Plate Tectonics and Earthquakes
Earthquakes happen when built-up stress in Earth's crust is suddenly released along faults. Since tectonic plates are always moving, stress constantly accumulates at their boundaries. Understanding where and why that stress builds tells you a lot about where earthquakes strike and how powerful they can be.
Plate tectonics and earthquakes
Most earthquakes occur at plate boundaries, where the interaction between moving plates creates stress in the surrounding rock.
- Convergent boundaries are where plates collide or one subducts beneath another. The dominant stress here is compression, which squeezes rocks together. Subduction zones produce some of the largest earthquakes on Earth.
- Divergent boundaries are where plates pull apart, like at mid-ocean ridges. The dominant stress is tension, which stretches and thins the crust.
- Transform boundaries are where plates slide horizontally past each other. The dominant stress is shear, which pushes rock in opposite directions on either side of the fault. The San Andreas Fault in California is a classic example.
At all three boundary types, the process follows the same basic pattern: stress accumulates in rock over time, the rock deforms and stores elastic energy, and when stress finally exceeds the rock's strength, the fault ruptures. That sudden rupture releases energy as seismic waves, which cause the ground shaking you feel during an earthquake.
Types of geological faults
Faults are classified by how the rock on either side moves, which directly reflects the type of stress acting on them. Two terms you need to know: the hanging wall is the block of rock above the fault plane, and the footwall is the block below it.
- Normal faults form under tensional stress (pulling apart). The hanging wall drops down relative to the footwall. You'll find these at divergent boundaries and rift zones, like the East African Rift.
- Reverse faults form under compressional stress (squeezing together). The hanging wall moves up relative to the footwall. These are common at convergent boundaries, such as the zone that built the Himalayan Mountains. A thrust fault is a special case: a reverse fault with a low angle (less than 45°), which allows large sheets of rock to be pushed long horizontal distances.
- Strike-slip faults form under shear stress. The blocks move horizontally past each other with little vertical displacement. Movement is described as right-lateral (dextral) if the far side moves to the right, or left-lateral (sinistral) if it moves to the left. The San Andreas Fault is a right-lateral strike-slip fault.
Elastic rebound theory
Elastic rebound theory is the foundational explanation for how and why earthquakes happen. It was developed after the 1906 San Francisco earthquake, when scientists noticed that the ground on either side of the fault had snapped back to a less deformed shape.
The earthquake cycle works like this:
- Tectonic plates move, and stress gradually accumulates in rocks along a locked fault.
- The rocks deform elastically, storing potential energy (think of slowly bending a stick).
- When stress exceeds the rock's frictional strength, the fault ruptures suddenly.
- The stored elastic energy is released as seismic waves, producing an earthquake.
- The rock snaps back toward its original, undeformed shape.
This process is sometimes called stick-slip motion. During the "stick" phase, friction locks the fault in place while stress builds. During the "slip" phase, stress overcomes friction and the fault moves suddenly. Then the cycle starts over.
Factors in earthquake occurrence
Several factors control where earthquakes happen, how often, and how large they can be.
Tectonic setting
- The type of plate boundary (convergent, divergent, or transform) and the rate of plate motion both influence earthquake frequency and magnitude.
- Most earthquakes occur at plate boundaries, but intraplate earthquakes can happen far from any boundary. The New Madrid Seismic Zone in the central United States is a well-known example. These are less frequent but can still be very powerful.
Fault characteristics
- Longer and deeper faults can produce larger earthquakes because more surface area ruptures at once.
- Faults with faster slip rates tend to have shorter intervals between earthquakes.
- Asperities, which are rough or locked patches along a fault surface, control how stress builds and where rupture begins.
Stress history
- Regions where a long time has passed since the last major earthquake may have accumulated significant stress, raising seismic hazard.
- Faster plate motion and stronger fault locking lead to more rapid stress buildup.
Crustal properties
- Rock type (lithology) and how rocks deform (rheology) affect whether stress is released in sudden earthquakes or through slow, gradual creep.
- Higher heat flow and the presence of fluids in the crust can weaken rocks and make faults more prone to slip.
Induced seismicity (human-caused earthquakes)
- Human activities can trigger earthquakes by altering stress conditions underground. Fluid injection from wastewater disposal or hydraulic fracturing can increase pore pressure along faults, reducing friction and promoting slip. Reservoir impoundment behind large dams and large-scale mining operations can also change local stress fields enough to trigger seismic events.