イギリスの地震学者のツイートから。
プレート境界などの明瞭な地質学的境界では、P波（縦波）が S波（横波）に、あるいは逆に S波が P波に変化することがあるのだそうです：
P-waves can convert to S-waves and vice-versa at sharp geological boundaries - they can be seen globally from deep quakes. They can also happen locally. Here is a clear s->p phase from a nearby tiny quake recorded on a seismometer sat atop a mid-ocean ridge plate boundary. Neat! pic.twitter.com/qiH1gGai95— Stephen Hicks 🇪🇺 (@seismo_steve) February 25, 2020
