Last September, those of us who live in Christchurch experienced a 7.1 earthquake. Since then, we have referred to that as ‘the big one’, confident that we would never again experience anything similar. In that September quake, there were no lives lost, but there were many houses badly damaged and destroyed. Within days, most of us were able to get on with our lives, although scratch below the surface and it was immediately obvious that many people were still struggling with the upheaval to their lives. It wasn’t a case of some people being tougher than others. Rather, it was a case of some of us having been spared the physical damage and others bearing the full brunt of it. To some extent, it depended on house construction methods and materials, but it also depended very much on location. There was some pride that we were a developed country with rigorous building standards that had served us well in time of need, and that the carnage was not a great deal more.
Well, everything has now changed. September 2010 was just the dress rehearsal and February 2011 is the real thing. Now, some five days later, I am trying to make sense of what has happened, and of why, once again, some of us have been spared and others have had their lives and property destroyed.
I do not intend to write here of the overwhelming human tragedy. Others can do that more eloquently. What I want to do is to try and make sense of the physical events. I do that as a layman; as someone trained in science but with no special expertise in geo-morphology. What I bring to the issue is an inquiring mind and perhaps some interpretive ability to communicate basic science to other lay people.
We read in the media that this was a 6.3 earthquake on the Richter scale. So that seems to be saying that it was smaller than the 7.1 September earthquake. In one sense that is true. The September eathquake had a shaking amplitude that was more than 6 times the February earthquake, and with a total energy release that was almost 16 times as great.
(For those who are interested in the maths of the above numbers, the Richter Scale is logarithmic to base 10; also the energy released is a function of amplitude to the power of 1.5. For more information and comparative data search for “Richter magnitude scale” on Wikipedia.)
The strength of the Richter scale (or more accurately the Modified Magnitude Scale) is that it provides an objective measure of the shaking amplitude and hence energy release. However, it provides no indication of the impact of an earthquake at locations other than the epicentre. Also, earthquakes can occur thoughout the lithosphere which forms the earth’s crust. So an earthquake that occurs at depth (such as 70-120 km ) will have only a small effect relative to one that occurs near the surface (such as 5-10 km).
The February earthquake has occurred right underneath the hill suburbs of Christchurch, between Lyttelton and the Heathcote Valley (i.e. almost right beneath the Lyttelton Tunnel) and hence less than 10 km from the CBD. Perhaps even more crucial was the shallow depth of only about 5 km, or maybe even a little less. So location and shallow depth have been the key issues.
The key measure of intensity at a particular location is the ground acceleration. This is measured in units relative to gravity ‘g’ (9.8m/s²). In September the highest recorded intensity was 0.8g in Christchurch and 1.26 g in Darfield. To put that in locational context, Christchurch was about 40km from the epicentre, whereas Darfield was probably about 10km. The September earthquake was at a depth of 10 km which is still very much in the shallow and hence damaging category. (Go to geonet.org.nz for more data.)
In this February earthquake, the maximum recorded intensity was 2.20g at the Heathcote Valley School. Pages Road in the eastern suburbs was the next highest at 1.88g. It is apparent that the ‘g’ force drops off rapidly at locations greater than about 10km from the epicentre. (Click on geonet map below.)
For earthquakes in many other parts of the world there is no ground acceleration (‘g’) data available. However, an internet search indicates that anything over about 1 g is regarded as huge. I can find no other data for an urban location as high as 2 g. This February earthquake really was the big one. The US Geological Service classes anything over 0.34 g as ‘severe’ with ‘moderate to heavy damage’, and their highest category of ‘extreme’, with ‘very heavy damage’ is for earthquake intensity greaterthan 1.24 g. (http://earthquake.usgs.gov/earthquakes/shakemap/background.php)
Although ground acceleration seems to be the best measure, even that does not tell the whole story. Duration of the shaking is also important when comparing different earthquakes.
My preliminary impression is that the damage in Christchurch correlates very closely with the measured intensities. The hill suburbs have been very badly hit, with the damage greatest in Sumner, Redcliffs, Mt Pleasant and slowly tapering off from there in a westward direction. I live on the most western part of the Port Hills (Kennedys Bush) and most of us in the west have come out of it OK. But even here at Kennedys Bush, there are houses that have lost their brick cladding, and some have considerable structural damage. Down on the flats it is the southern and eastern suburbs that have taken the brunt of it, and the devastation is huge. As for the CBD, well everyone has seen those pictures.
As the days slip by it is becoming apparent that Christchurch will never be the same. For some of us life is already getting back to normal, at least at a superficial level. We have beds to sleep in, we have electricity, and we have water. But for many thousands of others, the journey is going to be much rougher. It will take years to get this city together again.
( I am unable currently to credit the photo that heads this post, as it is not mine and I do not have the source. [Update: I have now seen this photo credited to Gillian Needham.] It appears to be taken from the lower to medium slopes of Cashmere and is looking directly down Colombo St. New Brighton is upper right. What appears to be smoke is predominantly debris and dust from the collapsing buildings. The photo has been taken in the immediate aftermath of the quake.)