Consideration of near-fault effects in New Zealand seismic hazard analysis and design spectra
DOI:
https://doi.org/10.5459/bnzsee.1743Abstract
This paper examines the manner in which near-fault ground-motion phenomena are considered in the probabilistic seismic hazard analysis underpinning the 2022 New Zealand National Seismic Hazard Model (NZ NSHM), and its subsequent codification in the draft Technical Specification TS1170.5:2004. Directivity is already implicitly considered in the 2022 NSHM, and thus the ‘baseline’ draft TS1170.5 spectra, before any additional near-fault factor is applied. Specific studies in NZ and California suggest that explicit modelling of directivity could result in increases of up to 15-20% for 2475-year return period SA(T = 3:0s) values for ‘directivity-prone locations’, but are more likely to be on the order of 10% when a weighted average of multiple directivity models is considered. In contrast, the NZS1170.5:2004 near-fault factor, N(T;D), results in a 36% increase for small source-to-site distances. Hence, either: (1) the NZS1170.5:2004 near-fault factor should be removed so that directivity is implicitly considered in the hazard, which is consistent with other international codes, and also consistent with the implicit treatment of deep sedimentary basin effects in the 2022 NSHM, or (2) a parametric revision of the near-fault factor is needed that results in approximately a four-fold reduction in its size. Obtaining a more precise quantification of directivity effects is complicated by multi-segment ruptures in contemporary seismic source models, and the large uncertainty in predicted directivity modification factors from the alternative existing models.
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