Scientists take new view of quakes

Destruction in Manchester Street in Christchurch's CBD. Photo / Brett Phibbs

The catastrophic Christchurch earthquakes have transformed the way scientists look at fault lines near urban centres, says an American journal.

Seismological Research Letters, published today, has committed an entire edition to the Darfield and Lyttelton earthquakes, with 19 papers from New Zealand and international scientists.

Its editors said the surprisingly large energy release and unprecedented ground motion were a cause for concern for other earthquake-prone urban centres built on soft soil.

SRL's editor-in-chief Jonathan Lees wrote: "Compared to the earthquake that destroyed much of Haiti, the scale of disaster in Christchurch may seem small.

"Christchurch, however, was constructed using much better technology and engineering practices, raising a very sobering alarm to other major, high-density western urban centres."

Guest editor Erol Kalkan said the San Francisco Bay Area and central Los Angeles also sat on geological features that could exaggerate or amplify ground motion.

"The question is how to apply or account for such significant, higher-than-expected ground motions, as seen in Christchurch, when evaluating the design of existing and new structures."

The February quake was weaker than the September tremor in terms of its Richter reading, yet it produced the strongest recorded ground acceleration in New Zealand's history - four times greater than the Japan quake a month later.

The strike-slip event with oblique motion (mostly side-to-side motion but occasionally up-and-down) moved at 2g, or twice the speed of gravity. This meant structures were shaken, and also lifted and dropped, twice as fast as a dropped rock would travel from a person's hand to the ground.

The simultaneous vertical and horizontal seismic shifts made it almost impossible for older buildings to survive.

GNS Science seismologist Bill Fry and colleague Matt Gerstenberger compared the stresses of the three largest earthquakes in the sequence to global events.

Dr Fry said: "We found the Christchurch earthquakes were relatively strong compared to their moment magnitude. The amount of energy released for the February event was larger than the amount of energy that would be released on the average same [magnitude] worldwide." He said this was because the fault was "strong" - its sides were held together by large amounts of friction - and ruptured quickly, releasing a lot of energy.