Introduction
Reconciliation of seismic data from various techniques necessitates development of new formats. Measurements of surface wave dispersion need to be cross-validated against source and station metadata to reference the appropriate seismic waveform and facilitate reproducible research. However, the analysis of large amounts of data from diverse catalogues is computationally inefficient with conventional ASCII and other text-based formats. We have devised reference seismic data formats (RSDFs) in consultation with the community to efficiently process diverse data sets typically used in seismic tomography. While a general definition of a surface wave format is desirable, practical considerations such as diverse processing techniques, computational expertise and utilization of legacy code prevent such an outcome throughout the community. RSDFs format guidelines encourage easy tracking of critical metadata information as header information that will work within existing processing schemes. Headers of RSDF ASCII files store all notes and details relevant to the analysis such as the radial reference Earth model and the associated phase velocity (PVEL) used for calculating the reference phase. Other metadata fields correspond to the assumptions used in calculating the various columns in the data such as those contributing to the predicted phase anomaly (Moulik & Ekström 2016, Appendix A). The format presented here includes the fields necessary for data reconciliation; other (meta)data specific to a processing scheme can be preserved at the discretion of the analyst.
References
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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: (1) controlling quality and salvaging missing metadata; (2) identifying discrepant measurements and reasons for discrepancies; (3) equalizing geographic coverage by constructing summary rays for travel-time observations and (4) 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.