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Two-dimensional array measurements of near-source ground accelerations

SEISMOGRAPHIC STATION DEPARTMENT OF GEOLOGY AND GEOPHYSICS UNIVERSITY OF CALIFORNIA, BERKELEY, CALIFORNIA 94720

Abstract

Two-dimensional array measurements of near-source accelerations have been conducted 6 km from the 5.6 M_L explosion Colwick at Pahute Mesa, Nevada Test Site. The three-component, nine-station array measured 400 m across with interstation spacing of 100 m. Wavenumber spectra and beam forming (stacking) were used to measure apparent velocities and directions of propagation of acceleration waveforms. The robustness of individual station accelerograms was measured with frequency-domain coherency estimates and cross-correlation functions. The dependence of interstation coherency upon station spacing and frequency band was also examined.

Wave propagation in Pahute Mesa can be characterized by P-SV and SH wave motion in a horizontally homogeneous medium for frequencies below 5 Hz. Strong incoherent scattered signals are present above 5 Hz on all components. The fraction of scattered energy can be estimated and treated as signal-generated noise. A strong SH pulse was observed to propagate across the array. Measured phase velocity, arrival time, and direction of propagation require cogeneration of SH with P-SV energy by the explosion Colwick. Weaker and more diffuse transverse acceleration signals arrive coincident with the P wave and during the time prior to S. These arrivals are interpreted as forward-scattered energy from the P wave. The small array permitted a refinement of the horizontally averaged S-wave velocity model for Pahute Mesa. The Mesa is characterized by a low velocity layer, 600 m thick, above the water table with a steep velocity gradient immediately below. The coherency measurements indicate the upper 1 km or more of Pahute Mesa is heterogeneous at scale lengths less than 1 km.

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