Centroid Depths of Mid‐Ocean Ridge Earthquakes: Dependence on Spreading Rate

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Abstract
From inversions of teleseismic P and SH waveforms we have determined the source parameters of 50 large earthquakes that occurred during 1962–1983 on slowly spreading mid‐ocean ridges. All events are characterized by predominantly normal faulting on planes that dip at approximately 45° and strike parallel to the local trend of the ridge axis. Centroid depths range from 1 to 6 km beneath the seafloor. The P waves from these earthquakes show strong water column reverberations, suggesting that fault rupture extended to the seafloor. Under the assumption that the centroid depth marks the mean depth of fault slip, earthquake faulting extended to a depth of 2–10 km for these earthquakes. The water depths inferred from the predominant periods of the water column reverberations constrain the epicenters of these earthquakes to lie within the relatively deep inner floor of the median valley. The maximum centroid depths of ridge crest earthquakes decrease with increasing spreading rate, and the maximum seismic moment may also decrease with increasing spreading rate. These results indicate a general decrease with spreading rate in maximum thickness of the mechanically strong layer beneath the ridge axis region. The concentration of large earthquakes within the deepest parts of the median valley and the depth extent of seismic faulting support lithospheric necking models for the origin of the median valley.