[1] The Hikurangi subduction margin, New Zealand, has not experienced any significant (>M w 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.Components: 20,324 words, 15 figures.

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The Canterbury region of the South Island of New Zealand straddles a wide zone of active earth deformation associated with the oblique continent-continent collision between the Australian and Pacific tectonic plates east of the Alpine fault. The associated ongoing crustal strain is documented by the shallow earthquake activity (at depths of <40 km) and surface deformation expressed by active faulting, folding and ongoing geodetic strain. The level of earth deformation activity (and consequent earthquake hazard) decreases from the northwest to the southeast across the region. Deeper-level subduction related earthquake events are confined to the northernmost parts of the region, beneath Marlborough. To describe the geological setting and seismological activity in the region we have sub-divided the Canterbury region into eight domains that are defined on the basis of structural styles of deformation. These eight domains provide an appropriate geological and seismological context on which seismic hazard assessment can be based. A further, ninth source domain is defined to include the Alpine fault, but lies outside the region. About 90 major active earthquake source faults within and surrounding the Canterbury region are characterised in terms of their type (sense of slip), geometry (fault dimensions and attitude) and activity (slip rates, single event displacements, recurrence intervals, and timing of last rupture). In the more active, northern part of the region strike-slip and oblique strike-slip faults predominate, and recurrence intervals range from 81 to >5,000 years. In the central and southern parts of the region oblique-reverse and reverse/thrust faults predominate, and recurrence intervals typically range from -2,500 to >20,000 years. In this study we also review information on significant historical earthquakes that have impacted on the region (e,g. Christchurch earthquakes 1869 and 1870; North Canterbury 1888; Cheviot 1902; Motunau 1922; Buller 1929; Arthurs Pass 1929 and 1994; and others), and the record of instrumental seismicity. In addition, data from available paleoseismic studies within the region are included; and we also evaluate large potential earthquake sources outside the Canterbury region that are likely to produce significant shaking within the region. The most important of these is the Alpine fault, which we include as a separate source domain in this study. The integrated geological and seismological data base presented in this paper provide the foundation for the probabilistic seismic hazard assessment for the Canterbury region, and this is presented in a following companion paper in this Bulletin (Stirling et al. this volume).

Nearly 200 historical accounts have been examined and analysed in order to determine the effects of the magnitude 8+ 1855 Wairarapa, New Zealand, earthquake. The documents examined include contemporary diaries, letters and journals, newspaper reports and articles, archives, memoranda and reports of the Wellington Provincial Government as well as later reminiscences, extracts from published scientific papers, books and other articles. Other than the published accounts of Sir Charles Lyell, who, in 1856, first recognised the importance of the earthquake as causing the greatest deformation and surface fault rupture then known, there has been no comprehensive account of the effects of the earthquake in the scientific literature until now. Much or the data is presented with extensive quotations from the source material, especially where conflicting accounts on important aspects have been found. All material is analysed with an understanding of the geographical, social and political conditions at the time. The reliability of the material is taken account of so that first-hand accounts, that have been recorded no more than several years after the earthquake, and in which there are no obvious inconsistencies or confusion with other earthquakes, are valued most highly. Using the historical accounts as the primary source of data, but also taking into account the results of more recent geological, geomorphological and seismological investigations of the deformation, many aspects of the earthquake are discussed in detail. These are mainshock magnitude and epicentre; felt intensity distribution: descriptive account of the effects of the mainshock on people (including casualties) and man-made structures by location throughout New Zealand (including a resume of contemporary building techniques): effects on the environment from strong shaking such as fissuring, liquefaction, spreading, subsidence and landslides, and from tectonically produced uplift, subsidence and faulting; biological effects; tsunami and seiche; aftershock occurrence and social response and recovery.

The 1968 Inangahua, New Zealand, earthquake occurred in the West Coast Basin and Range Province, northwest of the main plate boundary zone in northern South Island. At M S 7.4, it is not the largest known earthquake in the province, but it has been the subject of thorough seismological, geological, and geodetic documentation. Re interpretation of past observations and more recent data, in the light of new structural and tectonic theories, has produced a new source model for the earthquake. The data suggest that at least 4 m of reverse slip occurred on a fault plane dipping 45° to the northwest beneath the northern part of the Grey-Inangahua Depression, an area previously inferred to be on the footwall of major reverse faults bounding the ranges on either side of the depression. The seismogenic fault may have propagated north and south across older geological structures in recent times. Faulting within basement is occurring on pre-existing faults and is accommodating some of the compressional component of oblique relative motion across the plate boundary in northern South Island. Discontinuous coseismic fault ruptures are mainly interpreted as secondary features formed in response to widespread shortening within the sedimentary cover (flexural slip folding) imparted by the deeper primary faulting. Ongoing uplift across a late Quaternary fault trace at Manuka Flat possibly represents postseismic slip over the upper part or northern end of the 1968 rupture plane. Although the Inangahua earthquake source mechanism is consistent with the regional late Quaternary tectonic pattern, the regional rate of seismicity is high (perhaps representing clustering) in comparison with average fault slip rates and recurrence intervals in the province.