It has been observed that long-period seismographs respond to near-earthquakes in an unexpected manner. In order to explain this phenomenon true ground motion has been computed by multiple integration of the seismograph equation using an electric analog computer. It can be shown that after transit of the S-phase a permanent ground displacement (order of magnitude 10 μ) and true tilt (order of magnitude 10−2″) occur. Synthetic seismograms support these results.
These phenomena could be caused only by the “near-field” (amplitude decreasing according to r2) and not by the “wave field”.
Vertical seismometers are less suited to such studies. The helical spring of these instruments can be excited to almost undamped transverse oscillations which cause a second order disturbance.