Tuesday quake 'no aftershock'

16:00, Feb 27 2011

"That wasn't an aftershock," geologist Robert Yeats says of the earthquake that struck Christchurch with deadly force last week.

Professor Yeats, professor emeritus of geology at Oregon State University in Corvallis, has questions about whether last week's tremor was an aftershock of last September quake or a new quake in its own right.

"It might be a separate earthquake, part of a sequence of earthquakes. It is quite far from the Darfield aftershock cloud, and its fault plane solution is different," he said.

"However, there are a number of examples of earthquake sequences migrating along a fault plane.

"One rupture builds up strain on other parts of the fault, and causes other parts to rupture. Analogous to pulling buttons off a shirt."

Yeats says it generally takes many years before seismic activity can be considered a new quake rather than an aftershock of a previous one.

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"That's a point of debate among seismologists. But you can't paint all aftershock series with the same brush."

More detailed investigations would have to wait until the search and recovery operation was completed.

Although in the South Island the Hope, Marlborough and Alpine faults are better known, earthquakes near or under Christchurch were not unexpected.

A research paper commissioned by the NZ Earthquake Commission in 1991 predicted moderate earthquakes under the Canterbury Plains and Christchurch itself.

It also signalled liquefaction.

With the benefit of hindsight, Yeats said that perhaps a seismic survey of the Christchurch metropolitan area might have given some warning.

Tuesday's quake caused so much damage because the shaking was very intense.

Peak ground acceleration was up to 2.2 times gravity.

"Most cities in the world would be totally flattened by such acceleration," Yeats said.

He sung the praises of New Zealand's building codes following the earthquake in September.

He compared the massive loss of life from Haiti's similar magnitude quake to the "success" of Christchurch's 7.1 experience.

"You have to realise that New Zealand has some of the strongest building codes in the world, and those building codes are respected.

"That means you have loss of life, but it's in the dozens or maybe 100 or 200.

"If the same earthquake were to happen under a city of that size in a developing country, the number of deaths would be in the thousands, if not tens of thousands.

"Turkey, for example, had great building codes but that didn't keep tens of thousands of people from getting killed in the 1999 Izmit 7.6-magnitude earthquake.

"Because they weren't paying attention to those codes," Yeats said.

After many decades of studying earthquakes all around the world – from Haiti to Afghanistan – Yeats knows the power of hope and urges Cantabrians to keep theirs.

"Miracles happen every day in my world," Yeats said.

The Press