Periodic Reporting for period 2 - BREAK (Break-Through Rocks)
Reporting period: 2023-07-01 to 2024-12-31
The EU-funded ERC Advanced Grant BREAK is developing novel experimental laboratory techniques and models to unravel the mechanisms at the origin of earthquakes.
Deformation in Earth’s crust localizes onto faults that may rupture rapidly producing earthquakes or undergo slow aseismic slip. The detailed mechanisms that control the transition between the seismic and aseismic regimes and the onset of earthquakes remain unknown. These mechanisms control the geophysical processes preceding catastrophic failure, such as fracture development and strain localization on fault and in the rock volume surrounding them. The project BREAK provides quantitative laboratory observations of the full displacement field in rocks before and during fault slip and separate the aseismic and seismic components of it. We develop novel experimental techniques based primarily on simultaneous dynamic synchrotron X-ray microtomography imaging and acoustic emission data acquisition and analysis. The data reveal how slow and fast deformations develop and interact with each other in dry and wet crustal rocks under the stress, fluid pressure and temperature conditions at depths up to ten kilometres and characterize the production of fractures during earthquake nucleation and rupture propagation. We also develop methods to predict the time to failure from these signals using machine learning. We also compare the deformation microstructures produced in laboratory experiments with those of natural rock samples collected in California and Norway, where earthquakes have occurred. The overarching goal is therefore to progress toward a general model of the path to brittle failure in rocks by advancing knowledge of how fractures accumulate before and during both slow and fast earthquakes.
Deformation in Earth’s crust localizes onto faults that may rupture rapidly producing earthquakes or undergo slow aseismic slip. The detailed mechanisms that control the transition between the seismic and aseismic regimes and the onset of earthquakes remain unknown. These mechanisms control the geophysical processes preceding catastrophic failure, such as fracture development and strain localization on fault and in the rock volume surrounding them. The project BREAK provides quantitative laboratory observations of the full displacement field in rocks before and during fault slip and separate the aseismic and seismic components of it. We develop novel experimental techniques based primarily on simultaneous dynamic synchrotron X-ray microtomography imaging and acoustic emission data acquisition and analysis. The data reveal how slow and fast deformations develop and interact with each other in dry and wet crustal rocks under the stress, fluid pressure and temperature conditions at depths up to ten kilometres and characterize the production of fractures during earthquake nucleation and rupture propagation. We also develop methods to predict the time to failure from these signals using machine learning. We also compare the deformation microstructures produced in laboratory experiments with those of natural rock samples collected in California and Norway, where earthquakes have occurred. The overarching goal is therefore to progress toward a general model of the path to brittle failure in rocks by advancing knowledge of how fractures accumulate before and during both slow and fast earthquakes.
The project BREAK started with the development of novel experimental techniques that couples rock deformation and the use of X-rays to image deformations or measure directly stress and deformation inside samples. The main goal is to reproduce processes that occur at depth in the Earth’s crust where earthquake start, propagate and then stop. The work has been carried out through a collaboration between the University of Oslo and three beamlines at the European Synchrotron Radiation Facility (ID11 for X-ray diffraction, ID19 for ultra-fast imaging of shock wave in rocks, and BM18 for 4D in-situ imaging of rock deformation processes leading to system-size failure). We spent the first two years to develop the experimental techniques and publish the first results in a dozen of articles released in the best journals in geophysics. An important achievement has been to use machine learning techniques to predict the proximity to failure of rock cores while they are deformed until they break. Deformation occurs by the nucleation of microcracks in the samples and, as loading is increased, these cracks grow and coalesce until a system-size spanning fracture develop, leading to a laboratory earthquake. Using images of these fracture networks and a deep-learning algorithm we developed, we could show that not only the number of fractures controls the proximity to failure, but also how they are organized in space. Another important result is the development of a theoretical physical model of earthquakes with a minimal number of parameters. Such an approach is useful to unravel the basic mechanisms of earthquake propagation and arrest, and therefore identify which physical process provide a primary control on these processes.
This development of experimental techniques to reproduce earthquake processes in the laboratory has paved the way to several applications that we will pursue during the second half of the project. Particularly, important efforts will be made to measure the level of stress in situ in natural and experimentally deformed rocks. For this, we have adapted an existing rock deformation apparatus, called Hades, to beamline ID11 at ESRF. This beamline allows scanning a sample in three dimensions and measure everywhere tiny distortions of the crystal lattice of rock minerals. These distortions can be related to local heterogeneities of elastic strain that correspond to heterogeneities of stress. Preliminary measurements in natural samples and samples deformed in the Hades apparatus show that stress is indeed very heterogeneous at the grain scale and these heterogeneities may have recorded past deformation events such as past earthquakes. This method will be applied to natural rock samples for the first time and may provide a new tool available to the geoscience community in the future. Important discoveries are also expected in another set of experiments where shock waves are sent into rock samples, mimicking the propagation of an earthquake rupture into intact rocks. These shocks lead to a damaging of the rock and its transformation into a granular material. We have developed a technique to image this process with an ultra-fast camera (several million images per second), while the sample is under conditions of pressure of several kilometers depth. Overall, our experimental and theoretical works, validated by field data, will provide novel insights on the mechanisms at work during earthquakes.