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Earthquakes result from the sudden release of elastic strain energy accumulated along faults when shear stress exceeds frictional strength, a process described by the elastic rebound theory. The hypocenter (focus) is the point of rupture initiation at depth; the epicenter is its surface projection. Earthquake magnitude—measured on the moment magnitude scale (Mw)—is logarithmic: each unit increase corresponds to roughly 32 times more energy released. Shallow (< 70 km) earthquakes occur at all plate boundaries; intermediate (70–300 km) and deep (300–700 km) earthquakes occur only in subducting slabs where the cold, brittle lithosphere has not yet equilibrated to ambient mantle temperatures. The global seismograph network constrains fault geometry, stress drop, and rupture propagation within minutes of a major event.
Plotting the depth distribution of earthquakes along a cross-section perpendicular to a subduction zone reveals the Wadati-Benioff zone—the inclined slab of seismicity—which makes the geometry of subduction concrete. Comparing seismograms from near and far stations to understand how wave travel time changes with distance introduces the inverse problem central to seismology.
You already understand that Earth's lithosphere is divided into tectonic plates that move relative to one another at boundaries — divergent, convergent, and transform. Where plates interact, friction locks their edges together even as the plates continue to move, building up elastic strain energy in the rock over decades to centuries. Think of slowly bending a wooden stick: it stores energy as it flexes until it snaps. This is the elastic rebound theory proposed by H.F. Reid after the 1906 San Francisco earthquake: rocks on either side of a locked fault gradually deform, and when accumulated stress exceeds the frictional strength of the fault, they snap back to their undeformed shape, releasing energy as seismic waves.
The point underground where the rock first breaks is the hypocenter (or focus); the point on the surface directly above it is the epicenter. The distinction matters because earthquake depth dramatically affects the damage pattern. A shallow earthquake at 10 km depth concentrates its energy near the surface, producing intense shaking in a small area. A deep earthquake at 600 km depth spreads its energy over a much larger volume, producing weaker shaking at any single point. Earthquake size is quantified by the moment magnitude scale (Mw), which measures the total energy released based on the area of the fault that ruptured, the amount of slip, and the rigidity of the rock. The scale is logarithmic: a magnitude 7 earthquake releases about 32 times more energy than a magnitude 6, and about 1,000 times more than a magnitude 5. This is why the jump from a moderate earthquake to a great earthquake is not incremental — it is explosive.
The global distribution of earthquakes is not random; it traces plate boundaries with remarkable precision. Shallow earthquakes occur at all three boundary types: at divergent boundaries where plates rift apart, at transform boundaries where plates grind past each other, and at convergent boundaries where plates collide. But intermediate and deep earthquakes occur only at subduction zones, where cold, dense oceanic lithosphere plunges into the mantle. The subducting slab remains brittle enough to fracture down to about 700 km depth, producing an inclined plane of seismicity called the Wadati-Benioff zone. Below that depth, the slab has heated enough to deform plastically rather than fracturing, and earthquakes cease. Mapping these earthquake depths in cross-section reveals the geometry of the subducting plate — its angle, its extent, and where it may be tearing or bending — making seismology one of the most powerful tools for imaging Earth's interior structure.