Periodic Reporting for period 4 - RockDEaF (Dynamics of rock deformation at the brittle-plastic transition and the depth of earthquake faulting)
Reporting period: 2023-07-01 to 2024-06-30
The lithosphere is the thin outer shell of the Earth that supports the weight of mountains, plate tectonic forces, and stores the elastic energy that is released during earthquakes. The strength of the lithosphere directly controls the formation of tectonic plates and the generation and propagation of devastating earthquakes.
The strongest part of the lithosphere is where the deformation processes in rocks transition from brittle fracture to plastic flow. Earthquakes seem to stop below this transition, but we do not know exactly how deep they can propagate or nucleate. The transitional regime also marks the locus of the recently discovered low frequency earthquakes and tremors, which are currently not well explained.
Despite its fundamental importance, the transitional behaviour remains very poorly understood. In this regime, we still do not know how rock deformation processes and properties evolve with depth and, critically, time. We also do not know exactly where the transition occurs in nature, if and how it may move over time, and what are the prevailing conditions there.
The aim of this project is to provide unprecedented quantitative constrains on the key material properties and processes associated with deformation and fluid flow at the brittle-plastic transition, and arrive at a clear understanding of the prevailing conditions and the dynamics of fault slip at the depths where unconventional seismic motion (tremors, slow slip) is recorded in the lithosphere.
The project is interdisciplinary, at the interface between geology, materials science, and seismology.
We conducted laboratory rock deformation experiments at the high pressure and temperature conditions relevant to the transitional (or semi-brittle) regime, by developing state-of-the-art high pressure instrumentation combined with novel microstructural observation techniques. We determined the deformation mechanisms, the evolution of rock physical properties, the effects of fluids, on the dynamics of rock deformation across the brittle-plastic transition and on fault slip. Based on our laboratory data, we developed models that can be use to extrapolate the laboratory observations to natural conditions, and determine the geophysical signature of the brittle-plastic transition and its relationship to earthquake dynamics.
The strongest part of the lithosphere is where the deformation processes in rocks transition from brittle fracture to plastic flow. Earthquakes seem to stop below this transition, but we do not know exactly how deep they can propagate or nucleate. The transitional regime also marks the locus of the recently discovered low frequency earthquakes and tremors, which are currently not well explained.
Despite its fundamental importance, the transitional behaviour remains very poorly understood. In this regime, we still do not know how rock deformation processes and properties evolve with depth and, critically, time. We also do not know exactly where the transition occurs in nature, if and how it may move over time, and what are the prevailing conditions there.
The aim of this project is to provide unprecedented quantitative constrains on the key material properties and processes associated with deformation and fluid flow at the brittle-plastic transition, and arrive at a clear understanding of the prevailing conditions and the dynamics of fault slip at the depths where unconventional seismic motion (tremors, slow slip) is recorded in the lithosphere.
The project is interdisciplinary, at the interface between geology, materials science, and seismology.
We conducted laboratory rock deformation experiments at the high pressure and temperature conditions relevant to the transitional (or semi-brittle) regime, by developing state-of-the-art high pressure instrumentation combined with novel microstructural observation techniques. We determined the deformation mechanisms, the evolution of rock physical properties, the effects of fluids, on the dynamics of rock deformation across the brittle-plastic transition and on fault slip. Based on our laboratory data, we developed models that can be use to extrapolate the laboratory observations to natural conditions, and determine the geophysical signature of the brittle-plastic transition and its relationship to earthquake dynamics.
At the start of the project, a first series of laboratory tests were conducted to investigate in unprecedented level of detail the transition from localised (faulting) to distributed (ductile) deformation in crustal rock (using again marble as a representative rock type). We established a simple criterion that determines the partitioning of deformation between localised fault slip and off-fault, bulk ductile flow. The data highlight the key role played by strain hardening in the partitioning of deformation.
Since the start of the project in January 2019, extensive experimental work has been undertaken to upgrade an existing high pressure, high temperature rock deformation apparatus at UCL. Such work included setting up an ultrasonic monitoring system, the design and manufacture of new internal pistons, a new internal high temperature furnace, and a new control system. Since early 2021 the experimental apparatus is fully functional. In 2022 we completed a suite (around 100 tests) of high pressure, high temperature experiments aimed at analysing the brittle-plastic transition in calcite marble, notably looking into the role of grain size, microcracks, twinning and dislocations in the hardening behaviour of the material. Our dataset combines ultrasonic monitoring under lower-crustal deformation conditions, and showed the key role of grain size in the strength of rocks in the semi-brittle regime. We followed this study by a systematic work quantifying the evolution of key microstructural parameters during semi-brittle flow, which allowed us to determine the key state variables controlling semi-brittle strength.
In parallel with the experimental programme, we conducted theoretical work which lead to (1) a complete analysis of the role of microscale friction in the formation of microcracks in the semi-brittle regime, and (2) a first micromechanical model including feedbacks between brittle and plastic mechanisms, capable of reproducing key features of the stress-strain behaviour of rocks in the semi-brittle regime.
