This is the second post of a 3-4 part series on the Canterbury Earthquake.
Canterbury Earthquake, Pt II
Was this Canterbury’s ‘Big One’?
When I was shaken out of a deep slumber at 4:36 am last Saturday, I couldn’t help but think that ‘this is the big one’. The intensity of the shaking was certainly more than anything that I have experienced before. As I initially struggled to come to my senses, to even know if I was in the middle of a particularly vivid dream, it seemed that everything that wasn’t nailed down was jumping around. Over the next 20-30 seconds, the violent shaking and crashing escalated until it was nearly deafening. But not deafening enough to drown out the roar of the wave that was bearing down on us from the west at tremendous speed. A wave travelling through solid earth. When it arrived, the wave bore the entire house up upon its crest, and dropped us down the other side, as if we were afloat on some tempestuous solid sea. The 107 year old house flexed and groaned as the wave passed, protesting the immense strains that must have tested its solid wood-frame construction. The wave and its accompanying roar sped off towards central Christchurch, and the violent shaking and crashing resumed for a few more seconds. I don’t know exactly how long the main event lasted for, but at a guess, I would say that it was something like one minute. By then, as the shaking diminished, I knew that it wasn’t an Alpine Fault earthquake, which when it ruptures in the not-too-distant future, will generate strong shaking for several minutes. I will examine the likely impacts of the future Alpine Fault earthquake, and the implications of Canterbury’s earthquake for planning and preparing for the real ‘Big Event’ in another post in the near future.
A Previously Unknown Hazard Lurks Beneath the Canterbury Plains
Saturday’s magnitude 7.1 earthquake was centered near Kirwee, approximately 44 km west of Christchurch, at a depth of only 10 km. So why didn’t we know that this potentially damaging fault existed beneath the Canterbury plains, quietly building up enormous energy, so close to Christchurch?
The media has been portraying this as a ‘new fault’, which has not ruptured in the past. This is partly true in that this particular section of fault has not ruptured historically (i.e. within the last 200 years), and had not been previously identified. However, that does not necessarily mean that this fault hasn’t been active in the past. As noted by Dr. Mark Quigley, professor in Geological Sciences at Canterbury University, and one of the lead investigators on the ongoing assessment of the fault, it is possible that the recurrence interval on this fault is less than a few thousand years. Indeed, the modern day surface of the Canterbury plains is relatively young, having been steadily built out over the last 16,000 years, as the great braided rivers of Canterbury transport vast quantities of material from the Southern Alps. The Canterbury plains and the braided rivers that nourish them are, by nature, a very active landscape. So while this may indeed be a “new fault”, it is also very possible that it has ruptured in the past, but the surficial evidence of that rupture is no longer evident.
Seismically active faults are often initially identified by visual cues. The presence of a fault may be given away by its surface expression, such as fault traces (visible line of disturbance), or offset/displaced topography (such as river valleys). However, where these faults occur beneath or within very young and active surfaces, such as the Canterbury plains, their surface expressions may be periodically erased by active surface processes, such as deposition of gravels by braided rivers. Whenever the fault ruptures, its surface expression may be rewritten. Here is a typical view of the 20+ km surface expression of the fault following Saturday’s earthquake in Canterbury:
Geophysical Investigation Methods
Surface expression is not the only way to identify an earthquake fault. Geophysical investigation methods, including seismic refraction surveying, and ground penetrating radar (GPR) can show the subsurface structure of the earth. The seismic reflection profile shown below suggests a series of faults, many of which are blind (i.e. do not show any surface expression) in part of the northwest Canterbury Plains:
While geophysical survey methods allow scientists to look into the earth, these techniques are time consuming, labour intensive, and require specialized, often expensive equipment. For these reasons, areas of interest (e.g. surface fault traces) are often identified prior to any major geophysical investigation. It would take a tremendous concerted effort and a great deal of resources to undertake a detailed geophysical investigation of a region the size of Canterbury. Consequently, it is very possible (perhaps even probable), that faults buried beneath the ephemeral surface of the Canterbury Plains may not have been discovered yet.
Faults Beneath The Canterbury Plains Predicted by Scientists?
This is not to say that the presence of such faults hasn’t been predicted by scientists. An offshore fault system has been identified in Pegasus Bay, and may be related to the relatively well-established Marlborough fault system. The Pegasus Bay faults trend NE-SW, and likely extend under the Canterbury plains.
As mentioned by professor Quigley on his website, the presence of such hidden, or ‘blind’ faults were predicted by University of Canterbury scientist Dr. Jarg Pettinga and colleagues from GNS and Geotech Consulting Ltd in a report produced for the Canterbury Regional Council in 1998. However, predicting the existence of such faults is only the first step towards identifying their actual locations, and the likely recurrence intervals of associated earthquakes. Perhaps, Saturday’s powerful
earthquake in Canterbury will provide the impetus to get this important task underway, so that Cantabrians have a better idea of what to expect next time.