98 © CHARLES DAVISON 
so long as the volcano maintains its activity without notable 
change. Tectonic earthquakes differ in the following respects from 
volcanic earthquakes: (1) for the same intensity at the epicenter, 
they are felt over a much wider area; (2) though they sometimes 
occur in the immediate neighborhood of active volcanoes, they 
are as a rule quite independent of volcanic eruptions; and (3) 
great tectonic earthquakes seldom revisit the same district 
except at wide intervals of time. It will be seen later that these 
are not the only important differences between volcanic and 
tectonic earthquakes. For the present, however, it may be inferred 
that volcanic earthquakes as a rule originate in shallow foci which 
are confined to a limited region, while tectonic earthquakes spread 
from deeply seated foci that are subject to continual change. 
Professor Omori notices that, under his definition, tectonic 
earthquakes may occasionally be included—earthquakes which 
disturb large areas’ and are recorded by seismographs at very 
distant stations. For instance, about thirty hours before the | 
eruption of the Usu-san (north Japan) on July 25, 1910, an earth- 
quake occurred that was felt to a distance of 87 miles from the 
volcano and was recorded at a distance of 475 miles.2 The begin- 
ning of the great eruption of the Sakura-jima (south Japan) on 
January 12, 1914, was followed after a few hours by an earth- 
quake that damaged houses in Kagoshima and the surrounding 
country and was recorded in European observatories and probably 
in all parts of the world. Moreover, one month later, on February 
13, another strong earthquake took place during an eruption of 
the Iwo-jima, a volcano belonging to the same chain as the Sakura- 
jima. To these examples may be added the destructive Hawaiian 
earthquake of April 2, 1868, which originated in or near the southern 
part of Hawaii at nearly the same time as great eruptions of 
The occurrence of such earthquakes has long been recognized. For instance, 
G. P. Scrope, in his Considerations on Volcanos (1825), states that ‘those shocks .. . . 
which are felt to a considerable distance, are probably caused by new rents produced 
in the solid subjacent strata supporting or surrounding the mountain, and enter into 
that class of earthquakes which were discussed in a former chapter,” that is, of tectonic 
earthquakes (p. 155). 
2F. Omori, Bull. Imp. Earthquake Inv. Com., Vol. V (1911), p. 15; Vol. VI 
(1912), p. 9. 
3 Ibid., Vol. VIII (1914), p. 23; see also Nature, Vol. XCII (1914), pp. 716-17. 
