does not coincide with rupture of any other onland fault, and over 300 years separate the timing of the second most recent rupture on the Wellington fault (660-720 cal years B.P.) and the most recent rupture of the Ohariu fault (1060-1140 cal years B.P.). The most recent rupture of the Shepherds Gully/Pukerua fault is probably older than that of the Ohariu fault. The apparent nonclustering of surface rupture earthquakes in the Wellington region has been documented only for the on-land strike-slip faults. There are other possible seismogenic sources in the region, and thus important issues remain to be addressed regarding the history of large earthquakes in the Wellington region : (1) the seismogenic potential and earthquake recurrence interval of the subduction thrust beneath Wellington is not known ; (2) the timing of rupture events on the offshore portion of the Wairau fault is not known ; and (3) paleoseismic data are not available for the section of the Wellington fault north of the Wellington-Hutt Valley segment. Estimates of earthquake hazard in the Wellington region, for all return times greater than 50 years, that incorporate paleoseismicity data are between one and two Modified Mercalli (MM) intensity units higher than the hazard based solely on the historical seismicity catalog, and the hazard is spatially more variable. Using a deterministic attenuation model, the level of shaking hazard approaches near maximum values within a return time of ∼500 years, largely reflecting the recurrence interval (500-770 years) of surface rupture earthquakes on the Wellington fault. Inclusion of a plausible model for magnitude 8 subduction zone earthquakes does not affect the level of MM intensity in Wellington region at return times greater than 500 years but does make a small contribution to the hazard at return times between 50 and 500 years.

The accretionary margin of the Hikurangi forearc on the southeast coast of the North Island of New Zealand is part of the leading edge of the Australian plate, which is overriding the obliquely converging Pacific plate. We investigate the last 10 m.y. of deformation history of the innermost (western) quarter of the total width of the forearc through analysis of the sedimentologic and structural evolution of the Eketahuna area on the east coast of the North Island. The Eketahuna area is ideal for such a study because emergence of the margin in the Quaternary has exposed a complete late Neogene rock record. This record has allowed us to chronicle the strain history. From 10 Ma to about 2.5 Ma this forearc region was the locus of subsidence and marine deposition. In the latest Pliocene this part of the margin began to shorten through folding and reverse faulting, bringing an end to basin filling. The period of shortening was brief, and by the late Pleistocene, reverse faulting had ceased and was immediately succeeded by dextral strike-slip faulting, in some cases along the same faults. Presently, the dominant strain regime in the inner quarter of the forearc is strike-slip faulting. This structural history illustrates that, over time, the pattern of strain partitioning has changed in the Hikurangi forearc. The switch from crustal shortening to dextral shear along the major faults in this area in the last 1 m.y. may be a response to more than 10° of clockwise rotation in the southern Hikurangi forearc in Pliocene and Pleistocene time. This rotation is a consequence of the fact that the accretionary margin is undergoing continuous deformation between the obliquely converging Australian and Pacific plates in this area at the southernmost end of the Kermadec-Hikurangi subduction system. The inboard portion of this young accretionary margin is exceptionally well exposed today, probably in part because of the late Neogene subduction of relatively thick, buoyant crust of the Hikurangi-Chatham plateau.

A sequence of seven marine terraces of late Pleistocene age constitutes approximately 40% of the area of Mahia Peninsula in eastern North Island. The peninsula is one of the closest land areas to the Hikurangi Trough (approximately 80 km further east), where the Pacific plate is being subducted beneath the Australian plate. Terrace ages of 40, 59, 81, 106, 124, 176, and 212 ka are assigned by correlation with the marine terrace sequence of Huon Peninsula, Papua New Guinea. Paleontological data and amino acid dating of marine components of the 124 ka terrace at Mahia Peninsula provide a basis for correlation to the Huon Peninsula sequence. Loess and tephra stratigraphy constrain the ages of older and younger terraces. The marine terrace sequence is tilted to the WNW on the flank of the NNE trending Lachlan anticline. Uplift of at least 130 m of the circa 124-kyr-old shoreline indicates uplift rates of at least 1 mm/yr at the axis of the anticline. Younger marine terraces have been uplifted more rapidly (about 3 mm/yr), and there is a progressive increase in rate to the youngest Holocene shoreline. Structural contours normalized to the circa 124 ka terrace illustrate the tilt on the flank of the Lachlan anticline and on a secondary structure, the Aurora Point fold. Seismic profiling has confirmed the presence of a reverse fault (the Lachlan fault) at the eastern margin of the Lachlan anticline. The Lachlan fault is inferred to be responsible for growth of the anticline and deformation of the marine terraces. The relationship of the reverse fault to the subduction thrust, which separates the two crustal plates at about 20 km depth below Mahia Peninsula and dips about 6°–12° to the NW, is uncertain. However, the late Pleistocene and Holocene growth of the Lachlan anticline indicates substantial coupling across the plate interface, deforming the inner part of the frontal wedge of the Australian plate.

