Relations between earthquake distributions, geological history, tectonics and rheology on the continents.

This paper is concerned with the distribution of earthquakes, particularly their depths, with the temperature of the material in which they occur, and with the significance of both for the rheology and deformation of the continental lithosphere. Earthquakes on faults are generated by the sudden release of elastic energy that accumulates during slow plate motions. The nonlinear high-temperature creep that localizes such energy accumulation is, in principle, well understood and can be described by rheological models. But the same is not true of seismogenic brittle failure, the main focus of this paper, and severely limits the insights that can be obtained by simulations derived from geodynamical modelling of lithosphere deformation. Through advances in seismic tomography, we can now make increasingly detailed maps of lithosphere thickness on the continents. The lateral variations are dramatic, with some places up to 300 km thick, and clearly relate to the geological history of the continents as well as their present-day deformation. Where the lithosphere thickness is about 120 km or less, continental earthquakes are generally confined to upper crustal material that is colder than about 350°C. Within thick lithosphere, and especially on its edges, the entire crust may be seismogenic, with earthquakes sometimes extending into the uppermost mantle if the Moho is colder than 600°C, but the continental mantle is generally aseismic. Earthquakes in the continental lower crust at 400-600°C require the crust to be anhydrous and so are a useful guide or proxy to both composition and strength. These patterns and correlations have important implications for the geological evolution of the continents. They can be seen to have influenced features as diverse as the location of post-collisional rifting; cratonic basin formation; the location, origin and timing of granulite-facies metamorphism; and the formation, longevity and strength of cratons. In addition, they have important consequences for earthquake hazard assessment in the slowly deforming edges and interiors of continental shields or platforms, where the large seismogenic thickness can host very large earthquakes. This article is part of a discussion meeting issue 'Understanding earthquakes using the geological record'.