Open AccessMicroseismicity and tectonics at the Indian/Pacific plate boundary: south‐east Wellington province, New Zealand
Author(s) -
Kayal J. R.
Publication year - 1984
Publication title -
geophysical journal of the royal astronomical society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.302
H-Index - 168
eISSN - 1365-246X
pISSN - 0016-8009
DOI - 10.1111/j.1365-246x.1984.tb01950.x
Subject(s) - microearthquake , plate tectonics , induced seismicity , geology , traverse , seismology , tectonics , pacific plate , boundary (topology) , oceanography , geodesy , subduction , mathematical analysis , mathematics
Summary. The seismicity, structure and tectonics of part of the North Island plate boundary have been studied by means of a microearthquake traverse in the south‐eastern 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 four‐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. However, in vertical sections along the direction of the dip of the subducted plate 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 north‐west. 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 thrust‐fault 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 SW the zone is about 5 km thinner and 6° less steep than in the NE. On the landward side there is a bend in the Benioff zone at 25‐30 km depth; the dip of the zone sharply increases below this depth, where it enters the region of negative gravity anomaly.