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In the Balmuccia massif (NW Italy), a pseudotachylyte vein network (N068 trending) in a spinel lherzolite is interpreted as the product of frictional melting during a single Mw> 6 earthquake. The subvertical fault underwent a metric dextral coseismic displacement, raking 60°SW. The average width of the main slip surface is ~5 mm. A dense network of thin (20–200 μm) injection and ultramylonite‐like veins decorates the fault walls. In the injection veins, Raman microspectrometry mapping reveals pockets of still preserved amorphous silicate, containing ≈ 1% of structurally bound H2O. In the ultramylonite‐like veins, electron backscattered diffraction mapping reveals that ultrafine (0.2–2 μm) olivine grains exhibit a strong fabric with (010) planes parallel to shearing, consistent with temperatures above 1250°C during deformation and suggesting fast recrystallization from the frictional melt. The veins also exhibit pyroxene and recrystallized spinel, which proves that the earthquake occurred at a minimum depth of 40 km. The energy balance demonstrates that complete fault lubrication must have occurred during coseismic sliding (i.e., dynamic friction coefficient ≪ 0.1). Because of the low viscosity of slightly hydrated ultramafic liquids (≈ 1 Pa·s), we argue that lubrication was only transient, as the melt could rapidly flow into tensile fractures, which led to rapid cooling and promoted strength recovery and sliding arrest. Combined together, our observations suggest that this pseudotachylyte is the frozen record of a deep (>40 km) earthquake of 6 < Mw< 7. Its focal mechanism is deduced from the crystal preferred orientation due to late coseismic creep in ultramylonite‐like veins and deciphered by electron backscattered diffraction. In active mountain ranges, rocky massifs gradually rise to the surface under the combined effect of convergence and erosion. It is then possible to observe in the field the fossilized traces of earthquakes that have occurred several kilometers deep, tens of millions of years ago. Thus, it can be seen that contrary to what we experience on the surface, the rock melts when it tears. A magma forms and facilitates relative ground displacement. The traces discovered by the geologists are the solidified remains of this magma. Here by field observation and detailed analysis of samples in the laboratory, we describe the traces of an earthquake that occurred more than 40 km deep in the Earth's mantle. Its magnitude was greater than 6, the sliding on the fault greater than 1 m, and yet the trace is thinner than 5 mm. And, against all odds, the most talkative witnesses are extremely small: micrometric crystals, which formed from the magma upon the rupture, record crucial information unveiling its focal mechanism. We still do not entirely know what an earthquake exactly is, and nobody will ever be able to see what happens on the fault at depth during an earthquake. Hence, these fossils are key. The studied pseudotachylyte is the fossilized trace of an earthquake which occurred in the mantle, > 40 km deep; its magnitude was >6Coseismic total melting leads to complete fault lubrication; the ultramafic melt flows in “injection” veins, inducing fault restrengtheningMicrometric crystals, forming during fault restrengthening, record the sliding history, P‐Tconditions, and focal mechanism of the earthquake

The ability of crustal faults to compact and to pressurize pore fluids is examined by combining geological observations, petrophysical measurements (permeability, Pand Swave velocities, and porosity), and fully coupled hydromechanical modeling. A strike‐slip fault located in the Argentera‐Mercantour crystalline massif (southwestern French‐Italian Alps) was analyzed in the field. This mature fault belongs to a large active fault system characterized by a recurrent seismic swarm activity (Mw< 4) between 2 and 12 km depth. The studied exposure corresponds to a 50 m thick anastomosing fault composed of three types of rock: host‐rock gneiss, damage‐zone phyllonite, and core zone gouge. Laboratory measurements made at effective pressures ranging from 10 to 190 MPa show that the studied fault differs from the classical model and has a high‐porosity, high‐permeability, and low‐rigidity core zone surrounded by a low‐porosity, low‐permeability, and high‐rigidity damage zone with respect to the host rock. The hydraulic and elastic properties are controlled by different microstructures such as foliation, microcracks, and pores developed during the exhumation history of the massif and the reactivation of inherited low‐friction mylonitic foliation. Hydromechanical modeling is then used to investigate the spatio‐temporal evolution of the fluid overpressures across the fault zone elements in response to elastic compaction. Models demonstrate that fluid pressure can be developed and maintained temporally in the studied fault zone. This study concludes on the key role played by the hydromechanical properties of faults during compaction and provides an explanation for seismic swarm triggering and maintenance. Fault zone is characterized by heterogeneous hydraulic and elastic propertiesFluid overpressures can be developed and maintained in heterogeneous fault zonesElastic compaction is a viable mechanism to trigger seismic activity

Recent high‐velocity friction experiments on room temperature rocks reveal a dramatic weakening caused by extreme frictional heating and thermal degradation of the strength of microscopic asperity contacts on a slip surface, i.e., “flash” heating. Here we explore the effects of elevated ambient fault temperature, up to that at the base of the Earth's seismogenic zone, on the flash‐heating behavior of quartzite, Westerly granite, and India gabbro. The experiments demonstrate that elevated ambient fault temperature causes 1) an increase in the characteristic weakening velocity for flash heating, and 2) an increase in the friction coefficient at a given velocity above this characteristic weakening velocity. These unexpected results are reconciled with flash‐heating theory when temperature‐specific values of various material parameters are incorporated in existing constitutive descriptions of flash heating. Our results suggest that constitutive equations for flash heating can be extrapolated with confidence to the elevated ambient temperatures of the seismogenic zone. Experimental investigation of the effect of fault temperature on Flash heatingUnexpected frictional behavior supported by theory adapted for high temperatureFlash heating may play a second role in in the propagation of earthquake rupture

