ABSTRACT Fault surface roughness is a principal factor influencing fault and earthquake mechanics... more ABSTRACT Fault surface roughness is a principal factor influencing fault and earthquake mechanics. However, little is known on roughness of fault surfaces at seismogenic depths, and particularly on how it evolves with accumulating slip. We have studied seismogenic fault surfaces of the Gole Larghe Fault Zone, which exploit precursor cooling joints of the Adamello tonalitic pluton (Italian Alps). These faults developed at 9-11 km and 250-300°C. Seismic slip along these surfaces, which individually accommodated from 1 to 20 m of net slip, resulted in the production of cm-thick cataclasites and pseudotachylytes (solidified melts produced during seismic slip). The roughness of fault surfaces was determined with a multi-resolution aerial and terrestrial LIDAR and photogrammetric dataset (Bistacchi et al., 2011, Pageoph, doi: 10.1007/s00024-011-0301-7). Fault surface roughness is self-affine, with Hurst exponent H < 1, indicating that faults are comparatively smoother at larger wavelengths. Fault surface roughness is inferred to have been inherited from the precursor cooling joints, which show H ≈ 0.8. Slip on faults progressively modified the roughness distribution, lowering the Hurst exponent in the along-slip direction up to H ≈ 0.6. This behaviour has been observed for wavelengths up to the scale of the accumulated slip along each individual fault surface, whilst at larger wavelengths the original roughness seems not to be affected by slip. Processes that contribute to modify fault roughness with slip include brittle failure of the interacting asperities (production of cataclasites) and frictional melting (production of pseudotachylytes). To quantify the "wear" due to these processes, we measured, together with the roughness of fault traces and their net slip, the thickness and distribution of cataclasites and pseudotachylytes. As proposed also in the tribological literature, we observe that wearing is scale dependent, as smaller wavelength asperities have a shorter interaction distance and are consumed faster with slip than larger ones. However, in faults, production of cataclasites and pseudotachylytes changes the contact area of sliding surfaces by interposing a layer of wear products. This layer may preserve from wearing asperities that are smaller in amplitude than the layer thickness, thus providing a mechanism that is likely to preserve small amplitude/wavelength roughness. These processes have been considered in a new spectral model of wear, which allows to model wear for self-affine surfaces and includes the accumulation of wear products within the fault zone. This model can be used to generalize our results and contribute to reconstruct a realistic model of a seismogenic fault zone (http://roma1.rm.ingv.it/laboratori/laboratorio-hp-ht/usems-project).
In this contribution, we present a novel state-of-the-art experimental rotary shear apparatus (SH... more In this contribution, we present a novel state-of-the-art experimental rotary shear apparatus (SHIVA or Slow to HIgh Velocity Apparatus) capable of shearing samples at sliding velocities up to 10 m/s, accelerations of ∼ 40 m/s and normal stresses up to 50 MPa. In comparison with existing high speed friction machines, this apparatus extends the range of sliding velocities, normal stresses, sample size and, more importantly, accelerations.
Earthquakes are caused by fracture and frictional slip on faults. Friction is a key parameter in ... more Earthquakes are caused by fracture and frictional slip on faults. Friction is a key parameter in understanding the physics seismic source. Let's investigate friction under Earth crust conditions. Fault surface in dolomite, Southern Alps, Italy No direct access to the "earthquake engine"... Let's try to recreate it in the lab? Friction controls earthquake physics No direct access to the "earthquake engine" Earthquake simulation in the lab: friction machines Examples of experiments and typical results Lubrication processes and rock types Extrapolating the results: faults are weak during earthquakes Conclusions and future research
The determination of fault strength (rock friction sensu latu) at seismic slip rates (about 1 m/s... more The determination of fault strength (rock friction sensu latu) at seismic slip rates (about 1 m/s), is of paramount importance in earthquake mechanics, as fault strength controls rupture properties, stress drop, radiated energy and heat produced during slip. Given the lack of determination through seismological methods, elucidating constraints arise from experimental studies. Here we show that a review of the experiments (~400) performed in rotary shear apparatuses at slip rates of 0.1 - 1.3 m/s indicate a significant decrease in friction (up to one order of magnitude) for cohesive (silicate-, quartz- and carbonate- built) and non-cohesive (clay-rich and dolomite gouges) rocks. Low friction is concurrent to an increase in temperature in the slipping zone which triggers thermally-activated physico-chemical processes responsible for fault lubrication (decarbonation and dehydration reactions, flash heating, melt lubrication, etc.). Extrapolation of experimental data to natural conditions, suggests large coseismic stress drops (> 70 MPa) at earthquake nucleation depths (7 - 10 km), irrespective of fault rock composition and of the specific weakening mechanism involved. Such large stress drop estimates are consistent with dynamic stress drops obtained from seismic inversion data and geological studies.
