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Earthquakes occur when accumulated stress along faults exceeds rock strength, causing sudden slip and energy release. Focal mechanisms determined from seismic radiation patterns reveal the orientation of faults and the nature of faulting (normal, thrust, or strike-slip). Stress inversion from earthquake solutions maps regional stress fields.
From your study of plate boundary processes, you know that plates move relative to one another and that deformation concentrates at their boundaries. But plates do not slide smoothly past each other — friction locks fault surfaces together, and tectonic forces steadily build up elastic strain energy in the surrounding rock, much like compressing a spring. When the accumulated shear stress on a fault exceeds the frictional strength holding the fault locked, the fault ruptures and the rock on either side snaps to a new position, releasing the stored energy as seismic waves. This is the elastic rebound theory, and it explains why earthquakes are sudden and violent rather than gradual: energy that accumulated over decades or centuries is released in seconds.
The point where rupture initiates is the hypocenter (or focus), and the point on the surface directly above it is the epicenter. But an earthquake is not a point event — the rupture propagates along the fault plane, and large earthquakes can involve fault slip over hundreds of kilometers. The pattern of energy radiated during this rupture encodes information about the fault's geometry and the forces that drove it. Seismologists extract this information using focal mechanism solutions (also called fault-plane solutions or "beach ball" diagrams). By analyzing the first motion of P-waves recorded at seismograph stations surrounding the earthquake — whether the first arrival is compressional (push) or dilatational (pull) — they divide the radiation pattern into quadrants that reveal two possible fault planes and the orientation of the principal stress axes.
A focal mechanism with one compressional and one dilatational quadrant on each side of the beach ball indicates strike-slip faulting (horizontal motion), characteristic of transform boundaries. If the compressional quadrants sit at the top and bottom with dilatational quadrants on the sides, the mechanism indicates normal faulting (extensional), typical of rift zones. The reverse pattern — compression on the sides — indicates thrust faulting (compressional), typical of subduction zones and collision belts. These patterns connect directly to the stress-strain concepts you studied earlier: the orientation of maximum and minimum principal stress axes determined from focal mechanisms tells you which direction the lithosphere is being squeezed or stretched.
When seismologists compile focal mechanisms from many earthquakes in a region, they can perform stress inversion — a mathematical procedure that finds the single stress tensor best explaining all the observed mechanisms. This reveals the regional stress field: convergent plate boundaries show maximum compression perpendicular to the boundary, divergent boundaries show extension perpendicular to the rift axis, and transform boundaries show shear parallel to the plate motion vector. Importantly, Coulomb stress transfer — which you may have encountered — explains why one earthquake can trigger others: slip on one fault changes the stress field on neighboring faults, bringing some closer to failure and moving others further from it. This cascading stress redistribution is why aftershock sequences follow predictable spatial patterns and why earthquake hazard is not simply a matter of waiting for stress to re-accumulate on the same fault.