Shaun, New Hampshire asked:
I was on a business trip recently and experienced the earthquake, the 5.8 in Washington DC. My question is why do quakes occur that are nowhere near a tectonic plate boundary?
Dave - Yes, Washington DC is nowhere near a plate boundary. There's a variety of things and basically, anything which can apply large forces to the plates can cause earthquakes. You can get earthquakes a long way away from plate boundaries, actually still associated with them. So, in India, where you're creating the Himalayas and you’ve got a huge amount of compression, you can still get earthquakes hundreds of miles away from that plate boundary just because the compression and the crushing there is so immense. You can also get earthquakes when people build big reservoirs, so you put a load of water on the ground. That essentially loads it so the comfortable place where the ground wants to be is a bit lower, so the only way it can move there if it’s not squidgy like clay is by cracking, and so you get small earthquakes like that. You can get earthquakes as ice melts - we’re still getting earthquakes in Norway and actually, most of the earthquakes in Britain are probably associated with all the ice melting after the last ice age. In the north end of Britain, and in Norway, the ground is rising slowly actually a few centimetres a year, especially in Norway, and that eventually builds up some pressures and stresses in some places until it breaks. I think the one thing I vaguely heard about the Washington one is that it could also be sediment building up off the edge of the continent, building up on the ground that's pushing down and eventually, that could build up enough to create an earthquake.