Upper mantle deformation in the western US determined from seismic anisotropy; 2: Western North American plate boundary

Conference · Thu Apr 01 00:00:00 EST 1993 · Geological Society of America, Abstracts with Programs; (United States)

OSTI ID:5019162

Ozalaybey, S; Savage, M K ^[1]; Silver, P G ^[2]

---

1. Univ. of Nevada, Reno, NV (United States). Seismological Lab.
2. Carnegie Inst., Washington, DC (United States)

The western margin of the North American plate includes subduction, transform faulting and a migrating triple junction. The authors have measured polarization azimuths ([phi]) and delay-times ([delta]t) of split shear waves. Stations located close to the northern end of the San Andreas fault, near the San Francisco Bay area yielded well-constrained but azimuthally varying splitting parameters. These can be explained by a model consisting of two anisotropic layers: an upper layer with fast direction parallel to the strike of the San Andreas fault and a lower layer with E-W fast direction. Both layers have average delay-times of 1[+-]0.3 s. The authors have found that an east-west fast feature is also present beneath stations in the Sierra-Nevada, the Mojave Desert and the Los Angeles area. These latter measurements do not require more than one layer. The E-W fast layer diminishes near the southern edge of the Gorda plate. The authors interpret their measurements as caused by single or double layers of homogeneously, transversely anisotropic material with horizontal symmetry axes due to strain-induced preferred orientation of olivine in the upper mantle. They suggest that the fault-parallel fast layer is the result of finite strain associated with the transform plate motion between the North American and Pacific plates. The deeper layer with E-W fast direction can not be associated with known surface tectonic features. One possible mechanisms for this E-W fast feature is that it may be related to the passage of the trailing edge of the Farallon plate as the slab migrated northward beneath central California. The shear associated with the different motion between the slab and the asthenosphere may cause mineral alignment leading to shear-wave splitting.

• OSTI ID: 5019162
• Report Number(s): CONF-9305259-; CODEN: GAAPBC
• Journal Information: Geological Society of America, Abstracts with Programs; (United States), Vol. 25:5; Conference: 89. annual meeting of the Cordilleran Section and the 46th annual meeting of the Rocky Mountain Section of the Geological Society of America (GSA), Reno, NV (United States), 19-21 May 1993; ISSN 0016-7592
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
CALIFORNIA
GEOLOGIC FAULTS
SEISMIC SURVEYS
DATA ANALYSIS
ANISOTROPY
ATTENUATION
DEFORMATION
EARTH MANTLE
GEOLOGIC MODELS
GEOLOGY
GEOMETRY
ORIENTATION
PLATE TECTONICS
RESPONSE FUNCTIONS
SEISMIC WAVES
SUBDUCTION ZONES
WAVE PROPAGATION
DEVELOPED COUNTRIES
FUNCTIONS
GEOLOGIC FRACTURES
GEOLOGIC STRUCTURES
GEOPHYSICAL SURVEYS
MATHEMATICS
NORTH AMERICA
SURVEYS
TECTONICS
USA
580000* - Geosciences