What Causes Earthquakes
In between the molten core and surface crust of the earth, there is a mantle of liquid magma carrying semisolid plates. These plates hold the continents and the oceans and they are constantly in motion. This is the root cause of seismic activity from continental drift to volcanoes and earthquakes. Some plates are drifting away from each other, causing magma to seep up between the cracks to form new crust material. Others are sliding past one another or directly collide, one plate slipping beneath another to dissolve in the mantle. These kinds of plate movement cause earthquakes.
Plate movement works in geologic time, meaning it takes a very long time
for any noticeable change. For thousands of years, two plates will
grind together, straining and building pressure, until at last something
gives way and a plate will dip further beneath another or they will
jerk past each other. Everything sitting on the plates likewise feels
the reaction. This can result in mild tremors if the epicenter is too
far down to shake the crust much, or it can be a catastrophe if the
shifting of the plates cause half a city to go a few meters in one
direction while the other half goes in another.
While the cause of earthquakes is well understood, they remain difficult
to predict. Deformations of rock in seismically active areas, called
fault zones, can indicate where earthquakes are most likely to occur,
but the safest course is to simply be ready for the earth to move.
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