Procedure for calculating local magnitude, M_L

from Earthquakes by Bruce A. Bolt (W.H. Freeman and Company, 1988).

1. Measure the distance to the focus using the time interval between the S and the P waves (here S-P = 24 seconds).
2. Measure the height of the maximum wave motion on the seismogram (23 millimeters).
3. Place a straight edge between the appropriate points on the distance (left) and amplitude (right scales to obtain magnitude M_L = 5.0.