304 ANNUAL REPORT SMITHSONHAN INSTITUTION, 1950 



Detailed data concerning arrival times of waves at the reporting 

 seismological observatories are printed in the International Seis- 

 mological Summaries. In addition, these volumes contain calculated 

 values of the coordinates and depths of the earthquake foci and the 

 origin times of the shocks. They were formerly compiled at Oxford, 

 England, and now at Kew (Turner et al., 1923-50). The summaries 

 are based on the bulletins that are issued by most seismological sta- 

 tions. Some of these station bulletins contain, in addition to observed 

 times of various phases, the calculated amplitudes of the ground 

 motion. With this information it is possible to determine the size of 

 the earthquakes. The great importance for research of all such sta- 

 tion bulletins and international catalogs is obvious. 



There are now roughly 300 seismological stations with accurate time 

 service (at least to the nearest second) practically all over the world, 

 including South Africa, South America, New Zealand, Samoa, Aus- 

 tralia, and Madagascar in the Southern Hemisphere, and a much 

 denser network in the Northern Hemisphere. 



Until about 10 years ago the size of an earthquake could be esti- 

 mated only from the observed size of the area of perceptibility or of 

 damage or from changes found at the surface of the earth. Arbitrary 

 scales were applied to such data to find the intensity of a shock. For 

 example, in the scale used in the United States (Wood and Neu- 

 mann, 1931), intensity II indicates that the shock was felt only by 

 a few persons; intensity V, that it was felt by everyone, many were 

 awakened, some dishes were broken, etc.; intensity VIII indicates 

 slight damage in specially designed structures, considerable damage 

 in ordinary buildings, great damage in poorly built structures; and 

 intensity XII, the maximum, indicates destruction of all structures. 

 A scale of wholly different nature, based on instrumental data, was 

 devised by C. F. Richter ( 1935 ) . He defined magnitude of an earth- 

 quake at average (shallow) depth in southern California as the 

 common logarithm of the maximum trace amplitude expressed in 

 thousandths of a millimeter, with which the standard short period 

 torsion seismometer (period 0.8 second, magnification 2,800, damp- 

 ing nearly critical) would register that earthquake at an epicentral 

 distance of 100 kilometers. Magnitude 3/ =2 corresponds in shallow 

 earthquakes to a shock barely felt; a shock of magnitude 5 causes 

 minor damage; magnitude 7 is the lower limit of major earthquakes; 

 81/^ is the highest magnitude that has been determined from amplitude 

 data given in individual bulletins of seismological stations since 1904. 

 This magnitude scale was later extended by Gutenberg and Richter 

 (1936, 1942) to apply to shallow earthquakes occurring in other locali- 

 ties and recorded by other types of instruments. Gutenberg (1945a) 

 devised means for determining magnitudes of shallow earthquakes 



