Quakes are stone's throw from big faultlines
OPINION: Poor Seddon. Poor Marlborough. And, to a lesser extent, poor Wellington.
Frightening as the latest swarm of earthquakes has been for residents of a city with high-rise buildings, limited routes in and out and large areas of reclaimed land, the capital is a good 60 kilometres in a straight line away from their epicentres.
Seddon and Ward folk, however, have no such luxury of distance. They are feeling every single magnitude 2.5 to 3.0 jolt, still rattling through several times an hour and punctuating the gaps between the larger magnitude 4 and 5 aftershocks.
On the other side of Cook Strait, it's only the shakes above about magnitude 4.5 that are felt.
Thousands of Christchurch residents will sympathise with that constant state of heightened alert, that continual feeling of being on the edge that accompanies such bombardment. You keep one foot on the ground when you go to bed so you are ready to run for the door. Every night you eventually drift off to sleep after lying wondering yet again at what time you'll be woken by another big one.
But at least with the Canterbury quakes the previously unknown faults that lit up are not in the vicinity of the larger faults that can generate much bigger quakes of say magnitude 7.2 and above. From Christchurch, the Alpine Fault lies well away on the other side of the Southern Alps, while the Hope Fault, another likely source of a major quake, is a good distance north.
While the quakes in Canterbury continually kept redistributing crustal stresses between the newly discovered faults, they were unlikely to transfer that on to one of the long faultlines associated with the boundary between the Pacific and Australian plates quite capable of generating something far larger.
What worries me is the reverse is the case in that part of Marlborough now quivering as if the Earth itself is scratching a persistent itch. And some scientists must be wondering about this too, though none are saying so at the moment.
The small local faults that appear responsible for this latest swarm and the one last month - the London Hill and Hog Swamp faults (though the scientists are not apportioning blame, probably because the evidence is still not clear) - are not much further than a stone's throw away from the Awatere and Clarence faults.
These two are major splits in the crust, hundreds of kilometres long running right through the North Canterbury high country and Marlborough into Cook Strait, which one scientist has described as like Grand Central Station when it comes to the meeting place for faults.
If it's possible that the current Seddon quake activity is moving stress and adding it on to these major nearby structures, then that could be extremely worrying.
It really would be a case of poor Wellington, too.
- © Fairfax NZ News
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