Initial shear wave particle motions and stress constraints at the Anza Seismic Network

Abstract

S U M M A R YWe use focal plane solutions to constrain principal stress directions in the vicinity of six Anza Network stations which show evidence for shallow shear wave anisotropy in the vicinity of the Anza seismic gap region of the San Jacinto fault. Faulting near all stations is consistent with approximately N-S maximum compressive stress. Five of these stations show nearly N-S initial particle motion alignment, consistent with the anisotropy-stress relationship expected for stress-aligned microcracks. However, one station (KNW) has a well-defined preferred initial shear wave polarization direction of N40"W. Although this polarization direction differs dramatically from the local maximum compressive stress direction, it is consistent with the anisotropy expected due to a local alignment of anisotropic bedrock minerals, particularly biotite. Thus, anisotropy observed at this station most likely reflects a fixed, palaeostrain alignment of anisotropic minerals and/or microcracks and does not require a dependence on the current stress field.

Stress-induced upper crustal anisotropy in southern California

“…Shear wave splitting from local earthquakes has also been utilized and gives constraints on anisotropy in the upper crust. Extensive studies of shear wave splitting from local earthquakes have been conducted in southern California [ Aster et al , 1990; Aster and Shearer , 1992; Liu et al , 1997; Cochran et al , 2003, 2006; Boness and Zoback , 2004, 2006; Paulssen , 2004; Liu et al , 2008] and other active fault zone regions (e.g., Anatolian Fault [ Peng and Ben‐Zion , 2004]), which investigate the spatial distribution and possible temporal variations of crustal anisotropy.…”

“…In this model, the medium consists of vertically aligned, fluid‐filled microcracks parallel to the direction of maximum horizontal compressive stress σ H [e.g., Crampin , 1978, 1987; Leary et al , 1990], or the preferential closure of fractures in a randomly fractured crust [ Boness and Zoback , 2004]. The other category is structural anisotropy due to rock or mineral fabric, such as preferential mineral alignment [ Brocher and Christensen , 1990], rock fabric [ Kern and Wenk , 1990], and remnant features of paleostress [e.g., Aster and Shearer , 1992]. …”

Seismically invisible fault zones: Laboratory insights into imaging faults in anisotropic rocks

“…Anisotropy in fault zones is often inferred to result from a preferred fracture orientation due to the surrounding stress field [e.g., Evans, 1984]. Foliation or aligned minerals can also contribute to anisotropy within a region [Aster and Shearer, 1992 We use laboratory measurements on the Carboneras fault zone, a major regional strike-slip fault in SE Spain, to consider the potential effect of anisotropy on seismic imaging. The onshore portion of the fault zone has been mapped extensively [Faulkner et al, 2003;Rutter et al, 2012].…”

Relation of the 1992 Landers, California, Earthquake Sequence to Seismic Scattering

“…Rock fabric may be important and need not be fault-aligned if it records an earlier stress state (13), but rock fabric is typically subordinate to crack-induced anisotropy at the upper crustal depths of interest (14). Because of the approximate fourfold symmetry of scattering with respect to crack orientation, fault-normal 'cracks are an admissible altemative.…”