How real is an earthquake?

As far as I was concerned this week’s news of the week (in a less than good way) was the restating of the risk of an Alpine Fault earthquake event. From talking with other people, it seems this news flew under the radar for many. This is probably a good thing, because there isn’t anything we can do to prevent an earthquake, however probable or improbable it may be. Moreover, any advice on managing stress will likely tell you to focus on the things that you have power over, and earthquakes are well outside that category.

As background, the Alpine Fault runs up the spine of the South Island of New Zealand. It is the boundary between the Pacific and Australian plates – plate boundary faults are a big deal in the world of faults; they move the most. Another notable plate boundary is the San Andreas fault in California. As New Zealanders, and particularly South Islanders, we grow up knowing that we live in a place prone to earthquakes and experience small ones moderately regularly. We also have long known that a very big earthquake on the Alpine Fault, releasing stress as the two plates try to move relative to one another, is due or overdue. However, it is not until relatively recently that the risk of such an earthquake has been quantified. Further, the quantifications become ever more accurate as new information on frequency of past Alpine Fault events is gathered and a grouping of research organisations recently completed studies on a number of new sites, where previous estimates were based on reconstructing earthquakes that occurred at a single site in South Westland.

I found it really interesting how my brain responded to the revised risk, which has shifted from a 30% chance of an Alpine Fault earthquake in the next 50 years, to a 75% chance (I carried out my brain separation exercise in which one bit of me reacts while the other bit watches). How real did this news make an earthquake feel to me? What my reacting brain said about these numbers was the following. A 30% chance means it is quite likely that I will be dead before there is another major Alpine Fault earthquake. In contrast, a 75% chance means it is reasonably likely I will be alive during the next major Alpine Fault earthquake, although I could be quite old (my reacting brain seems to assume I will live until I am quite old). So the numbers made the possibility of a large earthquake feel quite real.

What also makes earthquakes feel real to me is having lived through the Canterbury Earthquakes; I can confidently say that I didn’t really know about what the experience of earthquakes is like until I lived through that period in Christchurch. As a result of the experience, I said that I really didn’t want to live through another major civil emergency. That wish was blown apart by having to live through the COVID emergency, together with everyone else. I scaled back to not wanting to live through another set of major earthquakes. I said this to a friend recently (who doesn’t live in Canterbury) and she said, ‘I’ve felt lots of earthquakes in Chile and New Zealand’, like it wasn’t such a big deal. However, what I don’t think she realised is that, it isn’t so much the earthquakes themselves that I don’t want to experience again (although I wouldn’t claim them to be fun in any way). There is a huge difference between experiencing an earthquake, and experiencing an earthquake disaster in my community. An earthquake can be scary and disorienting, however it is going to be over in maximum a few minutes (that would be a very bad one), and more likely seconds. An earthquake disaster in your community continues on for years, maybe for life.

What I particularly would rather not live through again includes:

  • Negotiating with an insurance company over what payments will be made to fix your house, while being told at every visit that your house is less damaged than the time before. We had an amazing, self-repairing house (which, sadly, was more damaged at every stage through the recurring 15,000 aftershocks).
  • Starting awake multiple every night, for a month or more after every major earthquake (of which there were four), as aftershocks hit your house like a sledgehammer against your bed.
  • Seeing the city you grew up in with gaping holes that used to be places people lived, worked and played, and seeing those holes remain for many years.
  • Seeing people you know ground down by the various stressors involved in broken houses, repairs or lack of them, insurance debates, lost jobs and damaged communities.
  • Living in a community where everyone is exhausted and where few are unscathed as a result of the event.

I tested whether my brain felt better about the fact that there is only an 82% chance that the earthquake will be over magnitude 8 (an ‘AF8’) on the Richter Scale. This means there is only a 62% chance of an earthquake over 8 in the next 50 years.

Earthquakes are quantified on the Richter Scale (geologists like Chris and me learn the Richter Scale in order to pass exams). Each step on the Richter Scale releases about 33x more energy than the step before. The Canterbury Earthquakes maxed out at 7.1 on the Richter Scale (and the Kaikoura event at 7.4), so were substantially smaller than many of the past earthquakes that have occurred on the Alpine Fault, estimated to be around 8 on the Richter Scale. An 8 is “likely to cause major damage to buildings, with structures likely to be destroyed, moderate to heavy damage to sturdy or earthquake-resistant buildings, damage in large areas and be felt in extremely large regions”.

One thing we did learn in the Canterbury Earthquakes, however, is that the Richter Scale is not nearly as important to the people feeling the earthquakes as the Modified Mercalli Scale (MM). The MM is very much about how real an earthquake feels, because this scale measures the felt intensity at a location, while the Richter is merely a measure of the energy released. What you feel at any location depends in a large part on how far you are from the epicentre of an earthquake, both horizontally and in terms of how deep the earthquake is. In Canterbury and Kaikoura the earthquakes were reasonably close to built-up areas and were very shallow – Canterbury earthquakes were around 5km deep and Kaikoura 15km (shallow earthquakes are 0 – 70 km deep, intermediate earthquakes are 70 – 300 km deep, deep earthquakes are at 300 – 700 km). These earthquakes were felt at maximum MMVIII, the description of which includes “general alarm, which may approach panic, people have difficulty standing, few buildings are damaged and some weak buildings are destroyed”.

This is one projection of the MM intensities that would be experienced resulting from an ‘AF8’. It made me feel a bit better because if we are in Queenstown we will ‘only’ experience an MMV and in Christchurch an MMVI. However, in another report it suggested MMVIII in Queenstown which made me feel less better again.

There are so many different projections as to how an Alpine Fault Richter 8 earthquake might play out across the South Island, that it is difficult to find any conclusive information on what intensity one might experience at any particular place. The picture above has one interpretation but there are many more. One thing that is certain is that the West Coast will be particularly badly off. If you want information on what might happen, one of the best sources of information is the AF8 website.

In conclusion…given that the possibility of an earthquake now feels so much more real to me, I had better work on our emergency kit in Christchurch. In Gibbston we are well set up to manage for a month or more in terms of water, food and a generator (and we will need to do so given that the valley will likely be isolated by bridges at each end collapsing). In Christchurch our food supplies and water supplies could well be improved. I can’t do anything about an earthquake, but I should do something about the things over which I do have control. And then I should put earthquake realities back into the hibernating component of my brain, just leaving a reminder in my calendar to freshen up the water supplies!


Published by janecshearer

I'm a self-employed life enthusiast living in Gibbston, New Zealand

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