How do plate tectonics cause earthquakes?

Plate motions driven by the Earth’s internal heat cause earthquakes when stress builds and is suddenly released along faults in the rigid outer shell. W. Jason Morgan of Princeton University helped formalize the plate tectonics framework that explains plates as rigid slabs moving over the ductile asthenosphere, driven by forces such as slab pull, ridge push, and mantle convection. Where plates interact, the relative motion concentrates strain in the crust and upper mantle. That strain accumulates until rocks fail, producing seismic waves that radiate away from the rupture.

Mechanics of plate boundaries

Different kinds of plate boundaries generate earthquakes by different mechanical processes. At convergent boundaries, one plate sinks beneath another in subduction zones and locks against the overriding plate. Over decades to centuries, elastic strain accumulates on the locked interface. Harry Fielding Reid of Johns Hopkins University described the elastic rebound mechanism following the 1906 San Francisco earthquake: when the locked fault slips, stored elastic energy is released and the crust snaps back to a new configuration. This process underlies the very large megathrust earthquakes that can exceed magnitude 8 and generate tsunamis. At divergent boundaries, tensional stresses pull plates apart, creating normal faults and shallower seismicity along spreading centers. Transform boundaries like the San Andreas Fault accommodate horizontal slip and produce frequent shallow, often damaging earthquakes when segments stick and then rupture. Seismological institutions such as the United States Geological Survey and the Incorporated Research Institutions for Seismology document these relationships and use seismic data to map fault geometry and past rupture behavior.

Seismic waves, propagation, and local amplification

When a fault ruptures, it radiates body waves and surface waves that travel through the Earth. The energy and frequency content of those waves depend on rupture length, depth, and slip distribution. Local geological conditions such as soft sedimentary basins can amplify seismic shaking, increasing damage at distances far from the source. Seismologists use recorded waveforms to infer rupture processes and to improve hazard models that guide building codes and emergency planning, an area of work led by practitioners including Lucy Jones of the United States Geological Survey.

Consequences for societies, cultures, and environments

Earthquakes produce immediate hazards including ground shaking, surface rupture, landslides, liquefaction, and tsunamis, which can devastate coastal communities. Beyond physical destruction, seismic events interact with cultural and territorial factors: building practices, population density, and historical land use determine casualty and loss patterns. In many Pacific Rim societies, long cultural memory of tsunamis has influenced settlement patterns and evacuation customs, while in urban areas modern engineering and zoning can reduce risk. Ecologically, large earthquakes can alter drainage, trigger slope failures that reshape habitats, and influence volcanic systems by changing stress fields.

Understanding plate tectonics and the mechanics of faulting is essential for realistic seismic hazard assessment and risk reduction. By integrating geological mapping, historical records, and modern seismology, scientists inform land-use policy, engineering standards, and public preparedness to mitigate the human and environmental consequences of earthquakes.