U r right Ms kaur, regarding cause of earth quake. What i meant is that when huge man made heavy structures are built near active seismic fault line then due to the weight PRBABILITY of one tectonic plate going under another and as a result earth quake increases. I never said that quakes r only due to this and never said that there was no earth quake in older days when there was not much development. As far as i know oil / gas rigs do not cause earth quake----depth of oil/ gas bearing sub-surface structures r nothing compared to the thickness of tectonic plates we r talking about. However oil / gas / coal mining activities do cause caving in of earth surface due to the vaccuam created if water injection / sand filling is not done along with extraction of oil / coal.
