How Earthquakes Work: Causes, Waves, and Measurement

What Causes Earthquakes

The immediate cause of most earthquakes is the sudden slip of rock along a fault, a fracture in the Earth crust where rocks on either side have moved relative to each other. Tectonic forces, primarily driven by plate motion, slowly build up stress in rocks over years, decades, or centuries. Rocks are elastic: they deform under stress but store that energy like a compressed spring. When the accumulated stress exceeds the frictional strength holding the fault locked, the rocks suddenly snap to a new position, releasing the stored energy as seismic waves. This process is described by elastic rebound theory, first proposed by Henry Fielding Reid after studying the 1906 San Francisco earthquake.

The point within the Earth where the rupture initiates is called the focus (or hypocenter). The point on the surface directly above the focus is the epicenter. Shallow earthquakes (focus less than 70 kilometers deep) tend to cause the most damage because the seismic energy has less distance to travel and less material to attenuate it before reaching the surface. Intermediate-depth earthquakes (70 to 300 kilometers) and deep earthquakes (300 to 700 kilometers) are associated with subduction zones, where cold, rigid oceanic lithosphere descends into the mantle and continues to generate stress at great depths.

Not all earthquakes are tectonic. Volcanic earthquakes result from magma movement beneath volcanoes. Collapse earthquakes occur when underground caverns or mine workings give way. Induced earthquakes can be triggered by human activities such as reservoir filling (the weight of water behind a new dam changes stress on underlying faults), deep injection of wastewater (which lubricates faults and reduces friction), and mining. The recent increase in earthquakes in parts of Oklahoma and other states has been linked to wastewater injection from oil and gas operations.

Seismic Waves

When a fault ruptures, it generates several types of seismic waves that travel through and along the surface of the Earth. Body waves travel through the Earth interior. P-waves (primary waves) are compressional waves that alternately squeeze and stretch the rock they pass through, similar to sound waves in air. They are the fastest seismic waves, traveling at 6 to 14 kilometers per second through rock, and they arrive at seismograph stations first. P-waves can travel through both solids and liquids. S-waves (secondary waves) are shear waves that move rock perpendicular to their direction of travel, like shaking a rope side to side. S-waves are slower than P-waves and, crucially, cannot travel through liquids, which is how geophysicists discovered that the Earth outer core is liquid: S-waves generated by earthquakes on one side of the planet fail to arrive at stations in the S-wave shadow zone on the opposite side.

Surface waves travel along the Earth surface and are generally responsible for the most destructive shaking during an earthquake. Love waves move the ground horizontally in a side-to-side motion perpendicular to the wave direction. Rayleigh waves produce an elliptical rolling motion, like ocean waves, that moves both vertically and horizontally. Surface waves are slower than body waves but typically have larger amplitudes and longer durations, making them the primary cause of structural damage and ground failure during major earthquakes. The difference in arrival times between P-waves and S-waves at a seismograph station allows seismologists to calculate the distance to the earthquake epicenter. Data from at least three stations are needed to triangulate the epicenter location.

Measuring Earthquakes

Earthquake size is described by two different measurements: magnitude and intensity. Magnitude quantifies the total energy released at the source. The moment magnitude scale (Mw), which has replaced the older Richter scale for scientific use, is based on the seismic moment, a measure that combines the area of the fault that ruptured, the distance the fault moved, and the rigidity of the rock. The scale is logarithmic: each whole number increase represents roughly 32 times more energy released. A magnitude 5 earthquake releases about 32 times more energy than a magnitude 4, and a magnitude 6 releases about 1,000 times more energy than a magnitude 4.

Intensity describes the effects of an earthquake at a specific location. The Modified Mercalli Intensity (MMI) scale ranges from I (not felt) to XII (total destruction). Intensity depends on distance from the epicenter, local soil and rock conditions, building construction quality, and earthquake depth. Soft, unconsolidated sediment amplifies ground shaking compared to solid bedrock, a phenomenon called site amplification or the basin effect. This explains why damage from the 1985 Mexico City earthquake was concentrated in areas built on the soft clay of a drained lakebed, even though the epicenter was over 350 kilometers away. Liquefaction occurs when saturated, loose sediment loses its strength during prolonged shaking and behaves like a liquid, causing buildings to sink, tilt, or topple and underground structures like pipes and tanks to float to the surface.

Earthquake Hazard Zones

About 80 percent of the world earthquakes occur along the Ring of Fire, a horseshoe-shaped zone encircling the Pacific Ocean where multiple tectonic plates converge. The Alpine-Himalayan belt, stretching from the Mediterranean through the Middle East to Southeast Asia, accounts for another 15 percent. The remaining earthquakes occur along mid-ocean ridges, within plate interiors, and along other boundary systems. Intraplate earthquakes, while less common, can be particularly dangerous because they occur in regions where building codes may not account for seismic risk. The New Madrid Seismic Zone in the central United States produced three of the largest earthquakes in North American history during 1811 and 1812, and remains a significant hazard.

Earthquake prediction remains one of the great unsolved problems in geophysics. Despite decades of research, no reliable method exists to predict exactly when, where, and how large the next earthquake will be. Seismologists focus instead on probabilistic seismic hazard assessment, which estimates the likelihood of ground shaking of a given intensity at a given location over a specified time period. These assessments inform building codes, insurance rates, and emergency planning. Earthquake early warning systems, which detect the initial P-waves from an earthquake and issue alerts before the more destructive S-waves and surface waves arrive, can provide seconds to tens of seconds of warning to nearby populations, enough time to take protective action, slow trains, and shut down industrial processes.