Seismic Constraints on Ocean‐Ridge Mantle Structure: Anomalous Fault‐Plane Solutions from First Motions

Summary Fault‐plane solutions from P ‐wave first motion studies of earthquakes on the crests of actively spreading mid‐ocean ridges often appear to have non‐orthogonal nodal planes. This anomaly, together with independent data from surface waves and shear waves from such events, is most simply explained as an effect of propagation of the P waves through the laterally heterogeneous mantle beneath the ridge. P ‐wave velocity models for the ocean‐ridge mantle are derived from temperature models, the phase diagram for wet peridotite, and the expected dependence of velocity on temperature and phase. Models that successfully account for the nodal plane non‐orthogonality are characterized by pronounced lateral temperature gradients and extensive partial melting at temperatures in excess of the anhydrous peridotite solidus, in agreement with models for the petrogenesis of tholeiitic basalts and with evidence for a region of very low Q beneath ridge crests. The depth to the top of the partially molten region is not well constrained, but the half‐width of the zone of extensive melting in the north Atlantic must be several tens of kilometres. A prediction of these calculations is a variation of the apparent non‐orthogonality of nodal planes with spreading rate for ridge‐crest earthquakes; the limited data are consistent with this variation. The most promising tests of proposed velocity models include long seismic refraction profiles and studies of S ‐wave polarization. Body‐wave travel‐time residuals observed teleseismically from ocean‐ridge earthquakes are predicted to be nearly independent of distance and azimuth and thus measurable only for events with independently determined origin times.