Can compliant fault zones be used to measure absolute stresses in the upper crust?

Abstract

Geodetic and seismic observations reveal long‐lived zones with reduced elastic moduli along active crustal faults. These fault zones localize strain from nearby earthquakes, consistent with the response of a compliant, elastic layer. Fault zone trapped wave studies documented a small reduction in P and S wave velocities along the Johnson Valley Fault caused by the 1999 Hector Mine earthquake. This reduction presumably perturbed a permanent compliant structure associated with the fault. The inferred changes in the fault zone compliance may produce a measurable deformation in response to background (tectonic) stresses. This deformation should have the same sense as the background stress, rather than the coseismic stress change. Here we investigate how the observed deformation of compliant zones in the Mojave Desert can be used to constrain the fault zone structure and stresses in the upper crust. We find that gravitational contraction of the coseismically softened zones should cause centimeters of coseismic subsidence of both the compliant zones and the surrounding region, unless the compliant fault zones are shallow and narrow, or essentially incompressible. We prefer the latter interpretation because profiles of line of sight displacements across compliant zones cannot be fit by a narrow, shallow compliant zone. Strain of the Camp Rock and Pinto Mountain fault zones during the Hector Mine and Landers earthquakes suggests that background deviatoric stresses are broadly consistent with Mohr‐Coulomb theory in the Mojave upper crust (with μ ≥ 0.7). Large uncertainties in Mojave compliant zone properties and geometry preclude more precise estimates of crustal stresses in this region. With improved imaging of the geometry and elastic properties of compliant zones, and with precise measurements of their strain in response to future earthquakes, the modeling approach we describe here may eventually provide robust estimates of absolute crustal stress.

Seismic and aseismic slip evolution and deformation associated with the 2009-2010 northern Malawi earthquake swarm, East African Rift

“…To evaluate whether this process indeed occurred along the 2009–2010 northern Malawi fault zone, we examine the effect of elastic moduli increase (elastic healing) in a fault‐zone that is subjected to tectonic shear and normal stresses in addition to the lithostatic stresses. Simple analysis (see also Hearn & Fialko 2009) shows that increase in the elastic moduli of a fault‐zone that is subjected to lithostatic stresses will lead to uplift rather than to subsidence of the fault‐zone surface. Fault‐normal stresses with an increase in the Young's modulus and a reduction in the Poisson's ratio due to healing can lead to subsidence only in the case of large compressional stresses.…”

Off-fault long-term damage: A condition to account for generic, triangular earthquake slip profiles

“…The 30 km long rupture plane where the coseismic slip occurs is represented by a zero-thickness interface (Figures 2a-2c). We surround this slip plane with a damage zone having different geometries and elastic properties (see below), extending over the entire depth of the model (depth extent of natural damage zones is poorly known, but might be large, extending across much of the brittle seismogenic crust [e.g., Fialko et al, 2002;Li et al, 2006;Barbot et al, 2009;Cochran et al, 2009;Hearn and Fialko, 2009;Griffith et al, 2012;Smith et al, 2013]), and stopping at the fault tips. We refer to the ''damage width'' in the direction perpendicular to the fault.…”

“…Although we do not discuss the complexities or formation of fault damage, which is beyond the scope of this paper, we simulate the preexisting long‐term damage and its “intensity,” by decreasing the elastic modulus of the medium around the fault. Such a decrease in elastic moduli as damage increases has been observed on natural rocks and faults, both during and immediately after earthquake ruptures [ O'Connell and Budiansky , ; Li et al ., ; Fialko et al ., ; Vidale and Li , ; Ben‐Zion et al ., ; Fialko , ] and around long‐term geological faults [ Li et al ., ; Gudmundsson , ; Faulkner et al ., ; Aydin and Berryman , ; Barbot et al ., ; Hearn and Fialko , ; Cochran et al ., ]. In that later case, damage is thus permanent [e.g., Cochran et al ., ].…”

“…As recognized for long by geologists, the growth and lateral lengthening of faults over geological time leaves behind the passage of their tips a wake of fractured, damaged rocks [e.g., Scholz et al ., ]. Even though various healing processes might occur at specific periods of the earthquake cycle and during the evolution of the fault [e.g., Brantley et al ., ; Moore et al ., ; Vidale and Li , ; Schaff and Beroza , ; Brenguier et al ., ; Mitchell and Faulkner , ], part of the damage (i.e., cracks and faults) is persistent, so that rock damage is cumulative and permanent [e.g., Chester and Chester , ; Li et al ., ; Sibson , ; Manighetti et al ., ; Dor et al ., ; Mitchell and Faulkner , ; Barbot et al ., ; Cochran et al ., ; Hearn and Fialko , ; Savage and Cooke , ; Savage and Brodsky , ; Griffith et al ., ; Smith et al ., ]. Furthermore, the cumulative, long‐term damage is heterogeneous, varying both across and along the fault; across the fault, the damage “intensity” (commonly taken as the crack density) decreases away from the fault [e.g., Chester and Logan , ; Vermilye and Scholz , ; Faulkner et al ., ; Wechsler et al ., ; Mitchell and Faulkner , ; Savage and Brodsky , ; Smith et al ., ], whereas, along the fault, the damage zone commonly enlarges in the direction of long‐term fault propagation [e.g., Manighetti et al ., ; de Joussineau and Aydin , ; Schlagenhauf et al ., ; Aydin and Berryman , ; Savage and Cooke , ; Faulkner et al ., ] (Figure b).…”

Mechanics, slip behavior, and seismic potential of corrugated dip‐slip faults

“…While some wavy surface traces of dip‐slip faults may be, in part, due to interference with surface topography, kilometer‐scale representations of natural fault systems commonly suggest considerable geometric complexity [e.g., Candela et al , 2011; Plesch et al , 2007]. At the outcrop scale, faults commonly exhibit geometrically complex damage zones [e.g., Kim et al , 2004] often with significant variations in host rock rheology [e.g., Dor et al , 2006; Fialko , 2004; Finzi et al , 2009; Hearn and Fialko , 2009; Sagy and Brodsky , 2009]. Despite these complexities, faults tend to localize the vast majority of deformation in a relatively narrow zone of slip along a nonplanar surface that is commonly striated [e.g., Resor and Meer , 2009; Sagy and Brodsky , 2009].…”