Models of the upper mantle beneath the central North Island, New Zealand, from speeds and anisotropy of subhorizontal P waves (Pn)

Abstract

The central North Island of New Zealand shows significant variations in Pn wave speeds over small distances, ranging from 8.5 ± 0.2 km/s to 7.4 ± 0.1 km/s over a distance of 150 km. A combination of national network seismometers, local volcanic seismic monitoring networks, and temporary deployments are used to collect arrival times from local events, during the period of 1990–2006. The data set consists of approximately 11,200 Pn observations from 3000 local earthquakes at 91 seismograph sites. We have created a method that allows us to model the predominant wavelength features of P wave speeds in the uppermost mantle, as well as estimating values of mantle anisotropy and irregularities in the crust beneath stations, using least squares collocation. The resulting model shows distinct variations in uppermost mantle Pn velocities. Velocities of less than 7.5 km/s are found beneath the back‐arc extension region of the Central Volcanic Region, and under the Taranaki Volcanic Region, indicating the presence of water and partial melt. The region to the east shows extremely high velocities of 8.3–8.5 km/s, where the P waves are traveling within the subducting Pacific slab. Slightly lower than normal mantle velocities of 7.8–8.1 km/s are found in the western North Island, suggesting a soft mantle. Pn anisotropy estimates throughout the North Island show predominately trench‐parallel fast directions, ceasing to nulls in the west. Anisotropy measurements indicate the strain history of the mantle. Null anisotropy measurements suggest an undisturbed mantle, suggesting that mantle beneath the western North Island is young.