An unusually large aftershock from the September 4 Darfield earthquake is almost certainly to blame for the devastating quake that struck Christchurch yesterday, according to geologists here and overseas who have studied the seismic data.
In the latest update from GNS Science seismologists said that Christchurch had been “comparatively lucky” with the location and timing of last September’s earthquake at Darfield, but that the location of the massive aftershock within 10km of the city and at a shallow depth of 5km during the middle of the day “has resulted in destruction, injuries and deaths”.
They add:
“Shaking intensity in the city was much greater for this earthquake than the magnitude 7.1 earthquake for any of its other aftershocks. The highest shaking was recorded at Pages Road Pumping Station at 188 %g, with readings of 127 %g at Heathcote Valley Primary School and 107 %g at Hulverstone Road Pumping Station. This is due to the proximity of the epicentre to the city and the shallow depth.
“Seismologically, this is classed as an aftershock because of its relationship to the ongoing activity since September last year. Its occurrence was always statistically possible, but the long time interval and slow decrease in general activity had made it less likely. Unfortunately, it has happened after all and in a location that has brought the worst result.”
Our colleagues at the Science Media Centre in London gathered these comments from experts based in the UK. Feel free to use these comments in your stories.
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Expert comments:
Ian Main, Professor of Seismology and Rock Physics at the University of Edinburgh said:
“Early projections are inevitably somewhat imprecise, and the actual outcome will depend on the local response of site conditions to ground shaking.
“The earthquake is very likely to be an unusually big aftershock, triggered by the much bigger Magnitude 7.0 September 4, 2010 Darfield, NZ earthquake. This earlier earthquake produced no fatalities, being further away from the main population centres. However, it has triggered several smaller (magnitude 5.0-5.8) shocks to the east of its fault tip in the Christchurch area.
“The mechanism of the Magnitude 6.3 earthquake yesterday is consistent with the regional plate tectonic deformation field, taking up compressional tectonic strain in the WNE-ESE direction, but occurring in an area where only a few earthquakes had been recorded historically.”
Dr Peter Stafford, RCUK Fellow and Lecturer in Modelling Engineering Risk, Department of Civil and Environmental Engineering, Imperial College London said:
“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 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.”
Dr David Rothery, Volcano Dynamics Group, The Open University said:
“From the news footage, it is apparent that the damage has occurred in a variety of ways. In much of Christchurch where the ground is flat and underlain by sand or silt, some structures have been shaken apart, causing upper stories to collapse onto the floors below. This is because soft ground magnifies how violently the surface shakes during an earthquake. In some places the ground liquified during the shaking, which is a hazard when soft ground is water-saturated, leading to localized flooding. This can undermine foundations, causing buildings to sink or topple. In some hilly suburbs, rock falls from steep slopes and cliffs cascaded onto properties below.
“I am relieved not to have heard reports of schools collapsing, which happens all too often in many parts of the world where building codes are routinely flouted. School rooms are bigger than rooms in houses, and so the ceilings and roofs need to be especially well-designed not to part company with their supporting walls during an earthquake. Christchurch’s schools seem to have withstood the shaking well, and the teachers and pupils knew how to behave – seeking shelter below tables and desks untl the shaking stopped, rather than rushing for the exits.
“Today’s magnitude 6.3 earthquake was a very large aftershock following the 3 September magnitude 7.0 earthquake. Its focus was at a very shallow depth of about 5 km, which doubtless contributed to the severity of the ground shaking that witnesses in Christchurch describe as more severe than in September. The rupture was also much closer to the city, and this too made it worse. Other, hopefully smaller, aftershocks are to be expected. These will be a hazard to people trapped in damaged and weakened buildings and their rescuers.”
Dr Dougal Jerram, Department of Earth Sciences, Durham University said:
“Christchurch sits on what is historically a tectonically active area where the Alpine Fault runs right across New Zealand’s South Island, associated with this are many fault segments
“What seems to have happened is that the pressure has built up on a particular fault segment with the epicenter much closer to the city itself.
