Today is the 4th anniversary of the main Christchurch earthquake on the 22nd February, 2011. With over $30 billion in economic costs and counting... as well as 185 fatalities, the city of Christchurch and its surroundings were shaken to their core over a long couple of years of damaging earthquakes.
The following is a summary of some of the events that unfolded by a young engineering geophysicist who is contributing to earthquake-report.com, and has also created in conjunction, the youtube channel CATNews.
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A link to a video summary of CATNews on the Christchurch 2010-2014 sequence of events.
About the Author: Andreas Schaefer is a PhD student at Karlsruhe Institute of Technology working in the field of tsunami and earthquake engineering and natural disaster risk assessment. He participated in the earthquake study tour to Christchurch in July 2014 with Prof. Dr. Michael Griffith of the University of Adelaide, investigating and studying the earthquake damage, the recovery process and new seismic design concepts.
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1. The Earthquake Sequence
In 2010, an earthquake sequence started in the Canterbury county of New Zealand, which was completely unforeseen. On September 4th, 2010 the Darfield earthquake hit the region with a moment magnitude of 7.1. It occurred on a fault of which no activity had previously been recorded based on paleoseismological observations for the last 16,000 years. Furthermore, not a single foreshock had been observed and thus this event can be classified as having been unpredictable based on our current knowledge. It occurred about 40 km to the west of Christchurch, in close proximity to the small town of Darfield. It caused major damage near the epicenter and slight to moderate damage in the communities of Christchurch, including massive liquefaction.
The whole area was hit by a large number of aftershocks which reached their peak with a subsequent secondary main shock with a magnitude of 6.3 on February 22nd in 2011. This event was considered a direct hit on the city of Christchurch since its epicentre was located close to the suburb port town of Lyttelton at a depth of 5 km and at about 9 km distance from the Christchurch CBD. Three more major events with magnitudes of about 6 +/- 0.4 were observed in June and December 2011 causing further damage. Based on tectonic observations, an earthquake propagation can be observed from west to east to north-east.
The February earthquake was by far the most damaging event of the whole sequence, leading to major destruction in multiple suburbs due to liquefaction of soft soils and along river banks, and moderate to severe damage to most of the CBD buildings. The maximum observed ground motion was 2.2 g for the vertical acceleration, measured at a school building in one of the suburbs, multiple times higher than the design accelerations of most of the buildings. In addition to ground shaking and soil liquefaction, rock falls from the hillsides were observed causing major damage on structures along the rock fall paths.
2. Damage
The damage during all earthquakes was wide spread, but the most severe damage occurred during the February event. Especially the destruction in the CBD was in economic terms unrecoverable (to date over $30 billion USD is the estimated cost of the sequence with some estimates reaching over $40 billion). About 80% of the whole city centre suffered damage to a level where retrofitting was considered infeasible. Except for a small number of buildings, the seismic design was successful, leading to almost no collapsed buildings, and leading also to many saved lives in comparison to other past worldwide earthquakes. Unfortunately, the small number of total collapses caused 185 fatalities. 115 fatalities and thus almost more than 60% of the total death toll was caused by the collapse of the CTV building. It was built in the 1980s and thus met the seismic code at the time, but mismanagement during construction including corruption and insufficient supervision of the building designer led to multiple mistakes during the design and construction process.
Similar conditions were also reported for the PGC building, whose collapse caused an additional 18 fatalities. Furthermore, the collapse of buildings and building elements of unreinforced masonry added another 42 fatalities (a good study by Lisa Moon can be read here). In general, these fatalities were caused by wall collapse and falling parapets when people left the building and were hit by falling bricks and debris.
Major losses were also reported from heritage buildings, including the Christchurch cathedral, which won't be rebuilt, after suffering severe damage including a total collapse of the cathedral's tower. The famous Christchurch Arts Centre did relatively well during the events, because of already ongoing retrofitting and the application of seismic design years beforehand, providing a good example of how to combine heritage preservation with successful seismic retrofitting. Critical elements could have been the multiple skyscrapers which experienced moderate to severe damage, being close to collapse, which was often indicated by a major inclination of the whole building due to column failure or delineation of structural elements in the lower levels.
Another significant factor causing severe widespread damage was liquefaction. A process which liquefies the soil while shaking removing any load bearing possibility and thus leading to massive destruction due to lateral movement of building parts, and destruction of infrastructure.
Liquefaction video by "Spongebob654"
Almost all suburbs along the Avon River close to the shore and some suburbs along the beaches were heavily affected. Damage was observed mostly at residential buildings and bridges. In addition to the liquefying effects, the shaking caused widespread sediment settlement, thus some location subsided more than 500 mm. Together with lateral spreading this leads to an increased hazard for future flooding and higher river levels. During the two major earthquakes in September and February, more than 60 bridges experienced severe damage mostly related to liquefaction which induced lateral spreading and subsidence of the river banks. In many cases, non-structural damage was observed in addition to structural damage, inducing damage to water, electricity and telecommunication infrastructure.
