December 2010 Christchurch earthquake

Boxing Day earthquake

UTC time	2010-12-25 21:30:15
Local date	26 December 2010 (2010-12-26)
Local time	10:30am NZDT (UTC+13)
Magnitude	Mw 4.7
Depth	5 km (3 mi)
Epicentre	43.55°S 172.63°E / -43.55; 172.63
Areas affected	Christchurch

On 26 December 2010 a M_w 4.7 earthquake occurred directly under the city centre of Christchurch, New Zealand, at a depth of 5 km (3.1 mi) or 4 km (2.5 mi). It caused "significant damage" to Christchurch and was part of the earthquake sequence beginning with the September 2010 Canterbury earthquake, and was followed by the 2011 Christchurch earthquake.

As the earthquake occurred on Boxing Day, it is commonly referred to as the Boxing Day earthquake. Businesses running their Boxing Day promotions at the time experienced lost revenues due to the earthquake, and responded by re-running Boxing Day sales on 12 February 2011.

Background

A magnitude 7.1 M_w earthquake occurred on 4 September 2010, and had, at the time, the biggest ground shaking ever to be recorded in New Zealand, at 1.25 times the acceleration of gravity. Several damaging aftershocks followed, including the Boxing Day earthquake, and the February 2011 earthquake, which caused 185 deaths and several billion dollars in damages.

Earthquake

The Boxing Day earthquake occurred at 10:30 am and had a moment magnitude of 4.7. It was located directly under the city at a depth of 5 km (3.1 mi) or 4 km (2.5 mi), with an epicentre near Barbadoes Street or 1.8 kilometres north west of Christ Church Cathedral. The maximum peak ground acceleration measured 0.48g, at Christchurch Botanic Gardens. By contrast, the M_w 7.1 September earthquake had peak ground accelerations in the central city ranging up to 0.3g. The Boxing Day earthquake did not last as long as the September one, which resulted in less damage. The strong ground shaking, despite the relatively low magnitude, was caused by the shallow depth and epicentre near the city centre. Strong ground shaking was limited to central Christchurch, due to the low magnitude. It was Christchurch's biggest earthquake magnitude since mid-November, and the 17th biggest since the sequence began in September.

The Boxing Day earthquake was followed by a 'swarm' of earthquakes in the same area, with 'two dozen' occurring in the first 36 hours, and over 30 occurring within three-to-four weeks. They mostly occurred within a 1 km² (0.39 sq mi) area, at depths of 3.5–7 km (2.2–4.3 mi), and "on a steeply dipping [strike-slip] fault with strike ~74°, with activity on that fault spread over a distance of ~2.5 km." Five of them had magnitudes equal or above M_L 4.0, and 20 were above or equal M_L 3.0.

Because the earthquake did not occur on the Greendale Fault that ruptured in the September earthquake, the Boxing Day earthquake does not fit the strict definition of an aftershock, which must occur on same fault. As it is likely that the Boxing Day earthquake was caused by the September earthquake, it can be 'loosely' regarded as an aftershock.

Damage and effects

The Boxing Day earthquakes caused "significant damage" to Christchurch. Building damage mostly occurred on buildings that had already been damaged in the September earthquake and its aftershocks. Of the 3,000 buildings that had been checked within a day of the earthquake (out of 5,000) in Four Avenues, 115 needed repairs.

Immediately after the earthquake, 40,000 residential properties lost power in Papanui, St Albans and Fendalton. It took about an hour for it to be mostly restored, and the power network did not experience significant damage. The earthquake also caused windows to break including in shop fronts, a few wall collapses, bricks and mortar to fall out of a few buildings including the building of Southern Encounter Aquarium in Cathedral Square, the top of a building to fall onto Manchester Street, a few cracks in buildings and a few bricks to loosen, broken sprinkler pipes, and masonry to fall onto a few streets. The damage was worst in Cashel Mall, "where an internal wall collapsed and building parapets crumbled", and some doors jammed which trapped people in buildings. Bricks and debris fell off a building and crashed through the roof of a restaurant in City Mall, crushing several tables. According to the owner, if the restaurant had been open, people could have died as a result. There was no damage to water, wastewater or stormwater networks. There were also no reports of injuries.

The Earthquake Commission treated the Boxing Day aftershock as a separate event to the 4 September earthquake. By 6 January, the commission had received 3,500 damage claims, and that rose to 7,000 by 14 January. By contrast, the September earthquake had 157,000 at that point in time, the 19 October earthquake had 2,139 claims on 6 January and the 14 November aftershock had 1,986. This was also more than the 6,200 claims received after the M_w 6.7 2007 Gisborne earthquake.

An expert panel report concluded that the aftershock "did not significantly reduce the earthquake resistance" of four major buildings that experienced building failure in the February 2011 earthquake: the CTV Building, the PGC Building, the Hotel Grand Chancellor building and the Forsyth Barr Building.

Response

Shortly after the earthquake hit, hundreds of shoppers were evacuated from the Westfield Riccarton shopping mall. The fire service received 200 emergency calls, mostly from alarm activations. Other calls included gas leaks, small fires, and people being trapped in lifts or stairwells.

Due to concerns about unstable structures, two blocks and several individual buildings were cordoned off, including in Cashel Street Mall. A section of Popular Lane was also closed. These cordons gradually shrank in the late afternoon, and were mostly gone by the morning of the following day. Police made three arrests relating to the cordons. A Civil Defence Emergency Operations Centre was set up at the council's Hereford Street building.

The earthquake occurred on Boxing Day, which is one of the busiest trading days of the year. A few shops had to close as a result, and retailers experienced a drop in revenue: Christchurch electronic transactions were down 10.7% from the previous year, which was more than the national drop of 6.7%. To recoup for these losses, Boxing Day sales were re-run on 12 February 2011. It was launched by Mayor Bob Parker and Earthquake Recovery Minister Gerry Brownlee at Cashel Mall, and received funding from the government, the city council and earthquake trusts.

At first it appeared that building damage would mean that the New Year's Eve celebrations in Cathedral Square had to find a new venue. But after structural assessments determining that there was no risk to the public, the council later decided that the celebrations would still take place there. It expected 15,000 attendees.

External links

• Photos of earthquake damage

• Boxing Day sale poster for the February 2011 'replay'