Giulio Di Toro

Friction falls towards zero in quartz rock as slip velocity approaches seismic rates

An important unsolved problem in earthquake mechanics is to determine the resistance to slip on faults in the Earths crust during earthquakes. Knowledge of coseismic slip resistance is critical for understanding the magnitude of shear-stress reduction and hence the near-fault acceleration that can occur during earthquakes, which affects the amount of damage that earthquakes are capable of causing. In particular, a long-unresolved problem is the apparently low strength of major faults, which may be caused by low coseismic frictional resistance. The frictional properties of rocks at slip velocities up to 3 mm s-1 and for slip displacements characteristic of large earthquakes have been recently simulated under laboratory conditions. Here we report data on quartz rocks that indicate an extraordinary progressive decrease in frictional resistance with increasing slip velocity above 1 mm s-1. This reduction extrapolates to zero friction at seismic slip rates of ∼1 m s-1, and appears to be due to the formation of a thin layer of silica gel on the fault surface: it may explain the low strength of major faults during earthquakes.

Recent advances in the understanding of fault zone internal structure: a review

Abstract It is increasingly apparent that faults are typically not discrete planes but zones of deformed rock with a complex internal structure and three-dimensional geometry. In the last decade this has led to renewed interest in the consequences of this complexity for modelling the impact of fault zones on fluid flow and mechanical behaviour of the Earths crust. A number of processes operate during the development of fault zones, both internally and in the surrounding host rock, which may encourage or inhibit continuing fault zone growth. The complexity of the evolution of a faulted system requires changes in the rheological properties of both the fault zone and the surrounding host rock volume, both of which impact on how the fault zone evolves with increasing displacement. Models of the permeability structure of fault zones emphasize the presence of two types of fault rock components: fractured conduits parallel to the fault and granular core zone barriers to flow. New data presented in this paper on porosity–permeability relationships of fault rocks during laboratory deformation tests support recently advancing concepts which have extended these models to show that poro-mechanical approaches (e.g., critical state soil mechanics, fracture dilatancy) may be applied to predict the fluid flow behaviour of complex fault zones during the active life of the fault. Predicting the three-dimensional heterogeneity of fault zone internal structure is important in the hydrocarbon industry for evaluating the retention capacity of faults in exploration contexts and the hydraulic behaviour in production contexts. Across-fault reservoir juxtaposition or non-juxtaposition, a key property in predicting retention or across-fault leakage, is strongly controlled by the three-dimensional complexity of the fault zone. Although algorithms such as shale gouge ratio greatly help predict capillary threshold pressures, quantification of the statistical variation in fault zone composition will allow estimations of uncertainty in fault retention capacity and hence prospect reserve estimations. Permeability structure in the fault zone is an important issue because bulk fluid flow rates through or along a fault zone are dependent on permeability variations, anisotropy and tortuosity of flow paths. A possible way forward is to compare numerical flow models using statistical variations of permeability in a complex fault zone in a given sandstone/shale context with field-scale estimates of fault zone permeability. Fault zone internal structure is equally important in understanding the seismogenic behaviour of faults. Both geometric and compositional complexities can control the nucleation, propagation and arrest of earthquakes. The presence and complex distribution of different fault zone materials of contrasting velocity-weakening and velocity-strengthening properties is an important factor in controlling earthquake nucleation and whether a fault slips seismogenically or creeps steadily, as illustrated by recent studies of the San Andreas Fault. A synthesis of laboratory experiments presented in this paper shows that fault zone materials which become stronger with increasing slip rate, typically then get weaker as slip rate continues to increase to seismogenic slip rates. Thus the probability that a nucleating rupture can propagate sufficiently to generate a large earthquake depends upon its success in propagating fast enough through these materials in order to give them the required velocity kick. This propagation success is hence controlled by the relative and absolute size distributions of velocity-weakening and velocity-strengthening rocks within the fault zone. Statistical characterisation of the distribution of such contrasting properties within complex fault zones may allow for better predictive models of rupture propagation in the future and provide an additional approach to earthquake size forecasting and early warnings.

