EARTHQUAKES IN NORTH AMERICA — GUTENBERG 305 



using amplitudes and periods of waves that had traveled through 

 the interior of the earth. He also extended the scale to include deep- 

 focus earthquakes (Gutenberg, 1945b). It is now possible to deter- 

 mine the magnitude of larger earthquakes within a few tenths of the 

 scale from seismograms at any well-equipped station. The relation- 

 ship between magnitude M of an earthquake and its energy E in ergs 

 is given roughly by the approximate equation log £'=12+1.8il/ 

 (Gutenberg and Richter, 1949). This holds for any focal depth. 

 The data concerning the magnitude and the instrumentally determined 

 epicenters and depths of foci of earthquakes provide the basis for 

 seismicity studies. 



Lists of earthquakes and other results of such an investigation of 

 earthquakes recorded over the period from 1904 to 1947 have been 

 published by Gutenberg and Richter (1949). Much of the following 

 information is taken from this book. 



The use of magnitudes for the first time provides reliable informa- 

 tion concerning the relative seismicity of all regions of the earth. It 

 eliminates the effects of density of population and of communication 

 facilities on the determination of intensities of reported earthquakes, 

 as well as effects of uneven distribution of seismological observatories 

 on seismicity patterns. If the magnitude of the earthquakes is not 

 considered, distorted appearance of seismicity maps may result from 

 an accumulation of many small shocks, which are plotted only in 

 regions well covered by stations with sensitive instruments. Thus, 

 Europe — which, except for the Mediterranean area, has a low actual 

 seismicity — has appeared on maps in the past as a region of relatively 

 high seismic activity. There are now five stations reporting magni- 

 tudes of earthquakes in their routine bulletins, but many more reg- 

 ularly furnish amplitude data required for the magnitude determina- 

 tion. Magnitude can be determined from a seismogram at any sta- 

 tion where instrumental constants are known and where a clear record 

 of an earthquake has been written, regardless of the distance or depth 

 of the shock. Magnitudes determined at different stations rarely 

 differ by more than 0.3 units from the average for a given earth- 

 quake. 



The outer part of the earth consists of relatively inactive blocks, 

 separated by active zones falling into four groups: (1) the circum- 

 Pacific zone, which includes about 80 percent of all shocks with origins 

 at a depth not exceeding 60 kilometers (about 40 miles), 90 percent 

 of the so-called intermediate shocks, which have their sources at 

 depths between 60 and 300 kilometers (about 40 and 190 miles), and 

 all deeper shocks (maximum observed depth approximately 400 

 miles). (2) The Mediterranean and trans- Asiatic zone, which in- 

 cludes nearly all remaining intermediate and large shallow shocks. 



