geek analysis

Following the earthquake, I was dogged by a persistent dizziness for most of the afternoon, and I think I have found the answer here: http://www.borderlands.com/newstuff/research/infra.htm

An extract from the link above:
"Earthquake infrasound manifests only at intermittent intervals, producing drastic and sustained negative modifications of consciousness. The human organism continues to reel under intermittent infrasonic assault for numerous reasons. After less than a five minute exposure to low intensity infrasound of 10 cycles per second, dizziness will last for hours. Infrasound of 12 cycles per second produces severe and long lasting nausea after a brief low intensity exposure." (emphasis added)

Most interesting phenomenon, albeit highly uncomfortable.