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.
Strong motion earthquake records
