Seismic refers to anything related to earthquakes or vibrations traveling through the Earth (or other planetary bodies). The word comes from the Greek “seismos,” meaning earthquake. When you hear about seismic waves, seismic activity, or seismic surveys, they all describe energy moving through rock and soil, whether from a natural earthquake, a volcanic eruption, or a deliberate pulse sent into the ground by scientists.
How Seismic Waves Move Through the Earth
When rock suddenly shifts along a fault line, or when an explosion sends a shockwave underground, the released energy travels outward as seismic waves. These waves come in distinct types, each moving differently and at different speeds.
P-waves (primary waves) arrive first. They compress and expand rock in the same direction they’re traveling, similar to how sound moves through air. In solid rock, P-waves travel at roughly 6 kilometers per second and can pass through both solid and liquid layers of the Earth. S-waves (secondary waves) follow, moving about 1.7 times slower at around 4 kilometers per second. Instead of compressing rock, they shake it side to side, perpendicular to their travel direction. S-waves cannot move through liquid, which is how scientists originally confirmed that Earth’s outer core is molten.
Surface waves arrive last but cause the most damage. Rayleigh waves roll the ground in an elliptical motion, creating both vertical and horizontal movement. Love waves slide the ground sideways, parallel to the surface. Because surface waves concentrate their energy near the top of the Earth rather than spreading it through the deep interior, they produce the violent shaking that collapses buildings.
What Causes Seismic Activity
Most seismic activity comes from tectonic plates grinding past, colliding with, or pulling away from each other. Stress builds along fault lines for years or centuries, then releases in seconds. Volcanic eruptions also generate seismic waves, as do landslides and large meteorite impacts.
Human activity can trigger earthquakes too. Injecting fluid underground increases pressure within fault zones, loosening them and making them more likely to slip. The U.S. Geological Survey has identified wastewater disposal wells as the primary driver of the recent increase in earthquakes across the central United States. These wells operate for long periods and inject far more fluid than hydraulic fracturing (fracking) does, which makes them more likely to induce quakes. Fracking itself produces mostly tiny “microearthquakes” too small to feel. Other triggers include filling large reservoirs, underground mining, and geothermal energy extraction.
How Earthquakes Are Measured
Seismometers detect ground motion using a simple principle: a suspended mass tends to stay still while the ground shakes beneath it. The difference between the stationary mass and the moving frame gets converted into electrical signals, displayed as wavy lines on a seismogram. Modern instruments use three sensors oriented in different directions to capture ground motion in all three dimensions.
Charles Richter developed the first magnitude scale in the 1930s to measure earthquakes in southern California, but it only worked reliably for certain distances and wave frequencies. As seismograph networks expanded worldwide, scientists created additional scales for different wave types, but each had limitations. The current standard is the moment magnitude scale (Mw), which calculates size based on the total area of the fault that slips and how far it moves. This approach works across the full range of earthquake sizes, from barely detectable tremors to the largest events ever recorded. For very large earthquakes, moment magnitude is the only scale that gives a reliable estimate.
All these scales are logarithmic. Each whole number increase represents roughly 32 times more energy released. A magnitude 7.0 earthquake releases about 32 times more energy than a 6.0, and about 1,000 times more than a 5.0. The largest earthquake ever recorded was the 1960 Great Chilean Earthquake near Valdivia, which reached magnitude 9.5 with a rupture zone stretching roughly 1,000 kilometers.
Early Warning Systems
Because P-waves travel faster than the more destructive S-waves and surface waves, there’s a brief window to send warnings before heavy shaking arrives. Systems like ShakeAlert in California use sensor networks to detect P-waves, transmit signals to processing centers, and push alerts to phones and infrastructure. In California, these alerts typically arrive five to eight seconds after an earthquake begins. That’s not much, but it’s enough time for trains to brake, surgeons to step back, and people to take cover.
Seismic Surveys and Exploration
Not all seismic activity involves earthquakes. Geologists deliberately send controlled vibrations into the ground and listen for the echoes that bounce back from underground rock layers. This technique, called seismic reflection, is one of the primary tools for finding oil, gas, and mineral deposits.
Different frequencies reveal different features. Low-frequency signals (below 20 Hz) can image deep structures like the boundary between Earth’s crust and mantle. Medium frequencies (20 to 60 Hz) map fault networks and rock contacts closer to the surface. High-frequency signals (above 60 Hz) can pinpoint specific ore bodies or mineralized zones. By combining data across these frequency bands, exploration teams build detailed three-dimensional maps of geology that would otherwise require expensive drilling to understand.
Seismic Engineering
Engineers design buildings and bridges to survive seismic forces using several strategies. Base isolation places flexible rubber or sliding bearings between a building’s foundation and its structure, allowing the ground to move while the building stays relatively still. The trade-off is that isolated buildings can sway with large lateral displacements, so they often incorporate damping systems to absorb that motion.
Tuned mass dampers take a different approach. A heavy weight, sometimes hundreds of tons, is mounted inside the building on springs or pendulums. When the building sways one direction, the weight swings the opposite way, counteracting the motion. Research on five-story base-isolated buildings has shown that combining both techniques, isolation bearings plus a tuned damper, significantly reduces displacement at the base while also controlling movement between floors. Newer designs replace some of that physical mass with mechanical gearing systems that achieve similar effects at lower weight.
Seismic Activity Beyond Earth
Seismic science extends to other worlds. NASA’s InSight lander, which operated on Mars from 2018 to 2022, carried a seismometer that detected hundreds of “marsquakes.” By tracking how seismic waves changed speed and direction as they passed through the Martian crust, mantle, and core, scientists mapped the planet’s interior structure for the first time.
One surprising finding was that meteoroid impacts on Mars produced seismic signals reaching much deeper than expected. Scientists had assumed most seismic energy from impacts would stay trapped in the Martian crust, which has properties that dampen vibrations. Instead, waves from a crater 21.5 meters wide traveled through a faster, deeper path in the mantle, reaching InSight’s sensors from much farther away than models predicted. This “seismic highway” through the mantle reshaped understanding of how energy moves through smaller rocky planets and provided clues about how all rocky worlds, including Earth, formed and evolved.