“This earthquake has also been particularly shallow, so despite measuring slightly less than the previous major earthquake in Christchurch, measuring only about 6.3 on the richter scale in comparison to the previous quake’s 7.1 magnitude, more of the earthquakes energy makes it to the surface causing far greater destruction, than if the quake was deeper underground.
“Other factors include the way in which the earthquake moves, whether it ruptures from side to side or vertically has a direct effect on the force of the quake.
“Critically the earthquake has also happened at a point when Christchurch has been at its busiest not in the dead of night when the city was empty as previously happened, which has tragically resulted in a much higher casualty level. The condition of some buildings in Christchurch after the previous earthquake might also have played its part.”
Dr David Robinson, Department of Earth Sciences, University of Oxford said:
“The earthquake occurred at around 12:50 local time (23:50 UK time) and was magnitude 6.3. It is likely that this earthquake is an aftershock related to the September 3rd Christchurch earthquake (Magnitude 7.0). The recent earthquake caused significantly more damage than the September earthquake because of its location – it was very shallow and was focused right underneath the city, whereas the September earthquake was further away.”
Professor Andreas Rietbrock, School of Environmental Sciences, University of Liverpool said:
“The New Zealand earthquake is part of the aftershock sequence of the magnitude 7.0 earthquake that the country experienced in September last year. The earthquake magnitude was estimated at 6.3 by the United States Geological Survey (USGS) and is so far the largest aftershock of this earthquake sequence. According to the USGS approximately six events, greater than or equal to, aftershocks of magnitude 5.0 have occurred since the September 2010 magnitude 7.0 earthquake. Whilst today’s earthquake is the largest aftershock so far it is difficult to predict if another large aftershock another magnitude 6.0, or one larger, will occur again in the near future.
“The earthquake can be explained, like the main shock in 2010, by regional plate boundary deformation associated with the interaction between the Pacific and Australia plates. The epicentre of the 2011 earthquake was located closer to the town of Christchurch which has resulted in more severe damage, despite its magnitude being much smaller than the previous year. The quake also occurred at shallow depths, which means the effects are felt much closer to the earth’s surface, causing more destruction to buildings and harm to people than an earthquake at deeper depths.”
Dr Elisabetta Mariani from the University of Liverpool’s School of Environmental Sciences is currently studying New Zealand’s Alpine Fault, one of the world’s largest fault lines in the world. Dr Mariani said:
“The latest earthquake to hit New Zealand may have occurred along a ‘blind fault’, which means that there is no expression of it at the surface of the earth that allows scientists to identify it. Our team has recently returned from New Zealand where we have been taking rock samples from the Alpine Fault, an area along the Southern Alps that is thought to rupture every 200 to 400 years, producing earthquakes of magnitude eight. In the labs in Liverpool we can look at the rock’s internal structure and simulate the stresses that it experiences under earthquake conditions. We hope that the data from this study can be used to understand the mechanics and dynamics of large earthquakes.”
Glenn Ford, a Seismic Analyst at the British Geological Survey said:
“This recent Christchurch earthquake was approximately 11 times smaller in energy release than the magnitude 7.0 Darfield earthquake on the 3rd Sept 2010. The Darfield earthquake was located about 70km further west.”
Dr Roger Musson, Head of Seismic Hazard at the British Geological Survey said:
“This earthquake produced exceptionally high ground shaking, more than the buildings were designed for and damage has been extensive. The death toll is expected to rise and will probably exceed 100.”
Alice Walker, Seismologist at the British Geological Survey said:
“Over 60 earthquakes of this size and above occur somewhere in the world every year.”
Dr Brian Baptie, Head of Seismology at the British Geological Survey said:
“This Christchurch earthquake is the most recent of an almost continuous series of aftershocks extending east towards Christchurch since the Darfield earthquake in September. “