Tectonic plates, fault lines, and the science of seismic movement
Earthquakes are the result of energy release from within the Earth. The lithosphere (crust and upper mantle) consists of seven major tectonic plates and several smaller ones, floating on the asthenosphere below, moving at rates of a few centimeters per year. When plates collide, separate, or slide past each other, enormous elastic stress builds up at plate boundaries. Rocks deform under pressure until the stress exceeds their strength — they fracture suddenly, releasing stored elastic energy as seismic waves that radiate in all directions. Main earthquake triggering mechanisms include: • Subduction zone earthquakes: one plate dives beneath another, reaching depths up to 700 km • Strike-slip earthquakes: plates slide horizontally, like the San Andreas Fault • Normal/reverse fault earthquakes: crustal stretching or compression causes rupture • Volcanic earthquakes: triggered by magma movement • Induced earthquakes: reservoir impoundment, mining, fluid injection Globally, about 500,000 detectable earthquakes occur annually — roughly 100,000 are felt, and about 100 cause damage. Over 90% occur along the Pacific Ring of Fire.