Mark Quigley: Seismic fear mongering or time to jump ship?

Dr Mark Quigley is Senior Lecturer in Active Tectonics and Geomorphology in the Department of Geological Sciences at the University of Canterbury, Christchurch, New Zealand.

Since the September Mw 7.1 earthquake, GNS scientists have been using aftershock statistics-based models to ‘forecast’ the expected range of aftershocks of given magnitudes.

Not to be confused with earthquake ‘predictions’, which require specific magnitudes, locations, depths, times, and methodological reproducibility estimates to be useful, this forecast model is based on a modified version of the long-established Omori’s Law for aftershocks, which states that the rate of aftershocks is proportional to the inverse of time since the mainshock.

Thus, depending on the values of parameters specific to certain regions, whatever the odds of an aftershock are on the first day, the second day will have approximately 1/2 the odds of the first day and the tenth day will have approximately 1/10 the odds of the first day. These odds can be summed over various time-scales, and the longer the time scale, the higher the probability, even though the probability decreases with time.

At present, these forecasts commonly look something like this:

“The expected number of aftershocks of magnitude 5.0 and above for the next month is 0-2, with an expected average of <1”.

Of course, one could dress this up differently using the same model applied over a full year, taking into account a reducing number of expected aftershocks, and the statement would look something like this,

“The probability of a magnitude 5.0 and above aftershock over the next year is ~90%”.

We have had 25 of these events since September, the frequency of which has declined systematically following our large earthquakes in Sept and Feb. So to say that there is a near certainty of an event occurring somewhere in this range in the next year is no surprising conclusion, because the unfortunate reality of aftershock sequences is that earthquakes decrease in frequency but not magnitude.

Remember also that this takes into account the entire aftershock zone, spanning an area from the eastern foothills of the Southern Alps to offshore east of Christchurch to Rangiora and throughout the Banks Peninsula; it doesn’t forecast the likelihood of one of these events occurring beneath your house. Large aftershocks have been recorded as far east as the Porter’s Pass area.

The probability of larger earthquakes (M>6) is a bit trickier, although the methodology behind the statement

“There is a 23 per cent chance of a magnitude-6.0 to 6.9 quake in the next year, dropping to a 10 per cent risk the following year” is the same.

Again, because this is averaged over the entire aftershock zone, this 23% probability for Canterbury becomes a 6% probability for Chch. “In rough terms, it means the quake probability for Chch has become similar to many other parts of NZ such as Wellington, Hawke’s Bay, Wanganui, and Poverty Bay where quakes are more frequent” (M. Gerstenberger and J. Callan, GNS).

To generate an earthquake of M 6-6.9, it is helpful to know whether there are faults that are long enough and ‘connected’ enough to be able to do this, and whether these faults have ruptured in big earthquakes in the past. One way to explore this to image faults in the subsurface using geophysical methods such as reflection seismic, gravity, and aeromagnetics. These can be combined with ‘relocations’ of aftershocks, which also help to define fault zones. The data processing in these techniques takes time but scientists are working on this furiously. The data will be released in due course.

I am not personally involved in this research and cannot state exactly when the scientists leading these projects will have a product that is ready for public consumption, but it will be soon. I am still working on the Greendale rupture! However, I will say that for the last 20 yrs or more, scientists have known that there are faults of sufficient length to generate magnitude 6 to 7 earthquakes in the broader Canterbury region and offshore. So it would not be a huge surprise, although it would be unfortunate, to discover that additional, similar structures exist in the Christchurch area.

In the meantime, please consider the following example, from what I feel is from an analogous setting (in some ways). This area is adjacent to a section of the San Andreas Fault (America’s version of our Alpine Fault) that has not had a major earthquake since 1812 (one segment) and 1680 (another segment), just like our Alpine Fault does not appear to have ruptured in a major earthquake since 1717.

On April 23, 1992, the Mw 6.1 Joshua Tree earthquake rocked the Californian desert east of the San Andreas Fault. Two months later, on June 28, 1992, the Mw 7.3 Landers earthquake occurred in the same region, with an epicentre located approximately 40km north of the Joshua Tree epicentre. Three hours after the Landers event, the Mw 6.2 ‘Big Bear’ aftershock occurred some 40km to the west.

On October 16, 1999, seven years after the Landers event, the Mw 7.1 Hector Mine earthquake occurred, with an epicentre some 40 km north of the Landers epicentre. Paleoseismologic estimates of the recurrence intervals of clusters of earthquakes in the Mojave Desert near the Landers rupture are in the range of 5,000 to 15,000 years (Rockwell et al., 2000), similar to expected range of recurrence intervals of active faults in our Canterbury Plains. So a situation like this is possible, although we would obviously prefer that the region settled down without the occurrence of any more big events.

Where to from here? We’ll do our best to provide the best scientific information possible. Wait for the information to come from scientists regarding the earthquake history, likely lengths, and ‘connectivity’ of faults in our region. Then take into account whether you want to occupy your time with fear of the next big one, which may or may not eventuate in the next few years or longer, or get on with your life while learning lessons about being prepared for earthquakes.