Darfield New Zealand 2010 (GNS)
In the early hours (4.35 am local time) of Saturday morning on September 4, 2010 an earthquake of Mw 7.0, 10 km deep struck the Canterbury Region of New Zealand in the South Island.
The epicenter (43.53 S, 172.12 E) was very near the small town of Darfield (population 1600) and ~40 km west of Christchurch (376,700 inhabitants at June 2010, Statistics NZ), New Zealand’s second largest city and main city of the Canterbury Region. In total 565,700 were living in the Canterbury Region at the time of the earthquake (NZ statistics).
A right-lateral strike-slip ground rupture was identified near the epicentre in the Canterbury Plain. It was named the Greendale Fault and had a length of 24-30 km striking east-west with maximum displacements of up to 5 m horizontally and 1.5 m vertically. The Canterbury Plains had no previous historical evidence of earthquakes. The earthquake rupture was in fact a complex process involving several fault segments (probably four), including stike-slip (in agreement with the Greendale Fault) as indicated by the teleseismic moment tensor (USGS) and blind thrust as indicated by seismological observations (GeoNet).
The event, felt throughout New Zealand, generated ground motion up to 1.26g (vertical component) with several readings well over 0.5 g, while the maximum intensity was estimated as MM IX in the areas near the fault rupture.
Probably the most striking features of the Darfield earthquake and of the Canterbury Seismic sequence that followed in general, were the multitude of aftershocks and the vast amounts of liquefaction phenomena that occurred. Aftershocks continued for several months after the earthquake, some strong enough to cause damage to already-weakened structures. The most severe were the Mw 6.2, on February 22, 2011 (the Christchurch earthquake), the Mw 6.0, on June 13, 2011 and the Mw 5.9, on December 23, 2011 (GNS Science).
During the September 4, 2010 Darfield earthquake the hardest hit areas were close to the coast and to the riverside areas, especially the town of Kaiapoi and the Christchurch suburb of Bexley. Those areas lie on the edge of the Canterbury Plain made up by coarser gravels in the western part and close to the coast (eastern part) by many layers of finer-grained sediment, such as sand, silt and clay characterized by a shallow groundwater level (0-2 m).
Liquefaction phenomena severely damaged buried lifelines (in particular water and wastewater pipelines), residential housing and other buildings, and to a lesser extent roads, railroad, bridges, embankments and levees. Liquefaction phenomena across the urban area of Christchurch resulted in widespread ejection of silt and fine sand (some of which contaminated by raw sewage) that created a significand demand in resources.
Generally, the building stock performed quite well to the ground shaking with the majority of damage suffered by old unreinforced brick masonry buildings, or brittle non-structural components (e.g chimneys). Structural damage occurred mainly on liquefied soils.
Swiss Re refers that a total of 100,000 buildings (out of which 500 were destroyed) and that 300-400 farm buildings were damaged. CATDAT based on 1,200 uninhabitable houses indicates that around 4000 people were made homeless (400 homeless were reported by Swiss Re).
No one died but over 2200 people were injured of which up to 93 were seriously injured. The lack of fatal incidents was in part attributed to the earthquake’s time of occurrence (streets were largely deserted), the fact that the epicentral area was sparsely populated (USGS’s PAGER estimated that 20,000 people were exposed to intensity VIII and 2,000 to intensity IX), the use of timber frame housing in the region and mainly to adherence to New Zealand's strict building codes.
In Christchurch city center many old unreinforced brick masonry buildings did suffer significant damage and the falling masonry hazard would have been a serious threat had the earthquake occurred during business hours.
The overall direct economic loss due to the Darfield earthquake ranges between 5.08 billion USD (SwissRe) to 6.5 billion USD (MunichRe and EM-DAT). The direct economic loss represented 3.6-4.5% of New Zealand’s GDP in 2010. Large part of this loss was insured: 4.45 billion USD for SwissRe, 5 billion USD for MunichRe.
Claims management continued more than a year after the earthquake, with paid claims reported by the EQC as of February 23, 2012 reaching 985 million USD (1182 million NZD), with 32.5% of the 156,656 accepted claims being closed and another 51.7% having received partial payment while 15.8% of the claims had not yet received any payment.
2256 injury claims were submitted to the ACC (Accident Compensation Corporation), but at least 500 of these occurred after the earthquake during the cleanup operations. The overwhelming majority of these were minor injuries.
There was extensive damage to unreinforced brick masonry buildings in the epicentral areas and in Christchurch CBD, but none collapsed entirely (2 collapsed during a M5.1 aftershock). Some of these masonry buildings were very badly damaged and had to be pulled down incl. 2 major buildings in Christchurch (the Manchester Courts and the Cecil House). Retrofitted masonry buildings performed much better.
By Sept. 15, 2010, in Christchurch CBD, 7% of 1151 ispected buildings were declared unsafe with demolition order under review.
In the rural-epicentral areas agricultural sheds were damaged but it was reported that only a few were likely to need to be completely rebuilt. The historic Homebush Stables building was badly damaged and had to be demolished.
More houses would have to be demolished due to the effects of liquefaction in Kaiapoi (30 to 40) and in Bexley (at least 60).
Four weeks after the earthquake, 6855 surveyed properties had land damage ranging between minor and very severe (Tonkin & Taylor, 2010).
595 out of 958 unreinforced masonry buildings existing in the Canterbury region were inspected and 91 (15%) had significant damage.
Two weeks after the earthquake, 2737 homes out of 220,000 Canterbury dwellings were unliveable and 3053 were not weather proof. However most of these homes were later repaired.
According to the EQC, approximately 141,000 building damage claims were accepted that were solely related to the September 4, 2010 earthquake. Claims refer to individual properties (not buildings).
Homeless people are estimated at ~7000 in the first two weeks with the number slowly falling as repairs were being carried-out. More than 250 people in welfare centres.
6.5 billion US$ overall loss incl. 5 billion in insured losses (Munich Re).
Indirect economic losses are reported in the 2011 Christchurch earthquake and refer to the cumulative effects of the 2010-2012 Canterbury earthquakes sequence.
By February 23, 2012 the EQC had paid 985 million US$ in claims related to the 2010 Darfield earthquake with 32.5% of the claims closed, while 15.8% had not yet received any payment.