A seismic velocity profile across the central South Island, New Zealand, from explosion data

Abstract

Data from a seismic experiment across the central South Island, New Zealand, were used to model the crustal structure of the Australia‐Pacific plate continent‐continent boundary to a depth of c. 40 km. Large explosions were detonated off each coast and in Lake Tekapo and recorded on 19 seismographs deployed in a line from Timaru to just south of Fox Glacier, and on the Pukaki Seismograph Network. In the upper crust, a clear difference was found between the seismic velocities in Permian and younger rocks, in agreement with findings from other studies. Within the Permian schists and greywackes, the P‐wave velocity is 5.8 km/s to 2.7 km depth and 6.0 km/s below, contrasting with 5.4 km/s in the Triassic greywackes which form the basement near Lakes Pukaki and Tekapo in the central part of the survey area. Strong second arrivals from a layer 24 km deep at the east coast, and dipping inland at 3.4°, were seen from both offshore shots. The true velocity of the arrivals was determined to be 7.2 km/s. Arrivals from a deeper refractor, with an apparent velocity of 8.3 km/s, were only evident on the West Coast shot record. If this refractor is assumed to have a true velocity of 8.0 km/s, a value previously determined from Pn arrivals from earthquakes, the refractor dip would be 2.9°, consistent with the overlying one. The thickness of the 7.2 km/s layer would be 11 km, giving an effective crustal thickness of 35 km at the east coast. The intercept time of the later arrivals from the West Coast shot and the delay of the first arrivals on the West Coast from the Timaru shot indicate that the average velocity above the refractors west of, and under, the Southern Alps is lower than in the east. The results can be explained by ray‐tracing models with a zone immediately east of the Alpine Fault c. 10 km wide and at least 10 km and possibly 30 km deep in which the velocities are 10% lower than in the adjacent rocks. The structural interpretation of the model, with a mid‐crustal detachment of the Pacific plate crust, has marked similarities to models of the European Alps, suggesting analogous roles for the Alpine Fault and the Insubric Line.

Keywords:

• seismic velocities
• South Island
• tectonic model
• Alpine Fault