Wairau Fault
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| Wairau Fault | |
|---|---|
 Map of the Marlborough Fault System with Wairau Fault in light purple | |
| Etymology | Wairau River |
| Country | New Zealand |
| Region | Marlborough |
| Characteristics | |
| Range | up to 7.7 MW[1] |
| Length | 200 km (120 mi)[1] |
| Displacement | 3.5 mm (0.14 in)/year[2] |
| Tectonics | |
| Plate | Indo-Australian, Pacific |
| Status | Active |
| Earthquakes | prehistoric |
| Type | Strike-slip fault |
| Movement | Dextral/convergent, east side up |
| Age | Miocene-Holocene |
| Orogeny | Kaikoura |
| New Zealand geology database (includes faults) | |
The Wairau Fault is an active dextral (right lateral) strike-slip fault in the northeastern part of South Island, New Zealand. It forms part of the Marlborough fault system, which accommodates the transfer of displacement along the oblique convergent boundary between the Indo-Australian plate and Pacific plate, from the transform Alpine Fault to the Hikurangi Trough subduction zone.[3]
Depending on the precise definition used, the Wairau Fault runs either from southern or northern end of 'The Bends' region. In the former case it is regarded as the Wairau segment of the Alpine Fault which means the Alpine Fault is about 800 km (500 mi) long.[4] In the latter case it is regarded as a separate fault and runs about 200 km (120 mi)[1] from near Lake Rotoiti to the edge of the continental shelf in Cook Strait near Cloudy Bay in the east.[1] To the west, the fault is a single strand but near Wairau Valley township, the fault splits into two strands. These two strands continue to within about 15 kilometres (9.3 mi) of the coast near Renwick. Further to the east only the southernmost strand can be detected[5] intermittently by lidar[2] and with offshore mapping.[2] It takes its name from the Wairau River, which follows the fault trace for most of its length on land.
Geology
The fault follows a valley where Pleistocene and Holocene deposits have been laid down over bedrock during the last two million years and where the river often obscures recent fault traces.[2] The underlying bedrock which is very rarely exposed along the fault line itself is to the fault's north the Dun Mountain, Maitai and Caples terranest and Triassic and Cretaceous greywacke otherwise.[2] The western fault zone width is about 300 m (980 ft) and contains several fault scarps up to 5 m (16 ft) high.[2]
Relationship to Alpine Fault
As said above, some have regarded it as a segment of the Alpine Fault. This does not concord with the rest of the Alpine Fault having a fairly predictable relatively short recurrence interval for major earthquakes of 291 ± 21 years[6] with good evidence for multi-segment rupture on many of these events over the last 2000 years.[7] The last rupture that may also have involved the Alpine Fault is about 2000 years ago which does not apply to the other active segments of the Alpine Fault (see timeline).
It has been stated that "Co-rupture of the Wairau and Alpine faults during great earthquakes occurs rarely (if at all) on millennial or longer timescales."[1]