Following the Hyogoken Nanbu earthquake (January 17, 1995, Mw=7.2), three drillholes were sunk through the Nojima Fault (Awaji Island, Japan). Textural and petrographic studies of the Geological Survey of Japan (GSJ) drill cores allow recognition of two deformation episodes. The first one is older than the deposition of the Middle to Late Eocene Kobe Group, corresponds to a left-lateral movement on the Nojima fault and is expressed by pseudotachylytes, kinking of biotite crystals in the low-strain rocks and an intense laumontite hydrothermal alteration. The second one displaces the basal unconformity of the Kobe group, corresponds to a right-lateral reverse displacement and is expressed at least by carbonate-filled hydraulic fractures and thin gouge zones. Different important deformation mechanisms are recorded by the fault rocks, but questions relating to the attribution of deformation and alteration features to one or other deformation episodes remain unresolved.

At the northern end of the Cap Corse peninsula, several klippes of ultramafic rocks (peridotite and serpentinite), among which the Monte Maggiore klippe is the least serpentinized one, rest upon continental-crust derived rocks (Centuri gneisses) and basic or metasedimentary schists (Schistes Lustrés). The Monte Maggiore ophiolitic klippe shares several characteristics with the Cima di Gratera klippe located 30 km further south. First, the two units are composed of a lherzolitic peridotite. Second, they record the same succession of metamorphic events. Third, in the Cap Corse tectonic pile, the two units occupy the highest structural position. Several differences are also observed. First, mafic rocks are significantly less abundant in the Monte Maggiore unit, where they are restricted to dykes cross-cutting the peridotite, than in the Cima di Gratera unit, where they constitute an entire sub-unit. Second, pyroxenite layers are more common at Monte Maggiore than at Cima di Gratera. Despite these differences, the Monte Maggiore and Cima di Gratera klippes can be considered as possible lateral equivalents of a single ophiolitic unit having covered the entire Cap Corse before subsequent erosion. Pseudotachylyte of seismic origin is newly discovered in the Monte Maggiore klippe. The host rock is a cataclastic serpentinized peridotite affected by a cataclastic foliation that is either flat-lying or steeply dipping. Pseudotachylyte fault veins are parallel to the host rock cataclastic foliation. The small lateral extension and the small thickness of fault veins along with frequent cross-cutting relationships suggest that the exposed pseudotachylyte most likely results from numerous small magnitude seismic events such as swarms or aftershocks rather than from large magnitude shocks. All these characteristics are also observed at the Cima di Gratera klippe where they are interpreted as the testimonies of a fossil intermediate-depth Wadati-Benioff zone at the time of subduction of the Ligurian Tethys oceanic lithosphere. Mineral assemblages that could constrain the depth of formation of the pseudotachylyte lack in the Monte Maggiore area. Despite this uncertainty, and given the similarities with the Cima di Gratera occurrences, the pseudotachylyte veins newly discovered at Monte Maggiore are tentatively related to the seismic activity linked with the subduction of the Piemonte-Ligurian oceanic lithosphere in Eocene times. This interpretation suggests that the fossil Wadati-Benioff zone could be traced further south in Alpine Corsica and further north in the Piemontese zone of the western Alps.

New geophysical data collected at the Aden‐Owen‐Carlsberg (AOC) triple junction between the Arabia, India, and Somalia plates are combined with all available magnetic data across the Gulf of Aden to determine the detailed Arabia‐Somalia plate kinematics over the past 20 Myr. We reconstruct the history of opening of the Gulf of Aden, including the penetration of the Sheba Ridge into the African continent and the evolution of the triple junction since its formation. Magnetic data evidence three stages of ridge propagation from east to west. Seafloor spreading initiated ∼20 Myr ago along a 200 km‐long ridge portion located immediately west of the Owen fracture zone. A second 500 km‐long ridge portion developed westward up to the Alula‐Fartak transform fault before Chron 5D (17.5 Ma). Before Chron 5C (16.0 Ma), a third 700 km‐long ridge portion was emplaced between the Alula‐Fartak transform fault and the western end of the Gulf of Aden (45°E). Between 20 and 16 Ma, the Sheba Ridge propagated over a distance of 1400 km at an extremely fast average rate of 35 cm yr−1. The ridge propagation resulted from the Arabia‐Somalia rigid plate rotation about a stationary pole. Since Chron 5C (16.0 Ma), the spreading rate of the Sheba Ridge decreased first rapidly until 10 Ma and then more slowly. The evolution of the AOC triple junction is marked by a change of configuration around 10 Ma, with the formation of a new Arabia‐India plate boundary. Part of the Arabian plate was then transferred to the Indian plate.