The structure of New Zealand from seismic evidence

Abstract

The New Zealand region is characterised by a crust 20--25 kms in thickness, and a sub-crust extending to at least 370 kms. These are separated by a transition layer extending from the base of the crust to a depth of about 100 kms. The crust is divided into blocks by steeply dipping faults. Some blocks are seismically active and others stable. The region is bounded by the Pacific Basin to the east, and the Tasman Basin to the west. Both of these areas have oceanic crusts about 5 kms thick. The sub-crust is traversed by a major wedge-shaped structure within which the deep focus seismicity is confined. This has been named the "Sub-Crustal Rift", and its activity is apparently an extension of that associated with the Kermadec Trench. It follows a roughly south-west course from the Bay of Plenty to Farewell Spit, when it turns abruptly southsouth-east, and finishes close to the Alpine Fault. Faults with both north-east and north-west trends are at present active. The most important of the northwest faults appears to run through Cook Strait. It is considered that the main crustal faults could have developed as the result of successive positioning above the Sub-Crustal Rift. Stable blocks have not been fractured in this way. There is some evidence for a difference in crustal structure on opposite sides of the Rift. The western portion is about 5 kms thicker, and contains a layer in which the P-velocities exceed 7 km/sec. No velocities above 61/2 km/sec have been observed in the eastern part. There is no evidence of arcuate structure. All the crustal and sub-crustal features appear to follow linear trends.

Gravity and crustal thickness in New Zealand

“…The reduction of this figure would demand either a reduction of the mean density of (he crust by at least 0-3 g/cm 3 , or a comparable reduction of the mean density within the upper mantle. However, the longitudinal wave velocities in the upper part of the crust as well as P n for the material immediately below the crust appear to be similar to those found in other continental areas of rhe world, though the velocity of 6-2 krnysec for Eiby's «intermediate" layer lying between 9 and 18 km in depth is less than the minimum of 6-5 krnysec generally found elsewhere; but to ascribe the source of the gravity deficiency to this layer would imply a reduction of its density by at least 0-55 g/cm 3 • For New Zealand earthquakes, Eiby (1958) has reported a wide variation in the Pn velocity above and below the «normal" 8 krnysec, but there is no suggestion of consistently low values.…”

Seismic wave velocities in the uppermost mantle beneath New Zealand

“…He observed that the Pn velocity is higher (8' 3 krn/s) along paths to the south-west of the epicentre of the 1966 Gisborne earthquake than it is along paths to the west (7'7km/s) and north-west (7·8km/s). In view of the average velocity of 8· 1 km/s found previously (Bullen 1939;Eiby 1958;Hamilton 1966), he interprets this difference as being possibly due to anisotropy.…”

Geological structure and volcanism of the Taupo-Tarawera district

“…The volcanic belt closely parallels the "sub-crustal rift" defined by Eiby (1959) on the basis of earthquake epicentres, but there is a marked absence of focal depths less than 100 km (pers. comm., Mr G. A.…”