Earthquake rupture dynamics frozen in exhumed ancient faults

Most of our knowledge about co-seismic rupture propagation is derived from inversion and interpretation of strong-ground-motion seismograms, laboratory experiments on rock and rock-analogue material, or inferred from theoretical and numerical elastodynamic models. However, additional information on dynamic rupture processes can be provided by direct observation of faults exhumed at the Earths surface. Pseudotachylytes (solidified friction-induced melts) are the most certain fault-rock indicator of seismicity on ancient faults. Here we show how the asymmetry in distribution and the orientation of pseudotachylyte-filled secondary fractures around an exhumed fault can be used to reconstruct the earthquake rupture directivity, rupture velocity and fracture energy, by comparison with the theoretical dynamic stress field computed around propagating fractures. In particular, the studied natural network of pseudotachylytes is consistent with a dominant propagation direction during repeated seismic events and subsonic rupture propagation close to the Rayleigh wave velocity.

Fault lubrication and earthquake propagation in thermally unstable rocks

Experiments performed on dolomite or Mg-calcite gouges at seismic slip rates ( v > 1 m/s) and displacements (d > 1 m) show that the frictional coefficient μ decays exponentially from peak values (m p ≈ 0.8, in the Byerlee9s range), to extremely low steady-state values (μ ss ≈ 0.1), attained over a weakening distance D w . Microstructural observations show that discontinuous patches of nanoparticles of dolomite and its decomposition products (periclase and lime or portlandite) were produced in the slip zone during the transient stage (d w ). These observations, integrated with CO 2 emissions data recorded during the experiments, suggest that particle interaction in the slip zone produces flash temperatures that are large enough to activate chemical and physical processes, e.g., decarbonation reactions ( T = 550 °C). During steady state (d ≥ D w ), shear strength is very low and not dependent upon normal stresses, suggesting that pressurized fluids (CO 2 ) may have been temporarily trapped within the slip zone. At this stage a continuous layer of nanoparticles is developed in the slip zone. For d >> D w , a slight but abrupt increase in shear strength is observed and interpreted as due to fluids escaping the slip zone. At this stage, dynamic weakening appears to be controlled by velocity dependent properties of nanoparticles developed in the slip zone. Experimentally derived seismic source parameter W b (i.e., breakdown work, the energy that controls the dynamics of a propagating fracture) (1) matches W b values obtained from seismological data of the A.D. 1997 M6 Colfiorito (Italy) earthquakes, which nucleated in the same type of rocks tested in this study, and (2) suggests similar earthquake-scaling relationships, as inferred from existing seismological data sets. We conclude that dynamic weakening of experimental faults is controlled by multiple slip weakening mechanisms, which are activated or inhibited by physicochemical reactions in the slip zone.

Gouge graphitization and dynamic fault weakening during the 2008 Mw 7.9 Wenchuan earthquake

The Longmenshan fault that ruptured during the 2008 Mw 7.9 Wenchuan (China) earthquake was drilled to a depth of 1200 m, and fault rocks including those in the 2008 earthquake slip zone were recovered at a depth of 575–595 m. We report laboratory strength measurements and microstructural observations from samples of slip zone fault rocks at deformation conditions expected for coseismic slip at borehole depths. Results indicate that the Longmenshan fault at this locality is extremely weak at seismic slip rates. In situ synchrotron X-ray diffraction analysis indicates that graphite was formed along localized slip zones in the experimental products, similar to the occurrence of graphite in the natural principal slip zone of the 2008 Wenchuan rupture. We surmise that graphitization occurred due to frictional heating of carbonaceous minerals. Because graphitization was associated with strong dynamic weakening in the experiments, we further infer that the Longmenshan fault was extremely weak at borehole depths during the 2008 Wenchuan earthquake, and that enrichment of graphite along localized slip zones could be used as an indicator of transient frictional heating during seismic slip in the upper crust.

Strain-insensitive preferred orientation of porphyroclasts in Mont Mary mylonites

Abstract The shape preferred orientation (SPO) of porphyroclasts was determined in high temperature mylonites. The porphyroclasts approach rhomboidal (sillimanite) or elliptical (garnet, plagioclase, sillimanite) shapes, and exhibit aspect ratios ( R ) as high as 11. Particles with R >3 are dominantly rhomboidal. The long axis of the best-fit ellipse defines a very strong SPO inclined at 5–10° to the mylonitic foliation, in an antithetic sense with respect to the shear direction. This angle is independent of R . The inclination of the long sides of rhomboidal sillimanite increases from 10 to 20° with decreasing R . In contrast, the short sides have a constant orientation of 15 to 17° irrespective of R and are parallel to extensional crenulation cleavage. Low aspect ratio (mainly elliptical) objects show low intensity SPO close to the shear plane. The two SPOs appear strain-insensitive. In the case of R R >3, a stable position is acquired. This is not explained by any of the current theoretical and experimental models of SPO.

