Roots of earthquake-prone faults brought to light
A new Nature Geoscience article sheds light on what happens deep down in the Earth’s crust where earthquakes initiate.
Many populated areas around the world are prone to earthquakes so understanding what controls the distribution and frequency of earthquakes is a top priority for the earth science community. Often, however, the controlling factors remain elusive because scientists have limited information about what happens deep down in the Earth’s crust where earthquakes initiate.
A new study led by professor Patience Cowie, Department of Earth Science, University of Bergen, Norway, published in Nature Geoscience, has shed light on this problem, and has shown how phenomena on the surface can be linked to the movement of rocks in the deep crust.
This is Cowie's popular summary of the study:
At a depth of about 15 km below Earth’s surface, where temperatures exceed several hundred degrees, the crust flows gradually under the forces imposed by the motion of the underlying mantle or the surrounding tectonic plates (like toothpaste flowing continuously by constantly squeezing the tube). However, at shallower depths, where temperatures are lower, there are faults in the crust that resist this flowing motion. Faults are often inactive 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 generating 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 in2009. In addition to the human cost, these earthquakes have left their mark on the landscape in the form of long scars, referred to as 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 the University of Bergen (Norway), Columbia University (USA), Birkbeck College and UCL (UK), and the University of Rennes (France).
“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 on rock”, 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 over geological time (thousands to millions of years) at the depths and temperatures where earthquakes nucleate. The timescales of thousands or millions of years are just too long to observe their motion.
The team overcame this practical difficulty by looking at the fault scarps, whose structures carry information about the historic movement of each fault. The data, collected by co-authors Gerald Roberts (Birkbeck College) and Joanna Faure Walker (UCL), were analysed to estimate the rate of deformation across the Apennines. “This thorough analysis gave many significant results, the most enigmatic of which was that variations in the rate of deformation (or “strain-rate”) of the faults are correlated to variations in the elevation of the topography, says Dr Faure Walker.
With co-author Philippe Steer, now at the University of Rennes but formerly a post-Doctoral researcher at the University of Bergen, Professor Cowie carried out theoretical work that allowed a link to be made between the field measurements and rock deformation experiments. By making this connection it became possible to show that the observed relationship between deformation rate and topographic elevation reflects the rheology of the deeper part of the crust even though the measurements were made at surface. “This tells us that the earthquake-prone faults in the shallow part of the crust are directly rooted into the flowing material at depth. Most of the flow will occur in narrow zones, called shear zones, which commonly form at the lower tip of earthquake-prone faults and extend downwards into the deep crust”, explains co-author Christopher Scholz (Columbia University). The two contrasting styles 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, ultimately, the average rate at which earthquakes will occur.
“A related conclusion is that a doubling topographic elevation corresponds to an eight fold increase in the deformation rate,” says Professor Roberts. This result alone provides a new framework for mapping out variations in seismic hazard in the region. For example, we can use this relationship for risk assessment in lower elevation areas where the deformation rates are low.
Link to the Nature Geoscience paper here.