What actually happens underground during an earthquake?

Introduction

Earthquakes are sudden, sometimes violent shaking of the ground that many people feel, but what actually happens below the surface is a complex, physical process. This article explains in plain English the forces, structures, and sequences that produce seismic events underground, and how scientists detect and interpret them.

What is an earthquake at its simplest?

An earthquake is the release of energy stored in the Earth’s crust that radiates outward as seismic waves. That energy release usually happens when rocks that are slowly pushed or pulled past each other suddenly move. The place underground where the movement starts is called the hypocenter or focus; the point on the surface directly above it is the epicenter.

Why does the ground store energy?

The outermost layer of Earth—made of rigid pieces called tectonic plates—moves slowly because of heat-driven convection in the mantle beneath. Where plates interact, they can slide past one another, collide, or move apart. At the edges of these plates and along faults (fractures in the crust), friction prevents smooth motion. Over years to centuries, stress builds in the rocks as the plates continue to push or pull. Rocks deform elastically to a point, like a bent stick storing energy, until they suddenly break or slip and release that stored energy.

Faults: the underground cracks that move

A fault is a fracture or zone of fractures where blocks of rock move relative to each other. There are several common fault types, named for the direction of slip:

Normal faults: occur where the crust is being pulled apart; one block moves down relative to the other.

Reverse (or thrust) faults: occur under compressional forces; one block is pushed up over the other.

Strike-slip faults: lateral motion where blocks slide horizontally past each other (the San Andreas Fault is a well-known example).

Most earthquakes happen on faults, but some deep events relate to other processes like volcanic movement or collapse.

What actually moves during a quake?

During a seismic rupture, a portion of a fault that was locked by friction suddenly slips. The rupture often starts in a small patch and spreads along the fault surface. The slipping area is called the rupture plane. Slip can be a few millimeters for very small quakes or several meters in large earthquakes. The sudden change in stress and position of the rocks produces seismic waves and often permanent offset of the ground.

Seismic waves: how energy travels from the focus

When rock breaks and slips, it sends out vibrations in several wave types:

P-waves (primary or compressional waves): these are the fastest seismic waves and compress and expand material in the direction they travel. They arrive first at seismographs and are the first waves people feel.

S-waves (secondary or shear waves): slower than P-waves, these move material perpendicular to their direction of travel and cannot travel through liquids. They usually cause stronger shaking than P-waves.

Surface waves: generated from the interaction of body waves with the Earth’s surface, these travel along the surface and often produce the most damaging, long-period shaking. Types include Rayleigh and Love waves.

The character of shaking at any location depends on the earthquake size, distance to the rupture, depth of the focus, and local ground conditions which can amplify or damp waves.

Depth matters: shallow versus deep earthquakes

Earthquakes occur at a wide range of depths. Shallow earthquakes (up to ~70 km deep) generally cause stronger surface shaking for a given magnitude because less energy is attenuated before reaching the surface. Intermediate (70–300 km) and deep (300–700 km) earthquakes occur within subducting plates well beneath the surface. Deep quakes usually affect a smaller area at the surface but can still be felt far from the epicenter because of the way waves travel through the mantle.

What happens to the rocks during rupture?

Rupture changes physical conditions locally and sometimes permanently. Stress drops near the fault and elastic strain stored in surrounding rocks is released, which can cause:

Permanent displacement along the fault, sometimes visible as ground offset or surface breaks.

Rock fracturing and the creation of new fracture networks that can alter fluid flow underground.

Changes in pore pressure where fluids (water, gas, or hydrothermal fluids) are present, which in some cases can promote further slip.

Aftershocks and stress transfer

After a main rupture, the crust adjusts to a new stress state. Smaller earthquakes called aftershocks commonly follow as the nearby rocks and other parts of the fault system settle into equilibrium. Aftershock rates typically decrease over time but can continue for months or years. Conversely, the stress change from one earthquake can slightly increase stress on neighboring faults, sometimes bringing them closer to failure.

Earthquakes and fault complexity

Faults are rarely simple planar surfaces. They often include branched segments, rough surfaces, and areas with different materials and frictional properties. These variations control how rupture propagates: the rupture can jump between segments, stop at barriers, or accelerate through weak patches. This complexity makes predicting the exact size or pattern of rupture difficult.

