New Zealand seismicity: Patterns revealed by the upgraded National Seismograph Network

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Abstract
The upgrade of the New Zealand National Seismograph Network in the late 1980s has enabled more accurate earthquake locations to be determined. The catalogue data for events occurring from January 1990 until the end of February 1993 show some new patterns that have not been identified in previous observation periods, and also confirm the persistence of some phenomena observed previously, such as the aseismic corridor through the Nelson region. The deep seismicity data show spatial patterns remarkably similar to those for higher magnitude events recognised by Reyners in 1989. The Hikurangi Benioff zone is marked by intense seismic activity at depths between 150 and 200 km beneath the Central Volcanic Region; it has a sharp discontinuity beneath northwest Nelson and it extends as far southwest as Westport. The Fiordland Benioff zone is distinctly more seismically active in its northern block, and activity is noticeably concentrated in a zone to the west of Lake Te Anau. Shallow earthquakes (depth <15 km) for the period 1990 to February 1993 outline the active eastern boundary of the Central Volcanic Region and an east‐west band running from Mt Ruapehu to Mt Taranaki. Earthquakes in this latter group also extend to deep crustal levels and they, and some recently recognised faults in this area, are probably related to a crustal discontinuity in this region. The Cape Egmont Fault Zone has been particularly active during this observation period. In the South Island, the 1990 Tennyson earthquake appears to have triggered activity along the Awatere Fault. The Alpine Fault is seismically quiet in the section from Harihari to Jackson Bay. A band of earthquakes lying to the east of the fault north of Harihari may represent activity associated with the Alpine Fault at depth although this cannot be confirmed by existing data. At the southern end of the Alpine Fault, two subparallel lineaments appear to form the boundaries of the western end of the Otago Range and Basin Province, but they are not associated with any known tectonic feature. In the Wellington region, the earthquakes shallower than 15 km are concentrated in the area between the Wellington and Wairarapa Faults. A group of earthquakes near Carterton possibly represents the interaction between two fault systems of different trends in that area. A very sharp northeast‐southwest trending boundary between the very active shallow seismicity of the Wanganui Basin and the aseismic Marlborough Sounds is probably linked to subduction. Earthquakes in the Benioff zone underlying the Wellington region show a gap beneath the Wairarapa Basin.