Bulletin of the Seismological Society of America

In the course of historical or statistical study of earthquakes in any given region i t is frequently desirable to have a scale for rating these shocks in terms of their original energy, independently of the effects which may be produced at any particular point of observation. On the suggestion of Mr. H. O. Wood, it is here proposed to refer to such a scale as a "magnitude" scale. This terminology is offered in distinction from the name "intensity" scale, now in general use for such scales as the Rossi-Forel and Mercalli-Cancani scales, which refer primarily to the local intensity of shock manifestation. The writer is not aware of any previous approach to this problem along the course taken in this paper, except for the work of Wadati cited below. Total original energies have been calchlated for a number of shocks, using seismometric and other data ; but such a procedure is practicable only for a limited number of cases, whereas it is desired to apply a magnitude scale to all or nearly all of the shocks occurring. Mr. Maxwell W. Allen states that he has for some time 'employed an arbitrary scale for rating large earthquakes, based on the amplitudes of earth motion calculated from the reports of distant stations. This laborious procedure is not far removed in principle from that adopted in the following discussion. Doubtless it has also occurred to others, but has failed of general application because of its paucity of dependable results. In the absence of any accepted magnitude scale, earthquakes have occasionally been compared in terms of the intensity on the Rossi-Forel or some similar scale, as manifested near the epicenter. Even when reliable information is Obtainable, this method is obviously exposed to uncertainties arising from variations in the character of the ground, the depth of the focus, and other circumstances not easily allowed for. In a region such as Southern California, where a large proportion of the shocks occur