Deep‐focus earthquakes and their implications

Abstract

No one any longer doubts that earthquakes occur at depths of several hundred kilometers. There is not the same unanimity in regard to the absolute depth of focus of these earthquakes, the relative merits of methods for measuring the depth, the character, and distribution of plutonic earthquakes; and especially is there disagreement as to their causes and mechanism and as to their geological implications.Several methods have been suggested for the measurement of depth of focus. Most of these are relative methods. Turner based his estimate of the relative depth of focus on the degree of earliness in the arrival time of longitudinal waves at the antipodes in the case of deep shocks as compared with normal earthquakes. Stechschulte took as his criterion the interval between the arrival of the direct longitudinal wave and that of the Walker reflection at the distance at which this interval reached its maximum. Scrase used Knott's calculations to determine the depth of focus from the departure of the travel‐time curves for deep earthquakes from those of normal earthquakes. Wadati has suggested a number of methods: First, the spacing of the isochrones or lines of simultaneous arrival; second, the time‐Intercept of the graph of the interval between the arrival of the longitudinal and that of the transverse waves against the absolute arrival time of the longitudinal waves; third, he used tables of calculated travel‐times from deep foci based on those of shallow earthquakes, each set of tables corresponding to a given depth of focus. Miyamoto determined the depth of focus by comparing the travel‐times of longitudinal and transverse waves reflected from the core of the Earth with those for normal earthquakes. Serlage uses the interval between the arrival of the direct longitudinal wave and that of the Walker reflection for direct calculation of the depth.