Stress release along a fault
Tectonic plates move slowly, grinding against each other. Along their edges, friction prevents smooth motion, so stress accumulates. When stress exceeds the strength of the rock, the fault suddenly snaps. The rocks shift, releasing energy as seismic waves that radiate outward in all directions.
The rupture happens in seconds. The fault move releases accumulated stress from years of plate motion. Stronger rocks can hold more stress before breaking, which is why larger ruptures occur on faults with less frequent earthquakes.
P-waves and S-waves arrive in sequence
Primary waves (P-waves) are the fastest and arrive first, traveling through rock and water. Secondary waves (S-waves) are slower and arrive after, traveling only through solid rock. The time gap between the two tells seismologists how far away the epicenter is. Far-field observers experience a gentle jolt from P-waves, then harder shaking from S-waves.