A team of earthquake geologists, seismologists, and engineering seismologists 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 applications. 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 distributed 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 consistency 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 seismichazard model.

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The Canterbury earthquake sequence triggered thousands of rockfalls in the Port Hills of Christchurch, New Zealand, with over 6,000 falling on 22 February 2011. Several hundred families were evacuated after about 200 homes were hit. We characterized the rockfalls by boulder-size distribution, runout distance, source-area dimensions, and boulder-production rates over a range of triggering peak ground accelerations. Using these characteristics, a time-varying seismic hazard model for Canterbury, and estimates of residential occupancy rates and resident vulnerability, we estimated annual individual fatality risk from rockfall in the Port Hills. The results demonstrate the Port Hills rockfall risk is time-variable, decreasing as the seismic hazard decreases following the main earthquakes in February and June 2011. This presents a real challenge for formulating robust land-use and reconstruction policy in the Port Hills.

A moment magnitude (M w) 6.2 earthquake struck beneath the outer suburbs of Christchurch, New Zealand's second largest city, on 22 February 2011 local time. The Christchurch earthquake was the deadliest in New Zealand since the 1931 M w 7.8 Hawkes Bay earthquake and the most expensive in New Zealand's recorded history. The effects of the earthquake on the region's population and infrastructure were severe including 181 fatalities, widespread building damage, liquefaction and landslides. The Christchurch earthquake was an aftershock of the M w 7.1 Darfield Earthquake of September 2010, occurring towards the eastern edge of the aftershock zone. This was a low recurrence earthquake for New Zealand and occurred on a fault unrecognised prior to the Darfield event. Geodetic and seismological source models show that oblique-reverse slip occurred along a northeastÁsouthwest-striking fault dipping southeast at c. 698, with maximum slip at 3Á4 km depth. Ground motions during the earthquake were unusually large at near-source distances for an earthquake of its size, registering up to 2.2 g (vertical) and 1.7 g (horizontal) near the epicentre and up to 0.8 g (vertical) and 0.7 g (horizontal) in the city centre. Acceleration response spectra exceeded 2500 yr building design codes and estimates based on standard New Zealand models. The earthquake was associated with high apparent stress indicative of a strong fault. Furthermore, rupture in an updip direction towards Christchurch likely led to strong directivity effects in the city. Site effects including long period amplification and near-surface effects also contributed to the severity of ground motions.

Attenuation relations are presented for peak ground accelerations (pga) and 5% damped acceleration response spectra in New Zealand earthquakes. Expressions are given for both the larger and the geometric mean of two randomly-oriented but orthogonal horizontal components of motion. The relations take account of the different tectonic types of earthquakes in New Zealand, i.e., crustal, subduction interface and dipping slab, and of the different source mechanisms for crustal earthquakes. They also model the faster attenuation of high-frequency earthquake ground motions in the volcanic region than elsewhere. Both the crustal and subduction zone attenuation expressions have been obtained by modifying overseas models for each of these tectonic environments to better match New Zealand data, and to cover site classes that relate directly to those used for seismic design in New Zealand codes. The study used all available data from the New Zealand strong-motion earthquake accelerograph network up to the end of 1995 that satisfied various selection criteria, supplemented by selected data from digital seismographs. The seismographs provided additional records from rock sites, and of motions involving propagation paths through the volcanic region, classes of data that are sparse in records produced by the accelerograph network. The New Zealand strong-motion dataset lacks records in the nearsource region, with only one record from a distance of less than 10 km from the source, and at magnitudes greater than Mw 7.23. The New Zealand data used in the regression analyses ranged in source distance from 6 km to 400 km (the selected cutoff) and in moment magnitude from 5.08 to 7.23 for pga, with the maximum magnitude reducing to 7.09 for response spectra data. The required near-source constraint has been obtained by supplementing the New Zealand dataset with overseas peak ground acceleration data (but not response spectra) recorded at distances less than 10 km from the source. Further near-source constraints were obtained from the overseas attenuation models, in terms of relationships that had to be maintained between various coefficients that control the estimated motions at short distances. Other coefficients were fitted from regression analyses to better match the New Zealand data. The need for different treatment of crustal and subduction zone earthquakes is most apparent when the effects or source mechanism are taken into account. For crustal earthquakes, reverse mechanism events produce the strongest motions, followed by strike-slip and normal events. For subduction zone events, the reverse mechanism interface events have the lowest motions, at least in the period range up to about ls, while the slab events, usually with normal mechanisms, are generally strongest. The attenuation relations presented in this paper have been used in many hazard studies in New Zealand over the last five years. In particular, they have been used in the derivation of the elastic site spectra in the new Standard for earthquake loads in New Zealand, NZS 1170.5:2004.