Mahia Peninsula is a prominent coastal landmark in eastern North Island and is the closest point of land in the North Island to the Hikurangi Trough, where the Pacific plate plunges beneath the subduction complex at the eastern margin of the Australian plate. Uplifted Holocene marine deposits of both estuarine and open beach affinities are found in many parts of the peninsula and provide the basis for Holocene tectonic characterization. Estuarine deposits record the later part of the postglacial transgression that culminated about 6500 years B.P. in New Zealand. The deposits have been differentially uplifted since that time at a rate of 2.5 ± 0.3 mm/yr in the central north coast area, decreasing to 0.7 ± 0.2 mm/yr about 6 km to the west. The coastal plain is characterised in many places by a stepped sequence of emergent shore platforms overlain by fossiliferous beach deposits. Extensive radiocarbon dating of samples from beach deposits shows that terraces in widespread parts of the peninsula are of five distinct ages. Each of the terraces is inferred to have formed in conjunction with a large prehistoric earthquake because of the stepped terrace morphology, clustering of ages on each terrace, differential uplift of terraces across the peninsula, and historic coseismic uplift events in this tectonic setting. Paleoseismic events of Mw 7.5–8.0 are estimated to have occurred approximately 250, ∼1600, ∼1900, ∼3500, and ∼4500 years B.P. Uplift distribution of the Holocene and late Pleistocene marine terraces shows that the peninsula lies on the west (gentle) flank of the active Lachlan anticline, which is cored by a major west dipping reverse fault (the Lachlan fault). Holocene active deformation at Mahia Peninsula and other coastal areas of eastern North Island is a continuation of structures developed in Pleistocene time in the landward part of the subduction complex adjacent to the Hikurangi subduction zone.

The Hikurangi subduction margin, New Zealand, has not experienced any significant (>Mw 7.2) subduction interface earthquakes since historical records began ∼170 years ago. Geological data in parts of the North Island provide evidence for possible prehistoric great subduction earthquakes. Determining the seismogenic potential of the subduction interface, and possible resulting tsunami, is critical for estimating seismic hazard in the North Island of New Zealand. Despite the lack of confirmed historical interface events, recent geodetic and seismological results reveal that a large area of the interface is interseismically coupled, along which stress could be released in great earthquakes. We review existing geophysical and geological data in order to characterize the seismogenic zone of the Hikurangi subduction interface. Deep interseismic coupling of the southern portion of the Hikurangi interface is well defined by interpretation of GPS velocities, the locations of slow slip events, and the hypocenters of moderate to large historical earthquakes. Interseismic coupling is shallower on the northern and central portion of the Hikurangi subduction thrust. The spatial extent of the likely seismogenic zone at the Hikurangi margin cannot be easily explained by one or two simple parameters. Instead, a complex interplay between upper and lower plate structure, subducting sediment, thermal effects, regional tectonic stress regime, and fluid pressures probably controls the extent of the subduction thrust's seismogenic zone.

[1] Early Holocene transgressive marine deposits infilling the Pakarae River paleovalley are used to extend the paleoseismic history of the Pakarae River locality, East Coast, North Island, New Zealand, back in time prior to eustatic sea level stabilization at ∼7 calibrated (cal) ka B.P. Paleoenvironmental evolution of the Pakarae River paleovalley from 7 to 10 cal ka B.P is reconstructed using sedimentology and biostratigraphy. Two estuarine units display sudden vertical transitions to floodplain sediments implying significant marine regressions and estuary abandonment. These regressions are attributed to coseismic coastal uplift events at ∼9000 and ∼8500 cal years B.P. A third uplift between 8500 and ∼7350 cal years B.P. is inferred from a significant difference between the amount of sediment preserved and the predicted sediment thickness according to the eustatic sea level curve. This study demonstrates the utility of the analysis of transgressive deposits and their paleoenvironmental characteristics for neotectonic investigations on active coasts.

[1] We investigate the causes of regional uplift along the Hikurangi Margin (North Island, New Zealand). Late Quaternary rock uplift rates and postglacial fluvial incision rates delineate broad changes in rock uplift pattern along the margin. Comparison with the Neogene rock uplift rates, the erosion surface on basement, and the margin morphology suggests that these uplift patterns are long-lived and relate to deep-seated subduction processes. We use two-dimensional numerical (finite element) models to place constraints on margin geometry and uplift rates resulting from subduction processes inferred to drive regional uplift. The models indicate that subduction of relatively buoyant and thick Hikurangi Plateau crust is the most likely mechanism for low rates (

