The post below is mirrored from Prof Dave Petley’s blog. He describes the Sendai earthquake from a geological perspective noting that these events were far from unusual taking into account the seismic history of the region.
The impact of the Sendai earthquake, and the media coverage of it, is undoubtedly extraordinary. This is a tragedy that has so many dimensions, ranging from tragedy (in so many cases) to ecstasy (as a survivor is found), including farce and the surreal. It plays on our fears of the breakdown of society; of catastrophic loss of all we hold dear; and of being trapped. The ongoing nuclear crisis brings back memories of the Cold War, a drum beat to which so many of us grew up. And the effects of the disaster continue to ripple outwards, and will do so for years to come, as the financial markets, insurance industry and energy generators strive to face the consequences. Of course for most of us life will go on essentially as before, but in every case at least some, often intangible, elements will have changed.
Against this background it is worth exploring this disaster from the perspective of our knowledge of the Earth system. It would be reasonable to surmise that such a catastrophic disaster in one of the best-prepared nations must have involved some unknown or unexpected element; some facet of earthquakes or tsunamis of which we were not aware. This is surely doubly the case with the nuclear reactors – these systems each cost literally billions of pounds to build, and pumping sea water into them in a desperate attempt to stabilise the temperatures is enough to ruin them forever, leaving someone with a huge bill to pay. Surely, from an investment perspective if not one of safety, the true risk of such an event will have been established. Thus, it could be argued that surely the earthquake and tsunami must be a freak or the consequence of something unknown?
Actually, I don’t think so. Indeed, as far as I can see this earthquake, and the resultant tsunami, are remarkably unsurprising. They are exceptionally large for sure, and they were not predictable, but they are not beyond the bound of human experience in any way that I can see. Let’s take the earthquake itself – the existence of the subduction zone to the east of Honshu is well-established. There have been many earthquakes on it in historic times. This is beautifully illustrated in a USGS publication entitled “Seismicity of the Earth 1900—2007, Japan and Vicinity“, which can be downloaded as a PDF. Download it, and send it to every journalist you know. And this is the key part of the map in that publication:
The line of the trench, marking the seabed expression of the junction between the tectonic plates is shown. The red dots are the larger earthquakes. The most interesting aspect is the yellow shaded areas, which show a part of the subduction zone that has undergone a large rupture event. Note the lack of stippled area to the south – i.e. this was an area that had not undergone a large rupture event in the last century – a so-called seismic gap. These are areas in which the stored energy can be high, and large earthquakes should be feared.
The Catastrophe Modelling company RMS have produced a map of the area of the fault that is thought to have ruptured, shown below:
Compare the maps – the recent rupture event has efficiently-filled the seismic gap. A rupture event of this size would be expected to generate a very large earthquake. The unruptured section of fault was clear from the USGS map. Thus, there is little of great surprise here.
Second is the shaking itself. Well, the RMS map above shows shaking intensities. MMI intensity of VII onshore a giant subduction zone earthquake is remarkably unsurprising, so nothing out of the ordinary there either.
So how about the tsunami? Well, in 1993 a magnitude M = 7.8 earthquake off Hokkaido generated tsunami waves that were up to 10 metres high. The event is well described in this article (despite the gaudy presentation), which includes this now awfully familiar scene of destruction:
Note that the tsunami broached the protective seawall around the village that was meant to provide a barrier against such events. This case is of course not alone – we know that large subduction zone earthquakes can generate large tsunamis from many historic and recent examples. As a society there is no excuse for our failure to recognise and prepare this.
So, actually, as far as I can see from a geological perspective there is nothing terribly surprising about this earthquake in terms of location, timing, magnitude or secondary hazards. Which of course acts to demonstrate that once again it is our preparedness that is at fault. Once again our knowledge of the hazard has failed to transfer into effective mitigation. This is so deeply frustrating!
Of course there is a crucial need for research into the earth science elements of this issue. In particular, the ongoing crisis in the nuclear facilities demonstrates that our ability to actually understand magnitude – frequency relationships for large, rare events is tragically weak. It is this failure that probably led to inadequate defence against the tsunami that occurred, with appalling consequences.