Dr. 0. Davison on Earthquake-Sounds. 67 



that the first recorded tremors may be the successors of those 

 which produce the preliminary earthquake-sound. The ob- 

 servations of British earthquakes show, however, that the 

 sound-vibrations are not confined to the earlier stage, but 

 that, like the ripples, they are also superposed on the slow 

 undulations which form the main feature of the principal and 

 end portions of an earthquake. 



Soine of the phenomena of earthquake-sounds, as we have 

 seen, are due to the neighbourhood, of the sound to the varying 

 lower limit of audibility. There are others, such as the time- 

 and space-relations of the sound and shock, for which a different 

 explanation is required, and whose origin is of a o-eolooical 



f I'll oo 



rather than a physical character. 



The theory which follows is based on the supposition that 

 the majority of non-volcanic earthquakes are due to the 

 gradual, but intermittent, growth of faults; the immediate 

 cause of the disturbance being the friction produced by the 

 slipping and rubbing of one of the adjoining rock-masses over 

 and against the other *. 



A fault-slip does not of necessity take place concurrently 

 all over the focus or instantaneously at any one point of it, 

 But, as a general rule, it certainly occupies a very short 

 interval of time, and at places near the epicentre the duration 

 of the sensible part of an earthquake must be mainly due to 

 the size of the focus and the finite velocity of the earth- 

 waves. 



The seismic focus is practically a surface inclined to the 

 horizon, and is often of great length in a horizontal direction. 

 In its simplest form there will be a central region of the fault- 

 surface where the relative displacement of the two rock-masses 

 is a maximum, and this will be surrounded by a region in 

 which the relative displacement is small, and gradually dies 

 away towards the edges. As the vibrations of great ampli- 

 tude are also of long period, we may consider that from all 

 parts of the focus there start together vibrations of various 

 amplitude and period, the large and slow undulations coming 

 mostly from the central region, and the small and rapid vibra- 

 tions from those which bound it. It is, I believe, from these 



* The principal facts in favour of the fault-slip theory are : — (1) The 

 elongated forms of the isoseisraal lines, the longer axes of which in any 

 district are, as a rule, either parallel or perpendicular to one another, and 

 parallel, or nearly so, to the chief lines of fault ; (2) the formation of 

 fault-scarps concurrently with violent earthquakes ; (3) the impossibility 

 of a oreat fault growing otherwise than by an almost infinite number of 

 slips ; and (4) the enormous excess of the number of earthquakes over 

 the number of faults in any one district. 



F 2 



