Further Note on the Depth of Xarthquake Foci.

Abstract

SUMMARY 1. Galitzin's direct observations of the “angle of emergence” at Pulkovo led him to specify three new “critical surfaces” at the moderate depths 106 km., 232 km., and 492 km. below the surface, in addition to the surface much lower already indicated by Wiechert. 2. Results obtained for the relative depth of earthquake foci suggest that they also collect about three chief values, without, however, giving any indication as to the absolute depth of any. 3. If we identify the three corresponding depths of focus with those of Galitzin's three surfaces and revise the calculated velocities near the surface accordingly, we (a) can get a value for the surface velocity of P agreeing with that observed at the Oppau explosion; (b) can get the value 207 km. for the depth of the average focus corresponding to the adopted tables. The depths of the three principal seats of earthquakes are of course those given above from Galitzin; (c) The destructive earthquakes are probably those originating in the uppermost layer, at a depth of 106 km. = 66 miles.