Abstract
SUMMARY The velocity structure of the uppermost part of the outer core can be investigated seismologically based on the differential traveltimes between the SKKS and S3KS waves (S3KS-SKKS). We develop a procedure for the automatic measurement and quality assessment of the S3KS-SKKS differential traveltime measurements. Four criteria with empirically determined thresholds are used to quantify the quality of the measurements in our automatic algorithm, which greatly simplifies the data quality control. Measurements obtained in the epicentral distance range of 120–140° display positive residuals relative to PREM with no obvious spatial dependency. Waveform simulations in both PREM and S40RTS models show that neither finite-frequency effects nor large-scale mantle heterogeneities can fully explain the magnitude of the observed residuals. Mantle heterogeneities have a significant contribution to the data uncertainty, but do not have sufficient effect on the mean finite-frequency correction. We thus conclude that the observed residuals in the S3KS-SKKS differential traveltimes after finite-frequency correction are mainly caused by the velocity structure in the uppermost outer core.
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