Abstract

Analysis of aftershock focal mechanisms of the October 1989, M 7.1 Loma Prieta earthquake reveals details of structure and deformation. Aftershocks below 4–6 km define an alignment plane oriented (131°, 65°). This alignment includes three planar segments defining a restraining bend in a blind strike-slip fault. Dextral strike-slip dominates in the south, and reverse-dextral slip dominates in the north, a distribution similar to slip during the main shock. Above this fault, dominantly reverse faults striking roughly 110° lie en echelon along the main fault trend.

In the 17 sets of spatially clustered aftershocks, a more complex substructure is defined by local planar hypocenter alignments and by preferred orientations of aftershock shear planes. The maximum shortening axes make angles with the normal to these planes that average 𝛉 ≈ 46° and 𝛉 ≈ 53°, respectively. These angles are incompatible with a weak fault, for which 0° ≤ 𝛉 ≤ 20°. Orientations of preferred aftershock shear planes resemble Riedel shears relative to local hypocenter alignment planes. The similarity of slip distributions inferred from the aftershock inversions and from the main shock implies that the stress drop during the main shock was incomplete and, thus, that the Loma Prieta fault is not unusually weak.

Triaxial brittle deformation is partitioned into a pair of plane strains rotated relative to each other about a common principal axis. The shape of the triaxial strain ellipsoid determines the angle of rotation. Evidence supports our micropolar model of brittle deformation, which predicts that the antisymmetric part of the seismic moment tensor records effects of block rotations.

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