A train thrown down by the earthquake that struck San Francisco on April 18, 1906 (via Wikimedia Commons).
Steve Connor writes in the Independent:
Some of the most violent earthquakes that have occurred unexpectedly in places with no recent record of tremors may be the aftershocks of previous earthquakes that took place decades or even centuries ago, scientists have discovered.
Earthquakes usually occur at the boundary of two or more tectonic plates — the massive chunks of the earth’s crust that grind slowly against one another. However, they can also occur many hundreds of miles from a fault line and it is these earthquakes that scientists believe may be the result of long aftershocks rather than background seismic activity.
A laboratory study that tested how tectonic faults work has found that the further away an earthquake is from such fault line, the stronger the likelihood that it could be the long aftershock of a previous earthquake that has taken many years to make itself felt in terms of a second series of violent ground movements.
The finding could explain many unexpected earthquakes in the centre of continental shelves, such as the disastrous quake in Sichuan in the heart of China in May 2008 which killed at least 68,000 people and injured up to 400,000 more. At 7.9 on the earthquake scale it was one of the most deadly quakes in history.
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