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Earthquakes are frictional instabilities caused by the shear stress decrease, that is, dynamic weakening, of faults with slip and slip rate. During dynamic weakening, shear stress depends on slip, slip rate, and temperature, according to constitutive laws governing the earthquake rupture process. In the laboratory, technical limitations in measuring temperature during frictional instabilities inhibit the investigation and interpretation of shear stress evolution. Here we conduct high velocity friction experiments on calcite‐bearing simulated faults, both on bare‐rock and on gouge samples, at 20–30 MPa normal stress, 1–6 m/s slip rate and 1–20 m total slip. Seismic slip pulses are reproduced by imposing boxcar and regularized Yoffe slip rate functions. We measured, together with shear stress, slip, and slip rate, the temperature evolution on the fault by employing an innovative two‐color fiber optic pyrometer. The comparison between modeled and measured temperature reveals that for calcite‐bearing faults the heat sink caused by decarbonation reaction controls the temperature evolution. In bare‐rocks, energy is dissipated as frictional heat, and temperature increase is buffered by the heat sink of the calcite decarbonation reaction. In gouges, energy is dissipated as frictional heat and for plastic deformation processes, balanced by the heat sink caused by the decarbonation reaction enhanced by the mechanochemical effect. Our results suggest that in calcite‐bearing rocks, a common fault zone material for earthquake sources in the continental crust at shallow depth, the type of fault materials (bare‐rocks vs. gouges) controls the energy dissipation during seismic slip. Plain Language Summary: During earthquakes, faults rocks lose strength, and therefore the ability to sustain shear stress as a consequence of slip, slip rate, and temperature resulting in dynamic weakening. The mathematical relationships between the decreasing strength and slip, slip rate, and temperature and the energy balance describing the partition of energy are of fundamental importance to model the propagation of an earthquake rupture. These relationships can be defined thanks to laboratory experiments that simulate seismic slip. Here, we tested calcite‐bearing fault rocks simulated as bare‐rock and gouges. During the experiments, temperature was monitored thanks to an innovative measuring system. Numerical models were done assuming all mechanical energy was converted into heat. By comparing all results above, we discovered that the mechanical energy in both bare‐rocks and gouges is converted to heat but limited by the occurrence of endothermic decarbonation reaction. In gouges, also an energy contribution for plastic deformation processes is required. Our work implies significant changes in the commonly accepted energy budget for earthquake propagation and show the importance of slip rate and temperature in driving together the dynamic weakening during seismic slip. Key Points: We measure temperature, shear stress, and slip rate to assess energy dissipation during seismic slip in calcite‐bearing fault materialsCalcite decarbonation compensates frictional heat production in all tested fault materialsMechanical energy is dissipated for frictional heat in bare‐rocks and for frictional heat and plastic deformation processes in gouges [ABSTRACT FROM AUTHOR]

Our knowledge of the mechanics of subduction zone megathrust earthquakes is hindered by the the limited number of observations due to the large discrepancy between the observational time and the earthquakes recurrence time, to lacking historical records and to the scarcity of instrumental coverage. Moreover, seismic and GPS inversions rely on indirect measurements which are taken away from the seismic source implying a limited knowledge of the elastic strain energy accumulated in the host rocks surrounding the fault before seismic slip and on the energy partitioning between on-fault and off-fault dissipative processes during seismic slip. Experiments allow us to decrease the observational time, increase the number of observations and control the boundary conditions. In particular, experiments performed with high velocity frictional apparatuses, which impose slip and slip rates typical of moderate to large magnitude earthquakes, provide a quite robust reproduction of on-fault seismic processes as attested by the similarity between the experimental and natural fault products.We tested materials sampled via scientific drilling in the sediment input to Costa Rican and north Sumatran subduction zones (IODP Exp. 334 and 362, respectively). In the case of Costa Rica these materials were sampled just a few km from the deformation front and drilling the toe of the system reveal that they build a small accretionary wedge at the front of an otherwise active erosional margin and may contain information about its past seismo and tsunamigenic history. In the case of Sumatra these materials were sampled at about 200 km from the deformation front and, although they are the same that are currently deforming in the shallow portions of the subducting interface, they may underestimate future diagenesis. To estimate the available energy to propagate seismic slip and respectively back - and fore - ward trace the history of the two subduction zones we performed experiments with a rotary shear apparatus on gouge layers made of the aforementioned materials: (1) under room humidity and temperature conditions; (2) controlling the normal stress, representative of depth < 1 km (i.e., 5 and 15 MPa), (3) observing the evolution of shear stress during a step at 10 µm/s slip rate, (4) observing the spontaneous evolution of slip and slip rate as the shear stress is increased stepwise and (5) observing the evolution of shear stress at constant 1 m/s slip rate for few meters of slip. The experiments allow us to achieve constraints on the energy dissipated during seismic slip in on-fault processes, including the available energy to propagate seismic slip (i.e., shear fracture energy). The experiments revealed that fault gouges at the shallow portions of the accretionary prism can store a relatively large amount of elastic strain energy, enough to promote seismic slip propagation when loaded near to the reactivation shear stress. The experiments may contribute to our understanding of fault stability and, in particular, to the debate on the possibility that seismic slip can propagate up to the trench during megathrust earthquakes, as proposed for the 2011 Tohoku-Oki Mw 9.0 earthquake. [ABSTRACT FROM AUTHOR]

