Dynamic seismic hazard model for New Zealand

Technical Abstract

A new dynamic seismic hazard model for New Zealand is created to add the effects of short-term hazard fluctuations due to earthquake triggering to the current Poissonian probability from the national probabilistic seismic hazard model. Prospective foreshock probability decay is modelled as a function of origin time and epicentral distance from the potential foreshock, and l, he magnitude difference between foreshock-mainshock pairs. Across the country, the distribution of mainshock magnitudes in the the set of foreshock-mainshock pairs follows the Gutenberg-Richter relationship with a b-value of 1.3. In the Taupo Volcanic Zone (TVZ), a region characterised by crustal extension and volcanism, the foreshock probability decreases with approximately 1/t2.4*0.5 and 1/7.3.62/0.03 . Elsewhere in the country, the decay with time and distance is smaller, at 1/t 1.620.2 and 1/7.2.710.2 . This is consistent with the higher attenuation of seismic energy likely to be experienced iii an area of active volcanoes. Combining this information with a generic aftershock model, fluctuating daily hazard maps for New Zealand are calculated, showing the regional probability distribution for peak ground accelerations of 0.05% or more being observed.

A methodology has been developed to test these ground motion forecasts against 40 years of strong motion observations in New Zealand. The Poissonian model underestimates the hazard at 0.05g level for the time period and locations in the dataset by a factor of two. The new dynamic seismic hazard model provides a closer match to the strong motion records, but with some overestimation (factor of three) of hazard on days of high probability and underestimation (factor of six) on days of low probability forecasts.