In 1811-1812 a series of large intra-plate earthquakes struck near New Madrid in the Mississippi valley, the largest were estimated to be between M7 and M8 (no instruments were around at the time so magnitude estimates are based on contemporary felt effect reports). The largest of these were probably bigger than the famous 1906 San Francisco earthquake. Since this time the New Madrid Seismic Zone has been quietening down but still has a much higher level of seismicity than most places in the USA (until 2009 the New Madrid seismic zone was the most seismically active area of the USA east of the Rockies, since then it has been overtaken by Oklahoma) Scientists estimate that such large earthquakes probably occur in this region every 500 years or so.
Most people in the UK would struggle to place Oklahoma on a map of the USA, however cultural references from cowboys to Steinbeck would ensure that most placed it somewhere in the middle, a long way from the west coast and California.
That causes a problem for geoscientists since we have spent the last 40 years or so trying to teach schoolboys like Ben that earthquakes are caused by plates moving, and that earthquakes occur wheer these plates meet.
Where are all the plates ?
In the USA there is a clear tectonic plate boundary running all the way down the west coast, where eth North American plate meets the Pacific plate. This explains earthquakes in California and volcanoes like Mount St-Helens. In fact Plate Tectonics is such a good theory that over 90% of the worlds can be explained with a relatively simple model of how the planet works.
Here is great video by IRIS about plate tectonics
However the world is a much more complicated place than any simple model can explain and we are left with a problem of trying to explain all the other earthquakes in the world. The UK, like Oklahoma is a thousand km from the nearest tectonic plate boundary, up until 2009 the UK and Oklahoma had very similar earthquake patterns, a few small earthquakes up to M3 each year and the occasional larger one. In the UK the largest one we know about is M6.1 in 1931 . Seismologists call these earthquakes intra-plate earthquakes (meaning they happen in the middle of plate rather than at the edge). The only explanation that we can give for these earthquakes is that the tectonic plates are not uniform rigid blocks but a complicated mix of older rocks all squashed together by past tectonic plate motions. This results in plates containing lots of old cracks and fractures some of which can be reactiviated by the present day stresses of eth plates moving.
