Kaikoura earthquake shake greater than Christchurch
Scientists have revealed incredible new insights into the 7.8
Kaikoura Earthquake, including the most violent ground shaking ever
recorded in New Zealand.
Readings taken at the North Canterbury
town of Waiau during the November event proved a new record for vertical
ground acceleration, reaching 3g, or 30 times the force an airliner
passenger feels at take-off.
Just-published research has detailed
the incredible ground motion generated by the midnight quake, which
triggered a tsunami and thousands of landslides, caused billions of
dollars of damage, shifted the South Island and left two people dead.
While
the November 14 quake erupted near the small North Canterbury town of
Culverden, scientists say it was Waiau, about 22 kilometres away, that
bore a New Zealand record for the most severe upward ground shaking.
GeoNet typically calculates the ground motion its
seismometers detect with what's called peak ground acceleration (PGA), a
measure often compared to acceleration due to gravity.
One
instrument at Waiau, since confirmed to have been functioning correctly,
showed a maximum vertical acceleration of about 3g, eclipsing the
previous national record of 2.2g produced by the February 2011
Christchurch Earthquake.
By comparison, passengers on airliners
feel accelerations of around 0.1g on take-off, while someone in a car
speeding up from 0km/h to around 100km/h in 10 seconds would experience
0.25g.
GNS seismologist Dr Anna Kaiser, the lead author of the
new study, said someone riding a rollercoaster would feel forces greater
than 1g; but they'd of course be strapped in.
Waiau School principal Mary Kimber said the power of the quake, which
shook bricks from her home and ripped shelves off their bolts, was
terrifying.
"I was actually cowering on the floor ... people
say, why didn't you get under the doorway? Well, you couldn't move, it
was just too strong."
Waiau Pub co-owner Lindsay Collins, who was
in Christchurch for all of the major Canterbury quakes, said the shake
was the most frightening he'd experienced.
Damage inside the Waiau Lodge Hotel following the earthquake. Photo / Belinda Feek "Everything just went everywhere: I was knocked off
my feet and everything was knocked off the walls," said Collins, whose
century-old pub sustained extensive damage.
"I managed to get out
of the bedroom after falling over bits and pieces, I got my son out of
his room, and we stood there for a good minute while the shaking was
going on ... it felt like a train."
Elsewhere, PGA readings of
more than 1g, and therefore higher than that due to gravity, were
recorded more than 100km away at Kekerengu (up to 1.2g) and Ward (up to
1.3g).
The total energy produced by the entire quake was
equivalent to that of 400 atom bombs, or enough to power every South
Island home for a year.
Its 8000 kiloton TNT equivalent was
around 11 times more than the 2010 7.1 Darfield Earthquake and about 60
times more than the twin quakes that shook Marlborough in 2013.
Science of the quake
Kaiser
said the Waiau recording had some "very unusual characteristics",
notably very strong upward acceleration compared with downward
acceleration.
"This suggests there could be a trampoline effect
going on, where the instrument is thrown upwards with very strong
acceleration and then it descends before encountering another very
strong push which sends it sharply back up again."
It was unclear
what had caused it, but it was possible that soft soil near the surface
interacting with stiffer material beneath may have worsened the
shaking.
Damage to the Waiau School pool observed immediately after the 7.8 Kaikoura Earthquake. Photo / File The Waiau instrument was also located virtually on top of the Humps Fault Zone, the first to rupture during the earthquake.
The
Ward and Kekerengu Valley instruments were located close to the
Kekerengu and Needles faults, where massive amounts of energy were
released during the rupture.
Kaiser said the ground motion at
these instruments, and also in Wellington, may also have been increased
by the "directivity effect", created when seismic energy was being
released at the same time as a rupture was unzipping along a fault.
"This
means if the rupture is coming towards you, energy can be stacked
together and you might experience stronger shaking than if the rupture
is moving away from you in the other direction, even if you are the same
distance from the fault."
But trying to pinpoint different
ground accelerations across the South Island was a complicated task,
because of the many different faults and soil types involved.
While
PGA rates were a good measure of how sharp the shaking was, there were
other important factors to consider, such as the permanent movement of
land, parts of the South Island were shunted 5 metres closer to the
North Island, along with the duration of ground shaking and the strength
of rolling-type motions, which could be enhanced by the presence of
deep or soft soils.
Destruction to the Satterthwaite home in Waiau. Photo / Supplied Kaiser said rolling-type motions were typically
experienced in places further away from the epicentre, and tended to
inflict more damage on taller buildings than smaller ones as was seen in
Wellington.
Researchers toil in quake's wake
Trying to unravel the mysteries of what was the most complex earthquake ever recorded remains a big job for GNS Science.
Around
45 staff at the institute are still spending nearly half their time
studying the quake, as part of national and international efforts.
As
the quake was one of the best-recorded event in history, yielding a
rich array of data, fresh insights across all facets of earthquake
science will likely be flowing in for years.
