During an earthquake, part of the released elastic strain energy is dissipated within the slip zone by frictional and fracturing processes, the rest being radiated away via elastic waves. While frictional heating plays a key role in the energy budget of earthquakes, it could not be resolved by seismological data up to now. Here we investigate the dynamics of laboratory earthquakes by measuring frictional heat dissipated during the propagation of shear instabilities at stress conditions typical of seismogenic depths. We estimate the complete energy budget of earthquake rupture and demonstrate that the radiation efficiency increases with thermal‐frictional weakening. Using carbon properties and Raman spectroscopy, we map spatial heat heterogeneities on the fault surface. We show that an increase in fault strength corresponds to a transition from a weak fault with multiple strong asperities and little overall radiation, to a highly radiative fault behaving as a single strong asperity.

Across the seismogenic zone, the transition from brittle to plastic deformation corresponds to a semibrittle regime where brittle fracturing and plastic flow coexist at high strength conditions. Thorough experimental investigations on brittle‐plastic transition are crucial to understand why natural faults behave in stable or unstable ways at varying crustal depths and why large earthquakes generally nucleate at the base of the seismogenic zone. To investigate semibrittle deformation in carbonates and the conditions promoting it, we reported here the results of experiments performed on Carrara marble saw cut faults in triaxial conditions. We studied the influence of the confining pressure (ranging between 45 and 180 MPa), axial loading rates (0.01 and 1 μm s−1) and initial fault roughness (smooth and rough) on fault (in‐)stability across the brittle‐plastic transition. We conclude that laboratory earthquakes may nucleate on inherited fault interfaces at brittle‐plastic transition conditions. The occurrence of laboratory earthquakes associated with increasing plastic deformation is promoted at high confining pressure, on smooth fault interfaces, or when the loading rate is slow. In a rather counterintuitive manner, increasing initial roughness promotes stable sliding and a larger amount of plastic deformation. Furthermore, we show that stable sliding tends to produce mirror‐like surfaces, while stick‐slips are associated with matte surfaces, on which the size of the asperities grows with increasing confining pressure. Finally, our results seem to reveal the influence of asperity hardness and melt viscosity on fault weakening.

We monitor dynamic rupture propagation during laboratory stick‐slip experiments performed on saw‐cut Westerly granite under upper crustal conditions (10–90 MPa). Spectral analysis of high‐frequency acoustic waveforms provided evidence that energy radiation is enhanced with stress conditions and rupture velocity. Using acoustic recordings band‐pass filtered to 400–800 kHz (7–14 mm wavelength) and high‐pass filtered above 800 kHz, we back projected high‐frequency energy generated during rupture propagation. Our results show that the high‐frequency radiation originates behind the rupture front during propagation and propagates at a speed close to that obtained by our rupture velocity inversion. From scaling arguments, we suggest that the origin of high‐frequency radiation lies in the fast dynamic stress‐drop in the breakdown zone together with off‐fault coseismic damage propagating behind the rupture tip. The application of the back‐projection method at the laboratory scale provides new ways to locally investigate physical mechanisms that control high‐frequency radiation.

Chemically activated processes of subcritical cracking in calcite control the time‐dependent strength of this mineral, which is a major constituent of the Earth's brittle upper crust. Here experimental data on subcritical crack growth are acquired with a double torsion apparatus to characterize the influence of fluid pH (range 5–7.5) and ionic strength and species (Na2SO4, NaCl, MgSO4, and MgCl2) on the propagation of microcracks in calcite single crystals. The effect of different ions on crack healing has also been investigated by decreasing the load on the crack for durations up to 30 min and allowing it to relax and close. All solutions were saturated with CaCO3. The crack velocities reached during the experiments are in the range 10−9–10−2 m/s and cover the range of subcritical to close to dynamic rupture propagation velocities. Results show that for calcite saturated solutions, the energy necessary to fracture calcite is independent of pH. As a consequence, the effects of fluid salinity, measured through its ionic strength, or the variation of water activity have stronger effects on subcritical crack propagation in calcite than pH. Consequently, when considering the geological sequestration of CO2 into carbonate reservoirs, the decrease of pH within the range of 5–7.5 due to CO2 dissolution into water should not significantly alter the rate of fracturing of calcite. Increase in salinity caused by drying may lead to further reduction in cracking and consequently a decrease in brittle creep. The healing of cracks is found to vary with the specific ions present.

A major part of the seismicity striking the Mediterranean area and other regions worldwide is hosted in carbonate rocks. Recent examples are the destructive earthquakes of L'Aquila M w 6.1 2009 and Norcia M w 6.5 2016 in Central Italy. Surprisingly, within this region, fast (≈3km/s) and destructive seismic ruptures coexist with slow (≤10 m/s) and nondestructive rupture phenomena. Despite of its relevance for seismic hazard studies, the transitions from fault creep to slow and fast seismic rupture propagation are still poorly constrained by seismological and laboratory observations. Here, we reproduced in the laboratory the complete spectrum of natural faulting on samples of dolostones representative of the seismogenic layer in the region. The transitions from fault creep to slow ruptures and from slow to fast ruptures, are obtained by increasing both confining pressure (P) and temperature (T) up to conditions encountered at 3-5 km depth (i.e., P = 100 MPa and T = 100 o C), which corresponds to the hypocentral location of slow earthquake swarms and the onset of regular seismicity in Central Italy. The transition from slow to fast rupture is explained by the increase of the ambient temperature, which enhances the elastic loading stiffness of the fault and consequently the slip velocity during the nucleation stage, allowing flash weakening. The activation of such weakening induces the propagation of fast ruptures radiating intense high frequency seismic waves.

No abstract

Fault weakening is a major phenomenon driving the dynamics of large earthquakes because the evolution of fault friction during slip controls dynamic stress drop and heat creation (Di Toro et al., 2006;Rice, 2006). As pointed out in Mair and Marone (2000), simple granular behaviors in the fault gouge cannot explain this phenomenon. Meanwhile, a large number of physical phenomena can promote fault weakening, including thermal pressurization (

No abstract