8 The amount of energy radiated from an earthquake can be measured using recent methods 9 based o... more 8 The amount of energy radiated from an earthquake can be measured using recent methods 9 based on earthquake coda signals and spectral ratios. Such methods are not altered by 10 either site or directivity effects, with the advantage of a greatly improved accuracy. 11 Several studies of earthquake sequences based on the above measurements showed 12 evidence of a breakdown in self-similarity in the moment to energy relation. Radiated 13 energy can be also used as a gauge to estimate the average dynamic stress drop on the 14 fault. Here we compute the dynamic stress drop, infer the co-seismic friction and estimate 15 the co-seismic heating resulting from the frictional work during events from different 16 main shock-aftershock earthquake sequences. We relate the dynamic friction to the 17 maximum temperature rise estimated on the faults for each earthquake. Our results are 18 strongly indicative that a thermally triggered dynamic frictional weakening is present, 19 responsible for the...
Viscous flow at high strain rates is a well-known deformation mechanism occurring in metals, but ... more Viscous flow at high strain rates is a well-known deformation mechanism occurring in metals, but has only recently been associated with the behaviour of natural fault materials during earthquakes in mobile belts. In particular, microstructures attributed to grain boundary sliding have been recognised in high velocity shear experiments where the recrystallized materials commonly have a nanometric grainsize.
Numerical simulations used to describe these features predominantly use standardised theoretical ... more Numerical simulations used to describe these features predominantly use standardised theoretical equations or experimental observations often assuming that their validity extends to all slip-rates, lithologies and tectonic environments. However recent rotary-shear experiments performed on a range of diverse materials and experimental conditions highlighted the large variability of the evolution of friction during slipping pointing to a more complex relationship between material type, slip rate and normal stress.
Fault zone structure over a wide range of scales strongly influences earthquake mechanics, includ... more Fault zone structure over a wide range of scales strongly influences earthquake mechanics, including the sites of earthquake nucleation and arrest, co-seismic strength and slip distribution, and the amount of energy expended during frictional heating and creation of wall-rock damage. ...
Recent seismological observations have highlighted increasing foreshock activity preceding the fa... more Recent seismological observations have highlighted increasing foreshock activity preceding the failure of large earthquakes [
The full-text may be used and/or reproduced, and given to third parties in any format or medium, ... more The full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-prot purposes provided that: • a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.
Abstract This chapter describes the behaviour of faults in the laboratory. The chapter is organis... more Abstract This chapter describes the behaviour of faults in the laboratory. The chapter is organised from small scale to large scale experiments, introducing the reader to general and less general observations of faulting and friction, and showing how these observations are linked to faulting processes occurring in nature. The first section introduces cm-scale friction experiments on gouge materials including the concept of rate-and-state friction, i.e., how velocity affects friction in the quasi-static regime. The following section is devoted to dynamic friction, i.e., observations of friction at high velocity as well as observations of dynamic rupture. The third section discusses the evolution of discrete faults and fault zones in up to meter-scale physical analogue experiments, their dependence on material properties and their significance for the study of large-scale tectonic structures. Finally, the various microstructural features and their possible link to fault stability obtained in the quasi-static regime will be discussed.
