Wave Velocities below the Mohorovičić Discontinuity

Abstract

The velocities of longitudinal waves immediately below the Mohorovičić discontinuity which have been found by various authors for continental regions decrease on the average from about 8.2 km/s, if the Mohorovičić discontinuity is at a depth of 30 km, to about 8.0 km/s, if it is at a depth of 50 km. The rate of this decrease in velocity of 0.011 ± 0.0016 km/s per km is statistically significant and exceeds appreciably the critical rate of about 0.0013 km/s per km, required for a low-velocity channel. The corresponding calculated average decrease in the velocity of transverse waves from about 4.64 km/s, if the Mohorovičić discontinuity is at a depth of 30 km, to about 4.56 km/s, if it is at 50 km (at a rate of 0.004 ± 0.0056 km/s per km) can be expected to represent average conditions under continents only if this decrease in velocity is considered in connection with that for longitudinal waves. For a depth of about 60 km, both calculated velocity-depth curves connect smoothly with those found by the author from the apparent velocities at the points of inflection of travel-time curves for earthquakes originating at various depths. The new results confirm the hypothesis that under continents the asthenosphere low-velocity channel starts at the Mohorovičić discontinuity.