| Hikurangi Margin | |
| Other Name: | Hikurangi Subduction Zone |
| Named For: | Hikurangi |
| Pushpin Relief: | 1 |
| Country: | New Zealand |
| Region: | North Island |
| Length: | 300km (200miles) |
| Displacement: | 6cm (02inches)/yr |
| Plate: | Indo-Australian |
| Status: | Active |
| Type: | Subduction |
| Age: | Miocene-Holocene |
| Embed: | New Zealand geology database (includes faults) |
The Hikurangi Margin (also known as the Hikurangi Subduction Zone) is New Zealand's largest subduction zone and fault.[1]
The Hikurangi Subduction Zone is an active subduction zone extending off the east coast of New Zealand's North Island, where the Pacific and Australian plates collide.[2] The subduction zone where the Pacific Plate goes under the Kermadec Plate offshore of Gisborne accommodates approximately 6cm/year of plate movement while off the Wairarapa shore this decreases to perhaps as low as 2cm/year.[1] It is the southern portion of the Tonga–Kermadec–Hikurangi subduction zone and its main feature is the Hikurangi Trough. The tectonics of this area can be most easily resolved by postulating between the Havre Trough to the east of the South Kermadec Ridge Seamounts, the Whakatane Graben and the Taupo Volcanic Zone on the North Island of New Zealand there is a continuation of the Tonga micro-plate into the Kermadec microplate which probably extends to Cook Strait.[3] The on land active fault systems would be consistent with the Kermadec Plate's unclear south western boundary being the North Island Fault System. The Kermadec Plate - Pacific Plate eastern boundary is the Hikurangi-Kermadec trench.[3]
The Hikurangi Plateau, a remnant of a large igneous province is being subducted under the North Island at the margin currently. The subducting slab's Wadati–Benioff zone is over 200km (100miles) deep at Tauranga and Mount Taranaki and more than 75km (47miles) deep under the Taupō Volcanic Zone.
Earthquakes of up to have been recorded on the Hikurangi Margin, generating local tsunamis, and earthquakes in the 9.0M range are thought to be possible.[4] The Ruatoria debris avalanche originated on the north part of the subduction zone and probably occurred around 170,000 years ago.[5] Multiple uplift earthquakes will have occurred in the locked areas of the fault but a good historical record does not yet exist. The Pacific Plate slab has earthquakes often associated with it under New Zealand and for example deep earthquakes at more than under Taranaki or more than under the North Island Volcanic Plateau are likely associated with the subducted slab as it goes deeper under the crust.
There are well characterised now slow slip events across the Hikurangi Margin [1] Hikurangi Margin slow slip events occur up to yearly at a shallow depth of less than 10km (10miles), and last for up to 6 weeks relieving stress on much of the fault.[6] For example the series of slow slip events between 2013-2016 involved moment release of approximately . [7] At least one of the well characterised events was very close to the trench.[8] On land parallel to the predicted fault line of the Hikurangi Margin are active faults which are not fully characterised and include the Parkhill Fault Zone near Cape Kidnappers, the Maraetotara Fault Zone, and the Flat Point Fault. The slow slip activity has been associated with on land a mud volcano eruption causing a significant landslip.[9]
Because it has been possible to examine the mechanical properties of the subducted ocean floor clays recovered by drilling into the subducted rock, it has been possible to develop a model that may explain both the slow slip events but also why large and relatively deep earthquake ruptures are propagated into the shallow areas of the subduction zone thus displacing the ocean floor and generating tsunamis.[10] The model suggests that shallow-depth subducted water-saturated clay-rich sediments, promote earthquake rupture propagation and slip.[10]
The Hikurangi Margin has the potential to produce notable earthquakes. Some significant earthquakes are:
There have been ten possible large subduction earthquakes identified over the past 7000 years before the above historic records along the Hikurangi margin. The last such pre history earthquake occurred ± 25 in the southern Hikurangi margin. An earthquake associated with a tsunami and at least 354km (220miles) of the margin rupturing, occurred between and .