ABSTRACT Fault surface roughness is a principal factor influencing fault and earthquake mechanics... more ABSTRACT Fault surface roughness is a principal factor influencing fault and earthquake mechanics. However, little is known on roughness of fault surfaces at seismogenic depths, and particularly on how it evolves with accumulating slip. We have studied seismogenic fault surfaces of the Gole Larghe Fault Zone, which exploit precursor cooling joints of the Adamello tonalitic pluton (Italian Alps). These faults developed at 9-11 km and 250-300°C. Seismic slip along these surfaces, which individually accommodated from 1 to 20 m of net slip, resulted in the production of cm-thick cataclasites and pseudotachylytes (solidified melts produced during seismic slip). The roughness of fault surfaces was determined with a multi-resolution aerial and terrestrial LIDAR and photogrammetric dataset (Bistacchi et al., 2011, Pageoph, doi: 10.1007/s00024-011-0301-7). Fault surface roughness is self-affine, with Hurst exponent H < 1, indicating that faults are comparatively smoother at larger wavelengths. Fault surface roughness is inferred to have been inherited from the precursor cooling joints, which show H ≈ 0.8. Slip on faults progressively modified the roughness distribution, lowering the Hurst exponent in the along-slip direction up to H ≈ 0.6. This behaviour has been observed for wavelengths up to the scale of the accumulated slip along each individual fault surface, whilst at larger wavelengths the original roughness seems not to be affected by slip. Processes that contribute to modify fault roughness with slip include brittle failure of the interacting asperities (production of cataclasites) and frictional melting (production of pseudotachylytes). To quantify the "wear" due to these processes, we measured, together with the roughness of fault traces and their net slip, the thickness and distribution of cataclasites and pseudotachylytes. As proposed also in the tribological literature, we observe that wearing is scale dependent, as smaller wavelength asperities have a shorter interaction distance and are consumed faster with slip than larger ones. However, in faults, production of cataclasites and pseudotachylytes changes the contact area of sliding surfaces by interposing a layer of wear products. This layer may preserve from wearing asperities that are smaller in amplitude than the layer thickness, thus providing a mechanism that is likely to preserve small amplitude/wavelength roughness. These processes have been considered in a new spectral model of wear, which allows to model wear for self-affine surfaces and includes the accumulation of wear products within the fault zone. This model can be used to generalize our results and contribute to reconstruct a realistic model of a seismogenic fault zone (http://roma1.rm.ingv.it/laboratori/laboratorio-hp-ht/usems-project).
In this contribution, we present a novel state-of-the-art experimental rotary shear apparatus (SH... more In this contribution, we present a novel state-of-the-art experimental rotary shear apparatus (SHIVA or Slow to HIgh Velocity Apparatus) capable of shearing samples at sliding velocities up to 10 m/s, accelerations of ∼ 40 m/s and normal stresses up to 50 MPa. In comparison with existing high speed friction machines, this apparatus extends the range of sliding velocities, normal stresses, sample size and, more importantly, accelerations.
Earthquakes are caused by fracture and frictional slip on faults. Friction is a key parameter in ... more Earthquakes are caused by fracture and frictional slip on faults. Friction is a key parameter in understanding the physics seismic source. Let's investigate friction under Earth crust conditions. Fault surface in dolomite, Southern Alps, Italy No direct access to the "earthquake engine"... Let's try to recreate it in the lab? Friction controls earthquake physics No direct access to the "earthquake engine" Earthquake simulation in the lab: friction machines Examples of experiments and typical results Lubrication processes and rock types Extrapolating the results: faults are weak during earthquakes Conclusions and future research
The determination of fault strength (rock friction sensu latu) at seismic slip rates (about 1 m/s... more The determination of fault strength (rock friction sensu latu) at seismic slip rates (about 1 m/s), is of paramount importance in earthquake mechanics, as fault strength controls rupture properties, stress drop, radiated energy and heat produced during slip. Given the lack of determination through seismological methods, elucidating constraints arise from experimental studies. Here we show that a review of the experiments (~400) performed in rotary shear apparatuses at slip rates of 0.1 - 1.3 m/s indicate a significant decrease in friction (up to one order of magnitude) for cohesive (silicate-, quartz- and carbonate- built) and non-cohesive (clay-rich and dolomite gouges) rocks. Low friction is concurrent to an increase in temperature in the slipping zone which triggers thermally-activated physico-chemical processes responsible for fault lubrication (decarbonation and dehydration reactions, flash heating, melt lubrication, etc.). Extrapolation of experimental data to natural conditions, suggests large coseismic stress drops (> 70 MPa) at earthquake nucleation depths (7 - 10 km), irrespective of fault rock composition and of the specific weakening mechanism involved. Such large stress drop estimates are consistent with dynamic stress drops obtained from seismic inversion data and geological studies.
