Seismic measurements in Wellington harbour

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Abstract
Seismic surveys in Wellington Harbour (Port Nicholson) show that the greywacke basement under the harbour is an undulating erosional surface which has been depressed to a depth of about 400 m in the northwestern part. For most of the harbour the basement lies at a depth of 200 to 300 m. The depression is likely to be a fault-angle depression caused by tilting in a westerly direction. No signs of faults or fault-block structures were found in the basement under the harbour. A ridge of basement rocks with high seismic velocity crosses the harbour and crops out at Somes Island and Miramar Peninsula. It is inferred that this ridge and other morphological features in the Wellington area aligned approximately north-south are not of tectonic origin but are erosional features. The seismic velocity structure of rocks in the direct vicinity of the transcurrent Wellington Fault, aligned along the northwestern side of the harbour, is not known in detail. The crush zone of the Wellington Fault is, however, likely to be about 500 to 1000 m wide under the harbour, and this zone has a subsurface slope about 30° to 50°. On land, near Thorndon and at Petone, the crush zone is about 200 to 400 m wide. The inferred widening of the fault zone under the harbour is probably a consequence of the curving of the Wellington Fault in this region.

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