MODERN SEISMOLOGY SCBASE 201 



times of travel of the compressional waves to stations nearly anti- 

 podal to the epicenter were consistently abnormal; if the focus 

 is deeper than normal the waves arrive at distant stations earlier 

 than the normal time. For some years, however, Turner's hypothe- 

 sis of deep focus was not generally accepted because he also main- 

 tained the possibility of abnormally shallow focus to account for 

 a few shocks in which the compressional wave arrived late at anti- 

 podal stations, and this implied that the normal focal depth is as 

 great as 250 kilometers. Independent evidence of the occurrence of 

 deep focus earthquakes was obtained by Wadati, who noticed that 

 some Japanese shocks showed an abnormal distribution of intensity 

 over the region in which they could be felt and that the time intervals 

 between the S and P phases were abnormally large near the epicenter. 

 The question was placed beyond doubt by the present writer, who 

 identified on the records of supposed deep-focus earthquakes certain 

 phases which could only be explained as reflections of P and S waves 

 near the epicenter ; such reflections may occur in normal earthquakes, 

 but it is only when the focus is deep that these additional phases 

 can be separated from the direct pulses. Another characteristic of 

 deep-focus earthquakes, recognized by Stoneley and others, is that 

 the surface waves are very weakly developed; this is in agreement 

 with theory which shows that the amplitudes of these waves should 

 fall off rapidly with depth. The more recent work on this class of 

 shock is mainly concerned with the construction of time-distance 

 curves applicable to various focal depths. The preliminary curves 

 prepared by the writer were obtained by applying appropriate cor- 

 rections for the effect of abnormal depth to the Zoeppritz-Turner 

 tables for normal earthquakes, and they therefore suffered from the 

 errors which were known to exist in these tables; improved curves 

 have been obtained from detailed studies of the records of individual 

 shocks. The occurrence of earthquakes at such depths as 500 kilo- 

 meters, equivalent to nearly a tenth of the earth's radius, is of great 

 importance, for it implies the existence of materials of great strength 

 at these depths. It is not definitely known what is the significance 

 of the observations which led Turner to attribute abnormally high 

 foci to certain earthquakes, but it is probable that the apparent 

 anomalies were due to certain phases being wrongly identified on the 

 records. 



EARTHQUAKE PERIODICITIES 



Although the primary causes of earthquakes lie within the earth, 

 it is quite possible that the actual time of occurrence of a shock is 

 determined by external factors. As a result of the gradual develop- 

 ment of stresses in the earth's crust dislocation has to occur sooner or 

 later, and the extra impulse which is sufficient to start the break 



