Scatter density plot of raw propagation phase anomaly measurements for 100 s R1 waves common to two techniques. Scatter points are colored based on high (red) or low (blue) spatial density. Strong agreement is found; minor discrepancies are from discrepant theoretical approximations, reference Earth models & processing schemes.
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Abstract: Global variations in the propagation of fundamental-mode and overtone surface waves provide unique constraints on the low-frequency source properties and structure of the Earth’s upper mantle, transition zone and mid mantle. We construct a reference data set of multimode dispersion measurements by reconciling large and diverse catalogues of Love-wave (49.65 million) and Rayleigh-wave dispersion (177.66 million) from eight groups worldwide. The reference data set summarizes measurements of dispersion of fundamental-mode surface waves and up to six overtone branches from 44 871 earthquakes recorded on 12 222 globally distributed seismographic stations. Dispersion curves are specified at a set of reference periods between 25 and 250 s to determine propagation-phase anomalies with respect to a reference Earth model.
Our procedures for reconciling data sets include:
- controlling quality and salvaging missing metadata
- identifying discrepant measurements and reasons for discrepancies
- equalizing geographic coverage by constructing summary rays for travel-time observations and
- constructing phase velocity maps at various wavelengths with combination of data types to evaluate inter-dataset consistency.
We retrieved missing station and earthquake metadata in several legacy compilations and codified scalable formats to facilitate reproducibility, easy storage and fast input/output on high-performance-computing systems. Outliers can be attributed to cycle skipping, station polarity issues or overtone interference at specific epicentral distances. By assessing inter-dataset consistency across similar paths, we empirically quantified uncertainties in traveltime measurements. More than 95 per cent measurements of fundamental-mode dispersion are internally consistent, but agreement deteriorates for overtones especially branches 5 and 6. Systematic discrepancies between raw phase anomalies from various techniques can be attributed to discrepant theoretical approximations, reference Earth models and processing schemes. Phase-velocity variations yielded by the inversion of the summary data set are highly correlated (R ≥ 0.8) with those from the quality-controlled contributing data sets. Long-wavelength variations in fundamental-mode dispersion (50-100 s) are largely independent of the measurement technique with high correlations extending up to degree ~25. Agreement degrades with increasing branch number and period; highly correlated structure is found only up to degree ~10 at longer periods (T>150 s) and up to degree ~8 for overtones. Only 2ζ azimuthal variations in phase velocity of fundamental-mode Rayleigh waves were required by the reference data set; maps of 2ζ azimuthal variations are highly consistent between catalogues ( R=0.6-0.8). Reference data with uncertainties are useful for improving existing measurement techniques, validating models of interior structure, calculating teleseismic data corrections in local or multiscale investigations and developing a 3-D reference Earth model.