The Star: March 16, 2023

The Star Thursday March 16 2023
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NEWS
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‘Unusual’ quake no
City2Surf returns to
pre-quake course
cause for concern
Big surf, hot sun at
national champs
Big turnout for
PRIDE Week
Fancy a game of
Ultimate Frisbee?
Brass band calls
out for cornets
• By Jaime Morton
SCIENTISTS HAVE picked up
an “unusual” magnitude 5.5
earthquake far off the east coast
of Christchurch.
But, while Monday afternoon’s
quake was unusual for its location,
a seismologist says it poses
no great cause for concern.
GeoNet recorded the
quake striking about 340km east
of Christchurch at 2.37pm.
It also appeared to have been
relatively shallow.
With no stations closer than
about 350km, calculating the
quake’s depth was difficult:
GeoNet was assuming it around
33km, while other agencies were
estimating 10km.
It happened along the
Chatham Rise, an area of ocean
floor stretching about 1000km
from near the South Island in the
west, to the Chatham Islands in
the east.
It’s considered part of the
sprawling, sunken Zealandia
continent that New Zealand sits
upon.
“We don’t record many earthquakes
in this spot, but there
are a handful in the catalogue,”
said duty seismologist Dr Jen
Andrews.
GeoNet’s catalogue, spanning
back to the 1940s, showed only
20 quakes larger than magnitude
4.5 in the area, along with just a
few hundred of any size.
Most had been centred closer
to the north and east – and much
closer to the Hikurangi Subduction
Zone, or on the Chatham
Rise-Hikurangi Plateau margin.
“However, the earthquake recording
capability over time will
vary significantly in this offshore
region, especially for smaller
events,” said Andrews.
“It’s fair to say we don’t record
many events like this.
“The Chatham Rise has been
studied as part of understanding
greater Zealandia, and it is made
up of continental crustal material,”
Andrews explained.
“Imaging by seismic surveys
show many fault structures, a
testament to its active tectonic
history over millions of years.”
Because they are offshore, the
fault structures weren’t fully
mapped, which made it hard
to pin the quake to any specific
rupture.
“This recent earthquake is
likely just the result of the local
tectonic stresses caused by the
nearby plate boundary,” Andrews
said.
“Similar to other regions of
New Zealand that aren’t very
close to the major fault zones,
infrequent moderate-sized earthquakes
are to be expected.”
She saw no reason for concern
about tsunami risk, nor further
investigation, which might be
warranted if more significant
activity was recorded there.
– NZ Herald
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