Monday 13 June 2011, Christchurch New Zealand was shaken by a series of strong tremors. The city is still recovering from two previous earthquakes that struck the region in September 2010 and February 2011.
According to the United States Geological Survey (USGS), the largest earthquake had a moment magnitude of 6.0 and occurred at a depth of just 9 kilometers. The USGS estimated that the quake’s epicenter was located 13 kilometers southeast of Christchurch at Lyttelton, the city’s port. The quakes collapsed dozens of buildings and knocked out power to thousands.
According to AIR, the city of Christchurch bore the brunt of the damage. Local authorities report that more than 75 buildings were tagged for demolition. There has been significant new damage to already weakened buildings and according to a statement from New Zealand’s Prime Minister, it is now expected that as many as 900 buildings in the city’s central business district (CBD) will need to be demolished for safety reasons as a result of the last two events.
According to AIR, the ground shaking in the CBD caused by this latest earthquake is similar to that of last September’s quake. The areas likely to experience the most severe damage are east and southeast of Christchurch. Given the good performance of New Zealand’s residential wood construction, further significant shake damage to these structures should be limited. However, AIR believes the severity of liquefaction damage in the city’s east and southeast will likely exceed that from the previous earthquakes, while in other areas, such as those near the city center and further north, liquefaction damage is likely to be less than that from the previous events. There have been reports of several sink holes opening up and several rock falls, mainly in eastern hills of Sumner, Taylors Mistake, Redcliffs, Morgan Valley.
Although the recent events were on a different fault from the event that struck Christchurch on February 22, they are part of the same aftershock sequence that was initiated by the Darfield earthquake, according to AIR. Aftershocks occur as a natural readjustment to the changes in stress caused by a quake and can last for many months or even years, depending on the size of the initial shock. New Zealand’s South Island, on which Christchurch is situated, has an active fault zone in the north, which includes a region known as “Porters Pass-Amberley Fault Zone” and two others, the Alpine fault zone and the Hope fault zone. According to AIR, most known historic earthquakes on South Island have occurred in this 200-mile wide collision plate boundary zone. The entire earthquake sequence is broadly associated with the regional plate boundary deformation as the Pacific and Australia plates interact in the central South Island.
Meanwhile, the Earthquake Commission (EQC) confirmed that it will be classifying Monday’s 6.0 tremors as new events. This is an important distinction for insurers who provide coverage for amounts above the EQC coverage; for reinsurers, the standard hours clauses used to define a loss occurrence would also treat these aftershocks as new events. At present, the EQC provides cover for damage to residential properties and contents from earthquakes up to a maximum of $100,000 and $20,000 for contents, along with defined coverage for damage to residential land. The complexity with this event derives from the many structures that are currently being repaired and that may have sustained additional damage, posing additional challenges for estimating incremental damage.
AIR continues to collect and analyze available data on this event. Updates will be provided as warranted.
Source : AIR Worldwide
