This is the question being answered by scientists working with Dr. Timm John from the Institute of Mineralogy at the University of Münster, using an innovative combination of field and laboratory work with numerical computer simulations.
The margins of the Earth's tectonic plates are characterized by a very high level of earthquake activity, which can originate at depths of just a few - or of several hundred - kilometres.
At places where converging plates override each another at their margins, earthquakes often occur at depths of more than 50 kilometres below the Earth's surface. "The mechanisms of 'shallow' earthquakes which occur at a depth of up to 50 kilometres are very well known," says John, "and they are attributable to failure in the tectonic plates displaying brittle behaviour. However, the causes of deeper earthquakes are still unclear today." As a rule, the behaviour of rocks under conditions at greater depth is ductile and not brittle. Conventional wisdom, however, says that here too brittle-like failure in the rock plays a role in connection with earthquakes.
During fieldwork in western Norway John and his colleagues from the Center for Physics of Geological Processes at the University of Oslo found shear zones in the rock they examined which are attributable to ductile deformation. In addition, there were earthquake fault zones present which the current state of research would interpret as brittle failure structures, but which, according to the scientists' latest insights, have other causes: "Our examinations showed that deeper earthquakes can in many cases be explained by shear heating of the rock," says John.
In such a case, the heating process which comes into being as a result of an initially very slow deformation of the rock along evolving shear zones leads to the rock becoming increasingly weaker and thus more deformable. In turn, this increasing deformation causes it to heat up. This process, which is self-amplifying, ultimately leads to the rock along a very thin zone becoming so hot that it begins to melt. The entire pent-up stress can then be released at seismic speed on this melted rock. The result is an earthquake.
"The simulations, which took into account the data gained in the field and in the laboratory, also showed," says John, "that the shear zones and the earthquake fault zones were formed by the same process. Very small differences in the rock properties decide whether a shear zone is formed or the deformation structure actually develops into an earthquake fault zone."
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