f SH-wave generator consisting of a vertical force and a pit in its vicinity

Contrary to conventional idea, predominant SH-waves are sometimes radiated from vertical impacts on the ground surface. Systematic field experiments were carried out to investigate the mechanism. The experiments revealed that the SH-waves are excited by waves incident on the boundary of a lower rigidity portion in the near-surface ground close to the impact positions. Accordingly inhomogeneities in the ground are considered to be the main cause of such SH-wave radiation that we encounter incidentally in usual seismic works using vertical forces as their sources.