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I am doing a research on earthquakes but I cannot fully understand what causes earthquakes.

"Caused by a sudden releases of energy by the Earth's crust, causing seismic waves which causes the ground to shake."

"Energy release associated with rapid movement on active faults is the cause of most earthquakes."

"Earthquakes are usually caused when rock underground suddenly breaks along a fault. This sudden release of energy causes the seismic waves that make the ground shake."

I have been puzzled by these statements and I really can't seem to understand.

Do these mean that energy is released by the earth's crust then produces seismic waves, then the seismic waves make the ground shake?

Does it also mean that earthquakes often most on faults because the release of energy and seismic waves occurs near or on a fault?

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  • $\begingroup$ Hello, and welcome to Stack Exchange. This is really too broad of a question for our Q&A format. You'll have better luck just reading introductions to earth science and seismology. $\endgroup$
    – Daniel Griscom
    Commented Jul 23, 2016 at 12:50
  • $\begingroup$ Hi Daniel. Sorry, but out of being very unable to understand, I went here in the hope to finally know. $\endgroup$
    – user6354
    Commented Jul 24, 2016 at 2:50
  • $\begingroup$ Have you tried Googling "what causes earthquakes"? Lots of material pops up for me... $\endgroup$
    – Daniel Griscom
    Commented Jul 24, 2016 at 3:12
  • $\begingroup$ I have tried, but I really don't understand. I am confused on how the connection between faults and energy. Does the fault break before the release of energy? Does the release of energy make the fault break? Is it the breaking of the rock underground that causes the energy release? Is it the fault that breaks? I have all these questions in my mind. $\endgroup$
    – user6354
    Commented Jul 24, 2016 at 3:30
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    $\begingroup$ Possible duplicate of How does one measure what causes earthquakes? $\endgroup$
    – Tom Au
    Commented Jul 28, 2016 at 1:51

1 Answer 1

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A lot of stress is accumulated along the fault by the time it breaks. For example, on the San Andreas, the Pacific plate is moving north with respect to the North American plate, but the fault resists this motion. As a result, a ton of elastic energy builds up around the fault as the stress increases.

Eventually, the stress becomes so much that the fault slips. The fault slipping releases the stored elastic energy, some of which is radiated as seismic waves. (Most of the energy becomes heat.)

Seismic waves "carry" stress and motion. When a wave first arrives, the rock at that location experiences a change in stress. This causes the rock at that spot to move slightly. But then, the rock at that spot has moved with respect to nearby rock. This differential motion of the rock creates stress, which then affects nearby rock similarly. So, the wave's stresses propagates motion in the rock, and motion in the rock propagates stress.

A few types of waves exist (P waves, S waves, and surface waves, in descending order of wave propagation speed). But the general idea behind them all is the same.

The ground therefore shakes when the waves arrive.

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