Analysis of Analog Airgun/Sonobuoy Records: A New Appraisal of the Variation ofUpper Crustal Seismic Velocity with Age

摘要

Analog sonobuoy records shot over 0 - 10 m.y. old crust along the East PacificRise have been digitally scanned, scaled, and analyzed for upper crustal velocity structure. Previous analyses of the original analog records E.G., Houtz and Ewing, 1976 suggested that the average velocity of layer 2A at the EPR is 3.4 km/s and increases with age to about 4.5 km/s in 40-m.y.-old crust. Later work Diebold and Carlson, 1993 suggests that arrivals interpreted by Houtz and Ewing 1976 as representing layer 2A headwaves are actually diffractions and/or converted shear waves. By digitally processing and analyzing many of the original analog records shot near the EPR, we have shown that (1) true layer 2A headwave arrivals are not present in these records, (2) the layer 2A retrograde branch reported in ESPs and other surveys cannot be resolved, and (3) layer 2B/3 velocities and average velocity gradients directly determined from the data range from 4.4 to 7.6 km/s and 1.1 to 2.2 /s, respectively. Because direct estimates of 'layer 2A' velocities are not possible, the linear gradient approximation for the hidden layer Ewing and Purdy, 1982 was applied; estimated velocities at the top of the basement section range from 2.0 to 3.1 km/s and velocity gradients are in the range 1.9 to 3.4/s. These results show considerable scatter, and no relationship between velocities in the uppermost part of the oceanic crust and age of the crust can be resolved.