Compute and Analyze Moment Tensors

The focal mechanism of an earthquake describes the inelastic deformation in the source region that generates the seismic waves. In the case of a fault-related event it refers to the orientation of the fault plane that slipped and the slip vector and is also known as a fault-plane solution. Focal mechanisms are derived from a solution of the moment tensor for the earthquake which itself is estimated by an analysis of observed seismic waveforms. The moment tensor solution is typically displayed graphically using a so-called beachball diagram. The pattern of energy radiated during an earthquake with a single direction of motion on a single fault plane may be modeled as a double couple which is described mathematically as a special case of a second order tensor (similar to those for stress and strain) known as the moment tensor. Our moment tensor inversion technique uses a combination of several seismic wave types, time windows and frequency bands carefully chosen based on event magnitude and station distance. Wave types include body waves, surface waves, mantle waves as well as the so-called ‘W-Phase’ (Kanamori and Rivera, 2008).