Scientists find rare mix of factors exacerbated the Christchurch quake -

17/03/2011 12:03 pm

Four seismic effects, including one that is rarely documented, combined to produce unusually strong ground-shaking in February's magnitude 6.3 earthquake in Christchurch, scientists have found.

The ground accelerations in Christchurch, the largest ever recorded for a New Zealand earthquake, were as much as four times higher than the highest accelerations measured in the magnitude 9.0 earthquake off the east coast of Japan on 11 March 2010.

The largest ground accelerations in Christchurch during the 22 February earthquake were more than twice the acceleration due to gravity.

Graphic showing the unusually high vertical ground accelerations in central Christchurch recorded during the magnitude 6.3 earthquake on 22 February 2011. Seismologists at GNS Science have found that four separate seismic effects combined to increase the intensity of the ground-shaking in Christchurch.

GNS Science Seismologist, Bill Fry, said the earthquake was so well documented by a network of seismic instruments around the greater Christchurch area it would provide many fresh insights that would change global scientific thinking about earthquakes. Most of the instruments in the Christchurch seismic network had been in place for a number of years.

"It's rare to have such a large amount of high quality data from a network of instruments so close to the epicenter of a damaging urban earthquake," Dr Fry said.

The data from these instruments had enabled seismologists to deepen their understanding of the mechanics of earthquakes. In time, this new knowledge would feed into a range of mitigation measures that would help New Zealand communities become more resilient to earthquakes.

The four dominant factors that contributed to the intense shaking in Christchurch were:

the high amount of energy released in the rupture of the fault
the direction that energy was released
a recently discovered trampoline-like interaction between geological layers under the city
the close proximity of the earthquake to the city.

Amount of energy released
The faults that failed in both the September magnitude 7.1 and February magnitude 6.3 Christchurch earthquakes were very strong, with high amounts of friction holding the two sides together..

"When a 'strong' fault breaks, it releases more energy than an equal-sized 'weak' fault would. Think about the difference between breaking a 1cm-thick sheet of Styrofoam and a 1cm-thick sheet of plywood, and the amount of jarring you would feel in your hands. The Canterbury crust is analogous to the plywood, but most earthquakes are more like breaking the Styrofoam."

In addition, seismologists have discovered that the earthquake rupture was faster than most earthquakes of this magnitude.

"In fact, the rupture traveled at about the same speed as some of the waves that were generated by it. This implies that as the waves from the first second or so of the rupture were traveling toward Christchurch, new energy was being released during the next couple of seconds, and these were added together to make the shaking particularly 'punchy'.

"This resulted in something analogous to the supersonic boom of a jet aircraft."

Direction of energy release
The previously unknown fault that ruptured under the northern edge of the Port Hills was sloping back like the back of a south-east facing lounge suite and pointing straight at Christchurch. When it ruptured, most of the energy was directed north-west toward the city.

Trampoline effect (also called 'Slapdown')
Seismic recordings show the dominant ground acceleration in Christchurch was oriented in the vertical direction (see graphic). In fact, higher accelerations were recording going up than down.

A recently discovered physical phenomenon explains this observation. As the huge early pulses of energy traveled through horizontal layers of the earth beneath Christchurch, the weaker upper layers traveled farther upward than the stronger lower ones, and so separated from them. When these upper layers fell back under gravity, they 'slapped' against the lower layers coming up again, producing very high impacts.

"These 'slapping' impacts generate energy that travels back up towards the surface, magnifying the whole process."

This effect also helped to explain the widespread liquefaction in Christchurch. As the layers separated, water in subsurface layers became less bound in soils. When the slapping occurred, large amounts of water was forced up to the surface.

Dr Fry said there had been isolated recordings of the trampoline effect in other parts of the world, but the data from Christchurch was unprecedented for its quality.

Proximity to Christchurch
The Christchurch earthquake occurred within kilometers of the central Christchurch leaving little time or distance for the seismic energy to attenuate before impacting on the city.

"We know that the top few metres of the ground in Christchurch played an important role in the shaking. But by studying the physics of the fault rupture, we now know that the intensity of the shaking was exacerbated by processes that occurred on the fault itself."

The Royal Society has posted a paper from the Office of the Prime Minister's Science Advisory Committee which answers many questions related to the Canterbury Earthquakes.

Listen to an interview with Martin Reyners and Bill Fry on National Radio (Audio from Thursday, 17 March 2011) .GNS seismologists explain why the Darfield and Christchurch earthquakes were so damaging and 'punchy' (duration: 13′04″) Download: Ogg Vorbis or MP3

A new video with Martin Reyners talking about the "Slap-down" effect with the Christchurch earthquake.

See a new video with Martin Reyners talking about the "Slap-down" effect with the Christchurch earthquake, here at source.

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