Scientists say the 7.1 magnitude quake that rocked Canterbury on Saturday morning came from a previously unknown fault, but that if recent history is anything to go by, it could be one of a series of 7+ magnitude quakes some of which are still to come.
Euan Smith Professor of Geophysics, Institute of Geophysics Victoria University of Wellington provided these comments to the SMC on various aspects of the earthquake:
“Like all earthquakes, Saturday morning’s M 7.1 earthquake did some things as expected but had some surprises as well.
“To put the earthquake in a historical perspective: this was the first occurrence of a major (magnitude 7 or greater) earthquake very near major urban areas since the 1931 M 7.8 Hawke’s Bay earthquake, which killed 256 people and injured thousands; and it was the largest earthquake onshore since the M 7.1 Inangahua earthquake in 1968, which killed two people.”
“The quake’s mobilisation of the Christchurch’s soils that had been identified as liquefaction prone, and the damage to roads, bridges and pipe infrastructure that resulted, were not surprising. Similarly the damage to unreinforced masonry structures (URM), including older brick chimneys, was no surprise. Both these risks have been identified in many studies. For example, chimney collapse was a major contributor to damage to housing in the 1931 earthquake.
“Similarly, the number and size of the aftershocks that the main shock has induced are, to date, in accordance with what we understand about aftershocks.
“For the surprisingly low casualties we can point to the good fortune of the earthquake occurring in the early hours of the morning. Had it happened at, say, 12:30 pm on a Friday, then the wreckage in the CBD, and damage to and within commercial properties elsewhere, must surely have produced a very much higher casualty rate that would have included deaths. However, we should also acknowledge the beneficial effect of New Zealand’s building codes, introduced after the 1931 earthquake, in reducing the damage to code-compliant structures, and their human occupants, in the Christchurch earthquake.”
“One ‘surprise’ that should not have been was the occurrence of the earthquake on a hitherto unknown fault. In fact the Earth science community knows that not all active faults in New Zealand have been identified. So the existence of this fault is an example of one of Donald Rumsfeld’s ‘known unknowns’.
“The occurrence of a major earthquake on a fault unknown before the event is by no means unprecedented. For example, the 1987 Edgecumbe, Bay of Plenty, earthquake occurred on a fault that was only recognised as such after the earthquake. The rapid sedimentation of many river plains means that the record of a fault’s past history may be buried quickly, leaving no trace of the fault for investigators.
“What is more of a surprise to all of us is that the Earth chose this fault to release some pent-up stress rather than a known active fault, the Porter’s Pass fault, less than 40 km away. How and why a ‘choice’ like this happens is not understood, but is the subject of ongoing research.”
“Several records show at least two large pulses. The USGS record has them about 20 sec long and 20 sec apart.
“I would guess that these two pulses represented continued rupture of the same fault, which would make them unlike Samoa’s two. But that’s a guess and modelling the data will need to be done to confirm it.”
Exploring the fault
“Seismological studies will in due course provide an image of the fault. But we can already make some estimates of its dimensions, and how much it moved on Saturday morning. The seismic moment of an earthquake is the product of the fault length, width, slip and shear modulus, a measure of rock resistance to shearing, usually about 3 x 1010 Newtons per square metre.
“A magnitude 7.1 earthquake has a seismic moment of about 6 x 1019 Newton-metres, and this is confirmed by GeoNet and USGS, who show that the fault was near vertical. If the earthquake broke all or almost all the brittle crust, the width will be in the range 10-15 km. Fault slip is often proportional to fault length. This means that the fault length was probably in the range 40-50 km long, and slipped an average of 2-4 metres. This in accord with the now observed surface offset of the fault.
“That a fault with these dimensions should be hidden until it reveals itself in an earthquake is a sobering thought for all those in the business of assessing a region’s earthquake hazard, and constitutes perhaps the second immediate lesson from this earthquake.”
Lessons from the quake
“The first lesson, for local authorities and policy makers, concerns what happened to the unreinforced masonry structures in this earthquake. Engineers in New Zealand have been warning of the danger posed by URM for decades. Some communities have taken action and required property owners to demolish such structures or to take remedial measures, such as retrofitting anti-seismic strengthening, to bring the structures up to an agreed level of performance in expected earthquakes.
“It is time for the rest to act. Owners of unreinforced masonry buildings, and chimneys, everywhere in New Zealand should be given a reasonable period of time – say 10 years – to demolish them or make them safe in future earthquakes. Action should be taken New Zealand-wide because nowhere is immune from earthquakes that could cause damage to these weak structures.
A trigger for a larger quake?
“Implementing such a policy may be urgent. In 1929 there occurred, in west Canterbury, a magnitude 7 earthquake which turned out to be the first of a series of seven major, magnitude greater than 7, earthquakes over the next 13 years. The series included the second and third largest earthquakes in European times – the M 7.8 Buller and Hawke’s Bay earthquakes. The series ended with two M 7.2 and 7.0 earthquakes in the Wairarapa in 1942.
“It is improbable that this occurrence of such large earthquakes in rapid succession was coincidental. It is more likely that the faults which broke during the series were all stressed and ready to break, and that the occurrence of successive earthquakes helped bring forward the occurrence of the next.
“There is no reason to think that such a series could not happen again. Equally there is no way of knowing whether or not Saturday’s earthquake has provided a trigger for more large earthquakes in the next few years.
“But if there are other faults that are ready to go, can we risk taking no action on a known and easily remediated peril – URM – and hope that that next earthquake also happens when people are safe in their houses?”