Liquefaction, landslides, and secondary effects

Seismic shaking influences more than just the fault itself. In water-saturated, loose sediments, strong shaking can temporarily reduce soil strength, causing it to behave like a liquid—a process called liquefaction. Buildings and infrastructure can sink or tilt as a result. Shaking on slopes can trigger landslides. Other secondary effects include changes to groundwater levels and release of trapped gases.

How do scientists study what happens underground?

Seismology combines observations and models to infer underground processes:

Seismographs record the arrival times and waveforms of seismic waves. By analyzing these records from many stations, scientists locate hypocenters, determine focal depths, and estimate the fault plane and slip direction.

Earthquake source inversion uses recorded waveforms to model how slip varied over the rupture plane in space and time, producing maps of slip amount and rupture speed.

Geodesy (GPS and InSAR) measures ground displacement before, during, and after earthquakes. These measurements constrain how the fault moved and how strain accumulates and releases.

Field geology documents surface rupture and offsets after shallow earthquakes, providing direct evidence of fault motion and helping to calibrate models.

Magnitude, energy, and intensity: different ways to describe shaking

Magnitude quantifies the size of an earthquake based on the energy released or the amplitude of seismic waves and is a single number for the whole event (examples: moment magnitude, Mw). Intensity describes the shaking and damage at specific locations and varies with distance and local ground conditions (examples: Modified Mercalli Intensity scale). Two earthquakes with the same magnitude can produce very different intensities across an area depending on depth, distance, and local geology.

Examples that illustrate underground processes

The 1906 San Francisco earthquake was a large strike-slip rupture along the San Andreas Fault: a long fault segment slipped, producing visible surface offset and widespread shaking because rupture reached the shallow crust.

The 2011 Tohoku (Japan) earthquake involved a large thrust rupture at a subduction zone. A huge area of the fault slipped, displacing the sea floor and generating a tsunami. Beneath the surface, massive slip and stress transfer occurred across a broad region.

The 1994 Northridge event in California was a relatively deep, blind-thrust earthquake (no surface rupture) with intense shaking because energy was released close to populated areas and focused by local geology.

What we still don’t fully understand

Seismologists have made great progress, but some things remain uncertain. Predicting exactly when and where a particular fault segment will rupture is currently not possible. Details of how rupture begins (nucleation) and why some ruptures jump long distances while others stop are active research topics. The role of fluids, fault healing between events, and the transition from slow slip to fast earthquakes are also areas of ongoing study.

How this knowledge helps communities

Understanding underground earthquake mechanics improves hazard assessments, building codes, and emergency planning. Mapping active faults and estimating the likely sizes and frequencies of earthquakes in a region lets engineers design structures to withstand expected shaking. Early warning systems use the detection of fast P-waves to provide seconds to tens of seconds of warning before the strongest shaking arrives, which can save lives and reduce damage when available.

Common misconceptions

Misconception: Earthquakes only happen on major plate boundaries. Correction: While many large earthquakes occur at plate boundaries, intraplate earthquakes can occur far from boundaries on pre-existing faults or due to human activities.

Misconception: Small earthquakes prevent big ones by releasing stress. Correction: Small quakes relieve a tiny fraction of regional stress. They may reflect ongoing adjustment, but they do not reliably prevent larger ruptures on the same fault.

Summary of the underground sequence

In simple terms, the underground sequence of a typical earthquake is: slow build-up of stress due to plate motions; elastic deformation and locking on a fault; sudden rupture and slip on a fault patch; emission of seismic waves (P, S, and surface waves) that travel through Earth; ground shaking and possible surface rupture; redistribution of stress causing aftershocks and longer-term changes in fault conditions.

Conclusion

Earthquakes are the result of stored elastic strain being released by sudden slip on faults underground. The process involves complex interactions among rock strength, fault geometry, fluids, and stress transfer, and it produces seismic waves that cause the shaking we feel. While scientists cannot yet predict exact earthquake timing, detailed study of ruptures, seismic waves, and ground deformation improves hazard understanding and community preparedness.