Another significant but often forgotten damage-causing secondary effect were rock falls and landslides. During almost all major events of the Christchurch earthquake sequence rock falls were observed. Many Christchurch suburbs are located at the flanks of the Port Hills, which originate back to the volcanic activity of the Banks peninsula, consisting of rock formations with basalt. Multiple rocks with more than 1m³ volume fell down the hills and crushed almost everything on their way. Since there is no building standard to protect buildings from rock falls, they are extremely susceptible, even if a direct hit is unlikely.
A boulder caused this hole in a house! (AGU)
In one example, a boulder simply passed through a whole house, leaving major holes behind in the walls and the house uninhabitable. The Bluff of the Port Hills has been closed for 4 years now with a repair bill of $20 million. The question of response remains open, as more than 10,000 rocks with a volume of more than 1m³ were indicated to be unstable in the Christchurch area. Furthermore hazard adoption led to unacceptable costs of construction, which led to abandoned houses and roads in hazardous areas. In addition, cliff falls in the suburbs of Sumner, Redcliffs and Mount Pleasant led to the destruction of multiple buildings along the cliff top, where the cliffs partially collapsed.
3. Reconstruction
Before the reconstruction took place, the society of Christchurch had to recover first, and therefore a lot of engineering effort was put in place to quickly tag buildings to tell people whether it was safe to return home or not. A green tag meant that a building is damaged, but safe to enter (or is undamaged), while yellow tag already indicates a damage state that an entering person should be aware of, and a red tag indicates that entering the building can be a threat to life and is unadvisable. Since major damage was also seen to the Christchurch wastewater infrastructure, more than 35,000 chemical toilets were distributed over the city. In the aftermath of the earthquakes and during the recovery it was decided that about 1,100 of the about 1,600 CBD buildings would have to be demolished and replaced, since retrofitting the buildings was infeasible. Of more than 300 bridges in the Christchurch area, 63 have experienced severe damage and will be replaced. Many of the liquefaction-affected suburbs will not be inhabited in the future and have been red zoned (deemed unfit for reconstruction), and also many locations along the hillside and the cliffs are now red zones where no further habitation is allowed.
Since the major shopping areas of Christchurch were destroyed, the community established the so-called Re:Start Mall in the city. An agglomeration of shipping containers in which shops have been located function as the new heart of the city centre. The area is filled with different shops, food places and architecturally designed to build up a nice atmosphere. Even being just a temporary solution, there are already voices calling to maintain the Re:Start Mall even after reconstruction of the CBD is finished. Today, the Christchurch CBD is the playground of architects and the community faces its responsibility of reconstruction appropriately redesigning city functions and investing in the future of the city. New building plans have been published, which prohibit the construction of skyscrapers. A famous, prior temporary, replacement building which will probably maintain part of the city is the Transitional Cathedral, also called Cardboard Cathedral, which is the current replacement for the destroyed Christchurch cathedral. It is mostly built from cardboard tubes and is now one of the most prominent landmarks in the city centre located close to the CTV memorial square.
Together with the local university, international engineering experts and companies, the lessons learnt from the earthquakes are being applied in developing new technologies for more earthquake resistant structures. Shifting the design criteria from not only saving lives but also to save costs in the future. Application of base isolation in multiple large scale structures like the new Christchurch Women's hospital, which was finished some years prior to the earthquakes and proved the quality of such technologies. Base isolation is becoming more commonplace also in commercial buildings like St. Elmo's building in the CBD. Many more technologies like special timber frame structures and reinforced concrete column bases have been developed and are currently applied in real world structures to lower the damage expected in the next event.
The total reconstruction will go on for at least the next 10 years regarding the total reconstruction of the CBD, retrofitting, repairing and replacing of an immeasurable number of residential buildings and rebuilding major parts of the bridge infrastructure. Thus, it will remain interesting to see the city evolve over the coming decade.
4. In Summary....
Even knowing that any deadly earthquake is a disaster, the earthquake sequence of Christchurch has shown that appropriate seismic design and community work during the aftermath lower the severity of such disasters. From an engineering perspective, the behaviour of the city's buildings was considered to be a success for earthquake engineering, since barely any buildings collapsed and most structures behaved within their design limits. But this makes the fatalities which occurred within the small number of building failures even more severe. Economically speaking it was still a complete disaster, since most of the damaged structures had to be demolished and replaced including the red zoned buildings. This led to the shift in design priorities from not only to be life saving in the future but also to save the structures.
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