[1] Elevated pore pressure can lead to reactivation of slip on pre-existing fractures and faults when the static Coulomb failure is reached locally. As the pressurized region spreads diffusively, slip can accumulate quasi-statically (paced by the pore fluid diffusion) or dynamically. In this work, we consider a prestressed fault with a locally peaked, diffusively spreading pore pressure field to study (1) conditions leading to the escalation of slip and nucleation of dynamic rupture and (2) rupture run-out distance before it is arrested. Nucleation appears in this model when the fault friction decreases from its peak value with slip, while arrest of dynamic propagation is imminent on aseismic faults (i.e., such that prestress t b is less than the residual fault strength t r at ambient conditions). When fluid overpressure is a small-to-moderate fraction of the ambient value of normal effective stress (and prestress is large enough for fault slip to be activated by overpressure), dynamic rupture always nucleates, and the nucleation length increases with decreasing prestress practically independently of the overpressure value. Transition from the ultimately unstable (t b > t r ) to the ultimately stable (t b < t r ) fault loading is marked by a strong increase of the nucleation length (∝1/(t b À t r ) 2 ) as t b approaches t r from above. For aseismic faults (t b < t r ), no dynamic rupture is nucleated at large fluid overpressures for all but the smallest values of prestress. The largest run-out distances of dynamic slip on aseismic faults correspond to overpressure/prestress just sufficient for slip activation. In such cases, the dynamically accumulated slip can lead to enhanced, dynamic fault weakening, resulting in a sustained dynamic rupture and generating a large earthquake. This is consistent with field observations when the largest injection-induced seismicity occurred after fluid injection ended.

[1] This study quantifies the excess pore pressure resulting from gas hydrate dissociation in marine sediments. The excess pore pressure in confined pore spaces can be up to several tens of megapascals due to the tendency for volume expansion associated with gas hydrate dissociation. On the other hand, the magnitude of excess pore pressure in wellconnected sediment pores is generally smaller, depending primarily on the hydrate dissociation rate and the sediment permeability. Volume expansion due to gas hydrate dissociation in well-connected pore spaces is related via Darcy's law to an increase in pore pressure and its gradient in sediment, which drives an additional upward fluid flow through the sediment layer overlying the gas hydrate dissociation area. The magnitude of this excess pore pressure is found to be proportional to the rate of gas hydrate dissociation and the depth below seafloor and inversely proportional to sediment permeability and the depth below sea level. The excess pore pressure is the greatest at low initial pressures and decreases rapidly with increasing initial pressure. Excess pore pressure may be the result of gas hydrate dissociation due to continuous sedimentation, tectonic uplift, sea level fall, heating or inhibitor injection. The excess pore pressure is found to be potentially able (1) to facilitate or trigger submarine landslides in shallow water environments, (2) to result in the formation of vertical columns of focused fluid flow and gas migration, and (3) to cause the failure of a sediment layer confined by low-permeability barriers in relatively deep water environments.Citation: Xu, W., and L. N. Germanovich (2006), Excess pore pressure resulting from methane hydrate dissociation in marine sediments: A theoretical approach,

Analytical models are used to compare the rates at which an isolated fracture and vertical, parallel fracture sets in hydrothermal upflow zones can be closed by silica precipitation and thermoelastic stress. Thermoelastic sealing is an order of magnitude faster than sealing by silica precipitation. In vertical fracture sets, both the amount of silica precipitation resulting from cooling and the total thermal expansion of the country rock may be insufficient to seal cracks at depth. These crack systems may ultimately close because the pressure dependence of silica solubility maintains precipitation during upflow even after the temperature gradient vanishes.

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