Fortunately, the most devastating earthquakes with magnitudes greater than 9 on the Richter Scale occur less than once in a human generation. Records show that when such strain is released there may be two or more as major faults adjust to the release by the first. That was the case for the Sumatra-Andaman earthquake (magnitude 9.1 to 9.3) of 24 December 2004 that created the Indian Ocean tsunamis. On 28 March 2005 it was followed by the magnitude-8.7 Nias earthquake to the south of the movement zone of the earlier event. Both occurred on the subduction zone that consumes the Indo-Australian plate obliquely, from SW of the Indonesian archipelago through the ocean floor west of the Nicobar and Andaman islands to link with the Himalayan subduction system. The last seismic event of such magnitude was beneath Alaska in 1964, before modern seismograph development. How such events propagate could only be guessed at by analogy with lesser earthquakes, so scientific interest in the seismograph records of these two and their analysis has been very high. The 20 May 2005 issue of Science devotes 22 pages to full accounts of the findings (Hanson, B. 2005. Learning from natural disasters; and 5 other papers. Science, v. 308, p. 1125-1146).
The Sumatran-Andaman earthquake involved movements of up to 20 m vertically that lasted about an hour, and thrusting “unzipped” the subduction zone over a length of around 1300 km, proceeding from south to north. The energy released was equivalent to that of 100 thousand one megaton nuclear explosions, or the energy used in the US in 6 months. It set up resonances in the entire Earth that are still reverberating, and changed the shape of the crust across a hemisphere by an amount measurable using high-precision GPS monitoring, which has raised global sea level by about 0.1 mm. Half a globe away, the surface waves from the earthquake triggered several minor shocks in Alaska in exact harmony with their passage. In social terms, the loss of 300 thousand lives resulted from the displacement of around 30 km3 of sea water by the movement of the faults. The prolonged event was complex, and one sobering feature is that in the northern part of its propagation it moved slowly, thereby failing to unleash yet more tsunamis: they would have devastated most of the coast of eastern India and the west of Myanmar and Thailand. Much of what occurred was unpredictable, and quite possibly the lessons learned here may not be directly applicable to future earthquakes of this magnitude, except for one: hazard assessment based on scaling up from lesser events underestimates enormously what actually happens. What the seismograph data will not do is help warn when similar events will occur elsewhere, with sufficient leeway to take measure that will mitigate effects.
Promising developments for forecasting lesser earthquakes
Although there are many places that are riskier, California is widely regarded as the earthquake capital of the world, mainly because so many people live there with such an economically huge infrastructure. At any rate, it is indeed the centre for the most advanced seismic forecasting based on far more data that are available for analysis than anywhere else. Until recently, forecasting was limited to the likely aftershocks following unpredictable large earthquakes. Seismologists of the US Geological Survey and at ETH in Zurich have developed an advanced modelling system based on the wealth of data (Gerstenberger, M.C. et al. 2005. Real-time forecasts of tomorrow’s earthquakes in California. Nature, v. 435, p. 328-331). Their model allows day-by-day calculation of probabilities for strong shaking (> Mercali Intensity VI), using the way in which seismic events cluster along different faults and monitored lesser movements that might presage a major fault break. These take the form of extremely graphic maps of hazard across the whole state. The system has been tested using historic data that preceded historic earthquakes.