174 



Dr. C. Davison — The Pendleton Earth-shake. 



(ii) Sound-Phenomena. — In earthquakes which disturb an area 

 of not more than a few hundred square miles the average percentage 

 of audibility is about 98, and the sound-area coincides with, or over- 

 laps, the disturbed area. Thus, in its audibility and contracted 

 sound-area, the Pendleton earth-shake differs from other shocks 

 with a small disturbed area. Nor does it differ less in the nature of 

 the sound, as will be seen from the next Table, in which the figures 

 represent percentages of the total number of comparisons to the 

 different types mentioned, for slight British earthquakes, the 

 Pendleton earth - shake, and similar earth - shakes of small and 

 approximately circular disturbed areas. 



The Pendleton earth-shake bears, indeed, a closer resemblance 

 to the earthquakes which are characteristic of volcanic regions. For 

 the sake of comparison, I reproduce in Fig. 2 the map of the 

 isoseismal lines of the Etnean earthquake of August 8, 1894,^ on 

 half the scale of the map of the Pendleton shake. The curve 

 marked A bounds the ruinous zone, in which buildings were 

 destroyed and several persons killed. The curve B bounds the 

 "very strong" zone, in which slight damage to buildings occurred; 

 the curve C the " strong " zone, in which the shock was strong 

 enough to make lamps, etc., swing; while, in the "slight" zone 

 indicated by the curve D, the shock was just strong enough to be 

 sensible. 



In both cases it is clear, from the rapid decline in intensity, that 

 the focus was situated at a very small depth. Secondly, in the 

 Pendleton earth-shake, it follows, from the nature of the sound, that 

 the focus was of larger dimensions than in the weak earth-shakes, 



' M. Baratta, " Intorno ai recenti fenomeui endogeni avvenuti nella regione 

 Etnea " : Boll, della Soc. Geogr. Ital., Oct. 1894. 



