Theoretical Seismograms for the Colombian Earthquake of 1970 July 31

Using Gilbert & Dziewonski's retrieved structural parameters, Earth model 1066A, their Qμ(r) model, and the moment rate tensor for the Colombian earthquake of 1970 July 31, we produce 75 theoretical seismograms in epicentral co-ordinates by superimposing all the normal modes (1105 modes) within a period range from 100·1 to 963·8 s. The computed seismograms are compared with the respective observed ones. Such a comparison is possible not only because all the modes in the frequency range are taken into account in the computation, but also because we have a realistic kinematic source mechanism at our disposal. A qualitative, though far from exact, reproduction of actual seismograms was successfully effected, indicating the moment rate tensor represents a reasonable source model for the Colombian earthquake. In particular, on the average we are able to reproduce the observed amplitudes to within 30 per cent. Among surface and body-wave phases identified, multiple S-waves can be identified up to 21 S at almost 8 h after the origin time on both the observed and computed co-latitudinal record sections.