(a) What is the ratio of the intensity of an earthquake wave as it passes a point 20 km from the source to the intensity as it passes a point 15 km from the source?

(b) What is the ratio of the amplitude of an earthquake wave as it passes a point 20 km from the source to the amplitude as it passes a point 15 km from the source?

If we assume the energy spreads out equally in
all directions, the intensity obeys the inverse square law. (a) Distance increases by a factor of 20/15 = 1.33
Intensity reduces by 1.33² = 1.77
Ratio of intensities is 1/1.77 =0.56
(b) The intensity is proportional to amplitude
squared, so amplitude is proportional to the square root of intensity. √1.77 = 1.33
Ratio of amplitudes is 1/1.33