Earthquake intensity attenuation pattern in the United States

Attenuation of earthquake intensities with epicentral distance was studied by analysing the intensity data for 39 earthquakes in the United States. Attenuation of MM intensity (I) with distance (Δ km) follows a simple relation of the type log I = log I0 − mΔ, where I0 is the intensity at the epicentre and m is a constant. Slope m is found to be inversely proportional to the square of the focal depth. Intensity attenuation pattern in the United States in general can be represented by a unified relation I/I0 = exp [−(0.8999/h2+ 0.0014)Δ] where 16km ⩽h⩽ 60km. Intensities were calculated with the help of this equation and a good agreement with the observed intensities were found. A comparative study has also been made between the attenuation relations applicable to India and the United States.