At least 34 quakes have been recorded west of Castlepoint in the last seven days - with the strongest measuring a magnitude of 4.2 in the early hours of Monday morning. Photo: GeoNet
Earth Sciences say they are monitoring a cluster of earthquakes centred off the east coast of the lower North Island this week.
At least 34 quakes have been recorded west of Castlepoint in the last seven days - with the strongest measuring a magnitude of 4.2 - at depths of about 20 kilometres.
On-call seismologist Sam Taylor-Offord said a concentrated sequence of events typically indicated a "much more intense" process than the usual background noise of seismic activity.
"You can think about it as something that is unfolding. It's a process.
"Every earthquake pushes a little bit of the stress that releases into the area around it and then that can create a cascade of increasing the stress in the rock surrounding it. That rock breaks - it increases the stress in the rock around it - that rock breaks and that's your sequence playing out," Taylor-Offord said.
But he said - along the line of subducting tectonic plates which characterised the fault along the east coast of the North Island - the quakes could also be associated with multiple "slow slip" events.
Subduction was the process where one tectonic plate was forced beneath another into the Earth's mantle.
"If you think of it as a very large earthquake that's happening but it's happening over weeks and months.
"It's still changing the stress in the surrounding area and in some places the earth breaks in a related way to that movement. That tends to break in a sequence. So that's one of the things that might be happening," Taylor-Offord said.
Taylor-Offord said the agency was looking into the pattern but it did not necessarily indicate an increased risk of a large quake in the area.
"Sometimes a sequence will precede a larger earthquake, sometimes nothing will come of it. Science is not quite at the point where we can say 'that one, not that one'," he said.
He said - on the flip side - it was not possible to infer that a quake cluster was indicating a gradual release of pressure which could ward off a larger quake.
"We have earthquakes like this all the time and - so far - they haven't stopped the larger earthquake coming.
"Perhaps a weaker fault [is] breaking but elsewhere there is a strong fault that is still accumulating that stress and will someday rupture in an earthquake. It's a fact of life," Taylor-Offord said.
He said the agency was also monitoring another cluster of just under 30 weak quakes centred south east of Seddon over the last month.
He said those quakes were more likely to be the remnants of aftershock sequences from the 2016 Kaikōura quake which was centred nearby.
Taylor-Offord said the activity was a reminder for people to stay prepared for a major event.
"Small earthquakes are good because they remind us that bigger quakes are possible. These are regions very close to the plate boundary where we have a lot of stress and a lot of strain and we expect large earthquakes in the future as we have seen in the past," Taylor-Offord said.
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