The science behind earthquakes and what makes them so dangerous | Just The FAQs

Every year, there are about 500,000 earthquakes – 100,000 of which can be felt on the Earth's surface. But how do earthquakes work? We explain.
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Following a series of earthquakes, the situation in Puerto Rico remains dire, with more than 2,000 people in shelters, nearly 1 million without power and hundreds of thousands without water as of Thursday.

According to the U.S. Geological Survey, there have been more than 950 earthquakes and aftershocks recorded on Puerto Rico since Dec. 31, though many were weak and could not be felt. More than 500, however, have been of magnitude 2 or higher.

The worst quake – a magnitude 6.4 that struck early Tuesday – killed at least one person, injured at least nine and caused the power outage.

So why have there been so many earthquakes in Puerto Rico over the past few days?

"Puerto Rico has been hit by what scientists call an earthquake swarm, which is a series of earthquakes rather than the usual pattern of one dominant earthquake followed by aftershocks," John Vidale, a seismologist at the University of Southern California, told USA TODAY.

"We don’t well understand why the larger earthquakes continue for a while," he said. "It may have to do with slow slip on faults in the area or perhaps the hydrology of the fault system."
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