Iran flows in response to Arabian push

Richard Walters is a postdoctoral scientist at the University of Leeds. He works on measuring and modelling regional deformation, and is co-funded by COMET and the Earthquakes without Frontiers consortium.
Understanding deformation where the Arabian and Eurasian plates converge

A long-standing problem in continental tectonics has been to understand what controls the distribution of deformation and hence earthquakes. As Arabia converges with Eurasia by a couple of centimetres each year, the country of Iran is slowly being squashed between two rigid tectonic plates, and recent measurements of deformation from GPS give us the opportunity to test competing ideas. This is an important societal as well as scientific issue –a large and growing population in Iran is exposed to a high degree of seismic hazard.

Modelling the deformation of Iran, we treated the Iranian lithosphere as a thin sheet of viscous fluid that flows under gravity as well as due to the push of the Arabian plate.  This model can predict both the GPS velocity field and patterns of earthquake types and locations.

It had been suggested that strength variations within the lithosphere are required to explain the lack of deformation in Central Iran, which has traditionally been thought of as a stronger, rigid region. Instead, we found this feature can be reproduced by taking into account gravity-driven flow of the lithosphere under its own weight.

Results from our numerical model of Iranian lithospheric deformation. Model boundaries are shown by the black lines. Deformation is driven by an ‘Arabian push’ on the southern boundary due to convergence between Arabia and Eurasia, and by variations in gravitational potential energy (GPE) of the lithosphere, shown by the background colour. Crustal velocities relative to a fixed Eurasia and measured by GPS are shown as black vectors, and predicted model velocities are shown by the white vectors.
Results from our numerical model of Iranian lithospheric deformation. Model boundaries are shown by the black lines. Deformation is driven by an ‘Arabian push’ on the southern boundary due to convergence between Arabia and Eurasia, and by variations in gravitational potential energy (GPE) of the lithosphere, shown by the background colour. Crustal velocities relative to a fixed Eurasia and measured by GPS are shown as black vectors, and predicted model velocities are shown by the white vectors.

We also developed a novel method for estimating seismic hazard where GPS velocity measurements are scarce.  We assumed that the motion of the elastic upper crust is driven by the viscous velocity field derived from our model, and calculated slip rates along major fault zones.

The predicted rates agreed well with independent, long-term estimates of slip rate, showing that this is a promising way of estimating seismic hazard from a physically-based model of continental deformation.

Reference

Walters, R. J., Houseman, G.A., England, P.C. (in prep.) Continuum deformation explains kinematics of continental convergence in Iran.

Centre for Observation and Modelling of Earthquakes, Volcanoes and Tectonics