Microseismic focal mechanisms: A tutorial

David W. S. Eaton

Techniques for characterizing seismic sources, borrowed from earthquake seismology, can provide useful information for microseismic studies. Various magnitude scales are used in earthquake seismology, the oldest of which (Richter magnitude) is specific to California earthquakes. All such scales are logarithmic and yield an estimated 30-fold increase in radiated seismic energy per unit magnitude increment. Simple models of fault displacement yield acceleration and displacement spectra from which seismic moments and corner frequencies can be determined. The corner frequency, in turn, may be used to estimate stress drop and source radius. The seismic moment tensor provides a general description of a source in terms of force couples, of which several types are of particular interest for microseismic studies. Double-couple sources describe slip on existing fracture surfaces, whereas tensile compensated linear vector dipole (CLVD) mechanisms describe a plausible mechanism for generation of new fractures within a competent rock mass. The role of stress transfer in earthquake occurrence has recently been recognized. Coulomb stress change provides a simple predictive model for discriminating between regions that move close to (or farther from) failure in the aftermath of an earthquake. Such models may have application to far-field induced seismicity associated with hydraulic fracturing.