The ‘second’ christchurch earthquake

The ‘second’ Christchurch earthquake, February 22, 2011 Dr Peter J. Stafford Willis Research Fellow RCUK Fellow / Lecturer in Modelling Engineering Risk Department of Civil & Environmental Engineering Imperial College London Note: Dr Stafford was born in Christchurch and has family and friends living in the city. He completed his undergraduate degree and his PhD at the University of Canterbury, in Christchurch. Widespread damage has been caused by the ground shaking induced by the ML6.3 magnitude earthquake located just to the south of Christchurch. The event is much closer, and much shallower, than the larger ML7.1 earthquake that occurred in September 2010. This is the primary reason why the levels of ground shaking that have been observed are so severe. Initial reports from GNS Science in New Zealand (a government research institute looking at Geological and Nuclear Sciences – GNS) indicate that the levels of shaking that were observed in and around Christchurch are significantly greater than the levels that structures are typically designed to withstand. It is therefore no surprise that many structures have suffered extensive levels of damage, and that some have collapsed. http://www.geonet.org.nz/var/storage/images/media/images/news/2011/lyttelton_pga/57159-‐1-‐eng-‐GB/lyttelton_pga.png

Images such as that above, taken from the GeoNet website, suggest that peak ground accelerations reached as high as 1.9 times the acceleration due to gravity (although it is not clear whether these are recorded in the horizontal or vertical direction – horizontal accelerations are those most relevant for estimating damage to buildings). By any measure, accelerations of this level are considered to be very large. For emergency response teams it is now crucial to ensure that survivors currently trapped in debris from collapsed buildings are extracted as soon as possible. Teams of people will be working through the night, in very trying conditions involving further aftershocks, but the likelihood of people surviving will reduce rapidly with time. This event is almost certainly an aftershock of the larger event that occurred in September. This large event caused damage to structures throughout the city that was thought to be primarily superficial (affecting what structural engineer’s refer to as non-‐structural elements, e.g., facades, in-‐fill panels, non-‐load-‐bearing walls and partitions etc). However it is too early to say whether some of the collapses that have occurred have resulted from undetected damage caused by the first major event. The recovery efforts following the first major event, while sustained and concerted, are far from compete and a lot of work remained in order to bring the city back to a normal state even prior to the more recent event that occurred this morning. The problem for many home-‐owners now will be associated with the uncertainty regarding how and when their homes will be repaired. The New Zealand Earthquake Commission (EQC) must again provide financial compensation for the losses sustained by the earthquake and this will place a heavy burden upon them. With estimates of direct losses on the order of 2 billion NZD resulting from the first event, it is likely that this new, more damaging, event will send direct losses many times above this level. And, of course, the indirect cost to businesses is also likely to be very significant. Aftershocks of this size are to be expected following a major earthquake. As a general ‘rule-‐of-‐thumb’ the largest expected aftershock in a sequence will typically have a magnitude that is roughly one unit of magnitude lower than the mainshock. Note, however, that a unit change in magnitude relates to an approximately 30-‐fold difference in the amount of energy that is released from the earthquake source. Aftershock events are defined as being dependent upon the mainshock. However, it is not always clear whether an earthquake is really an aftershock or not. The relevance of this point is that in many countries, insurance policies are tied to covering one ‘act-‐of-‐God’ type event per annum. If the damage caused by the aftershock is deemed to be part of the main event then one may be covered by insurance, if the event is deemed to be independent then coverage may not be guaranteed. The implications of this are great for events such as this where the aftershock is going to be responsible for significantly greater degrees of loss than the mainshock event. Fortunately, for those affected by this event, their coverage through the EQC is not structured in this way. A very good idea of the types of events that the citizens are experiencing now can be seen from the website below that plots the aftershocks that have occurred over recent times. http://www.christchurchquakemap.co.nz/

The ‘second’ christchurch earthquake

by petergnz on Feb 22, 2011

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