Frictional heating in faults during seismic slip induces a temperature increase on the rubbing contacts and within the slipping zone which activates deformation processes and chemical reactions resulting in dramatic decrease of the frictional resistance and, in turn, decrease of the frictional heating itself, in a negative feedback loop. An independent measurement of temperature is required to correlate frictional strength with temperature and to constrain the deformation processes and associated chemical reactions activated during seismic slip. So far, in-situ temperature measurements performed at high acquisition rates (from kHz to MHz) and high spatial resolution (<< 1 mm²) to detect, for instance, flash heating and weakening mechanisms (micro- to milli-second in duration over contact areas << 1 mm²) were impossible to achieve due to the low acquisition rate and large thermal inertia of traditional techniques (i.e., thermocouples). As a consequence, temperatures in the slipping zone sheared at seismic slip rates (1 m/s) were often modelled numerically using the frictional power (shear stress x slip rate) dissipated on the slipping zone as an input parameter, but by making poorly constrained estimates about (1) the partitioning of the dissipated power between frictional heating and wear processes or, worse, (2) other energy sinks that would result in the buffering of the temperature increase (e.g., endothermic reactions). Here we reproduced earthquake slip in the laboratory via rotary shear experiments performed on Carrara marble rock samples, slid at 10 and 20 MPa effective normal stresses, 1 and 3 m/s slip rates, and 10 and 5 m of total displacement, respectively. During the experiments, the bulk temperature of the slipping zone was measured at acquisition rate of 1 kHz with an optical fiber conveying the radiation from the hot rubbing surfaces to electric signals using a two-color pyrometer. The measured temperatures were synchronized with the friction coefficient measurements. In the experiments, the friction coefficient dynamically decreased to a minimum (µ = 0.1-0.03) and at the same time the bulk temperature increased up to ca. 500-600 °C, in a time interval of 250-500 ms. At this stage the slipping zone of the Carrara sample was already decarbonating. Our data provide the first direct measurements of decarbonation reaction temperatures during seismic slip in carbonate-built rocks. These and future similar experiments will shed light on the mechanics of carbonate-hosted earthquakes, a main hazard in the Mediterranean and other areas worldwide. [ABSTRACT FROM AUTHOR]

One moderate- to large-magnitude earthquake (M > 6) nucleates in Earth's crust every three days on average, but the geological record of ancient fault slip at meters-per-second seismic velocities (as opposed to subseismic slow-slip creep) remains debated because of the lack of established fault-zone evidence of seismic slip. Here we show that the irreversible temperature-dependent transformation of carbonaceous material (CM, a constituent of many fault gouges) into graphite is a reliable tracer of seismic fault slip. We sheared CM-bearing fault rocks in the laboratory at just above subseismic and at seismic velocities under both water-rich and water-deficient conditions and modeled the temperature evolution with slip. By means of micro-Raman spectroscopy and focused-ion beam transmission electron microscopy, we detected graphite grains similar to those found in the principal slip zone of the A.D. 2008 Wenchuan (Mw 7.9) earthquake (southeast Tibet) only in experiments conducted at seismic velocities. The experimental evidence presented here suggests that high-temperature pulses associated with seismic slip induce graphitization of CM. Importantly, the occurrence of graphitized fault-zone CM may allow us to ascertain the seismogenic potential of faults in areas worldwide with incomplete historical earthquake catalogues. [ABSTRACT FROM AUTHOR]