Small earthquakes have a lot of energy in the higher end of the frequency scale, which is rapidly dissipated. Larger earthquakes have more low-frequency energy

Small earthquakes have a lot of energy in the higher end of the frequency scale, which is rapidly dissipated. Larger earthquakes have more low-frequency energy, so they go further. There's no simple equation, they just use regression to fit the attenuation as a factor of 1/r, 1/r^2 (...), and for magnitude, and for frequency. You can start here: http://earthquake.usgs.gov/hazards/about/attenuation.php http://earthquake.usgs.gov/hazards/about/attenuation.php