What modifications were involved in applying the scale to worldwide earthquakes?
RICHTER: You’re quite rightly pointing out that the original magnitude scale which I published in 1935 was set up only for southern California and for the particular types of seismographs in use there. Extending the scale to worldwide earthquakes and to recordings on other instruments was begun in 1936 in collaboration with Dr. Gutenberg. This involved using the reported amplitudes of surface waves with periods of about 20 seconds. Incidentally, the usual designation of the magnitude scale to my name does less than justice to the great part that Dr. Gutenberg played in extending the scale to apply to earthquakes in all parts of the world. HS: In looking at worldwide earthquakes, did Gutenberg explore other possible magnitude scales? RICHTER: Yes, without any significant contribution from me, he later worked out a version of the magnitude scale that used the measured amplitudes and periods of body waves, that is P (primary), S (secondary), and PP (P waves that are reflected at the Earth
