Parameters and observations

The determination of the source mechanism of earthquakes consists in finding the parameters of the model used in its representation, whose number depends on its complexity. The localization of the source is given by four parameters (φ, λ, h, and t0), the coordinates of the epicenter, depth of focus, and origin time (section 15.2). Generally, in determinations of focal mechanisms, these parameters are assumed to be known, although in some methods some (the focal depth) or all (the centroid) are determined anew. The size given by the magnitude M is independently determined, but, in many methods, the seismic moment M0 is evaluated as part of the mechanism.

The simplest models of source mechanism are those with point sources. For a point shear dislocation, the orientation of the mechanism is given by the angles φ, δ, and λ (section 15.1). For the equivalent double-couple force model, the orientation is given by those of the X and Y axes, that is, by the angles ⊗x, φy, and φy, or by the P and T axes, angles ⊗P, φP, and φT (section 16.2). If the source is not a shear fracture, it can be represented by five components of the moment tensor Mij (for a deviatoric source) or six if there are changes in volume (Chapter 17). Including its size, a shear-fracture point source or DC source is given by four parameters (M0, φ, δ, and λ) plus four for its location, that is a total of eight parameters.