[1] Fluvial incision rates are increasingly being used as a proxy for rock uplift rates for inland settings. We test the validity of this approach by comparing incision rates and rock uplift rates along the Hikurangi Margin, New Zealand. Postglacial fluvial incision rates were calculated by measuring the difference in altitude between the Last Glacial Maximum (LGM) terrace tread (18 ± 2 calendar ka) and the present river bed for 10 major rivers (catchment areas ∼1400–5900 km2). Incision rates have thus been measured over a half glacial cycle. They also represent average rates over the last 18 kyr, during which time variations are likely due to processes such as knickpoint retreat. Rock uplift rates have been estimated from (1) altitude differences between the treads of pairs of fluvial fill terraces (∼55 and ∼18 calendar ka) assumed to have formed at the same altitude (i.e., rock uplift relative to the geoid), (2) present-day altitudes of Late Neogene paleoshoreline marker horizons (1.6–5.7 Ma) (i.e., rock uplift relative to sea level), and (3) present-day altitudes of Neogene mudstones for which maximum depth of burial has been calculated from porosity (i.e., rock uplift relative to sea level). These rock uplift rates overestimate tectonic uplift rates because they do not take into account isostatic compensation due to erosion. Mean incision rates range up to 11.7 mm yr−1, and mean rock uplift rates range up to 4, 1, and 0.9 mm yr−1 for the three methods, respectively. We have made some qualitative and semiquantitative observations about nontectonic controls on postglacial incision rates and find that localized influence only is exerted by rainfall, base level, and bedrock lithology, while negative or no correlations are shown with LGM fluvial fill thickness and stream power. Incision rates and uplift rates display a rough correlation, with incision rates typically 1.5–5 times uplift rates. This relation suggests that tectonic uplift is likely to exert an important control on fluvial incision in the Hikurangi Margin.

Well-preserved mid-late Holocene coral reefs are exposed in low coastal cliffs in the vicinity of the Madang lagoon on the north coast of Papua New Guinea. Results from U/Th and 14 C dating of corals, surveying, and field mapping indicate several major changes in relative sea level over this period. Specifically, there is evidence for a relative sea level fall of ≥4.5 m about 3000 calendar years B.P., followed by relative sea level rises of ∼1.5 m about 2400 calendar years B.P. and ≥0.5 m about 1200 calendar years B.P. and a subsequent relative sea level fall of ≥3 m some time in the past 1000 years. Since regional eustatic sea levels are believed to have been dropping gradually over this time frame, these observed changes in relative sea level are interpreted as reflecting alternating tectonic uplift and subsidence. Furthermore, the detailed structure and age relationships of the coral deposits indicate that both uplift and subsidence occurred rapidly, most probably as coseismic events with vertical displacements of 0.5 to 4.5 m. These events may be related to rupture on NW-SE trending reverse faults which have been mapped in the nearby Adelbert Range and possibly on NE trending cross faults which have been inferred from seismicity. This interpretation implies a much greater degree of tectonic instability and potential seismic hazard in the region than previously recognized, although the inferred coseismic vertical displacements are shown to be consistent with present-day local seismicity. In a broader context, the study illustrates how detailed analysis of vertical changes in coral reef structure and assemblages may be used as a sensitive indicator of changing relative sea level, capable of resolving century timescale events and reversals.

[1] The 1957 Gobi Altay M8.3 earthquake in southern Mongolia was associated with the simultaneous rupture of several faults, including the Gurvan Bulag reverse fault, which is located about 25 km south of the main strike-slip Bogd fault. Our study of paleoseismic excavations across the Gurvan Bulag fault suggests that the penultimate surface rupture occurred after 6.0 ka, most likely between 2.6 and 4.4 ka, and a possible earlier rupture occurred after 7.3 ka. Our interpretation of the stratigraphic relations in one of the exposures suggests that at least five earthquakes have generated surface rupture of the Gurvan Bulag fault since the abandonment of an ancient alluvial fan surface. Luminescence dating of sediment associated with this surface indicates that it formed either 26.6 ± 2.1 ka or 16.1 ± 2.0 ka. These data imply that the recurrence intervals for surface faulting on the Gurvan Bulag and Bogd faults are similar, on the order of several thousands of years, but that the penultimate surface ruptures of the two faults did not occur during the same earthquake.

Both the South Island of New Zealand and the Transverse Ranges of California have (1) a major right-slip fault marking the plate boundary, (2) a left-stepping bend in the fault resulting in ranges and basins bounded by active reverse faults that strike 45° counterclockwise to the regional strike of the main fault, (3) a set of right-slip faults on one side of the constraining bend, with most of the total displacement concentrated On the fault stepped farthest left from the continuation of the fault on the other side of the bend, and (4) intersections between the set of faults and the main fault within the bend not present or poorly defined. The faults with most of the displacement (Alpine-Wairau, San Andreas) may pre-date the constraining bend, The reverse fault province southeast of the Alpine fault is separated from that fault by the Southern Alps, which may be underlain by crust that is too hot and too weak to sustain major reverse faults. Average reverse fault recurrence intervals in New Zealand and California are 10³ to 2 × 104 years as compared to less than 10³ years for the right-slip master fault, Seismicity is high but patchy in the Nelson-Buller region of the northern South Island, low but patchy in the Transverse Ranges, and very low in Central Otago in the southern South Island, all reverse fault provinces. Seismicity patterns in both regions, poorly related to overall late Quaternary fault distribution, correspond largely to individual rupture events on portions of Quaternary faults, and the seismic inactivity of many of these faults may simply represent quiescence between seismic faulting with long recurrence intervals. The geology and Seismicity of the three reverse-fault provinces support the suggestion of C. R. Allen [1975] that late Quaternary fault history is a better long-term guide for assessment of large-magnitude earthquake hazard than instrumental and historical Seismicity for faults with long recurrence intervals.