And just for extra completeness:
2010 Canterbury earthquake magnitude 7.1, 10 km deep and 40 km from Christchurch: 16 times the magnitude, twice the depth and four times as far away. Zero deaths, 2 serious and 100 total injuries.
Firstly let me thank Dopers for their compassion and sympathy. It can be easy to ignore tragic events far away.
New Zealand like Japan (and California) is situated on the edges of tectonic plates. We have volcanoes and earthquakes. Generally the volcanoes provide a spectacle once in a while, and every few years the ground trembles. That’s about it. No real danger or drama. The earthquakes tend to be felt up to 200 miles away and I didn’t notice either of our recent events (300 miles away).
However in my 55 years I have seen at least 3 major earthquakes (and many smaller ones) but this event is one in one hundred years. Absolutely tragic. The only recent precedent is the Napier earthquake in 1931 where 257 people died and the foreshore lifted 5 feet out of the sea.
So why did a “small” earthquake cause so much damage?
The brutal fact is this fault-line movement was not small. No way. It was massive.
The problem and misconception lies with the Richter Scale and the more recent Moment Magnitude Scale. Both are geological measures which are designed to give scientists consistent information across the world regarding the energy involved.
A deep seismic shift can go unnoticed at the surface if the vibrations are soaked up by the surrounding earth strata. But a shallow hiccough barely worth noticing on the Richter meter can move houses and destroy lives. Think of landslides in South America - a slight vibration combined with rain and entire towns are torn apart.
NZ has strict earthquake building codes which is why our houses are so expensive. Despite the violent September 2010 quake, no modern houses actually fell down. Cracks yes, but they remained intact and secure while the ground under them opened and reshaped.
Christchurch was settled in the 1840s and contains many Victorian and Edwardian stone buildings. Its a lovely city, probably one of my favourites along with San Francisco and Vancouver. Christchurch is not known for earthquakes which is why we are all so shocked.
The historic buildings are being strengthened over time but - and this is a guess - I think the work was not sufficient to withstand a dramatic shock only 3 miles deep. That is very shallow. Additionally some modern buildings have also collapsed which indicates the energy released surpassed any design limits.
Magnitude scales measure energy released. You’d be interested in the ground motion parameters at the site, which are affected by type of 'quake, distance, intervening geology (how the motion is attenuated) and site characteristics (local soils).
Quite a few factors are involved. Some of which have been mentioned.
Firstly, Christchurch is not situated in an area known for earthquakes even though all of the country presents some risk. The fault which became active on Sept 4 last year was previously unknown to exist. It is now clear that a lot of energy was stored in rock layers not far below the surface. It is likely that the system has remained locked for centuries – longer than human habitation.
Secondly, Christchurch is located on the canturbury plains – a large alluvial plain formed from rock sand and shingle carried down by river from the Southern Alps. The plain is some 450km by 150km. It is not unusual for settlements to be built in such an area – many cities in valleys near rivers have some similar material. The problem is that this kind of substrate has the effect of magnifying the amplitude of seismic waves.
Thirdly, and related to the above is the relatively unusual situation that has arisen in Christchurch with respect to liquifaction. This simply means that with enough shaking the shingle becomes like a liquid. Imagine vibrating a tray of gravel. Alternatively think of what happens when you stand in wet sand and vibrate your feet or wat happens when you are pouring concrete and vibrate the mix. Anything that is dense or has large particle size tends to sink. Smaller light particles rise to the surface. In Christchurch however it is apparent that the substrate is not uniform. That means that the liquifaction effects are uneven. Many buildings have been damaged when one portion of the foundation has sunk more than the rest. I have heard of one house in the September quake that was severely damaged while the neighbours were unscathed. It turns out that there was an old stream bed 10 metres below the foundation of the house. It is really not easy to know these facts ahead of time and the fact that Chch isn’t (until now) known for earthquakes meqant that these factors were not considered as it was built.
Fourthly, much of the CBD consists of very old buildings. Christchurch is NZ’s oldest city with many stone and brick buildings built in the days befor steel reinforcing was in common use. If a similar event occurred in London or Paris there would be similar devastation.
Fifthly, there have been a large number of powerful aftershocks following the September quake. Many buildings have been weakened by the incessant hammering. There have been shallow earthquakes of magnitude 4-5 every couple of days and sometimes more frequently since september last year. That sort of thing will wear anything down eventually. It certainly made the city more susceptible to damage for this latest big aftershock.
Sixthly, as has been mentioned, this aftershock was extremely shallow for such a large tremor. The energy was released close to the surface and was not able to dissipate. One seismologist said that the earthquake hit the bullseye in terms of its position and magnitude to cause maximum damage.
Seventhly, some of the damage seen in residential areas and around Port Lyttleton has been the result of rockfall. Parts of the city lies at the base of some rather steep hills – the remnant of an extinct volcano. Although this is bedrock and relatively rigid and not prone to the same liquifaction, the large boulders that have been dislodged have been able to roll a considerable distance and cause a large amount of damage in some cases.
Eighthly, and this is a factor that impacts the human toll, the time that the earthquake struck was significant. On September 4 the quake struck at 4 in the morning when most people were safe in bed in their (mostly wooden) homes. These are generally quite robust in a quake due to NZ’s good building codes and also the materials used which allow a lot of flex before collapse. The Feb 22 quake happened in the middle of the working day while people were in their offices or cafes or tourists at the cathedral or wherever. At least one bus was crushed by a falling wall with a large number of fatalities.
Finally, I am led to believe that the tremor was of significant duration – in excess of minute. The motion was not simply back and forth but had vertical and horizontal components of movement. This can cause maximum damge as the shaking continues.
I’m sure that I have omitted something, but in summary, it was the combination of factors that made this situation so unexpected and so tragic.
The timing of the quake- just before 1pm in the afternoon- meant that there were a lot of people at work and out and about, which didn’t help.
The other thing is that even though it was well known that Christchurch was near a fault line, there hadn’t been any notable earthquakes in the nearly 160 years the city had been settled, so even though modern buildings were reinforced against earthquakes, it’s my understanding that no-one actually expected it to be a necessary precaution. Obviously the buildings were built properly, but just not with the same anti-earthquake measures you’d find in Tokyo, for example; at least from what I’ve been led to believe.