standard Christchurch earthquake secondary hazards

The recent earthquake that struck Christchurch earlier this week is known so far to have killed 113 people, with hundreds reported missing.  The greatest concern is that the death toll will continue to rise in the days ahead.  There appears to be a number of factors involved that led to devastation worse than the Canterbury earthquake last year.  Director of IHRR, Dave Petley, has covered the event extensively on his blog — On the causes of the high levels of loss in the Christchurch earthquake — identifying the high accelerations of the earthquake (up to 220% g) that would exceed the design strength of many buildings leading to excessive damage.  Also, the fact that the high magnitude shallow earthquakes within Christchurch can cause more damage than deep earthquakes.  This map from Geonet shows an overwhelming number of shallow earthquakes in and around Christchurch in the past 60 days:

Not far from the city of Christchurch, after shocks dislodged a number of boulders the size of buses that swept through the town of Lyttleton:

Three cyclists caught on Evans Pass in Lyttelton during the earthquake dodged boulders the size of buses as they ran for their lives, and they fear a jogger may have been killed.

Dave Curtis, along with Andre Chappell and another friend, were “terrified” as rocks ranging from the size of ovens to buses crashed down around them.

They said they suspected a jogger who had been running close by may have been killed by falling boulders. Two feared crushed by boulders in Lyttlelton.

Large boulders literally crashed through people’s homes as shown in these photos from The Press:

These forms of hazards are examples of how what happens after the earthquake can be nearly as deadly and damaging as the earthquake itself, but how people can prepare for something as sudden and powerful as boulders dropping from high elevations is daunting to say the least.  Other forms of secondary hazards have also taken place, especially landslides.  This short video of a landslide in Sumner, just south of Christchurch, gives a strong impression of just how intense these hazards actually are:

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Aerial photos of the landslide and soil liquefaction caused by the earthquake are available on this site.

Researchers in New Zealand have expressed concern about more landslides occurring in the area and about soil liquefaction, where soil loses strength and stiffness due to the stress of an earthquake causing buildings to literally sink into the ground:

Dr Mark Quigley, a geology lecturer at the University of Canterbury, said: “These are landslides that have the potential to carry houses down with them, or have run-outs into populated areas.

“There have been rocks the size of cars which have come down, and some of them have damaged houses – one has gone right through a house.

“There are still numerous boulders which have the potential to come down.”

Professor Peter Malin, director of the University of Auckland’s Institute of Earth Sciences and Engineering, said: “With the decay of the Darfield event, many of us would have breathed a sigh of relief – until Tuesday.”

The quake was a “strike-slip event with oblique motion”, meaning the earth moved mostly side-to-side but occasionally up-and-down.

Institute associate director Eylon Shalev said the vertical acceleration of the earth, at 1.9 times the acceleration of gravity, was far greater than the sideways movement.

“Anything you put at nearly 2g – it’s like lifting a building and dropping it to the ground.”

Professor Malin said the simultaneous vertical and horizontal seismic shifts made it almost impossible for buildings to survive.

“It’s a blow from below that compresses things up, then a shove sideways, and very little can withstand that.”

The earth under Canterbury is still shaking out the stress of the fault, with more than 70 aftershocks measured since Tuesday, four of them magnitude 5 or greater.

Seismologists said 4000 aftershocks had occurred since the September quake, but now the “clock had been reset” Cantabrians could expect months more of tremors.

Geologists have reported that the liquefaction in the city was worse than during last year’s tremor.

Many suburbs have seen houses sink into the ground as the shaken soil turns to wet mush. ‘Christchurch earthquake: Rockfall threat still strong, say experts.’ NZ Herald


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