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Summary: To appear in Bull. Seism. Soc. Am., Vol. 95, No. 2, April 2005 1
Ambiguity of the Moment Tensor
by Jean-Paul Ampuero1
and F. A. Dahlen
Abstract An earthquake on a fault separating two dissimilar ma-
terials does not have a well-defined moment density tensor. We
present a complete characterization of this bimaterial ambiguity in
the general case of slip on a fault in an anisotropic medium. The
ambiguity can be eliminated by utilizing a potency density rather
than a moment density representation of a bimaterial source.
Introduction
Earthquake seismology is being nourished by a growing body of observational
constraints on the structure of fault zones, including geological field studies of
exhumed faults (Chester et al., 1993), classical and guided-wave fault-zone to-
mographic studies (Catchings et al., 2002; Thurber et al., 2003; Ben-Zion, 1998;
Li et al., 2000), high-resolution microearthquake relocations (Got et al., 1994;
Waldhauser and Ellsworth, 2002), and active fault-zone drilling projects such
as the San Andreas Fault Observatory at Depth, the Corinth Rift Laboratory
and the geophysical boreholes into the Nojima Fault Zone (Ohtani et al., 2000).
These new constraints are driving fundamental investigations of fault-zone con-
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