Attenuation of weak ground motions

Technical Abstract

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: Eastern North Island Shallow, Eastern North Island Deep, Central Volcanic Region Shallow and the Central Volcanic Region Deep. Earthquakes were classified as deep if they occurred below 33km. equations have been fit to the data for each of the regions using two different attenuation models.

Attenuation rates were found to be greatest in the shallow Central Volcanic Region. Eastern North Island Deep attenuation rates were similar to Eastern North Island Shallow rates. The lowest attenuation rate was found for earthquakes within the Central Volcanic Deep Region that were recorded in the Eastern North Island. This is consistent with a low rate of attenuation in the subducting Pacific plate.

Although the measured attenuation rate is comparable to some strong-motion relations, the absolute level of strong-motion attenuation curves greatly differs from those of the weak-motion, particularly at low magnitudes. This may be due to a number of factors. Unlike models within the literature which used near-field strong-motion data, far-field weak-motion data has been used for this study. A second possible explanation for the difference in absolute level is a change in frequency characteristics between large and small earthquakes. Most importantly, differences may results from the use of different magnitude scales in the weak-motion and strong-motion models. The magnitude scale difference appears to be the primary reason for the difference in absolute level between the weak-motion and strong-motion relations. This implies that while some terms in the derived attenuation equations are different for strong-motion models, the attenuation rate term should be comparable.

A variation of peak ground acceleration with changes in direction of travel of the seismic waves is evident within each of the regions. Within the Eastern North Island, the attenuation rate is lowest in the direction of 30-60degrees from North, which is roughly along the strike of the subducting Pacific plate. A similar azimuthal dependence was also noted within the deep Central Volcanic Region, while the direction of lowest attenuation for waves originating in the shallow CVR is 5degrees