How to Compare the Strength of the Seismic Waves

Calculate the magnitude of the surface waves. Surface waves reach the seismic station after body waves. The standard formula for measuring surface waves is MS = log10 (A/T) + 1.66 log10 (D) + 3.30. There are variations of these standard formulas that account for variations in specific geographic regions. While surface waves are of a lower frequency than body waves and arrive after body waves, they are responsible for most of the damage from earthquakes. Deeper earthquakes cause less damage than earthquakes closer to the surface. There are two types of surface waves: Love waves, named a British mathematician (1911), and Rayleigh waves, first identified in 1885. Love waves are fastest and move the ground side to side, whereas Rayleigh waves roll like the waves across the surface of the ocean.

Analyze the waveforms produced by the seismograph. Factors to consider are the direction of the fault movement and the orientation of the fault. Fault geometry is determined from waveforms that are recorded at various distances and azimuths from the earthquake. (The azimuth is the horizontal angle of the observer's bearing in surveying.)

Calculate the seismic moment. The formula for calculating the seismic moment is MO = µS<d>. In this formula µ is the shear strength of the faulted rock, S is the area of the fault, and <d> is the average displacement on the fault. Seismic moment is considered to provide a truer measure of the earthquake size than magnitude, because it is based on fault geometry and observer azimuth. Additionally, seismic moment has the advantage of not having an intrinsic upper boundary in its calculation.