Sunday, 20 May 2007

Rockin' down Shake Highway 45

MT TARANAKI is soundly asleep, but you could still be shaken awake by a quake.
That’s according to seismologist Steve Sherburn, who is an expert on Taranaki’s tremors.
In the past 60 days there have been more than 30 shallow shudders on the coast, with about 25 of those in the Okato area, 26km south-west of New Plymouth, on the west coast of New Zealand's North Island.
Many have been too light to make the GeoNet website “recent quakes” section, but they do appear as splodges on a map that makes coastal Taranaki look like a teenager with severe acne.
New Plymouth looks worse, as if the pimples have become infected. While there has been less activity beneath the city, the quivers have been bigger.
Sherburn, from GNS Science in Taupo, says the cluster of quakes is normal for us. “The pattern that we see is fairly typical of what’s normal for Taranaki.”
That’s because of the Cape Egmont Fault Zone, which is a line of faults that run north-east and south-west off Cape Egmont. They are mostly offshore where there are no recorders, but the faults come onshore at the extreme west of the bump.
The fault belt heads towards Nelson and up towards New Plymouth, so the lines would look like 2o’clock on a watch face.
Our region is shaken between 200 and 300 times a year. “Some years there will be a lot more than that.”
New Zealand’s most active area is Matata in the Bay of Plenty, where there have been more than 700 earthquakes since November last year.
In Taranaki, we are having a normal year, says Sherburn, who has been one of Taranaki’s major fault-finders. A few years back he set up 70 seismic recorders around the region. For six months he monitored the movement of the ground and learnt something that will reassure those who fear our volcano could suddenly erupt.
In the whole time he took those recording, there were only three earthquakes under the mountain and they were “bog standard fault shakes”.
Sherburn is absolutely certain they had nothing to do with moving molten rock or gaseous liquids – nor have any quakes since.
“When volcanoes are waking up they produce a characteristic earthquake.”
These are, rather obviously, called “volcanic earthquakes” and their movement is not like a fault jolt.
“It’s more of a rocking than a sharp shaking,” he says.
It can, therefore, be picked up instantly by a seismologist, which is good news for those living in the shadow of Mt Taranaki.
“There is no evidence that it’s waking up,” he says. “Because we are not getting any now, hopefully when it does start to wake up, we will notice and, hopefully, get quite a long period of warning.”
But don’t start relaxing yet – there’s still the matter of the moving plates and our fault belt. Perhaps our surfing route could be renamed Shake Highway 45?

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