Shallow subduction tectonics in the Raukumara peninsula, New Zealand, as illuminated by earthquake focal mechanisms

The Raukumara Peninsula affords an excellent opportunity to study the subduction process, as subduction of the buoyant Hikurangi Plateau on the Pacific plate has resulted in exposure of the forearc above the shallow part of the subduction thrust. Here we report on the focal mechanisms of 117 earthquakes of M-L 2.4-4.9 and shallower than 80 km, recorded during a 5-month deployment of 36 portable seismographs on the peninsula. Mechanisms have been constrained using both first motion polarity data and amplitudes of seismogram envelopes. Downdip tensional strain predominates in the subducted plate, with Taxes of events in both the upper and lower planes of the dipping seismic zone generally paralleling the local dip of the zone. Trenchward extensional strain is seen in the uppermost part of the overlying Australian plate, in line with geodetic and geological results. This can be related to extension and gravity sliding of surficial rocks due to uplift of the Raukumara range resulting from underplating of subducted sediment. There is a marked change in earthquake mechanisms along strike in the lower part of the overlying plate and at the plate interface. A concentration of low-angle thrust events at the plate interface in the northeastern half of the peninsula suggests that the plate interface is less coupled there than to the southwest. This along-strike change in plate coupling corresponds closely to a change in the crustal structure of the overlying plate and also to a change in tectonic rotation domain determined paleomagnetically.