Crustal structure from P- and S-wave analyses: ocean bottom seismometer results in the north-east Pacific

An 80-km seismic refraction line was recorded on an array of three ocean bottom seismometers located north-west of the northern tip of Explorer ridge and parallel to Revere-Dellwood fracture zone in the northeast Pacific Ocean. The explosion data were supplemented at shorter distances by seismograms obtained using a 16-litre airgun as source. The combined use of rotated SV component and polarization filtered record sections enabled identification and timing of the refracted S-wave on most sections. The travel time—distance relations for both P- and S-waves are interpreted in the intercept time (tau) and ray parameter domain using tau inversion to give extremal bounds for velocity—depth curves. A linearized inversion technique is also applied to give the smoothest velocity—depth profiles consistent with the travel time data. Amplitude analysis using disc ray theory synthetic seismograms further refine the P-wave velocity—depth models. The final P- and S-wave velocity—depth profiles show a general increase of velocity with depth and no distinct structural discontinuities, not even at the Mohorovičić discontinuity. From the velocity profiles, values of Poisson's ratio as a function of depth are determined. In the upper crust, these vary considerably among the three data sets, but in the lower crust they are more consistent, ranging from 0.26 to 0.31 with an average value of about 0.29. Across the M-discontinuity, the values, which are not well constrained, decrease to between 0.25 and 0.26. Better resolution of the velocity—depth profiles is required to determine Poisson's ratio sufficiently well for a meaningful geological interpretation to be made. We find a normal oceanic crustal thickness of approximately 6.5 km and an anomalously low Pn velocity of 7.4 km s−1. The existence of an abnormally thick crust (8—10 km) on the opposite side of the ridge in the Explorer plate, determined in other studies, contrasts markedly with the results of this research. Such a contrast lends support to the proposals that spreading along Explorer ridge is asymmetric and that the complex structure and thick crust are the result of compressive interaction between the young, small Explorer plate and the older, larger North America plate.