Roots of earthquakes explained

Many populated areas around the world are prone to earthquakes, so understanding what controls their distribution and frequency is a top priority for the earth scientists. However the root causes often remain elusive because scientists have limited information about what happens deep down in the Earth's crust. A new study by a team including Joanna Faure Walker (UCL IRDR), published in Nature Geoscience, has shed new light on this problem, and has shown how phenomena on the surface can be linked to the movement of rocks deep down in the Earth's crust.

At a depth of about 15 km below Earth's surface, where temperatures exceed several hundred degrees, the crust flows gradually, driven by the motion of the underlying mantle and the surrounding tectonic plates (like toothpaste flowing continuously under a constant hand squeezing). However, at shallower depths, where temperatures are lower, there are faults in the crust that resist this flowing motion, often over very long-periods of time (hundreds or even thousands of years), before abruptly slipping during an earthquake. This is similar to a rubber band that progressively stretches before finally snapping.

The contrast between the deep and the shallow styles of crustal deformation, and in particular how one may control or interact with the other, is thought to play a key role in the nucleation of earthquakes.

The study was conducted in the central and southern Italian Apennines where the Earth's surface has been repeatedly shaken and devastated by major historical earthquakes, including the magnitude 6.3 L'Aquila earthquake, which claimed the lives of over 300 people in 2009. In addition to the human cost, these earthquakes have left their mark on the landscape in the form of long scars, or fault scarps. It is the deeper structure of these faults, at depths where earthquakes nucleate, that has been revealed by this collaborative work, which brought together researchers at UCL, Birkbeck, the University of Bergen (Norway), Columbia University (USA) and the University of Rennes (France).

A fault scarp along a hillside in the Italian Apennines. Fault scarps are scars left behind by historic earthquakes Credit: Joanna Faure Walker (UCL IRDR)

"What is exceptional about this study is that we can demonstrate for the first time that these fault zones deform exactly as predicted by laboratory experiments", explains Patience Cowie (University of Bergen) the first author of this study. Up to now, no direct measurements existed on the deformation style ("rheology") of rocks at the depths and temperatures where earthquakes nucleate. The timescales of thousands or millions of years are just too long to observe their motion.

In order to infer the faults' histories, the team instead looked at the fault scarps, whose structures carry information about the historic movement of the fault. Dr Joanna Faure Walker (UCL Institute of Risk and Disaster Reduction) analysed these faults to estimate the rate of deformation across the Apennines using the surface displacement and slip direction indicators present along the fault scarps.

"This thorough analysis gave many significant results, the most enigmatic of which was that variations in the rate of deformation of the faults are correlated to variations in the elevation of the topography," says Dr Faure Walker.

Theoretical work by the team linked the field measurements and laboratory experiments on rock. By making this connection it became possible to show that the observed relationship between deformation rate and topographic elevation reflects the gradual flow of the deeper part of the crust even though the measurements were made at surface.

Damage caused by the 2009 L'Aquila earthquake Photo: Joanna Faure Walker (UCL IRDR)

"This tells us that the earthquake-prone faults in the shallow part of the crust are directly rooted into the flowing material at depth," says Prof Cowie. "Most of the flow will occur in narrow zones, called shear zones, which form at the lower tip of the earthquake-prone fault and extend downwards into the deep crust."

Thus the deep and shallow types of crustal deformation exert controls on each other to determine where the shear zones are located and the rate at which the earthquake-prone faults are slipping and. This, ultimately, determines the rate at which earthquakes will occur.

A related conclusion is that a doubling of the topographic elevation corresponds to an eight fold increase in the deformation rate. This means faults located in lower elevation areas that have not ruptured in historical times could in fact still be active but with longer periods of quiescence between successive earthquakes due to their lower deformation rate.

Notes

The research appears in a paper published in the journal Nature Geoscience, entitled "Viscous roots of active seismogenic faults revealed by geologic slip rate variations"

High-resolution images

Fault scarp in the Italian Apennines

Damage in L'Aquila following the 2009 earthquake

These images can be reproduced freely providing the source is credited

Researcher profiles

Joanna Faure Walker

Science contact

Joanna Faure Walker
UCL Institute of Risk and Disaster Reduction
020 3108 1108
j.faure-walker@ucl.ac.uk

Media contact

Oli Usher
UCL Faculty of Mathematical and Physical Sciences
020 7679 7964
o.usher@ucl.ac.uk

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