Microseismicity in the Tararua—Wairarapa area: depth-varying stresses and shallow seismicity in the southern North Island, New Zealand

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Abstract
Results from previously unpublished microearthquake field studies in the Tararua-Wairarapa area during 1971–72 demonstrate important differences between crustal seismicity shallower than 20 km and intense, regionally extensive, underlying seismicity in the 20–40 km depth range. As earlier observed in the Marlborough region, and more recently in the southernmost North Island, composite focal mechanisms in the Tararua-Wairarapa area indicate markedly different stress orientations above and below 20 km. Compressional transcurrerit faulting in the upper crust contrasts with extensional normal faulting at depths of 20–40 km. The layer 20–40 km deep appears to be far more seismically active than the upper crust and is interpreted to mark the uppermost part of the subducted Pacific plate. Accordingly, the data here provide valuable control for locating the plate interface, for modelling elastic deformation at a convergent plate boundary, and for assessing fault activity and shallow seismicity in the southern North Island.