A Microearthquake Study in a Subduction Zone: Southeast Wellington Province, New Zealand

Abstract

The seismicity, structure and tectonics of part of the North Island plate boundary have been studied by means of a microearthquake traverse in the southeastern part of Wellington province. The short-term microseismicity observed in this survey is similar to the long-term macroseismicity. About 200 microearthquakes have been located in a 4-week survey. Microearthquakes occur mostly within the depth range 20-40 km, defining a band of activity which is fairly continuous and uniform along the strike direction of the subducted Pacific plate. A break in seismic activity is observed below 40 km depth. In vertical sections along the direction of the dip of the subducted plate, on the other hand, systematic differences in microearthquake activity are observed between the supposedly locked and unlocked regions of the subducted plate. The sections show further how the foci deepen progressively towards the northwest. For a few subcrustal earthquakes (deeper than 40 km), permanent as well as temporary networks record strong phases between P and S, and these are identified as having been converted from S to P or P to S at the Mohorovičić discontinuities of the Indian and Pacific plates. The Pn arrival-times from 7 shallow earthquakes have been used to determine the Pn.velocity in the uppermost mantle. The values are 8.64 (+0.13) km/s in the coastal part of the region and 8.90 (±0.13) km/s in the Wairarapa region. A value of 8.22 (±0.15) km/s between Wellington and the Wairarapa region is interpreted as due to differences in velocity structure above the refractor. Seismic velocity-ratio (γ) is studied by means of the Wadati plot. 49 earthquakes of depth 2-40 km, each with at least four P and S arrivals are used in this analysis. The area as a whole is found to be characterised by a γ value of 1.74-1.76 for earthquakes of depth 12-40 km, except in a 50 km long section near Wellington, where γ is low at 1.60. This low γ has been attributed to the fault zones in the region. A small change of γ is observed between the upper crust (≤5 km) and lower crust (≥12 km), but there is no change of γ between the lower crust and uppermost mantle. Focal mechanisms of 115 well-located microearthquakes are examined in relation to the structure and tectonics of the region. The mechanisms are related to the plate interface geometry in a manner consistent with that found for other regions of plate convergence: microearthquakes in the overriding plate are associated with thrustfault mechanisms, while those in the subducted plate exhibit normal faulting. The stress pattern is also consistent with the tectonics of the region: the T-axes of the normal-fault mechanisms are parallel to the dip of the subducted plate, whereas the P-axes of the thrust-fault mechanisms are nearly in the E-W direction. This is consistent with the suggestion that the plates are locked in the coastal part of the region. In addition, small but systematic differences in the thickness and dip of the Benioff zone are observed: in the southwest the zone is about 5 km thinner and 6° less steep than in the northeast. 0n the landward side there is a bend in the Benioff zone at about 25 km depth; the dip of the zone sharply increases below this depth, where it enters the region of negative gravity anomaly.