Earthquakes have inspired fear and awe since beings on Earth possessed the capacity to feel these emotions. Earlier mystical explanations for earthquakes have been replaced in our rational era by natural mechanistic explanations informed by observational science.
Stresses in the rock that make the Earth are usually in a state of equilibrium--all the forces acting on a chunk of rock are balanced by opposing forces. The overall stress levels can increase slowly as the rock deforms slowly, but nothing dramatic happens, nothing accelerates. Occasionally that equilibrium is disturbed--usually because the strength of a weak spot in the rock (a "fault") is exceeded, or because someone sets off a big explosion underground.
During the temporary loss of equilibrium, the rock on either side of the fault, or around the explosion-generated cavity, accelerates until it relieves the pent-up elastic stresses, then decelerates and stops moving and equilibrium is re-established. The acceleration and deceleration of the rock, and perhaps the grinding of the rock along the fault -- all driven by the collapse of elastic strain -- generates seismic waves that radiate outward, sharing a fraction of the energy released by the rupture with the rest of the world.
The rest is just the working out of details. But oh, what fascinating details!
In this area of our web site you can explore:
• how plate tectonics works
• learn about earthquake waves
• how earthquakes are located
• how earthquakes and their impacts are measured (Magnitude and Intensity)
• and find other excellent resources to learn about and prepare for earthquakes
In other areas of the web site we discuss Pacific Northwest earthquake hazards, the different types of earthquakes that occur here, and how you can reduce your risk of losses from future earthquakes.