Earthquake magnitude as a function of intensity data in California and Western Nevada

Using published intensity data for California and western Nevada earthquakes, from 1950 through 1971, magnitude was related to maximum reported intensity, area of perceptibility, and the areas enclosed by various isoseismal lines. The relations developed are

where A_(VI), for example, is the area, in square kilometers, shaken at intensity VI or greater. Previously unpublished isoseismal maps, for earthquakes in the 1940's, are presented and were used to test these relations. The tests show that magnitudes estimated from I_max may be underestimated by up to 1.3 magnitude units, but that magnitudes estimated from various isoseismal areas are generally within 0.5 units of the published values.