Kinematically consistent models of viscoelastic stress evolution

Abstract

Following large earthquakes, coseismic stresses at the base of the seismogenic zone may induce rapid viscoelastic deformation in the lower crust and upper mantle. As stresses diffuse away from the primary slip surface in these lower layers, the magnitudes of stress at distant locations (>1 fault length away) may slowly increase. This stress relaxation process has been used to explain delayed earthquake triggering sequences like the 1992 Mw = 7.3 Landers and 1999 Mw = 7.1 Hector Mine earthquakes in California. However, a conceptual difficulty associated with these models is that the magnitudes of stresses asymptote to constant values over long time scales. This effect introduces persistent perturbations to the total stress field over many earthquake cycles. Here we present a kinematically consistent viscoelastic stress transfer model where the total perturbation to the stress field at the end of the earthquake cycle is zero everywhere. With kinematically consistent models, hypotheses about the potential likelihood of viscoelastically triggered earthquakes may be based on the timing of stress maxima, rather than on any arbitrary or empirically constrained stress thresholds. Based on these models, we infer that earthquakes triggered by viscoelastic earthquake cycle effects may be most likely to occur during the first 50% of the earthquake cycle regardless of the assumed long‐term and transient viscosities.

Complex Near‐Surface Rheology Inferred From the Response of Greater Tokyo to Strong Ground Motions

“…In viscoelastic models, the recovery timescale τ is related to the viscosity η as τ = η / G . We can crudely estimate a viscosity of the shallow soils by assuming a value for G of 50 MPa (typical of shallow clays, Ishihara, ) and taking τ = 10 days, which gives a viscosity of about 4 × 10 13 Pa · s. Such a value is much lower than values estimated for lower crustal and upper mantle materials (10 18 to 10 21 Pa · s, DeVries & Meade, ).…”

Strain Partitioning and Present‐Day Fault Kinematics in NW Tibet From Envisat SAR Interferometry

“…Low‐viscosity layers, potentially transient, have been evidenced by interferometric synthetic aperture radar (InSAR) measurements on some parts of the Tibetan Plateau from far‐field postseismic displacements (Huang et al, ; Ryder et al, , ; Wen et al, ) or by lake surface and level changes (Doin et al, ). However, they suffer from trade‐offs between the viscosity and the depth of the deformation and from our poor knowledge of the steady state rheology of the crust at longer time scales (e.g., DeVries & Meade, ; Leloup et al, ).…”

Connecting depth limits of interseismic locking, microseismicity, and large earthquakes in models of long‐term fault slip

“…Coupling of faults in an elastic lithosphere with a viscoelastic substrate in the context of seismic cycles was first introduced by Savage and Prescott [], building on the work of Nur and Mavko []. More advanced numerical models were able to tackle three‐dimensional problems with complex fault geometry [e.g., Smith and Sandwell , ] and include stress‐controlled fault creep by considering more realistic stressing [e.g., Li and Rice , ; Johnson and Segall , ] or laboratory constitutive laws such as rate‐and‐state fault friction or inelastic bulk rheology [e.g., Johnson and Segall , ; Smith and Sandwell , ; Hetland et al , ; Takeuchi and Fialko , ; DeVries and Meade , ]. With the coseismic ruptures approximated as imposed instantaneous slip confined to the seismogenic zone, these models typically focus on the postseismic and interseismic responses of the fault zone between large earthquakes.…”