S waves generated by small seismic sources

Abstract Observations of S waves from small earthquakes recorded at near distances indicate that the relative S wave generation extends over a wide range. Similar measurements from underground nuclear explosions indicate a ratio of S to P wave generation to be below the observed values for 70 % of the earthquakes studied. The observation of S waves from small events at teleseismic distances is made difficult by interference of microseisms in the period range where S waves may be expected. The observation is also made difficult by the physical requirement that two horizontal component seismographs are required to obtain good resolution of S wave motion. Some measurements have been made at the University of Michigan by means of three-component seismometers in arrays to obtain better resolution of S wave motion. The results of these measurements suggest that improvements in S wave signal/noise ratios similar to those obtained for P waves are possible. The use of S waves from small events at teleseismic distances should not be considered of value as a method of detection. It does, however, offer promise as an added method of identification for small seismic events.