The highly active Sumatran Subduction Zone has produced more than four great earthquakes in the last decade. The first of these was the giant Mw 9.2 Sumatra-Andaman earthquake that ruptured on 26 December 2004. This devastating event was followed by three others – the Mw 8.6 Nias-Simeulue quake in 2005, the Mw 8.4 Bengkulu earthquakes in 2007, and the Mw 7.7 Mentawai tsunami-earthquake in 2010.

In order to understand why so many great earthquakes originate from this region, we have to measure the strength of the rocks in the earth’s lower crust and upper mantle. Our new research, published today on 8 March 2018 in Nature Communications, highlights a ground-breaking new approach to how the strength of these rocks can be assessed. 

The technique we developed allows us to measure the physical properties of rocks in the mantle and to monitor how they behave across both space and time. Our study illuminates a large region of very weak rocks beneath the active volcanoes along the Sunda arc, and shows how deformation in that region was accelerated by the great megathrust earthquakes in the last decade. 

The way in which rocks in the earth’s mantle and crust move and deform in response to stress plays a crucial role in the global distribution of seismic and volcanic hazards. The strength of the rock controls tectonic processes like continental drift and earthquakes. In catastrophic earthquakes, strong rocks release stress within tens of seconds to minutes, while weaker rocks can release stress over days, months, years, or even decades.

These longer-term motions are capable of redistributing tectonic stress, loading existing faults, bringing them closer to failure, and putting people’s lives at risk. However, rocks do not have the same strength everywhere. They vary not only between broad regions, but also at the local scale. It is difficult to measure these local variations, and even more challenging to monitor how they fluctuate in time. 

The recent earthquakes in the Sumatran Subduction Zone not only allowed vast portions of the tectonic plates to move, releasing huge amounts of stress – the 2004 event alone produced as much energy as 150,000 Hiroshima nuclear bombs within 10 minutes – but they also stressed the crust and upper mantle, redistributed the tectonic stress, and generated ongoing surface displacement. This displacement was measured by GPS and tide gauges in the subsequent years and decades. 

In our study, we took advantage of the large stress perturbations from these multiple great earthquakes, coupled with high-resolution measurements of surface deformation to directly image the spatial and temporal evolution of deformation in the mantle beneath Sumatra. 

This powerful technique allowed us to monitor how the strength of the rock varies both in space and time, which we measured by examining how the viscosity in the mantle wedge evolved over time.

For example, the viscosity found at the Sumatra mantle wedge shows a very low value immediately after a great earthquake, indicating that the rocks are weak, but as time progresses the rocks become stronger and the viscosity increases, until they eventually reach a constant strength – the same strength that they had before the earthquake. Our findings suggest that the mantle rocks have a transient behaviour and their strength evolves with time, as a direct response to the stress perturbation of these great earthquakes. 

For the first time, we have directly imaged the complexity of post-seismic deformation of the lower crust and upper mantle across space and time. Our findings illuminate the transient behaviour of the rock strengths. Our simple images and representations of the rock properties hundreds of kilometres below the earth’s crust (as shown in the figure above) reveal how the earth deforms in response to large stress changes.

The technique presented in our study connects the surface geodetic measurements to rock properties at tens to hundreds of kilometres beneath the surface. This method highlights a new potential for geodesy to shed light on the possible mechanisms of the ductile distributed deformation.

Our findings bring us one step closer to building a model of the mechanical coupling between the lithosphere-asthenosphere systems. Such a model could provide invaluable insights for future seismic hazard assessment.

(Source of Thumbnail Image: DFAT/AusAid/Flickr)