Systematic non‐double‐couple components of earthquake mechanisms: The role of fault zone irregularity

Abstract

Several recent studies have revealed statistical correlations between earthquake mechanism type and associated non‐double‐couple components in catalogs of seismic moment tensors. This systematic behavior may result either from biases in the solutions due to Earth structure effects in different tectonic regimes, or from source radiation effects. For certain large non‐double‐couple events, detailed analyses have shown that multiple subevents with different fault orientations produce the non‐double‐couple radiation. We generalize this idea and simulate non‐double‐couple components (NDCC) resulting from subfaults with variable geometry, showing that the statistical behavior of the NDCC in moment tensor catalogs can be generally accounted for by such source complexity. Assuming that an earthquake fault consists of many subfaults with random fluctuations about some mean geometry, the total moment tensor for failure of the system under a regional stress state can be represented by the sum of moment tensors of the subfaults. The sign of the NDCC in the composite moment tensor directly reflects the stress state applied to the fault system, and the NDCC amplitude is expected to be systematic over a wide range of cumulative seismic moment (Mo ) given the basic fractal nature of fault systems. These simulation results are consistent with the global behavior of the NDCC reported in the Harvard centroid moment tensor catalog for 1977–1991. Parameters such as the applied stress state, parameterization and randomness of the subfault geometry, and seismic moment distribution among the subfaults affect the predicted NDCC amplitude. Change in the parameters as a function of subfault seismic moment controls the shape of the NDCC ‐ Mo relationship. Regional variations of the NDCC ‐ Mo relationship may thus reflect regional variations in the fault zone parameters, raising the possibility that the regional NDCC behavior may be used to infer stress state and subfault distribution in various source regions.