ON SEISMOLOGICAL INVESTIGATIONS. 



97 



Assuming the alternation in frequency in these two districts to be 

 real, an explanation for the same may possibly be found in the seasonal 

 alternation in the flow of ocean currents, the measured oscillations of sea- 

 level, and the chani»es in the direction of barometric gradients — phenomena 

 which are inter- related. 



In summer-time oft' the coast of Japan the Kuro Siwo or Black Stream 

 runs further northward in that season than in winter ; while Dr. Omori 

 tells us that although barometric pressure may be low at this time, this 

 decrement in load is more than compensated for by the increased height 

 in oceanic level. 



For the third region the summer frequency for the land area is nearly 

 identical with that of the West Pacific. 



V. Jielationshi]) of Large Earthquakes to each other and to Volcanic 



Eruptions. 



In any two districts, or in the same district, no apparent time- 

 relationship between the occurrence of successive megaseisms has yet been 

 formulated. If, however, we consider groups of large earthquakes which 

 have occurred in a particular district, we may find a yearly decrease in 

 their numbers, as in district A, while in districts like F and K there may 

 be an inci'ease. The time-relationship between after-shocks is well known. 

 A striking illustration of the connections between a large earthquake and 

 small shocks in a neighbouring district, together with displays of volcanic 

 activity, occurred on January 31, 1906. On that date the coast of 

 Colombia was inundated, islands sank at the mouth of the Esmeralda 

 River, a cable was broken, and the volcano Cumbal erupted. These 

 changes on the flanks of the Cordilleras were followed by disturbances in 

 the Antilles. 



Many small earthquakes occurred, and on February 16 several of 

 them were sufficiently severe to damage masonry. Mount Pel^e and 

 La Soufriere showed signs of increased activity, and eight, if not nine, 

 cable interruptions were recorded. 



Here we have another illustration of the remarkable relationship 

 which exists between the hypogenic activities of these parallel folds, a 

 disturbance in one being accompanied or followed by a response in the 

 other. ^ 



VI. Earthquakes and Changes in Latitude. 



The following table is a continuation of one published in the ' British 

 Association Report,' 1903, pp. 78-80 : — 



1906. 



' See Brit. Assoc. Bej>., 1902, p. 73. 



