Christchurch Earthquake

First Posting | Summary
Posting Date: September 3, 2010, 2:45:00 PM

An earthquake struck 55 kilometers west northwest of Christchurch, New Zealand, at 4:35 am local time (16:35 UTC) Saturday morning. Early images show toppled chimneys and parapets, and the collapse of many masonry infill walls. New Zealand’s Ministry of Civil Defence and Emergency Management has activated the national emergency response centre. Significant damage to infrastructure is feared, especially to sewer and water facilities in Christchurch. Orders have been issued to conserve water. Christchurch is the second largest city in New Zealand (population of 340,000 according to the 2006 Census) and the largest on the South Island. The quake, initially estimated at a Richter magnitude of 7.4 by GeoNet, a hazard monitoring system in New Zealand, occurred at the boundary between the Pacific and Australia plates. The USGS reported a moment magnitude of 7.0 for this event. Focal depth has been estimated at 12 kilometers, making this a shallow event. Aftershocks are continuing to occur, including a magnitude 5.7 aftershock that struck 20 minutes after the initial temblor. The main shock was felt for about 40 seconds, according to local reports. The Pacific Tsunami Warning Center has said there is no tsunami danger from today’s onshore event. The predominant construction types in New Zealand are wood frame, unreinforced masonry (URM) and reinforced concrete frame. The majority of older buildings are URM, which is highly vulnerable to ground motion. Throughout Christchurch, residents have reported broken glass, structural damage to houses and toppled contents. Masonry facades on many buildings have collapsed. One resident reported that a row of shops was completely demolished. In anticipation of looting, police have sealed off parts of the Christchurch’s central business district. It is still in the very early in the aftermath of this earthquake and downed telephone and power lines over a large area have made communications difficult. Half of Christchurch’s central business district is without power, as is most of the north of the city. In rural Canterbury, about 80 miles to the west of Christchurch, almost all electricity has been disrupted according to an employee at Orion Energy. Power outages and damage have been reported as far away as Dunedin, more than 350 kilometers to Christchurch’s south. People trying to flee the seaside suburbs of New Brighton and Sumner have caused traffic jams on local roads. Because roadways have been damaged throughout the region, police have asked residents to stay at home. Engineers are being sent to examine bridges for structural damage. In Avonside, near Christchurch, local residents reported that the Avon River overflowed its banks at certain places after the quake caused a surge in the river this morning. The overflow has led to flooding in nearby homes. More than 14,000 earthquakes impact New Zealand each year, but only around 20 have a magnitude in excess of 5.0. Today’s event occurred in the southeastern foothills of the Southern Alps, in central New Zealand. The epicenter was on the Porters Pass-Amberley Fault Zone (PPAFZ), which forms the southern part of the collision zone along the Pacific-Australia plate boundary. The Pacific plate subducts under New Zealand’s North Island, along the Hikurangi Trench. This subduction zone terminates about 100 miles north of Christchurch, which is on the South Island, and the plate boundary becomes a collision zone that extends across the South Island. There is an active fault zone in the north of the South Island that includes the aforementioned Porters Pass – Amberley fault zone, as well as the parts of the Alpine fault zone and the Hope fault zone. Most known historic earthquakes on the South Island occurred in this 200-mile wide collision plate boundary zone. These historic quakes include three events between Mw 6.8 and Mw 7.1 in 1881, 1888 and 1929 along the Hope fault zone, about 40 mile north of this event, and two more recent events (M 7.1 and 7.8 in 1968 and 1929, respectively) in the Buller region, about 100 miles north of today’s earthquake. Since instrumental record keeping began, the largest magnitude earthquake to strike New Zealand was the 1931 magnitude 7.8 Hawke’s Bay earthquake. This event prompted a significant revision of the country’s building codes. Over the years, New Zealand Building code has advanced significantly, and its current building code is one of the most stringent in the world. Once again, it is in the early aftermath of this event, whose parameters may yet change as reports continue to come in from seismic networks around the world. Insurance take-up rates in New Zealand are high for both residential and commercial lines. The AIR earthquake team is collecting and analyzing the available information and will provide updates as warranted by events. ALERT subscribers will be notified when loss scenarios generated using the AIR Earthquake Model for New Zealand are available.

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Posting Date: September 3, 2010, 2:45:00 PM