Wairarapa Fault Research Articles

Magnetotelluric measurements in the Wellington region

Abstract Magnetotelluric measurements have been made at three sites on a northeast-southwest profile across the Wellington region. Apparent resistivity curves and induction arrows for the three sites have been calculated. Significant differences occur in the amplitude of the apparent resistivity curves between two sites to the east of the Wairarapa Fault and a site just inland from the west coast. Along with a reversal of short period real induction arrows, this suggests the existence of a major boundary in electrical resistivity structure, probably associated with either the Wellington or Wairarapa Faults. One-dimensional modelling of the apparent resistivity curves indicates that the two sites to the east of the Wairarapa Fault have a conducting layer at the surface. At the eastern site, the thickness of this layer agrees with the depth to the top of the subducted Pacific plate ascertained from seismic studies. In contrast, to the west of the fault, the surface layer has a resistivity a factor of 1000 times greater.

Velocity structure of the Wellington region, New Zealand, from local earthquake data and its implications for subduction tectonics

Summary. The technique of inversion of arrival time data from local earthquakes to determine hypocentres and velocity structure simultaneously has been applied to the Wellington region, New Zealand. The subducting Pacific plate lithosphere, which here has a thicker than normal and somewhat continent-like crust, lies at 20-40 km under the area. The overlying Indian plate is broken by several major faults, one of which separates it from the accretionary border. The data used were P and S arrival times for 93 local earthquakes, four explosions, and three regional events as recorded on the 12 station telemetered Wellington seismograph network. The inversion procedure involved multiple iterations with progressively decreasing damping and an approximate, but quite good, three-dimensional ray tracing scheme. A major problem is the choosing of appropriate velocity blocks and assessing the effect of possibly inaccurate choices on the derived velocities. Starting with simple models and gradually increasing the complexity leads to a good appreciation of the information in the data and its limitations. The final velocity model is two-dimensional (plus station terms), the block boundaries drawn on the basis of seismicity and results of preceding simpler models. Both P and S velocities were determined with good resolution. This model reduces the variance of the travel-time residuals by 60 per cent compared to the one-dimensional model now used for routine processing and greatly increases the self-consistency of hypocentre determinations. The results show that the crust east of the Wairarapa fault, part of the accretionary border, has relatively low velocity, especially for S-waves, and high heterogeneity compared to the crust of the Indian plate proper. The probability of anisotropy in the mid- and lower-crust of the Indian plate complicates the interpretation, especially of a velocity reversal for S-waves from 15 to 25km depth. The gently dipping band of relatively intense seismicity parallel to the plate interface is found to be a region of high seismic velocity (an average of 7.37 km s-1 for P) and is interpreted as the lower crust of the subducting Pacific plate, the upper crust having been accreted to the plate boundary further east. Events at greater depth are occurring in mantle material with a P-wave velocity of 8.69km s-l and are probably related to bending of the subducted lithosphere.

Uniformity of vertical faulting for the last 7000 years at Lake Poukawa, Hawke's Bay, New Zealand

Abstract Rates of fault movement are commonly assumed to be uniform. Analysis of the progressive increase in offset, by the Wairarapa Fault, of dated tephras within peat at Lake Poukawa, shows this to be true. A uniform vertical faulting rate of 0.2 mm/year is determined. Age of each offset of the fault is estimated giving a recurrence time of 800-900 years. The constant rate of faulting indicates that the next major earthquake affecting the fault at Poukawa will be in no more than 500 years time. This does not, however, exclude the possibility of other earthquakes in this region.

Gravity models of the Wairarapa region, New Zealand

Abstract Two-dimensional and three-dimensional gravity models of the Mesozoic basement topography in the Wairarapa show that the major feature is the Wairarapa Trough, aligned SSW—NNE through the area. The Trough has two arms, one north of Eketahuna and the other south of Carterton. All three features are fault-angle depressions, bounded to the west by the West Wairarapa, Wellington, and Dry River Faults, respectively, and reaching their greatest depths at these faults. A structural contour map of the Mesozoic surface in the Wairarapa shows that the basement is covered by more than 3000 m of Cenozoic material. The West Wairarapa Fault is shown to be reverse in the south, with a dip as low as 15°. The other faults are apparently near-vertical.

Some Problems of Zealand Geomorphology

Wellington Fault belongs to the major fault belt which extends NE-SW through New Zealand, Namely, Alpine Fault of South Island extends along the northwestern side of the Southern Alps and the fault branches at the north end of the South Island into the Wangamoa, Wairau, Awatere, Clarence, Kekerengu, Hop-kaikoura and Port Pass Faults. And so, the Weillington Fault is regarded as the continuation of the Awatere Fault, and the Wairarapa Fault is regarded as the continuation of the Kekerengu Fault. These faults are mostly active and even in the historic past of 120 years, numerous earthquakes took place associated with crustal deformations. Wellington Fault has a boldly defined, fairly straight course and can readily be traced north-east for 150 miles from Cook Strait through Wellington City and along the west of the Wellington Harbour and Hutt Valley. Wellington Fault is a clockwise transcurrent fault and upthrow of the crustal blocks bordering this fault has raised the hill country to the west of it, while downthrow and buckling of the land to the east has helped to form the Port Nicholson and Hutt Valley depressions. At the upper levels of the western side of Wellington City, there are remains of Peneplains which are called the 'K' Surface by C. A. Cotton. Perhaps the 'K' Surface can be classified into some surfaces.The periglacial deposits were found by C. A. Cotton and M. T. Te Punga for the first time at Wellington. The periglacial deposits of the South Island in New Zealand were found by Jane Soons, Mr. Brockie and others.Two periglacial deposits-stratified scree and stratified solifluction tongue-were classified by Jane M. Soons in South Island. But, there are many problems on the periglacial geomorphology in New Zealand.