Hawke’s Bay is one of the most seismically active regions of New Zealand and in the 160 years since substantial written records began, several large and damaging earthquakes have occurred. Most notably the earthquake of 1931 changed the landscape, the cities and has remained a prominent feature in Hawke’s Bay’s living memory. Hawke’s Bay experiences many smaller earthquakes each year, but another large earthquake can occur at any time.
National seismic hazard model for 2010 showing peak ground acceleration expected with a 475 year return period
Hawke’s Bay is located on the Australian Plate, about 150 km west of the Hikurangi Trough, which marks the subduction boundary between the Pacific and Australian Plates. At this latitude, the Pacific plate is plunging beneath the Australian plate at about 42 mm/year.
Hawke’s Bay is located on the Australian Plate, about 150km west of the Hikurangi Trough.
Hawke’s Bay location above the subduction interface means that it is within a zone of high deformation, and as a consequence has many earthquakes. These earthquakes can cause ground shaking, liquefaction, surface rupture, lateral spread and other ground damage, regional subsidence or uplift along with tsunami, landslides and rockfalls. Learn more about these earthquake hazards at GNS Science.
Hawke's Bay sits above the subduction zone where the Pacific Plate is moving under the Australian Plate
A fault line is a fracture along which the earth’s crust has moved. There are 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, so these are harder to recognise. The Active Fault Map shows generalised traces of active surface faults in the Hawke's Bay region. Active faults are those faults that have moved within the last 125,000 years. While no surface traces of active faults have been mapped in the Napier and Hastings city areas, this is because historic floods and development have covered them over. Scientists believe both cities have 'buried’ or ‘blind’ fault sources including the large fault source that caused the Hawke’s Bay earthquake of 1931, but they are currently unable to map them. The subduction interface between the Australian and Pacific plates is the largest offshore fault in our region.
Liquefaction occurs when waterlogged sediments are agitated by an earthquake, typically larger than magnitude 6. As a result, the soil behaves like a liquid, has an inability to support weight, and can flow down very gentle slopes. Buildings can sink and underground pipes may rise to the surface. When the shaking stops, groundwater is squeezed out of the ground causing flooding, which can leave areas covered in mud. Hawke’s Bay has several areas with sediment of high liquefaction susceptibility and numerous earthquake sources capable of generating an earthquake large enough to cause liquefaction, and there were numerous reports of liquefaction following the 1931 earthquake.
Low-lying areas in the region, especially those near the coast, and reclaimed land are particularly susceptible. Liquefaction susceptibility maps for Hawke's Bay and Napier/Hastings updated by GNS Science in 2017.
You can find out if liquefaction or fault lines might affect your property by visiting www.hbhazards.co.nz and entering your property address.
Based on a national seismic model shown above, scientists have estimated Hawke’s Bay might experience the following shaking intensities for different return periods in the future. Our risks include earthquake prone buildings which have been identified within our main cities & towns, and although residential dwellings are considered a low risk, damage may be widespread requiring re-housing. Infrastructure networks are also at risk and there are also risks to our future social and economic wellbeing.
In the past most moderate to large earthquakes in Hawke's Bay have been on shallow (<45 km deep) faults, but a few, such as the June 1921 Hawke’s Bay earthquake, were caused by rupture at greater depths. The largest historical earthquakes affecting Hawke’s Bay are listed in this Table.
HISTORICAL EARTHQUAKES MAGNITUDE > 7 AND FELT MM INTENSITIES OF 7 OR GREATER
You can find out more about these earthquakes on Geonet
Cleaning up after the Hastings 5.9 earthquake on 25 August 2008 (Photo courtesy of HB Today)
WHAT CAN YOU DO?
Most people in Hawke’s Bay will survive a large earthquake with some loss, but some people will be severely affected. Action you take now can help reduce damage to your home and business and help you survive. Practice Drop, Cover and Hold and learn more here
MISLEADING ADVICE 'TRIANGLE OF LIFE'
There is an email circulated by a self-professed ‘rescue expert’ suggesting earthquake advice contradictory to what Civil Defence and the NZ Earthquake Engineering Society recommends. Although discredited in the US, where it originated, the advice continues to resurface causing confusion about what to do, which can result in people getting seriously injured or killed in an earthquake. If you want to know more about why the 'Triangle of Life' is not the best advice you can visit the USA Red Cross website response
CASE STUDY: 1931 Hawke's Bay earthquake