Infrasound is very low frequency sound that people cannot hear. However, it can generate strange perceptions in people. I was just thinking that perhaps earthquakes cause infrasound...

http://www.timeenoughforlove.org/sav...Sensations.htm
Soundless Music Shown to Produce Weird Sensations
Sun Sep 7, 7:09 PM ET
By Patricia Reaney

MANCHESTER, England (Reuters) - Mysteriously snuffed out candles, weird sensations and shivers down the spine may not be due to the presence of ghosts in haunted houses but to very low frequency sound that is inaudible to humans.

British scientists have shown in a controlled experiment that the extreme bass sound known as infrasound produces a range of bizarre effects in people including anxiety, extreme sorrow and chills -- supporting popular suggestions of a link between infrasound and strange sensations.

"Normally you can't hear it," Dr Richard Lord, an acoustic scientist at the National Physical Laboratory in England who worked on the project, said Monday.

Lord and his colleagues, who produced infrasound with a seven meter (yard) pipe and tested its impact on 750 people at a concert, said infrasound is also generated by natural phenomena.

"Some scientists have suggested that this level of sound may be present at some allegedly haunted sites and so cause people to have odd sensations that they attribute to a ghost -- our findings support these ideas," said Professor Richard Wiseman, a psychologist at the University of Hertfordshire in southern England.

In the first controlled experiment of infrasound, Lord and Wiseman played four contemporary pieces of live music, including some laced with infrasound, at a London concert hall and asked the audience to describe their reactions to the music.

The audience did not know which pieces included infrasound but 22 percent reported more unusual experiences when it was present in the music.

Their unusual experiences included feeling uneasy or sorrowful, getting chills down the spine or nervous feelings of revulsion or fear.