Double-couple source

Summary

A double-couple source is a point source made of two force couples of equal and opposite moment. So its net force and net torque vanish. It is the point-source equivalent of shear slip on a small fault, and so the standard model of an ordinary tectonic earthquake. Its moment tensor has eigenvalues $M_0$, $0$, $-M_0$, where $M_0$ is the seismic moment. The eigenvectors give the tension, null and pressure axes of the earthquake.