Is the seismic moment tensor ambiguous at a material interface?

Abstract

S U M M A R Y The moment tensors are unique and describe the body force equivalents of rupture processes in any medium including faulting at a material interface, defined as the contact of two media with non-zero velocity or density contrasts. From a practical point of view, however, the moment tensor inversion of sources near or at a material interface is more involved than if the medium is smooth in the source area. First, the moment tensors of sources characterized by the same displacement discontinuity display jumps when the source crosses the interface. Consequently, themoment tensors become sensitive to the source location. If the source lies near the interface, the location into an incorrect half-space can introduce errors in the moment tensor. Second, if the source lies at the material interface, some of the spatial derivatives of the Green’s function are, in general, discontinuous and the radiated wave field must be calculated using a generalized representation theorem. Third, themoment tensors are functions of averaged elastic parameters known from effective medium theory. The theory implies that shear faulting at a material interface in isotropic media is represented by the standard double-couple moment tensor. The scalar seismic moment is calculated as a product of the displacement discontinuity across the fault, the fault size and the effective rigidity at the fault. The effective rigidity is the harmonic mean of rigidities at the individual sides of the fault.