Quakes eat up the heat

More than half of the heat generated by friction in earthquakes could go to towards endothermic reactions, a study by geoscientists in Japan and Taiwan has shown. The conclusion suggests that chemical reactions ought to be more seriously considered when modelling the dangerous events.

Earthquakes are among the most deadly natural hazards, yet are also some of the most poorly understood. They tend to occur at the ’fault zones’ between two tectonic plates in the Earth’s crust, and start as small cracks which quickly grow in a chaotic fashion. During these events a huge amount of energy bursts free - perhaps hundreds of megaJoules per square metre - in the form of seismic waves, fractures and heat. To gain a better understanding of the underlying mechanics, geoscientists need to know how the energy is distributed between these forms.

But according to Yohei Hamada of Osaka University and colleagues, geoscientists have not given enough attention to the proportion of heat energy that can be consumed by chemical reactions. 

They have studied the composition of three rock samples from Chelungpu in Taiwan, a fault zone that in 1999 brought a 7.3 magnitude quake to the nearby town of Chi-Chi causing thousands of deaths and billions of dollars of damage. The researchers found that the shallowest sample, from an area likely to be closest to the true fault, contained far less calcite, smectite and kaolinite than the surrounding areas. These minerals can react away through thermal decomposition and dehydroxylation given sufficient heat.

Using numerical analysis, Hamada’s group estimated that it must have taken energy of 0.43MJ/m² to supply such endothermic reactions. This corresponds to 0.79 per cent of the heat energy thought to have been produced by the Chi-Chi quake. Although this value is fairly small, the researchers point out that there were fairly low concentrations of the reactants to begin with. For fault zones in limestone or mudstone, in which the minerals are much more abundant, they calculate 50 per cent or more frictional heat could disappear in endothermic reactions. This, they say, would keep the temperature from rising too much in the fault zones.

Nevertheless, Dan McKenzie, a geoscientist at the University of Cambridge, thinks most endothermic reactions in earthquakes would lead to some sort of melting. ’The presence of melt on ancient fault planes is common: such melts are known as pseudotachylites,’ he says. ’They are believed by most people to have a dramatic effect on seismic energy release, because of their effect on the friction.’

Jon Cartwright