A “phase-equalization-and-sum” method was applied to long-period data from vertical-component instruments of the LASA array to obtain dispersion data for mantle Rayleigh waves. With this method, group velocities in the period ranges 50 to 300 sec and 50 to 160 sec were determined for mantle waves from moderatesized earthquakes (M≈ 6) in Mongolia and near Taiwan, respectively, by using recordings which are individually dominated by background noise. Advantages of this method are: (1) Signal-to-noise amplitude ratio is increased by 8 db for periods of 50 to 100 sec, 10 db in the 100- to 200-sec-period range, and 5 db at 500-sec-period; (2) fidelity is thought to be high because long-period surface waves show almost perfact correlation between sensors after phase equalization; and (3) deviations due to each station are expected to be reduced by 1 / √̄N times, where N is the number of stations used. In addition to the usefulness of the phase-equalization-and-sum method, the reliability of LASA long-period digital data is demonstrated.

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