Low‐ to high‐velocity frictional properties of the clay‐rich gouges from the slipping zone of the 1963 Vaiont slide, northern Italy

The final slip of about 450 m at about 30 m/s of the 1963 Vaiont landslide (Italy) was preceded by >3 year long creeping phase which was localized in centimeter-thick clay-rich layers (60–70% smectites, 20–30% calcite and quartz). Here we investigate the frictional properties of the clay-rich layers under similar deformation conditions as during the landslide: 1–5 MPa normal stress, 2 × 10^(−7) to 1.31 m/s slip rate and displacements up to 34 m. Experiments were performed at room humidity and wet conditions with biaxial, torsion and rotary shear apparatus. The clay-rich gouge was velocity-independent to velocity-weakening in both room humidity and wet conditions. In room humidity experiments, the coefficient of friction decreased from 0.47 at v 0.70 m/s: full lubrication results from the formation of a continuous water film in the gouge. The Vaiont landslide occurred under wet to saturated conditions. The unstable behavior of the landslide is explained by the velocity-weakening behavior of the Vaiont clay-rich gouges. The formation of a continuous film of liquid water in the slipping zone reduced the coefficient of friction to almost zero, even without invoking the activation of thermal pressurization. This explains the extraordinary high velocity achieved by the slide during the final collapse.

Seismicity preceding volcanic eruptions: New experimental insights

A variety of seismic signals representing different physical mechanisms precedes volcanic eruptions. The most meaningful signals are high- and low-frequency earthquakes and volcanic tremor that have tentatively been related to fracturing and magma transport in the volcanic edifice. We provide experimental support for this association by reproducing magma migration while recording seismic signals. Opening fractures emit high-frequency acoustic events, while the switch to low frequency and harmonic tremor accompanies the flow of the melt in the fractures. Discerning between these seismic signals in nature can significantly refine volcanic hazard evaluation.

Pseudotachylytes and Earthquake Source Mechanics

Destructive earthquakes nucleate at depth (10 to 15 km), therefore monitoring active faults at the Earths surface, or interpreting seismic waves, yields only limited information on earthquake mechanics. Tectonic pseudotachylytes (solidified friction-induced melts) decorate some exhumed ancient faults and remain, up to now, the only fault rocks recognized as the unambiguous signature of seismic slip. It follows that pseudotachylyte-bearing fault networks might retain a wealth of information on seismic faulting and earthquake mechanics. In this chapter, we will show that in the case of large exposures of pseudotachylyte-bearing faults, as the glacier-polished outcrops in the Adamello massif (Southern Alps, Italy), we might constrain several earthquake source parameters by linking field studies with microstructural observations, high-velocity rock friction experiments, modeling of the shear heating and melt flow, and dynamic rupture models. In particular, it is possible to estimate the rupture directivity and the fault dynamic shear resistance. We conclude that the structural analysis of exhumed pseudotachylyte-bearing faults is a powerful tool for the reconstruction of the earthquake source mechanics, complementary to seismological investigations.

Relating high-velocity rock-friction experiments to coseismic slip in the presence of melts

The dynamic strength (τ f ) of faults during coseismic slip is a major unknown in earthquake mechanics, though it has crucial influence on rupture properties, dynamic stress drop, radiated energy and heat produced during slip. In order to provide constraints on τ f , High-Velocity Rock Friction Experiments (HVRFE) are conducted on natural rocks with rotary shear apparatuses, reproducing slip (several meters) and slip rate (0.1-3 m s -1 ) typical of large earthquakes. Among the various weakening mechanisms possibly activated during seismic slip, we focus on melt lubrication. Solidified, friction-induced melts (pseudotachylytes) decorate some exhumed seismic faults, showing that melt can occur on natural faults, though its frequency is still a matter of debate. In the presence of melt, τ f undergoes an initial strengthening stage, followed by a dramatic weakening stage (thermal runaway). Field estimates based on pseudotachylyte thickness and experimental measures of τ f suggest large stress drops once thermal runaway is achieved. These estimates of τ f are compatible with large dynamic stress drops and high radiation efficiency, as observed for some earthquakes. Moreover, the threshold for the onset of thermal runaway might explain differences between the mechanics of small (M < 4) and large earthquakes. A simple mathematical model coupling melting, extrusion and thermal diffusion reproduces some observed experimental features such as the duration of the weakening stage and the convergence to a steady-state.