Radial modes from the great 1994 Bolivian earthquake: No evidence for an isotropic component to the source

We investigate a possible isotropic component in the source of the Bolivian earthquake of 09 June 1994 through a study of the radial modes _pS₀ (p = 0, 1) excited by the event. Our study departs from previous similar attempts by making use of a time series of sufficient duration (over 116 days) to sample the shape of the spectral line of the fundamental radial mode ₀S₀, and by combining measurements of two radial modes, thereby lifting the trade-off between deviatoric and isotropic sources of excitation. Our result (an isotropic component of 0.5±1.3% of the main double-couple) fails to identify a significant change in volume in the source of this, the largest deep earthquake ever recorded.