December 2021 ESRFnews
on rock samples from a deep fault zone in western Norway, to show that earthquake ruptures can be so fast they not only break the rock, but also melt it (Sci. Adv. 5 eaaw0913). Now, however, Renard wants to understand what hap-
pens before rocks fracture. For BM18 he has designed a very large rock deformation apparatus, ZEUS, that will be able to put rocks the size of pieces of board chalk under unprecedented compression for synchrotron experi- ments. Most synchrotron X-rays would struggle to pene- trate the centimetre-thick walls of ZEUS; they would be absorbed before ever reaching the samples, which is why, for the past 50 years, most experiments of this kind have had to rely on fracture sounds detected with microphones. But the extremely brilliant X-rays of the ESRF EBS fun- nelled into BM18 will pass through the apparatus with lit- tle attenuation, allowing Renard, together with his ESRF colleague Benoît Cordonnier, to unearth the weak signals that precede rupture (see box below Bigger is better ). The researchers hope that the BM18 experiments will
help them identify the quiet seismological signals that precede real earthquakes. This project won t enable us to predict earthquakes, says Renard. But if we can demon- strate that the joint analysis of acoustic-emission signals and X-ray microtomography data can be used to predict dynamic rupture in our experiments, we will have discov- ered an important lead towards earthquake prediction. Renard s project officially starts in January, although
he does not expect the ZEUS apparatus to be built before the end of next year. In the meantime, he and Cordon- nier have other, complementary, experiments to perform at the ESRF: ultrafast imaging at the ID19 beamline to understand how water can strengthen or weaken faults; and X-ray diffraction at ID11 to deduce the strain at the tip of ruptures. The ESRF has the world-leading capa- bilities needed to perform the experimental programme, says Renard. n
synchrotron X-rays provide a whole new level of insight and here, the ESRF EBS leads the pack. As the world s brightest synchrotron X-ray source, the ESRF EBS is able not only to probe deep into rock samples, but also and perhaps more importantly to see through the metal apparatus required to generate huge pressures. BM18, a new ESRF-EBS flagship beamline that is due to open to users next year, will be able to accommodate compression apparatus of unprecedented size for the study of very large samples at the highest X-ray energies. At BM18, we will be able to study rocks at greater pressure than at any other synchrotron X-ray facility, says François Renard, a geo- physicist at the University of Oslo in Norway who in April was awarded the project BREAK, an advanced grant from the European Research Council to study the origins and precursors of earthquakes at the ESRF.
The way rocks fracture in earthquakes is complex. The simplest picture is that an earthquake originates along a fault plane where two tectonic plates slip past each other. From there, fractures propagate outwards, forking as they go to create branched structures throughout the rock, and sending out shock waves, which manifest as the ground shaking. As Renard s previous ESRF research has shown, however, this picture is a little too simple. In 2019, his group used X-ray tomography at the ESRF s ID19 beam- line to expose a competition between microfractures in compressed rock, whereby some develop into larger frac- tures while others fizzle out soon after they have begun (PNAS doi:10.1073/pnas.1902994116). In another study that year, the researchers used the same technique at ID19
When it comes to studying fracture propagation in rocks, size matters. Rock grains have sizes on the order of a fraction of a millimetre, which means that to see how fractures propagate, geophysicists ideally need samples of centimetre-size or more. Then, to generate earthquake- like compressions of up to 800 MPa, they need some considerable apparatus. Fortunately, BM18 is big very big. Some 220 m long, the ESRF s forthcoming flagship beamline will be able to accommodate sample setups with an area of more than 3.5 m2 (up to 2.5 m vertically by 1.5 m horizontally), and a mass of 300 kg. It will perform hierarchical imaging (a voxel size of 100 μm down to 1 μm) of medium to large objects using the propagation phase- contrast technique, with good contrast up to 350 keV; with propagation distances up to 35 m, high levels of phase contrast will be possible even at maximum energy and low resolution.
At BM18, we will be able to study rocks at greater pressure than at any other synchrotron X-ray facility
BIGGER IS BETTER
FR A N Ç O IS R E N A R D
B E N O ÎT C O R D O N N IE R
François Renard is now looking at the weak signals that are produced before the rock fractures.