New Zealand has suffered a large aftershock following Septembers initial quake of 7.1. During this first event, which was ten times larger than the quake this month, there was one indirect fatality, yet so far the death toll from February's quake stands at 147 and 1/3rd of the buildings in Christchurch need to be demolished and rebuilt.
There is some dispute over whether this event should be considered a quake in its own right, or a significant aftershock. It is by no means the only earthquake to occur since last september, there have been 40 events between magnitude 4.5 to 5.0 and 14 between magnitudes 5.0 and 5.6. As of Sunday night, there have been five aftershocks between 5.0 and 6.3. The graph below (captured as a static image on 27/02, but available to view through this link as a live graph) shows that the fault zone is clearly still settling, though the amount of energy being released on a daily basis is reducing, the levels of seismicity are still above normal.
As a case study, this quake raises some questions about developed countries and their level of earthquake preparedness. New Zealand has every attribute to make sure that people are as safe as possible in earthquakes, they have a long running history of large quakes in the area, roughly one every 55 years. They have emergency services, earthquake insurance, modified buildings, all the benefits of a developed country and suffered an earthquake nearly ten times larger four months ago. In comparison to Haiti, where there had not been an earthquake in 200 years, they are one of the lowest ranked countries in the HDI ratings, and there was not a single seismometer installed in the region. So, why is it that Christchurch has suffered such significant damage? In simple terms, this was the worst type of earthquake, all the elements came together in an event that could not fail to be destructive, it was shallow, close to an urban center, the middle of the day and occurred on already weakened soil. The city center ground motions were between 50 to 100% stronger in February than September.
As a follow up to the discussion on P-Waves and their shadow zones before half term, here is the theoretical P-Wave travel time map from the USGS for this quake:
For this quake, the shadow zone is represented by the two thick black lines, and included large parts of North America, Africa and Russia, this does not mean that the quake was felt everywhere else, only that it could be recorded by a seismometer in regions outside of the shadow zone.
Factors to consider:
This was a shallow earthquake, at 5km, it is in the uppermost layer of sediments, often loose/unconsolidated rocks that can amplify earthquake L waves. The Haiti Quake was at 13km, last Septembers was 10km, and the Chile event of last year was at 35km, when a staggering 8.8 killed 521 victims (Chile is similar to New Zealand in terms of history and preparedness). Why this is important is that the more material the quake waves travel through, the more energy they loose, if the quake is shallow, none of the energy is lost through dissipation. Hence, the L waves that hit Christchurch had lost none of their energy when they struck.
This quake was close to Christchurch, only 10Km from the Center of the largest city in the South Island with a population of nearly 400 000, and a density of 863.5/ square kilometer.
Time of Day
This occured at the worst time of day, just after midday means that people were concentrated in the city centre and in office blocks and schools. Some of the worst groupings of fatalities were in the international school and the CTV buildings. The last quake happened at 4am, meaning the population was dispersed in their houses.
There were 29 buildings in Christchurch classed as "Non-Ductile", and at least two of the collapsed buildings, the PGC and CTV buildings were among those that collapsed with the worst fatalities. These were buildings that predated the modern building codes. Non ductile means concrete with no element of flex, therefore more likely to collapse as these buildings cannot accommodate wave movement. Many of the older buildings were buttressed after September and stayed upright, but many church spires have collapsed, including the Cathedral (it has also collapsed in 1888 and 1901)
Nature of the Boundary
Subduction zones hold the potential for the largest quakes as pressure is released by the springing back of the upper plate. It also means that large magnitude quakes can occur at shallow depth.
Christchurch sits on Alluvial sediments which are notorious for liquefaction, there has been extensive liquefaction in some suburbs with both water and raw sewage rising up to the surface. Many areas are cordoned off and the level of pollution will be high.
Overall, this quake is an example of how a combination of natural factors, and the location of a city have combined to make this a very devastating event, even though the quake has a relatively small magnitude in comparison to other recent events. Wherever this quake happened, the death toll and devastation would have been high as it was so shallow and so close to the city centre, proving it is not always the poorest countries that suffer, and that engineering and earthquake preparedness can only do so much in terms of mitigation.
The current situation is that no one has been found alive since wednesday afternoon, the city will need extensive rebuilding and all earthquake proofing needs re-assessing and possibly upgrading. The main hope now will be that there are no further large magnitude aftershocks to worsen this disaster as too many buildings have been overstressed by frequent tremors.