8 The amount of energy radiated from an earthquake can be measured using recent methods 9 based o... more 8 The amount of energy radiated from an earthquake can be measured using recent methods 9 based on earthquake coda signals and spectral ratios. Such methods are not altered by 10 either site or directivity effects, with the advantage of a greatly improved accuracy. 11 Several studies of earthquake sequences based on the above measurements showed 12 evidence of a breakdown in self-similarity in the moment to energy relation. Radiated 13 energy can be also used as a gauge to estimate the average dynamic stress drop on the 14 fault. Here we compute the dynamic stress drop, infer the co-seismic friction and estimate 15 the co-seismic heating resulting from the frictional work during events from different 16 main shock-aftershock earthquake sequences. We relate the dynamic friction to the 17 maximum temperature rise estimated on the faults for each earthquake. Our results are 18 strongly indicative that a thermally triggered dynamic frictional weakening is present, 19 responsible for the...
Viscous flow at high strain rates is a well-known deformation mechanism occurring in metals, but ... more Viscous flow at high strain rates is a well-known deformation mechanism occurring in metals, but has only recently been associated with the behaviour of natural fault materials during earthquakes in mobile belts. In particular, microstructures attributed to grain boundary sliding have been recognised in high velocity shear experiments where the recrystallized materials commonly have a nanometric grainsize.
Numerical simulations used to describe these features predominantly use standardised theoretical ... more Numerical simulations used to describe these features predominantly use standardised theoretical equations or experimental observations often assuming that their validity extends to all slip-rates, lithologies and tectonic environments. However recent rotary-shear experiments performed on a range of diverse materials and experimental conditions highlighted the large variability of the evolution of friction during slipping pointing to a more complex relationship between material type, slip rate and normal stress.
Fault zone structure over a wide range of scales strongly influences earthquake mechanics, includ... more Fault zone structure over a wide range of scales strongly influences earthquake mechanics, including the sites of earthquake nucleation and arrest, co-seismic strength and slip distribution, and the amount of energy expended during frictional heating and creation of wall-rock damage. ...
Recent seismological observations have highlighted increasing foreshock activity preceding the fa... more Recent seismological observations have highlighted increasing foreshock activity preceding the failure of large earthquakes [
The full-text may be used and/or reproduced, and given to third parties in any format or medium, ... more The full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-prot purposes provided that: • a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.
Abstract This chapter describes the behaviour of faults in the laboratory. The chapter is organis... more Abstract This chapter describes the behaviour of faults in the laboratory. The chapter is organised from small scale to large scale experiments, introducing the reader to general and less general observations of faulting and friction, and showing how these observations are linked to faulting processes occurring in nature. The first section introduces cm-scale friction experiments on gouge materials including the concept of rate-and-state friction, i.e., how velocity affects friction in the quasi-static regime. The following section is devoted to dynamic friction, i.e., observations of friction at high velocity as well as observations of dynamic rupture. The third section discusses the evolution of discrete faults and fault zones in up to meter-scale physical analogue experiments, their dependence on material properties and their significance for the study of large-scale tectonic structures. Finally, the various microstructural features and their possible link to fault stability obtained in the quasi-static regime will be discussed.
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