There’s often a fine line to be walked when you’re asked to talk about future earthquake risks in the media. People are looking for what scientists can’t provide: firm predictions of where the next big earthquake is coming from, and when. Meanwhile scientists have to downplay the hiding-to-nothing that is the specific prediction game and communicate what we do know: the areas that are most at risk from large and damaging earthquakes, so that people and emergency planners are not taken by surprise when they inevitably occur – be it next month or next decade. It’s tricky: talk up the dangers too much, and you might spark an unneccesary panic. Don’t go far enough, and you store up trouble for the future in the form of an unaware and unprepared populace.
Pointing to any one corner of the Earth as the location of the next Big One is not a winning game. Take a map of the world’s most active plate boundaries and throw a dart; where it lands is as good a guess as any.
…the next Big One might not be within the lifetime of any individual alive today but is very likely to occur within the lifetime of many of the buildings being constructed today.
What we know for sure is that preparedness remains our best defense against devastating earthquakes.
It’s perhaps no surprise that this particular seismologist is so good at talking about earthquake risks and predictions: she is, after all, the author of an excellent and highly readable book on the subject. However, if you want demonstration of just how easily one can go astray in this particular field, Simon Winchester, the author of a number of popular geoscience books, provides one with this rather less good article in Newsweek. It’s unpromisingly titled ‘The Scariest Earthquake Is Yet to Come’, but since titles are the first casualty of the editor’s red pen, I’ll overlook that. But following a few paragraphs describing the March 11th Japanese earthquake and tsunami – the standard “we are all helpless against the caprice of Nature” angle – we get this:
For this event cannot be viewed in isolation. There was a horrifically destructive Pacific earthquake in New Zealand on Feb. 22, and an even more violent magnitude-8.8 event in Chile almost exactly a year before. All three phenomena involved more or less the same family of circum-Pacific fault lines and plate boundaries—and though there is still no hard scientific evidence to explain why, there is little doubt now that earthquakes do tend to occur in clusters: a significant event on one side of a major tectonic plate is often—not invariably, but often enough to be noticeable—followed some weeks or months later by another on the plate’s far side…Now there have been catastrophic events at three corners of the Pacific Plate—one in the northwest, on Friday; one in the southwest, last month; one in the southeast, last year.
That leaves just one corner unaffected—the northeast. And the fault line in the northeast of the Pacific Plate is the San Andreas Fault, underpinning the city of San Francisco.
This assertion is problematic in a number of ways. Firstly, putting the magnitude 6.3 Christchurch earthquake – even last year’s magnitude 7 – in the same class as the events in Chile and Japan is wrong. The Christchurch quake released thousands of times less energy than either of those events, and was only so damaging due to its close proximity to Christchurch.
Secondly, the idea that megaquakes may beget other megaquakes is still at the ‘interesting speculation’ stage. In other words, there is still no ‘hard scientific evidence’ that there’s an effect to explain at all. Even if there is, it’s considerably more complicated than ‘look to the opposite side of the plate!': when you’re considering the far-field effect of megaquakes, the other side of the planet would be just as susceptible as the other side of the plate. The state of stress in the crust is a far more important factor than any particular relative location.
Thirdly, the San Andreas Fault is probably not physically capable of producing much more than a magnitude 8 earthquake. The reason subduction thrusts are the source of such large earthquakes is because of their shallow dip, which results in a large area of the fault being above the depth where rocks become too warm and weak to rupture brittlely in earthquakes. Because the San Andreas Fault is almost vertical, the area that can potentially rupture is smaller, and the upper limit on the size of earthquake it can produce is also smaller. An earthquake on the San Andreas, which is on land, also wouldn’t produce a tsunami, which was what caused the real damage in Japan.
This isn’t to say a magnitude 8 earthquake isn’t a very serious future hazard for California. But to argue that it would be more ‘scary’ than what we witnessed a couple of weeks ago is pushing it a bit. To argue that this horror is imminent is borderline irresponsible – there is no scientific basis for stating the risk of a ‘Big One’ in California is any greater than it was a month ago. The same is true of the arguably much more scary Cascadia subduction zone to the north – which can potentially produce a magnitude 9 earthquake, and will produce a tsnuami when it does so. We know that both of these faults will rupture at some point in the future, and people need to be aware of that. But claiming we’re in some period of extra-special risk right now is, to put it bluntly, just making stuff up.
In conclusion: Susan Hough: take a bow. Simon Winchester: don’t. In any sense of the word.