Automatic Measurement and Quality Control of S3KS-S2KS Differential Traveltime and the Influence of Mantle Heterogeneity

Abstract:

The detailed velocity structure of the topmost outer core can be investigated seismologically based on the differential traveltimes between the S2KS and S3KS waves (S3KS-S2KS). We develop an automatic algorithm for the measurement and quality control of S3KS-S2KS data. Four criteria with empirically determined thresholds are used to quantify the quality of the measurements. Our automatic algorithm greatly reduces human inspection involved in the data quality control. Measurements obtained from records of events during 2000-2019 in the epicentral distance range of 120˚-140˚ display positive residuals of less than 0.5 s relative to PREM after finite-frequency correction, with no obvious temporal and spatial dependency. Based on the sensitivity kernels for S3KS-S2KS differential traveltimes, the mean value of our dataset with globally distributed path coverage is less likely to be affected by heterogeneities in the mantle. Further simulations using AxiSEM3D show that for ray paths travelling through the LLSVP, the corresponding mantle corrections tend to be positive for relatively large epicentral distances. Nevertheless, mantle effects are generally smaller in magnitude than finite-frequency corrections, and neither can fully explain the magnitude of the observed residuals. We thus conclude that the observed positive residuals in S3KS-S2KS mainly come from the velocity structure, e.g. a lower velocity relatively to PREM or anisotropy, in the topmost outer core.