Hydromechanical coupling in response to earthquakes: on the possible consequences for aftershocks

Pierre Gavrilenko (1)

1 GR - Géosciences Rennes

Numerous observations indicate that fluids circulate in the crust after an earthquake, with time constants comparable to those of the aftershocks. This paper provides new viewpoints concerning the relationship between the fluid flow and aftershocks. The fault on which the main quake occurs is modelled using a rectangular dislocation surface that is embedded in a 3-D poroelastic half-space. The main event is modelled by prescribing a given amount of displacement over the dislocation surface which results in deformation throughout the surrounding crust. The fluid redistribution that then ensues is modelled using the equations of poroelasticity. We show that this process can lead to time-dependent weakening effects. The results are analysed in terms of how changes in a Coulomb failure function (ΔCFF) evolve through time. The spatial and temporal distributions of aftershocks triggered by ΔCFF are modelled on a statistical basis. We test our model in various situations. The spatial distribution of aftershocks is depicted in various tectonic environments and we show that the fluid flow seemingly provides a convincing mechanism for the triggering of aftershocks. Both off-fault aftershocks and aftershocks close to the surface rupture have been modelled. We have also investigated the condition under which the Omori decay law (and its associated time exponent p) is expected and we show that the permeability should vary within a window of less than two orders of magnitude in order to match the observed value of p≈ 1. With seismic data from the Northridge event (1994), we show that a crustal permeability of 7.5 × 10−15 m2 allows us to describe quite well the time dependence of the aftershock. However, the permeability should vary proportionally to L2, where L is the scale length over which the fluid-pressure equilibration occurs that is dependent on the size of the main event, in order to obtain a similar decay law (p≈ 1) at every scale (main-event magnitude). Finally, the link with the post-seismic hydrological observations is established and we show that the fluid flow predicted in the model of the Northridge event is consistent within an order of magnitude with field observations reported by Muir-Wood & King.

Mots clés: aftershocks; fluid flow; hydrological anomalies; poroelastic coupling; stress transfer

Domaines: Géophysique [physics.geo-ph]

Pierre Gavrilenko. Hydromechanical coupling in response to earthquakes: on the possible consequences for aftershocks. Geophysical Journal International, 2005, 161 (1), pp.113-129. ⟨10.1111/j.1365-246X.2005.02538.x⟩. ⟨hal-00077885⟩