A new seismic hazard model for new Zealand

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 modem 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.