Checking the pulse of the Hikurangi subduction zone
Recording the heartbeat of the Hikurangi margin became easier this year when scientists installed instruments across southern Hawke’s Bay, Tararua and Wairarapa to better understand earthquakes there.
The project will enable scientists to determine whether there are any observable changes in the Hikurangi subduction zone that could be used as future monitoring tools to better forecast large earthquakes. The project is called PULSE – Physical processes UnderLying Slow Earthquakes – and it consists of a network of nearly 50 temporary seismic and geodetic (GPS) instruments to record the slow slip earthquakes that occur every five years or so in this region.
Earthquake Monitoring | Taking the PULSE of the Hikurangi subduction zone transcript
Over a couple of weeks, a team of around 10 scientists
have deployed more than 50 instruments to record signals
related to slow slip earthquakes.
The PULSE project
is an important step towards better understanding earthquake
hazard in New Zealand, and the subduction processes that are
happening along the Hikurangi subduction zone along the East Coast
The Porangahau region is particularly interesting because
these slow slip events happen approximately every five years
or so in this region.
The last event was triggered in 2016 and
November after the Kaikoura earthquake.
And so we've been expecting that there would have been
a slow slip sometime in 2021, about five years after 2016.
The slow step events themselves happen so slowly that
they don't produce vibrations and ground shaking,
and that means that we can't record them
or know that they're happening with normal seismic instrumentation.
The only way that we've been able to observe them and map out
where and when they happen is by deploying GPS equipment
So these are sensors that sit at the ground surface
and record their position, and that can tell us how points
on the earth surface are moving really slowly through time.
So while these sorts of events don't produce any noticeable ground shaking
in themselves, they can produce and trigger small earthquakes.
So the other type of instruments that we can install are
these seismometers, these earthquake recorders.
We bury the sensors in the ground, and we leave them
there for a few months, and they basically record any kinds of
vibrations that travel through the crust.
When we record these small earthquake signals,
we're able to do a much better job of telling where these earthquakes are happening.
And that can tell us something about how these slow slip events
are changing the state of stress in the crust
and how they're triggering the seismicity.
These slow slip events do happen on other subduction zones around the world,
but New Zealand and the Hikurangi in particular has really been a magnet for world
leading science in terms of slow slip event understanding in recent years.
And this is because the Hikurangi is in a
really unique setting to be so close and
so shallow beneath our feet underneath the North Island.
And the observations that we make are much clearer then if
we were trying to observe things much deeper down in the earth.
The type of fault motion that we see in these slow slip events
is exactly the same as what we see in these large
damaging tsunamigenic subduction zone earthquakes.
And so we want to try and understand that if the driving forces
behind these two different slip modes, the slow slip and the damaging slip,
- if the driving forces are the same, then can we
use the much more frequent slow slip earthquakes
to better understand how the fault is driven to failure.
And this deployment wouldn't have been possible
without the generous help of many of the landowners
along New Zealand's east coast.
Thank you for letting us put one of these instruments on your land.
Earthquake Monitoring | Taking the PULSE of the Hikurangi subduction zone
The PULSE project (Physical processes UnderLying Slow Earthquakes) is a GNS Science led, newly funded Marsden project that joins a range of other efforts to better understand the Hikurangi Subduction zone.
Slow slip events are common along the Hikurangi subduction zone, where the Pacific Plate is subducting under the Australian Plate. As the name suggests, the energy is discharged slowly, over weeks to months, not through a sharp jolt (as for a typical earthquake).
They occur when the boundary between two tectonic plates becomes temporarily ‘unstuck’ and they begin creeping past each other for a period of time, sometimes weeks or even months.
Humans can’t feel slow slip earthquakes and they are also too slow to be picked up by seismometers. They can only be recorded with special GPS equipment that can measure the slow movement of the land.
Analysing the small earthquakes occurring both before and during a Pōrangahau slow slip event is a fantastic opportunity to test our ideas around how these slow slip events happen
“We’ve observed previously that the behaviour of small earthquakes changes both prior to and during slow slip earthquakes, but not in enough detail to fully understand why”.
“New data collected in this project, from additional instruments, will greatly improve our understanding of how and why these slow earthquakes occur so regularly, and what causes them to happen in the first place”.
The PULSE project is in collaboration with Te Herenga Waka—Victoria University of Wellington and is part of a Royal Society Te Apārangi Marsden-funded programme.
