What causes earthquakes?
The short answer is that earthquakes are caused by faulting, a sudden
lateral or vertical movement of rock along a rupture (break) surface.
Here's the longer answer: The surface of the Earth is in continuous slow
motion. This is plate tectonics--the motion of immense rigid plates at the
surface of the Earth in response to flow of rock within the Earth. The
plates cover the entire surface of the globe. Since they are all moving
they rub against each other in some places (like the San Andreas Fault in
California), sink beneath each other in others (like the Peru-Chile Trench
along the western border of South America), or spread apart from each other
(like the Mid-Atlantic Ridge). At such places the motion isn't smooth--the
plates are stuck together at the edges but the rest of each plate is
continuing to move, so the rocks along the edges are distorted (what we
call "strain"). As the motion continues, the strain builds up to the point
where the rock cannot withstand any more bending. With a lurch, the rock
breaks and the two sides move. An earthquake is the shaking that radiates
out from the breaking rock.
People have known about earthquakes for thousands of years, of course, but
they didn't know what caused them. In particular, people believed that the
breaks in the Earth's surface--faults--which appear after earthquakes, were
caused *by* the earthquakes rather than the cause *of* them. It was Bunjiro
Koto, a geologist in Japan studying a 60-mile long fault whose two sides
shifted about 15 feet in the great Japanese earthquake of 1871, who first
suggested that earthquakes were caused by faults. Henry Reid, studying the
great San Francisco earthquake of 1906, took the idea further. He said that
an earthquake is the huge amount of energy released when accumulated strain
causes a fault to rupture. He explained that rock twisted further and
further out of shape by continuing forces over the centuries eventually
yields in a wrenching snap as the two sides of the fault slip to a new
position to relieve the strain. This is the idea of "elastic rebound" which
is now central to all studies of fault rupture.
Dr. Gerard Fryer
Hawaii Institute of Geophysics & Planetology
University of Hawaii, Honolulu, HI 96822