Weak motion attenuation of peak ground acceleration in the North Island, New Zealand
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Attenuation relations using weak ground motion recordings have been determined using data from the New Zealand National Seismograph Network and several temporary seismograph deployments. Models have been developed for earthquake sources in four regions: the Eastern North Island deep and shallow regions and the Central North Island (CNI) deep and shallow regions. Deep events were those with hypocenters below 33 km. Regression coefficients have been determined using the attenuation models of Joyner and Boore (1981) and Molas and Yamazaki (1995). The anelastic attenuation rates in the Eastern North Island expressions are comparable to that of Joyner and Boore (1981), suggesting that weak motion attenuation can be used to estimate variations in strong motion attenuation. However, the absolute level of the strong-motion attenuation curves greatly differs from those of the weak-motion. The anelastic attenuation rate for the shallow CNI is of the order of two to three times that observed for the Eastern North Island. The lowest attenuation rate was found for events within the deep CNI, whose ray paths did not cross the shallow Central North Island region. This is consistent with a low rate of attenuation in the subducting Pacific plate. Azimuthal dependence of PGA is evident within each of the regions. Within the Eastern North Island, the attenuation rate is lowest in the direction of 35-55° from North, which is roughly along the strike of the subducting Pacific plate. A similar azimuthal dependence was also noted within the deep CNI region, while a slightly different minimum direction (5°) was determined for the shallow CNI region.