A new moment-tensor decomposition for seismic events in anisotropic media

Investigating the mechanisms of small seismic sources usually consists of three steps: determining the moment tensor of the source; decomposing the moment tensor into parameters that can be interpreted in terms of physical mechanisms and displaying those parameters. This paper concerns the second and third steps. Two existing methods-the Riedesel-Jordan and Hudson-Pearce-Rogers parameters and displays-are reviewed, compared and contrasted, and advantages and disadvantages of the two methods are discussed. One disadvantage is that neither method takes into consideration the effect of anisotropy on the interpretation. In microseisms, anisotropy can be important. A new procedure based on the biaxial decomposition of the potency tensor is introduced which explicitly allows for anisotropy and interprets the moment tensor in terms of an isotropic pressure change and a displacement discontinuity on a fault. It is shown that this interpretation is always possible for any moment tensor whatever the anisotropy. To compare the pressure change with the displacement discontinuity, it is useful to be able to determine the volume change from the pressure source in any medium. This depends on the embedded bulk modulus, which differs from the normal bulk modulus. The embedded modulus in isotropic media is well known and the equivalent anisotropic result is derived in this paper. Interpreting a seismic source in terms of the volume change due to a pressure change and a displacement discontinuity on a fault allows a simple 3-D graphical glyph to be used to display the interpretation.