A Global Comparison of Mid-Ocean Ridge Processes from Seismic Anisotropy

Abstract:

As oceanic plates form it is thought that they will inherit an internal anisotropic fabric due to basal shear, developing a fast seismic axis parallel with the seafloor spreading direction. Studying this process in-situ however has proven difficult given the lack of seismic instrumentation over the oceans. In this study we instead utilise the worldwide distribution of seismicity along mid-ocean ridges (MOR), as well as the source-side shear wave splitting technique, to characterise seismic anisotropy and therefore upper mantle dynamics beneath MOR earthquakes. To achieve this we collate over 80 “null” seismic stations distributed globally that appear lie above effectively isotropic upper mantles. These then allow us to then isolate the anisotropic signature beneath the earthquake or source, instead of beneath the receiver, by using direct teleseismic S phases.

Globally on average we find that around 40% of measurements from ridge earthquakes produce splitting of the shear wave, with the other 60% recording a null result (i.e. lack of splitting), likely reflecting complex processes happening beneath MORs. Of the shear wave splitting seen, the magnitude is moderate (delay times average 1.2s), suggesting considerable seismic anisotropy beneath some MOR locales. The distribution of nulls versus splits is not homogeneous however, with localised pockets of coherent nulls or coherent splits with similar fast directions. The seismic properties appear to change abruptly over relatively short distances, such as across adjacent transform faults, or on different limbs of a triple junction, suggesting local ridge processes control the anisotropy rather than the larger scale mantle flow.