A SEARCH FOR SPLIT SKS WAVE-FORMS IN THE NORTH-ATLANTIC

To test a prediction of the anisotropic properties of the upper mantle beneath the central North Atlantic, we measure splitting of the waveforms of SKS phases recorded at WWSSN stations in the Azores (PDA), and on Bermuda (BEC). Because the analysis of individual, analogue records does not provide sufficient resolution, we have developed a new technique for representing the confidence limits that permits a straightforward estimation of the average splitting beneath a station from a group of events. If events from a variety of different azimuths are recorded, the average splitting parameters can be tightly constrained. We found five records for PDA and eight for BEC that have good signal-to-noise ratio and a prominent SKS phase. Only one of the phases at PDA shows evidence of splitting; the average anisotropy must be small if it exists at all. At BEC, the average anisotropy is significant with a fast direction between NW-SE and N-S and a time delay between the two split components of about 1 s. Both of these observations are significantly smaller than predicted by a model of the upper mantle anisotropy based on SS-S differential traveltimes. The absence of systematic anisotropy at PDA may be attributable to a complex flow pattern in the upper mantle at this site near the Eurasian-African plate boundary in the vicinity of an active hotspot. At BEC, the observed direction is consistent with the prediction and both the direction and the magnitude of the average anisotropy are indicative of a sublithospheric source. To check the reliability of these results from analogue records, we have performed a similar analysis for station KIP on Oahu where digital records are available for comparison. The results from digital and analogue records are similar. At KIP, the splitting is more variable than expected for uniform anisotropy, with the fast directions for individual records ranging from about N26-degrees-E to about N83-degrees-E and with variable time delays. Similar variability is observed at all three stations. The average anisotropy at KIP is well-constrained, however, with a time delay of about 0.6 s and fast direction N75-degrees-E. The fast direction coincides with the fossil sea-floor spreading direction beneath the Hawaiian islands, suggesting that the dominant source of anisotropy beneath KIP is lithospheric.