(1) The effect a very attractive woman (or group of women) has on the 'girth' of male members in her (or their) vicinity. This Girthquake can be measured on the 'Dichter' scale- from one up to ten, and is a handy way of discussing the attractive potential of women. Can be accompanied by rattling of glass on table to indicate violence of initial tremor, or at higher ends of the 'Dichter' scale by 'Star Trek' style synchronized pitching from side-to-side whilst trying not to spill drink, as if floor is swaying.
(2) (noun) A way of describing a previous evening, or party- if said event led to the conquest of an improbably attractive woman, or if the ratio of such women to men was unusually high- said event/evening can be described as being a 'Girthquake'. In the event of such an evening leading to a secondary event (such as an afterparty) where a further woman (or cluster of extremely attractive women) who rate highly on the Dichter scale is discovered, then the term 'Aftercock' can be substituted.
MAN1: "G...g....g.......GIRTHQUAKE!" (has spotted woman who scores on Dichter scale- remember optional rattling of glass on table to indicate violence of initial tremor)
MAN2: (adopting scientific expression) "In my opinion at least a 7 on the Dichter scale. Be careful, there is a strong chance of an 'Aftercock'"
(stands, switches to Roger Moore expression and approaches woman)