Structure and deformation associated with the 1989 Loma Prieta Earthquake aftershock sequence
Robert J. Twiss - Geology Dept., University of California at Davis, Davis, CA 95616 email: twiss@geology.ucdavis.edu
Jeffrey R. Unruh - William Lettis & Associates, 1777 Botelho Dr., Suite 262, Walnut Creek, CA 94596 email: unruh@lettis.com
Alignments of aftershock hypocenters of the October 1989 M 7.1 Loma Prieta earthquake reveal details of the structure, and inversion of spatially-localized sets of focal mechanisms defines the orientation and shape of the instantaneous strain ellipsoids.
Aftershocks define a hierarchy of four orders of faults: The first order is a single plane fit to all hypocenters on the main trend; the second order comprises three planar alignments that define a restraining bend in a blind fault with an upper tip line between 4 and 6 km depth, above which is a shallow zone of faults en echelon along the main fault trend. Third order structures comprise multiple alignments of hypocenters within spatially-localized aftershock sets. Fourth order structures comprise the preferred nodal plane from each aftershock focal mechanism.
Inversion solutions generally have a common maximum shortening axis d3 oriented [06°, 019°]. Maximum lengthening axes d1 define a girdle with subhorizontal and subvertical maxima. The slip direction on a plane is the direction of maximum resolved shear for an inversion solution. On the restraining-bend structure, slip varies from dominantly strike-slip in the south to reverse-dextral in the north, which is comparable to slip during the main shock. In the shallow zone, slip is strongly reverse.
Triaxial brittle deformation is accommodated by a partitioning into a pair of plane strains rotated relative to each other about a common principal axis. Here, that axis is generally d3, which implies a triaxial flattening instantaneous strain. This partitioning shows that multiple solutions from field shear-plane-and-slickenline data may record a partitioned triaxial strain, rather than a temporal evolution.
For faults of orders 1 through 4, respectively, the angle q between the d3 solutions and the fault- normals average approximately 29°, 39°, 46° and 53°. Thus the fourth-order faults have a Coulomb-fracture orientation and a similar relation to the third-order faults as Riedel shears have to their associated shear plane. The angles for the two lower orders of structure are consistent with convergent strike-slip.
The q angles for third- and fourth-order faults, and the similarity between the aftershock and the main shock slip fields, indicates that the Loma Prieta fault has a normal strength for crustal rocks.
