How to observe fault injections in real time Faults in the ground are known to deform in response to procedures such as wastewater injection that change the pore pressure. Guglielmi et al. took a crack at monitoring this process in real time with a controlled fluid injection into an inactive fault (see the Perspective by Cornet). Reactivating the dead fault induced aseismic slip, which triggered small earthquakes. These observations can inform models of how friction is related to slip rate. The technique can also be applied to field-scale monitoring of seismicity-inducing wastewater injections. Science , this issue p. 1224 ; see also p. 1204

[1] Oscillations in stress, such as those created by earthquakes, can increase permeability and fluid mobility in geologic media. In natural systems, strain amplitudes as small as 10À6 can increase discharge in streams and springs, change the water level in wells, and enhance production from petroleum reservoirs. Enhanced permeability typically recovers to prestimulated values over a period of months to years. Mechanisms that can change permeability at such small stresses include unblocking pores, either by breaking up permeability-limiting colloidal deposits or by mobilizing droplets and bubbles trapped in pores by capillary forces. The recovery time over which permeability returns to the prestimulated value is governed by the time to reblock pores, or for geochemical processes to seal pores. Monitoring permeability in geothermal systems where there is abundant seismicity, and the response of flow to local and regional earthquakes, would help test some of the proposed mechanisms and identify controls on permeability and its evolution.

[1] Pore fluid pressure plays an important role in the frictional strength and stability of tectonic faults. We report on laboratory measurements of porosity changes associated with transient increases in shear velocity during frictional sliding within simulated finegrained quartz fault gouge (d 50 = 127 mm). Experiments were conducted in a novel true triaxial pressure vessel using the double-direct shear geometry. Shearing velocity step tests were used to measure a dilatancy coefficient (e = Df/Dln(v), where f is porosity and v is shear velocity) under a range of conditions: background shearing rate of 1 mm/s with steps to 3, 10, 30, and 100 mm/s at effective normal stresses from 0.8 to 20 MPa. We find that the dilatancy coefficient ranges from 4.7 Â 10 À5 to 3.0 Â 10 À4 and that it does not vary with effective normal stress. We use our measurements to model transient pore fluid depressurization in response to dilation resulting from step changes in shearing velocity. Dilatant hardening requires undrained response with the transition from drained to undrained loading indexed by the ratio of the rate of porosity change to the rate of drained fluid loss. Undrained loading is favored for high slip rates on low-permeability thick faults with low critical slip distances. Although experimental conditions indicate negligible depressurization due to relatively high system permeability, model results indicate that under feasible, but end-member conditions, shear-induced dilation of fault zones could reduce pore pressures or, correspondingly, increase effective normal stresses, by several tens of megapascals. Our results show that transient increases in shearing rate cause fault zone dilation. Such dilation would tend to arrest nucleation of unstable slip. Pore fluid depressurization would exacerbate this effect and could be a significant factor in generation of slow earthquakes, nonvolcanic tremors, and related phenomena.

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Magmatic intrusions can initiate and sustain massive and catastrophic volcano collapse. Their role is twofold, involving both driving and resisting forces. First, flank stability is diminished by magmastatic and magma overpressures, and steepened slopes, that accompany intrusion. Second, excess pore pressures in potential failure zones can be generated as a result of intrusion-related mechanical or thermal straining of the rock-fluid medium, pressurized retrograde boiling in high level magma chambers, or hydrothermal fluid circulation. Also, earthquakes may aid collapse through inertial forces and shaking-induced pore pressure generation. These excess pore pressures reduce the sliding resistance, as shown for wedge-shaped slide blocks for selected cases. The destabilizing influence of mechanically induced pore pressures is maximized as the intruded width, or corresponding overpressure, of the intrusion is increased. The destabilizing influence of thermally induced pore pressures is conditioned by the severity of thermal forcing, ratios of thermal and hydraulic diffusivities, and the time required for the fluid pressure disturbance to propagate outwards from the intrusion. Retrograde boiling and hydrothermal circulation overpressure mechanisms may be evaluated by similar models. Failure initiation does not imply sustained failure; in some cases, enhancement of pore pressures through deviatoric shearing, frictional heating, or runout over compressible saturated alluvium or marine sediments may be necessary following slide initiation to maintain the impetus of flank failure for long runout. Models are examined for oceanic volcanoes of shallow flank inclination and for terrestrial composite volcanoes with considerably steeper flanks. Les intrusions magmatiques peuvent provoquer et entretenir l'effondrement massif et catastro-phique d'un volcan. Elles le font en faisant intervenir à la fois des forces d'entraînement et des forces de résistance. Pour commencer, les surpressions magmastatiques et magmatiques et l'accentuation des pentes qui accompagnent l'intrusion réduisent la stabilitè des versants. Deuxièmement, la pression des pores pent devenir excessive dans les zones d'effondrement potentiel sous l'effet des contraintes mécaniques ou thermiques imposées au milieu roche-fluide par l'intrusion, du bouillonnement rétrograde sous pression dans les réservoirs de magma de haut niveau, ou de la circulation du liquide hydrothermal. Les séismes peuvent, eux aussi, contribuer à l'effondrement en produisant des forces d'inerde et des secousses qui font augmenter la pression des pores. L'excès de pression réduit la résistance au glissement, comme le montrent les exemples de blocs de glissement en coin illustrés dans Particle. Plus l'intrusion est large ou plus la surpression correspondante est forte, plus Paugmentation mécanique de la pression des pores a un effet déstabilisant. L'effet déstabilisant de Paugmentation thermique de la pression des pores dépend de l'importance du refoulement thermique, des ratios de diffusivité thermique et hydraulique, et du temps de propagation de Paugmentation de pression. On pent utiliser des modéles semblables pour évaluer le bouillonnement rétrograde et les mécanismes de surpression de la circulation hydrothermale. L'amorce d'un effondrement n'est pas nécessairement suivie d'un effondrement entretenu: dans certains cas, Peffondrement des versants, une foisamorcé, ne pent se poursuivre que si la pression des pores augmente sous Peffet d'un cisaillement déviateur, d'un réchauffement produit par frottement, ou d'un écoulement sur des alluvions ou des sédiments marins saturés et compressibles. Let auteurs examinent des modèles pour des volcans océaniques à pentes faibles et pour des volcans terrestres composites à versants bien plus raides.