1. A method of analysing an underground seismic event comprising: receiving and measuring seismic waves emitted by the seismic event; and converting the measurements of the seismic waves into a value for magnitude of explosion or implosion, a value for magnitude of displacement discontinuity at a plane, the direction of a normal to that plane, and a direction of displacement; where the two said values and the two said directions compose the whole moment tensor describing the seismic event.