STRUCTURE OF THE EARTH — HODGSON 283 



are in operation, reporting regularly to each other and to a central 

 station at Oxford, England, where an agency under the joint auspices 

 of that university and of the International Union of Geodesy and 

 Geophysics determines the position of all locatable shocks and pub- 

 lishes a statistical summary of the data. We now know that the most 

 active seismic regions of the world may be defined as a belt about the 

 Pacific with an offshoot zone across Asia and Europe north of India 

 and along the north shore of the Mediterranean to meet another belt 

 running north and south through the relatively shoal water of the 

 mid-Atlantic. We know, further, that all earthquakes are not con- 

 fined to these more active belts. We have very good reason for say- 

 ing that there is no region of the earth in which a major earthquake 

 might not take place. This international service was begun in 1895 

 and has been growing steadily ever since. Canada has participated 

 since 1897. 



How then can seismic stations determine from their records where 

 an earthquake has occurred ? It would take much too long to sketch 

 the slow degrees by which men learned to construct seismographs and 

 deduce the method of interpreting from their records the distance 

 from the recording station to the source of an earthquake. The ab- 

 stract mathematical foundations of the theory were laid as early as 

 1849, but it was not until 1889 that, by chance, the first record of an 

 earthquake was made at a distance. At that time an earthquake in 

 Japan was found to have been recorded on a very sensitive instrument 

 at Potsdam, Germany, set up for the purpose of determining to what 

 extent the land itself has tides similar to, though smaller than, those 

 caused on the sea. Not until 1900, however, was such a distant earth- 

 quake record interpreted to yield a value of the distance from station 

 to origin. 



Before explaining how distance can be read from the record, we 

 must know just a little about the instrument known as a seismograph. 

 Suppose someone asked you to describe a lady's hat — not some par- 

 ticular hat — just a hat. That difficulty is comparable with an attempt 

 to describe the appearance of a typical seismograph. Such instru- 

 ments vary in size from one you could hold in your hand to one which 

 stands more than 6 feet high and weighs 20 tons. One thing, how- 

 ever, seismographs have in common. A relatively heavy weight is 

 held very delicately suspended from a framework which is, itself, 

 in firm contact with the earth. When the earth moves, the weight 

 stays behind as you tend to do when the streetcar starts suddenly. 

 The relative motion of the weight and the frame attached to the earth 

 is recorded continuously and gives the record. When the earth is at 

 rest the record line is straight. When a shock impulse arrives, the 

 line is deflected. At regularly timed intervals, usually once a min- 

 ute, a slight motion is given the recording mechanism so that a small, 



