Buried faults make assessing Hawke's Bay's earthquake risk complex

Hawke's Bay's is already at risk from a potential "megathrust" magnitude 8.5 earthquake but, in a region awash with active faults, more faults remain unmapped directly beneath Napier and Hastings cities.

A report of natural hazards in the region, compiled with help from GNS Science, said Hawke's Bay is one of the most seismically active regions of New Zealand, adding that in the 160 years since substantial written records began, seven large and damaging earthquakes have occurred - most notably the earthquake of 1931 - which changed the landscape.

Hawke's Bay experiences many smaller earthquakes each year, but another large earthquake - magnitude 6 and over with the potential to result in liquefaction - could occur at "any time".

Potentially Hawke's Bay's most damaging risk lies in its location near the Hikurangi subduction zone, where the Pacific tectonic plate dives (subducts) under the Australasian plate.

Subduction zones like Hikurangi, which is 150 kilometres off the Hawke's Bay coast, produce the planet's biggest and deadliest earthquakes — also referred to as megathrust earthquakes — as well as devastating tsunami.

It is arguably New Zealand's most significant active fault and is capable of generating a magnitude 8.5 earthquake that, in addition to widespread ground shaking, is also likely to produce a tsunami, coastal uplift and subsidence, landslides and liquefaction.

There are also numerous active faults in Hawke's Bay onshore and offshore.

Many are surface faults where a rupture that initiated at depth has broken through to the surface and left a visible fault trace. Others are buried or "blind" faults that slip at depth but do not rupture to the ground surface.

The situation in Napier and Hastings urban areas is further complicated because historic floods and development have covered over suspected active faults. That means those faults have not been able to be mapped.

Scientists believe both cities have buried or blind fault sources including the large fault source that caused the Hawke's Bay earthquake of 1931.