Scientists inspect ground raised into
a wall near Waiau by the November 7.8 M earthquake. Photo / Kate
Pedley, University of Canterbury A big focus has been on the Kekerengu Fault, which
generated the longest continuous rupture during the earthquake,
stretching about 90km from Mt Manukau in the seaward Kaikouras to the
Needles Fault in Cook Strait.
It collectively encompasses the Kekerengu Fault, the Jordan Thrust, the Upper Kowhai Fault, and the Needles Fault.
One
point of interest is the Clarence Valley where five faults intersect:
Investigations into this quintuple intersection area will improve the
understanding of how multiple faults interact in three dimensions during
a single earthquake.
With science now suggesting that these
complex ruptures are more common in large earthquakes in New Zealand
than previously thought, improving the understanding of how these
ruptures occur will help New Zealand be better prepared for future
events.
Mapping and analysing the thousands of landslides the quake caused has also been a priority for the scientific effort.
Landslides
have blocked main roads, dammed water ways and damaged large areas of
land and critical infrastructure throughout the impacted area of the
upper South Island.
Most occurred in either the weak, young mudstones and sandstones or in the harder, older greywacke rocks.
A picture taken immediately after the quake shows a large landslide blocking State Highway 1, near Kaikoura. Photo / File A major impact had been the development of landslide
dammed lakes along water courses, and significant work had been done to
evaluate whether these could fail and what the flood impacts might be.
Further,
the earthquake had loosened and mobilised debris, which could be more
susceptible to failure in the future and affect river catchments for
some time.
Scientists say the landslides appear to cluster around
areas of observed fault rupture, rather than close to the earthquake
epicentre as normally expected, making the interaction between
landslides and fault ruptures an important area of research in the
future.
GNS soil mechanics scientist Dr Jon Carey said a range of
technology was being used to study the landslides and attempt to
measure their stability.
This included satellite imagery, aerial
photography, laser scanning and drones equipped with technology that
could reveal structural changes in landslides.
"What we are
trying to do is collate as much information as possible to create a
really big database, and compare this earthquake to previous and future
ones," Carey said.
A picture of the toe of the Leader
River landslide dam, taken by University of Canterbury scientists last
year. Photo / Kate Pedley "We'd like to be able to plug [the databases] into
modelling approaches to try to predict how bad future events might be in
certain locations and what landslide impacts might look like."
With
around 15,000 quakes since the November 14 event, scientists have
statistically forecast a 10 per cent probability of an aftershock of
more than 7.0 in the next year, a 68 per cent probability of a 6.0-6.9
quake, and a 99 per cent probability of five to 26 quakes ranging
between 5.0 and 5.9.
New study to shed light on Auckland quake risk
Meanwhile, a new study is centring on known and potential faults in an area close to New Zealand's largest city.
In
a newly-funded project, an international team of scientists led by the
University of Auckland's Dr Jennifer Eccles is working to understand the
dynamics of the slow-moving Hauraki Rift.
The geological
structure contains the active Kerepehi Fault, which runs between
Matamata and north towards Waiheke Island, and lies only 30km east of
Auckland and Hamilton.
"The Hauraki Rift is not as active as
areas around Canterbury and the north of the South Island that have
suffered recent large earthquakes," Eccles said.
"However, the
Christchurch earthquakes in particular highlighted the need to research
and better understand slower moving geological structures that have the
potential to pose a significant natural hazard risk to high population
areas and infrastructure." Yet the rift remained poorly understood, and there had been no significant scientific research into the seismic area in decades.
The
team would be specifically working to understand the geology and
activity of the Kerepehi Fault, recently revealed to consist of a belt
of many faults, in a wide zone of segments that could potentially
experience a large quake every 1000 years.
Eccles and her
colleagues would also try to identify blind faults or activity on mapped
faults currently deemed inactive across the Hauraki Plains and Auckland
city.
The team are using regional GPS measurements to monitor
how the region is stretching through time and are building a model of
the ground to understand its properties and composition from using
reflected seismic waves.
"These are waves of vibrational energy
that travel through the Earth's layers and interact with the
subsurface," Eccles explained.
"These vibrations are recorded and
measured at various locations and depths to build an understanding of
faults at depth including possible fault lines previously undetected."
"These
projects will improve the scientific community's knowledge of the
Kerepehi Fault and the wider Hauraki Rift and what seismic movement is
taking place within this area.
"They also contribute to building a
bigger picture of seismic activity in New Zealand and helps our
communities become more prepared for and resilient to a natural disaster
event."
A 2013 GNS report into potential impacts on Auckland
looked at a trio of scenario earthquake sources: two were nearby active
faults, the Wairoa North and the offshore segment of the Kerepehi fault,
and the third was the more distant Kermadec Subduction Zone.
This was equated to the 526,000 buildings in the region, valued at $309 billion and housing 1.4 million people.
In
the first two scenarios, large quakes could cause up to 53 and up to 26
deaths in the region respectively, along with $1.9b and $1b in damage.
The
new study has received a $70,000 grant from the Earthquake Commission's
Biennial Grants Programme and $75,000 as part of the EQC University
Post-Graduate Grant Programme.
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