New technology to be installed off the East Coast will give better insights into off-shore quakes like that which caused these cracks near the East Cape last week. Photo: Ali Rewi
Scientists are to create an underwater "earthquake observatory" near the North Island's East Coast that will give detailed information about off-shore quakes like the 7.1 that shook the North Island last week.
Researchers say the lab could allow them to understand big undersea quakes much better and faster, which is crucial to issuing timely tsunami warnings.
The project, a collaboration of the 26-nation International Ocean Discovery Programme (IODP), will see New Zealand become one of a handful of countries to have the sub-seafloor technology.
International scientists will next year drill three holes of up to 1.5km deep in the seafloor in different parts of the Australian tectonic plate, which overlies the Pacific plate, about 40km east of Gisborne.
Scientists aboard the US-based research ship Joides Resolution will insert a range of instruments into one of the holes to make continuous physical and chemical measurements inside the plate boundary zone.
The instruments will be in place for at least the next decade, and will detect any changes from earthquake or other types of tectonic events in the region.
The ship will also collect sediment and rock from the drill holes for analysis.
A key focus for the more than 50 scientists involved in the project will be to learn how temperatures and pressures at depth, and the types of rocks being subducted - or pushed under the plates - might influence earthquakes and slow-slip events where the Pacific plate is being forced under the Australian plate.
The expedition's leader, GNS Science geophysicist Laura Wallace, said the project's findings would almost certainly have global significance.
"It has the potential to significantly boost the understanding of the mechanics of subduction zone faults and the earthquakes that occur on them," Wallace said.
The main aim of the project was to improve the understanding of slow-slip or silent earthquakes, which were features of the subduction zone east of the North Island.
But Wallace said it would also have multiple benefits in understanding a range of plate-boundary phenomena.
Over the past three years, seafloor sensors have been deployed off the Gisborne coast to monitor slow-slip activity and earthquakes on the Hikurangi subduction zone east of the North Island.
On loan from Japanese and US institutions, the instruments sit on the seafloor for 12 months recording offshore quake activity with much greater precision than land-based instruments because of their proximity to the action.
After a year they are retrieved by a ship and the data is recovered.
The ship then deploys fresh instruments at the same place.
"These seafloor instruments have given us tremendous insights into the workings of this complex plate-boundary fault zone, but the information is always retrospective," said Wallace.
He said that after last week's earthquake northeast of East Cape, there was a case to be made for data from the seafloor instruments to be sent onshore in real-time using fibre-optic cable.
"A continuous quake-monitoring capability east of the North Island would be enormously helpful in speeding up the analysis and understanding of complex offshore earthquakes that might have generated a tsunami."
The distances between offshore quake epicentres and land-based seismometers meant it was difficult for New Zealand's current national network, operated by GeoNet, to quickly learn the depth, location and magnitude of offshore earthquakes, Wallace said.
Knowing this kind of information with confidence was crucial to issuing timely tsunami warnings.
The new project, one of five back-to-back IODP expeditions the research ship will do in New Zealand waters starting late next year, follows a recent study that showed three or four large tsunamis have struck the East Coast in the past 1200 years.
Scientists estimate the tsunamis were between 9m and 12m high, and would have travelled inland many hundreds of metres in low-lying areas.
The last big one struck as recently as 300 years ago.
However, scientists knew from past fieldwork and from modelling that 5m-plus tsunamis could strike anywhere along the East Coast.
The research, led by GNS Science, also showed three large earthquakes were probably centred north of Gisborne in the past 1800 years.
Each quake was thought to be around magnitude 7.2 and each lifted coastal land by about 3.5m.
The earthquakes happened 1800, 1200, and 400 years ago, an average of 750 years apart, suggesting the next quake was probably not due yet.
As part of the work, scientists dug a 90m-long trench at right angles to Puatai Beach north of Gisborne and geologists looked for evidence of past quakes in the trench walls.
Radiocarbon-dated shells and wood in the trench walls were used to establish the dates of the three earthquakes and two tsunamis.
The researchers found a number of offshore faults in the region, and any of them could have produced the tsunamis.
It was also possible they came from faults farther afield, such as near South America.
Ruptures on the Gable End Fault targeted in the study - did not coincide with the dates of the tsunamis, meaning that they were generated by other faults.
Although the tsunamis would have affected the entire East Coast of the North Island, it was hard to estimate from a single study how large a particular tsunami would have been at various places along the coast.
Scientists have known for a long time that the East Coast is seismically active and can experience earthquakes of magnitude 7.0 and above, and the study had provided an insight into their timing and magnitude.
Coastal sites that provided good evidence of tsunamis were rare, so such hard data added to overall body of knowledge on earthquakes and tsunamis in this part of New Zealand.
GNS Science said the research did not increase the level of hazard for Gisborne and the East Coast, as the new findings were consistent with existing modelling.
Scientists expected to share detailed findings with authorities in November.