Asymmetry in elastic properties and the evolution of large continental strike‐slip faults

Abstract

[1] We use geodetic studies to quantify several cases of significant asymmetry in interseismic and coseismic effects along large continental strike-slip faults using simple two-dimensional edge dislocation models. We first show that asymmetric elastic loading characterizes the present Main Marmara Fault, a portion of the North Anatolian Fault along the northern margin of the Sea of Marmara. The ratio of asymmetry there is about 10. This ratio is even larger, about 30, along the northern Sumatra fault near lake Toba caldera. We then examine two profiles near Point Reyes and Point Arena across the northern San Andreas Fault that have been previously proposed as affected by asymmetry both in interseismic and coseismic effects. We show that an asymmetry ratio of 1.6 in interseismic loading exists near Point Arena, with the southwest side of the fault being more rigid than the northeast one. On the other hand, we do not find significant asymmetry for the Point Reyes profile that was previously described as highly asymmetric. We examine coseismic motion during the 1906 earthquake along the same two profiles. Ratios of 1.2 and 1.7 are found for the Point Arena and Point Reyes profiles, respectively. We discuss the possible causes of asymmetry. Contrasts in seismic velocity in the brittle portion suggest ratios generally not exceeding 2.5 for the dynamic rigidity in the upper brittle section. Larger ratios may involve other complex causes such as differences between static and dynamic rigidities, contrasts in rheology in the deeper creeping sections, and postseismic transients. We conclude that asymmetry should be systematically included within the parameters to be inverted when dealing with the mechanics of largescale strike-slip faults.Citation: Le Pichon, X., C. Kreemer, and N. Chamot-Rooke (2005), Asymmetry in elastic properties and the evolution of large continental strike-slip faults,

Interseismic strain accumulation and the earthquake potential on the southern San Andreas fault system

“…However, the inferred magnitude of the rigidity contrasts exceeds high-end estimates from seismic tomography of a factor of 2-2.5 24,25 . The total inferred contrast across the San Andreas and San Jacinto faults is about a factor of 5 to 10, significantly larger than any seismically determined lateral variations in elastic moduli, but comparable to inferences based on geodetic data from other localities 15 . An alternative possibility is that the Quaternary fault traces are not indicative of the fault positions below the brittle layer.…”

“…Asymmetric patterns of interseismic velocities have been reported elsewhere on the SAF and other major strike-slip faults 15,20,21 , and often attributed to lateral variations in crustal rigidity 14,15 . Significant (a factor of two) variations in the effective shear modulus were also reported for kilometer-wide damage zones around several faults in southern California 11,22 .…”

“…Possible explanations for the concentrated deformation on one side of a fault include acrossfault contrasts in the effective shear modulus of the host rocks 14,15 , postseismic relaxation in the presence of lateral variations in the effective viscosity of the substrate 16 , multiple sub-parallel shear zones 17 , and a non-vertical fault geometry 18,19 . Postseismic transients are an unlikely explanation given the large time lapse since the presumend last great earthquake on the southern SAF.…”

Fault slip rates, effects of elastic heterogeneity on geodetic data, and the strength of the lower crust in the Salton Trough region, southern California

“…Previous work in the Ventura Basin of southern California [ Donnellan et al , 1993; Hager et al , 1999] has shown that an elastically weak sedimentary cover can obscure locking depth and slip rates estimates based on geodetic data. Thus lateral and vertical variations in elastic and viscous properties in the Salton Trough may be important in influencing the form of surface deformation [ Le Pichon et al , 2005] and therefore our interpretation of geodetic data.…”

Southern San Andreas‐San Jacinto fault system slip rates estimated from earthquake cycle models constrained by GPS and interferometric synthetic aperture radar observations

“…The simplest approach relies on two-dimensional cross-sectional models of geodetic velocity profiles, which allow examination of slip partitioning or material heterogeneity for specific sections of the plate boundary [Malservisi et al, 2001;Johnson and Segall, 2004;Le Pichon et al, 2005;Argus et al, 2005;Hilley et al, 2005;Fay and Humphreys, 2005;Schmalzle et al, 2006;Fialko, 2006;Johnson et al, 2007]. For the SJF-SAF system, the elastic deep slip-driven fault model of Fialko [2006] finds slip rates of 25 ± 3 and 19 -21 mm/a for the SAF and SJF, respectively, whereas the elastic and viscoelastic models of Fay and Humphreys [2005] find rates of about 22-23 and 14-15 mm/a for the SAF and SJF, respectively (Table 1).…”