Satellite view shows Raoul Island before and after quakes, tsunami

Footage shows tsunami waves hitting New Zealand. Video / Claudia Maaka / Waiapu Civil Defence / Gina Pewhairangi / Dean Purcell

Satellite images appear to show a raft of fresh landslides around Raoul Island, near where 7.4 and 8.1 earthquakes struck on Friday morning.

It comes as GeoNet has published data from tidal gauges, showing how a tsunami reaching up to 40cm high was measured at Great Barrier Island soon after the events.

This afternoon, University of Auckland coastal geomorphologist Dr Murray Ford tweeted a before-and-after satellite view of Raoul Island.

The first decent cloud free image of Raoul Island after the 8.1 quake and tsunami last week. Lots of landslides. @NZNavy HMNZS Canterbury anchored just offshore with @gnsscience and @MetService team. Images captured by @planetlabs pic.twitter.com/Pa9Sqdp182

— Murray Ford (@mfordNZ) March 10, 2021

Visible in the latest image, recorded by US Earth imaging company Planet Labs, showed numerous landslides on the island, which is about 1000km northeast of New Zealand.

MetService and GNS Science were currently at the island checking monitoring equipment, after embarking aboard the HMNZS Canterbury on Monday.

The initial 7.4 quake, which struck near the island at 6.41am on Friday, appeared to have knocked out tidal gauges at the island.

This week in a mission led by @MetService and supported by @NZNavy our @gnsscience staff are landing on Raoul Island to repair links to vital earthquake, tsunami and volcano sensors impacted by Friday's earthquakes: https://t.co/zpl0gYUDdr pic.twitter.com/3NlNkHw83e

— grahamleonard (@grahamleonard) March 9, 2021

That was followed by a larger 8.1 quake, recorded at 8.28am at the same site.

Those events in the Kermadec Islands are held to be unrelated to quakes that were recorded around New Zealand's East Cape earlier that morning, beginning with a 2.27am 7.3 jolt.

In a blog post published today, geological hazard monitoring service GeoNet described how these events all produced tsunami that were later recorded at sites hundreds of kilometres away.

"Our instruments recorded the tsunami from the M7.3 East Cape earthquake reaching a maximum amplitude [wave height] of ~30-35cm at Lottin Point (East Cape) and ~10-20cm at Great Barrier Island," GeoNet said.

"This was closely followed by tsunamis generated by the M7.4 and M8.1 Raoul Island earthquakes."

"These waves overlapped to produce a tsunami between 35 and 40 cm in amplitude at the Great Barrier Island tsunami gauge and recorded at many other gauges around New Zealand."

Source / GeoNet

Gauge data showed the largest tsunami waves occurred just after the 8.1 quake.

"Interestingly, we can also see the activity lasted several days. It's worth noting that tide gauges just show the tsunami heights at specific locations," GeoNet reported.

"Tsunami heights vary significantly along the coast so other locations may have experienced larger tsunamis."

GeoNet pointed out that, although a few centimetres didn't seem like much, tsunami were nothing like wind-generated waves.

"In a tsunami, the whole water column is moving with the tip of the iceberg being the wave on top, often surging faster than you can run."

Last Friday three large earthquakes occurred offshore Aotearoa New Zealand. They all produced tsunami that overlapped and were recorded all around New Zealand. We have the latest science on the tsunami for you here: https://t.co/zP7M6saeD7 #eqnz pic.twitter.com/9kiOoXwMyH

— GeoNet (@geonet) March 10, 2021

Speeds could vary from 10 to 30km/h at the coast, to 400 to 800km/h on the open ocean.

"Surges come in non-stop for around five minutes or more, and then recede just as fast for a similar period of time, acting more like a very rapid tide than a wave," GeoNet said.

"This type of back-and-forth pattern (and sometimes weird eddies) continues for several hours (if the tsunami was generated close by) to days (for tsunami that has travelled across the Pacific).

"For someone fishing on the rocks, that relentless surge of water might be enough to knock them off their feet and not let them back up."

GNS Science has launched a survey about peoples' tsunami observations, which could be completed here.

Expedition at tsunami risk zone

Meanwhile, a state-of-the-art underwater remotely operated vehicle (ROV) is helping a team of scientists better understand another big risk area for tsunamis.

The Hikurangi Subduction Zone is a major offshore fault where the Pacific Plate dives – or subducts – westward beneath the North Island, and represents our largest geological threat.

Scientists believe the subduction zone has the potential to unleash "megathrust" earthquakes larger than magnitude 8, such as those which created tsunamis that devastated Indonesia in 2004 and Japan 10 years ago this week.

During a five-day voyage aboard Niwa's research vessel Tangaroa, scientists from GNS Science, Niwa and University of Washington will use the ROV to download the latest data from a recently installed seafloor observatory.

An underwater remotely operated vehicle, pictured being lowered from the RV Tangaroa, is helping a team of scientists understand the Hikurangi Subduction Zone earthquakes. Photo / Laura Wallace

They'll also retrieve instruments earlier deployed along the ocean bottom.

Voyage leader Dr Laura Wallace, of GNS Science, said scientists couldn't wait to see what the data revealed - especially about long-lasting "slow slip" earthquakes known to have occurred in the region over past years.

"We're also keen to learn more about last week's 7.1 magnitude earthquake near East Cape, which was only about 100 km away from the offshore observatories," Wallace said.

"We hope this data will help us figure out why these slow slip earthquakes are occurring on the Hikurangi subduction zone and to better understand the processes that occurred in the East Cape earthquake."

The two earthquake observatories were installed beneath the seafloor by the research vessel JOIDES Resolution more than three years ago and have been actively recording changes in the Earth's crust due to earthquakes and slow slip earthquakes off the coast of Gisborne since then.

This was the first time the Canada-based ROV, called ROPOS, will be operating in New Zealand waters, to download the information from the Hikurangi subduction zone observatories.

ROPOS was operated on Tangaroa by a team of eight engineers from the Canadian Scientific Submersible Facility.

It was expected to take about one to two hours to download the three years' worth of data from the observatories, once the robot plugged itself into the observatory.

Scientists aboard the Niwa vessel Tangaroa watch a live feed from the ROPOS remotely operated vehicle as it moves through the ocean. Photo / Dr Jess Hillman, GNS Science

Scientists were interested in understanding the relationship between earthquakes and slow slip earthquakes.

Slow slip earthquakes appeared to occur every one to two years off New Zealand's east coast.

Unlike a normal earthquake, which released built-up stress suddenly, a slow slip event unfolded over a longer period – anything from days to weeks to months.

ROPOS would also be used to retrieve 16 seafloor instruments, which have been measuring the rate of water flowing out of the seafloor and collecting water for chemical analyses.