We also devoted a significant effort to developing new laboratory methods to measure in situ fluid pressures during rock deformation. This new technology allowed us to conduct work demonstrating how dilatancy (the increase in crack volume occurring prior to and during faulting) can lead to dramatic fluid pressure drop during rock failure, providing the first direct experimental evidence of the so-called "seismic sucction pump" concept established in the 1980s. In follow up work we demonstrated experimentally that (1) dilatancy has the potential to stabilise shear rupture and fault slip in rocks, which had been suspected in theory but never observed so far; (2) post-rupture fluid pressure changes lead to substantial fault slip, which had never been reported before.
Overall, the work from the project lead to 17 publications in internatial peer-reviewed journals, with 9 more manuscripts submitted or in advanced stages of preparation. We communicated results in major international conferences. Our work has lead to major technical progress in the field of experimental geophysics, with the development of ultrasonic monitoring techniques at elevated pressure and temperature, and in situ fluid pressure monitoring sensors for use in high pressure rock deformation apparatus. In the course of the project, we generated unique, systematic datasets documenting the behaviour of rocks across the brittle-plastic transition and the role of fluids on the dynamics of faulting, and developed new physical models to quantify and upscale our results.
Since the start of the project in January 2019, extensive experimental work has been undertaken to upgrade an existing high pressure, high temperature rock deformation apparatus at UCL. Such work included setting up an ultrasonic monitoring system, the design and manufacture of new internal pistons, a new internal high temperature furnace, and a new control system. Since early 2021 the experimental apparatus is fully functional. In 2022 we completed a suite (around 100 tests) of high pressure, high temperature experiments aimed at analysing the brittle-plastic transition in calcite marble, notably looking into the role of grain size, microcracks, twinning and dislocations in the hardening behaviour of the material. Our dataset combines ultrasonic monitoring under lower-crustal deformation conditions, and showed the key role of grain size in the strength of rocks in the semi-brittle regime. We followed this study by a systematic work quantifying the evolution of key microstructural parameters during semi-brittle flow, which allowed us to determine the key state variables controlling semi-brittle strength.
In parallel with the experimental programme, we conducted theoretical work which lead to (1) a complete analysis of the role of microscale friction in the formation of microcracks in the semi-brittle regime, and (2) a first micromechanical model including feedbacks between brittle and plastic mechanisms, capable of reproducing key features of the stress-strain behaviour of rocks in the semi-brittle regime.
We also devoted a significant effort to developing new laboratory methods to measure in situ fluid pressures during rock deformation. This new technology allowed us to conduct work demonstrating how dilatancy (the increase in crack volume occurring prior to and during faulting) can lead to dramatic fluid pressure drop during rock failure, providing the first direct experimental evidence of the so-called "seismic sucction pump" concept established in the 1980s. In follow up work we demonstrated experimentally that (1) dilatancy has the potential to stabilise shear rupture and fault slip in rocks, which had been suspected in theory but never observed so far; (2) post-rupture fluid pressure changes lead to substantial fault slip, which had never been reported before.
Overall, the work from the project lead to 17 publications in internatial peer-reviewed journals, with 9 more manuscripts submitted or in advanced stages of preparation. We communicated results in major international conferences. Our work has lead to major technical progress in the field of experimental geophysics, with the development of ultrasonic monitoring techniques at elevated pressure and temperature, and in situ fluid pressure monitoring sensors for use in high pressure rock deformation apparatus. In the course of the project, we generated unique, systematic datasets documenting the behaviour of rocks across the brittle-plastic transition and the role of fluids on the dynamics of faulting, and developed new physical models to quantify and upscale our results.
The project lead to substantial progress beyond the state of the art. From a technical standpoint, we developed a new high pressure (1 GPa), high temperature (700C) triaxial apparatus with ultrasonic transmission capability, which is unique in the world. We also designed a new type of transducer to measure in situ, local fluid pressure in high pressure triaxial apparatus. Both of those new techniques allowed us to make major progress in our understanding of rock deformation mechanisms under crustal conditions.
Firstly, we contributed a unique, systematic dataset documenting the relationship between mechanical behaviour and microstructure evolution in the so-called "semi-brittle" regime, in conditions representative of the middle to lower crust. We used this new dataset to develop a new model for the rheology of rocks in the semi-brittle regime, including the concurrent operation of multiple grain-scale mechanisms (frcaturing, twinning and dislocation glide).
Secondly, we discovered that rock failure and fault slip leads to massive fluctuations in the pressure of the fluid saturating the pore space of the rock. During failure, the formation and linkage of microcracks had been thought to lead to fluid pressure drop, but this effect had never been observed experimentally until now. In this respect, our experimental work constitutes a major achievement, validating directly a long-standing theoretical model, and bringing quantitative constrains on key model parameters that allowed us to upscale the results to the field scale.
Firstly, we contributed a unique, systematic dataset documenting the relationship between mechanical behaviour and microstructure evolution in the so-called "semi-brittle" regime, in conditions representative of the middle to lower crust. We used this new dataset to develop a new model for the rheology of rocks in the semi-brittle regime, including the concurrent operation of multiple grain-scale mechanisms (frcaturing, twinning and dislocation glide).
Secondly, we discovered that rock failure and fault slip leads to massive fluctuations in the pressure of the fluid saturating the pore space of the rock. During failure, the formation and linkage of microcracks had been thought to lead to fluid pressure drop, but this effect had never been observed experimentally until now. In this respect, our experimental work constitutes a major achievement, validating directly a long-standing theoretical model, and bringing quantitative constrains on key model parameters that allowed us to upscale the results to the field scale.