Sep. 28, 2007 Researchers analyzes over a thousand aftershocks of the 1989 Loma Prieta earthquake, which occurred along the San Andreas Fault system in northern California.
It determines details of the fault structure from aftershock alignments at different scales in the fault zone, and the authors used the slip directions on local groups of aftershock faults to map the variations in deformation throughout the fault zone. The main fault is about 60 km long, and is defined by three segments that form a 20° bend, shaped like an open 's'.
Aftershock slip was dominantly in a horizontal, west-side-north sense on the Southern Segment, a mixture of west-side north and west-side-up on the Central Segment, and a complex distribution of slip directions characterized by west-side-north with a large to dominant component of west-side-up on the Northern Segment. These three segments of the fault do not reach the surface, but are overlain by a Shallow Zone comprising another set of faults that are distributed along the main trend of the three fault segments but are oriented at an angle to them.
The slip on faults in the Shallow Zone is dominantly west-side-up. These zones of the fault all have a substructure of smaller faults at length scales of several kilometers to several tens of meters on which the deformation actually accumulates. The patterns of slip and the relation between the orientations of these smaller faults and the deformation, can be interpreted to show that the Loma Prieta fault zone had the strength of normal crust when the earthquake and its aftershocks occurred.
This conclusion contradicts numerous previous studies, which inferred that the fault was abnormally weak. The detailed analysis of the aftershock deformation has also provided evidence for a newly recognized mechanism of brittle deformation by which a three-dimensional deformation is split into two two-dimensional deformations, both of which occur within the same volume of rock at the same time.
This provides a new model for understanding this type of brittle deformation in the Earth's crust. Constraints on the rotations of fault blocks in the shear zone also can be inferred from the seismic data, a result that supports a model we have proposed previously but that is not predicted by classical seismology.
Reference: Structure, Deformation, and Strength of the Loma Prieta Fault, Northern California,U.S.A., as Inferred from the 1989-1990 Loma Prieta Aftershock Sequence, Robert J. Twiss and Jeffrey R. Unruh, Geology Department, University of California at Davis, Davis, California 95616, USA., Geological Society of America Bulletin, Pages 1079-1106.
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