The surface is always moving
Earth’s outer shell is broken into large tectonic plates that move very slowly. Their boundaries contain many faults: fractures where blocks of rock can move relative to each other. Some faults lie far from plate boundaries too.
Why movement can happen suddenly
Friction can lock parts of a fault even while surrounding rock continues to deform. Stress builds. When the force is sufficient to overcome the resistance, the fault slips and releases stored energy. That energy travels outward through the ground as seismic waves.
What causes the shaking
As seismic waves pass, the ground moves. The shaking people feel depends on factors including the size and depth of the earthquake, distance from the fault, and the local ground conditions. Not every earthquake can be felt, and an earthquake can be followed by aftershocks.
What scientists measure
Seismometers record ground motion. Scientists use those recordings to identify when and where an earthquake occurred and estimate its magnitude. Magnitude and experienced shaking are related, but they are not the same measure.
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