Yol. 56.] COKNISH EAKTHOJTAKES OF 1898. 7 



There can be little doubt that the earthquakes of March 29th and 

 April 1st were connected with one and the same fault, for their iso- 

 seismal axes are parallel, and the line joining their centres is nearly 

 parallel to these ases. Proximity in time of occurrence and disturbed 

 area render probable a similar connexion for the third shock ; for it 

 is more likely that the change of stress brought about by such com- 

 paratively small movements should precipitate a slip in the same 

 fault than in another fault of the same system. Assuming this to 

 be the case, we see that the areas of successive slips were subject 

 to a continual easterly advance along the fault. 



Judging from the dimensions of the disturbed area, the focus of 

 the first shock cannot have been much more than a mile in length, 

 its centre lying beneath a point a short distance N. 33° W. of the 

 centre of the disturbed area. The focus of the second shock 

 was probably not less than 5, and not more than 7, miles long, 

 its centre being about 1 mile E. 33° N". of the former. It must, 

 therefore, have included the focus of the first earthquake.^ The 

 position and magnitude of the third focus are indeterminate ; but 

 its length must have been very short, and, as it lay about 2 miles 

 from the second centre, it must also have been included within the 

 second focus, unless it was situated at a different level of the fault- 

 surface. 



Besides the focal transference along the strike of the fault-surface, 

 there is also evidence of translation in the direction of its dip. Each 

 of the first two earthquakes consisted of a double series of vibrations. 

 Nothing can be learned from the double series on March 29th, except 

 that there must have been two separate slips ; but with regard to 

 that on April 1st the evidence is unusually clear. Corresponding 

 to each series of vibrations there was a distinct focus, that of the 

 earlier shock being at the higher level. 



Discussion. 



Prof. Sollas welcomed this valuable addition to our knowledge of 

 the connexion between faults and earthquakes. The Author had 

 no doubt shown a wise reticence in abstaining from any expression 

 of opinion as to the actual depth of the foci below the ground. The 

 speaker could not help thinking that all earthquakes which are 

 produced by the bumping of the country as it falls along a surface 

 of faulting must originate at comparatively slight depths, not greater 

 probably than 5 miles. When a depth of 30 miles has been assigned 

 to the position of the focus, either the disturbance had some other 

 cause than faulting, or inferences resting on the presumed angle of 

 emergence of the wave at different points of observation were mis- 

 taken. The refraction of the wave as it passed from layers of the 

 earth's crust of greater to those of less elasticity would of itself lead 

 to an exaggerated estimate of the depth of the focus. 



Prof. Milne and Mr. Teall also spoke. 



