East Coast
Gisborne sits on the geological equivalent of a “bowl of jelly”, 37 times bigger than Mt Everest, scientists have discovered.
Analysis of seismic waves from the 2016 Kaikoura earthquake has enabled scientists to identify a “wedge-shaped” body of softer rock and sediment under the seafloor on the East Coast, GNS Science says.
Estimated to be between 5km and 12km thick, it can cause slow swaying movements lasting several minutes in the Gisborne region during large earthquakes.
“A rough analogy for the type of rocks within this body are the crumbling cliffs that can be seen at places such as Tolaga Bay,” a GNS Science spokesman said.
A study just published in the Journal of Geophysical Research – Solid Earth and led by seismologist Yoshi Kaneko of GNS Science, says the sediment body measures about 300km long by 150km wide and it is probably the largest such body under the North Island.
Scientists already knew that softer rocks were present at the Hikurangi subduction zone (which lies off the Gisborne coast), but this is the first time they have been able to estimate the approximate size and shape of the body.
The study found during the Kaikoura earthquake, the sediment body resulted in slow swaying motions in the Gisborne region, lasting up to eight minutes.
A summary of the study on the American Geophysical Union website states the long-duration shaking also triggered a large slow slip event (sometimes called silent earthquakes because they happen over a prolonged period of time) on the Hikurangi subduction zone offshore Gisborne, more than 600km away.
“Our result has important local and global implications for ground shaking hazard in areas with similar geological characteristics to those in the northeastern North Island, and improves understanding of processes that can trigger slow slip events and earthquakes on subduction zones.”
GNS Science told the Gisborne Herald the long slow swaying motion occurs independently of quake location.
“In fact, scientists had noticed it in previous quakes — the magnitude 6.8 in 2014 for instance.
“However, the 2016 Kaikoura quake is the first time they have done a detailed investigation. Their efforts were boosted by the presence of about 40 ocean bottom seismometers placed on the seafloor off the Gisborne coast.”
Dr Kaneko said the swaying may not have been felt by people here as their senses would have been preoccupied by the shorter more violent shaking from the quake.
“However, the swaying motion was recorded by both on-shore and offshore seismic instruments.”
A network of seismic instruments has been present on the seafloor east of the East Coast for several years, part of a large international science programme led by GNS Science.
The aim is to improve the understanding of the Hikurangi subduction zone, where the Pacific plate is subducting under the Australian plate.
“It’s a bit like a bowl of jelly,” Dr Kaneko said.
“When you jiggle the bowl a bit, the jelly moves even more. In this case, the stiffer ‘bowl’ is the Pacific Plate subducting under the North Island and the sediment wedge is above the plate.
“We were curious about the unusual behaviour of seismic waves from the Kaikoura earthquake in this region and that prompted us to try to identify the cause.”
As “long-period” seismic waves travelled north through the Earth, they slowed down dramatically and started reverberating off Hawke’s Bay north of Napier.
“Based on the way the seismic waves behaved, we were able to model the body of softer rock and sediment.”
Dr Kaneko said it meant the region was susceptible to long, slow swaying during large earthquakes — perhaps even from earthquakes as far away as the Kermadec region.
He said that the slow swaying effect would only be pronounced in large earthquakes — those above magnitude 7.
Dr Kaneko also advises these sedimentary rocks are ubiquitous in the East Coast region, and can be seen at the surface in a number of places.
“So you don’t have to drill down very far to encounter them.”