The measurement of the intensity of an earthquake using the amplitude of seismic waves. As the amplitude depends on the depth of the earthquake focus, the distance of the recording station from the focus, the travel path, and local geology at both the source and receiver, such magnitude estimates need to be constrained by several determinations. At any given recording station, the magnitude (M) of a shallow earthquake is given by the equation: M = log(A/T) + 1.66 log Δ + 3.3, where A is the maximum amplitude, T is the period, and Δ is the epicentral angular distance between the earthquake and receiver. For deeper earthquakes, the magnitude is given using 20-second-period Rayleigh waves by M = log(A/T) + af Δh + b, where h is the depth of the focus, and a and b are empirically determined constants for each seismic station.