Abstract

F-k analyses of short-period recordings of both explosion and earthquake sources at the high-frequency NORESS array indicate secondary arrivals from a near-receiver source about 25 to 30 km southwest of the array. Use of residual seismograms, derived by subtracting the beamed record from each array channel, improved the identification of the same scatterer. F-k power difference plots, obtained by subtracting (after normalization) the f-k power versus slowness estimates for the initial P window from those for the latter windows, provided nearly identical results. The secondary seismic source appears to be short-period surface waves, Rg, generated by the scattering of incident P waves in the region of Lake Mjosa, 27 km southwest of NORESS, where there is about 1 km of relief from the bottom of the lake to the top of an adjacent hill. Polarization analysis of an explosion recorded on three-component elements of NORESS also suggested a secondary phase with polarization characteristic of Rg and arrival time and azimuth consistent with those derived from the f-k analysis. Near-source scattering is investigated by f-k analysis of data from reciprocal arrays formed by interchanging the roles of source and receiver. Application to closely spaced Yucca Flat (NTS) shots recorded at several stations provides evidence for RgP scattering near the northwestern and other basin boundaries of the Yucca Valley. It seems that deterministic locations of near-receiver PRg and near-source RgP scattering can be obtained by f-k techniques.

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