Kaikōura earthquake

The 2016 Mw 7.8 Kaikōura earthquake was a complex sequence of ruptures that included 12 major faults and nine lesser faults. The initial rupture started on a fault near Culverden and then jumped from fault to fault as it moved at a speed of 2 km/s. The ruptures travelled 170 km along the South Island’s north-east coast. Seismic energy was released for nearly 2 minutes.

Experts believe this may be a world record for the number of faults rupturing during a single earthquake. More faults may be found as scientists continue to collect and analyse data.

Rather than treating small faults as capable of only generating small earthquakes, we now need to understand that small faults can sometimes join up in the same earthquake and create larger earthquakes.

GNS Seismologist Bill Fry

Scientists have recorded more than 16,000 aftershocks in the 6 months following the event. The quake also triggered three slow-slip events in the Hikurangi subduction zone. The first slow ‘afterslip’ began immediately after the quake and lasted for several weeks. It released energy equivalent to a Mw 7.3 earthquake. A 2-week slow-slip event between Gisborne and Hawke's Bay was equivalent to Mw 7.1, and a third event beneath Kapiti Coast and the Marlborough Sounds was equal to Mw 7.0.

Significant damage

The earthquake caused significant damage to homes, businesses, primary production and infrastructure. There were two fatalities, and dozens of people were treated for injuries. Fortunately, the earthquake struck just after midnight. Popular tourist destinations – such as the Ohau Point seal colony, which was buried in a landslide – were deserted at the time.

State Highway 1 and the KiwiRail Main North Line between Picton and Christchurch were severely damaged. Coastal uplift has changed both the coastline and the seafloor. Land Information New Zealand is analysing the seafloor to map any new hazards.

Damage was not limited to the Kaikōura area. Buildings in Wellington and Lower Hutt were damaged, with a number of them demolished as a result.

Tsunamis

GNS Science has tide and tsunami gauges installed along the New Zealand coastline. The gauge near Kaikōura measured a 2.5 m drop in water level 25 minutes after the earthquake, indicative of a tsunami. The water level then rose about 4 m from its lowest point, with a series of waves continuing for several hours. Interestingly, the tsunami gauge measuring the water levels was actually raised up by 1 m due to the seabed uplift. Scientists also used onshore debris to measure the height of the tsunami. Goose Bay showed the highest run-up height of 6–7 m.

Landslides

Landslides caused significant damage and disruption to an area covering about 10,000 square kilometres. Scientists have mapped more than 6,000 of the 10,000 landslides in detail. They’ve discovered that the landslides are clustered along the actual fault lines rather than around the earthquake’s epicentre – as was the conventional thinking.

About 200 debris dams were also created. Scientists believe the dams consist of 12 million cubic metres of rock that fell in the Kaikōura Ranges and Hapuku River area. Scientists used a combination of physical measurements and lidar to predict the possibilities of downstream floods if or when the dams fail. Heavy rains have caused some of the dams to breach.

A seismically active area

The Kaikōura earthquake occurred on the plate boundary between the Australian and Pacific plates. This is a seismically active area, responsible for creating the Southern Alps and the Kaikōura Ranges. The Alpine Fault – a transform boundary, where the plates meet and slide horizontally past each other – is to the south of where the ruptures occurred. The Hikurangi subduction zone – where the Pacific plate moves under the Australian plate – is to the north.

Moving the land in many directions

The ground shaking was the most violent ever recorded in New Zealand. As a result, the landscape was significantly altered. GNS Science reports that the earthquake:

• pushed the rocky seabed up by as much as 8 m, forming a vertical barrier between the shore and the sea in some places
• moved parts of the South Island 5 m closer to the North Island
• moved the land from Kaikōura to Cape Campbell north-west by up to 6 m
• raised some parts of the region by up to 8 m.

Changing earthquake research worldwide

Observations – from sensors in place at the time and subsequent data gathering – make this one of the best-recorded large earthquakes in the world. Scientists used drones, helicopters, satellite radar information and on-site surveys to measure offsetting fence lines and railway lines. NIWA used its research vessel to map the six fault lines that crossed from the land into the sea.

The results of the data from this complex earthquake are making scientists reconsider the way they look at earthquake hazards along plate boundary zones.