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 Previously unpublished isoseismal maps, for earthquakes in the 1940's, are presented and were used to test these relations.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 Imax 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.