Calculation of static deformation following the Bolivia Earthquake by summation of Earth's normal modes



The vertical ground displacement associated with the 9 June 1994 great Bolivia earthquake is calculated by summation of Earth's normal modes. Because of the great depth of the earthquake, the contributions to the surface displacement field from modes of low radial and angular orders predominate. Convergence is fast, and the truncated mode sum provides a good estimate of the total global displacement field. The vertical displacement is calculated to be on the order of 1 cm within a few hundred km of the epicenter, and 1 mm at distances as great as 1500 km. Normal mode seismograms are calculated for a location near the epicenter, and the static offset is seen to occur primarily between the P and S phases, consistent with the effect of a near-field term. The predicted seismograms are in qualitative agreement with the recently reported observations of seismically recorded static displacements on the BANJO broadband array [Jiao et al., this issue]. The static offset calculated in this study is smaller than the observations and predictions reported by Jiao et al., probably owing to uncertainties in the observations and in their approximate calculations, which are based on dislocation theory in an elastic halfspace.