Lateral segmentation of the subducted plate at the Hikurangi Margin, New Zealand: Seismological evidence

Abstract

Seismological evidence for lateral segmentation of the subducted Pacific plate at the Hikurangi Margin, New Zealand, includes: 1. (1) the distribution of seismicity and stresses in the dipping seismic zone; 2. (2) the extent of rupturing of large historical interplate earthquakes and the patterns of seismicity accompanying these events; 3. (3) the distribution of recent shallow seismicity; and 4. (4) the stress pattern at shallow depth. Six down-dip tears in the subducted plate are suggested by the seismological data. These are located at the southern end of the Kermadec Trench, northern Hawke's Bay, southern Hawke's Bay (the best-de-fined tear), the Cook Strait region, central Marlborough and the Kaikoura region. Such a model of down-dip fractured regions in the subducted plate proves useful in interpreting a significant episode of seismic strain release which occurred in the central part of the Hikurangi Margin in 1980. Segmentation is manifest by “tearing” earthquakes (i.e. strike-slip faulting in the plane of the subducted plate with the fault plane oriented down the dip of the plate) and these must be distinguished from interplate “unlocking” earthquakes (thrust faulting events with a fault plane coinciding with the plate interface) and “bending” earthquakes (normal faulting events with a T-axis directed down the dip of the subducted plate) in an interpretation of the present tectonics of the margin.