A model of deep crustal fluid flow following the M_w = 8.0 Antofagasta, Chile, earthquake

Abstract

[1] We develop a model to test the hypothesis that a high fluid pressure pulse and subsequent fluid flow caused a spatially extensive increase in the V P /V S ratio following the M w = 8.0 Antofagasta, Chile, subduction zone earthquake. The postseismic anomaly appeared within a 50-day period inside the forearc region of the Andes continental crust. We model this anomaly with a poroelastic medium that combines the fluid flow response to mean stress changes from slip along a dislocation plane, with a pore pressure pulse initiated by the coseismic rupturing of a seal separating hydrostatic-lithostatic fluid pressure conditions in the hanging walls and footwalls of the subduction zone, respectively. Variations in seismic velocity due to porosity and pore pressure changes are calculated using Gassmann's formula and the empirical law of critical porosity. We show that the slip-induced perturbation of the mean stress field is insufficient to explain the anomaly, but the release of lithostatic pressurized fluid trapped below the rupture plane (within the oceanic crust) can explain the transient changes in seismic velocities. Our preferred model requires a very large intrinsic permeability of around 1 Â 10 À13 m 2 , suggesting a mechanism of a high-amplitude pressure pulse propagating through a highly permeable fracture system of the lower continental crust.

Effects of pore collapse and grain crushing on ultrasonic velocities and Vp/Vs

“…The ratio V p / V s is a quantity frequently used in seismology and V p / V s anomalies have sometimes been recorded before and after earthquakes. For example, after the Antofagasta earthquake, an anomaly of V p / V s was observed [ Husen and Kissling , 2001; Koerner et al , 2004]: V p / V s increased from 1.72 to a significant mean value of 1.77. Husen and Kissling [2001] suggested that the anomaly was fluid driven.…”

An aseismic slip pulse in northern Chile and along‐strike variations in seismogenic behavior

“…Besides bulk viscoelastic processes, poroelastic processes have also been inferred as a primary source of postseismic deformation for shallow strike‐slip earthquakes [e.g., Peltzer et al , 1996; Jónsson et al , 2003]. Subsurface fluid flow following the 1995 earthquake is suggested based on independent evidence [ Husen and Kissling , 2001; Koerner et al , 2004] and may also contribute to postseismic deformation. A simple calculation for northern Chile (using plausible values for the drained and undrained Poisson's ratio [ Rice and Cleary , 1976]) indicates that the direction of deformation is opposite that observed and the amplitude is probably an order of magnitude too small [ Pritchard , 2003].…”

Evidence for triggering of the Vogtland swarms 2000 by pore pressure diffusion

“…Besides for borehole fluid injections, pore pressure diffusion (described by either the coupled or uncoupled equations) as triggering mechanism has been proposed for various case studies: e.g., for reservoir (large artificial lakes) induced seismicity [ Howells , 1974; Talwani , 2000], for water table changes in large basins connected with microseismicity [ Costain and Bollinger , 1991; Lee and Wolf , 1998], for volcanic seismicity in Italy [ Saccorotti et al , 2002], and for aftershocks of large earthquakes [ Nur and Booker , 1972; Bosl and Nur , 2002; Jonsson et al , 2003; Shapiro et al , 2003; Miller et al , 2004; Koerner et al , 2004]. …”