Seismology is the study of earthquakes and the seismic waves that travel through and around the Earth. It’s a branch of geophysics that uses vibrations in the ground to understand everything from why earthquakes happen to what the deep interior of a planet looks like. While earthquakes are the headline application, seismology also plays a role in oil and gas exploration, construction safety, and even studying the interior of Mars.
What Seismic Waves Actually Are
Seismic waves are vibrations caused by the sudden movement of materials inside the Earth. The most familiar trigger is slip along a fault during an earthquake, but volcanic eruptions, landslides, avalanches, explosions, and even rushing rivers produce them too. These waves radiate outward from the source, traveling through rock, soil, and water, and they carry information about every layer they pass through.
There are two main types of waves that travel through the body of the Earth. P-waves (primary waves) arrive first because they’re faster. They compress and stretch rock in the same direction they’re moving, similar to how a slinky bunches up and spreads apart when you push one end. S-waves (secondary waves) follow behind, shaking the ground perpendicular to their direction of travel, more like the side-to-side motion of a shaken rope. The USGS compares the pair to lightning and thunder: the P-wave is the flash, the S-wave is the rumble. The time gap between them tells seismologists how far away the earthquake occurred.
Surface waves arrive after both body waves. They travel along the Earth’s outer layer and typically cause the most damage during an earthquake because they produce larger, slower rolling and shaking motions near the surface where people and buildings are.
How Seismologists Detect Ground Motion
The core instrument is the seismometer, a sensor that detects ground motion, paired with a recording system (together called a seismograph). Seismometers work on a simple physical principle: inertia. A heavy mass suspended inside the instrument tends to stay still while the ground, and the instrument’s frame, move around it. The difference between the stationary mass and the moving frame is the signal. Modern research seismometers are fully electronic and record motion in all three dimensions simultaneously.
A newer technology called distributed acoustic sensing, or DAS, is changing the scale of what’s possible. Instead of placing individual seismometers one by one, DAS turns ordinary buried fiber-optic cables into thousands of virtual sensors. A small device called an interrogator, roughly the size of a microwave oven, sends pulses of laser light down the cable and measures tiny changes in the light that bounces back. When seismic waves stretch or compress the cable, those changes show up in the data. In one demonstration, scientists at Lawrence Livermore National Laboratory operated roughly 8,000 sensors along 80 kilometers of existing fiber-optic cable between San Francisco and Sunnyvale. The technology picked up vehicle traffic, passing trains, and a small earthquake with impressive clarity. Because it repurposes cables already in the ground, DAS dramatically increases sensor density at a fraction of the cost of traditional networks.
Measuring Earthquake Size
You’ve probably heard of the Richter scale, but seismologists largely moved past it decades ago. Charles Richter developed his scale in the 1930s to measure earthquakes in southern California using nearby seismograph stations. It worked well in that narrow context, but as global seismograph networks expanded, it became clear that the Richter formula was only reliable for certain distances and wave frequencies.
The replacement is the moment magnitude scale (Mw), which measures something more physically meaningful: the total energy released by an earthquake. It’s calculated from the area of the fault that slipped, how far it slipped, and the rigidity of the rock. For moderate earthquakes the two scales give similar numbers, which is why news reports still casually say “Richter scale.” But for very large earthquakes, moment magnitude is far more accurate. It doesn’t max out or lose reliability the way older scales do, so it works across the full range of earthquake sizes.
Magnitude tells you how much energy was released at the source. Intensity is a separate concept that describes how strong the shaking felt at a specific location. A single earthquake has one magnitude but can produce wildly different intensities depending on distance, local soil conditions, and building construction.
Can Scientists Predict Earthquakes?
No. A true earthquake prediction would need to specify a date and time, a location, and a magnitude. No scientist or agency has ever successfully predicted a major earthquake, and the USGS does not expect that to change anytime soon. The physics of fault systems are too complex and too poorly observable at depth for that kind of precision.
What does exist is earthquake early warning, which is a fundamentally different thing. Early warning systems detect an earthquake after it has already begun, using the fast-moving P-waves recorded by nearby sensors to calculate the expected shaking and send alerts before the slower, more destructive waves arrive. This can provide seconds to tens of seconds of advance notice, enough time to take cover, slow a train, or pause a surgical procedure. Seismologists can also estimate the probability that a region will experience a significant earthquake within a given number of years, which informs building codes and hazard maps. That’s forecasting, not prediction.
Human Activities That Cause Earthquakes
Not all earthquakes are natural. Induced seismicity refers to earthquakes triggered by human activity, and the most prominent cause in recent years has been the deep underground injection of wastewater from oil and gas operations. Oklahoma saw a dramatic increase in earthquake rates after large volumes of saltwater, a byproduct of extraction, were pumped into deep disposal wells. The injected fluid raises pressure in rock formations over months or years, eventually reducing friction on existing faults enough to trigger slipping.
Hydraulic fracturing (fracking) itself rarely causes felt earthquakes. The key distinction is that fracking operations inject fluid at high pressure for short periods to crack rock near a well, while wastewater disposal wells operate for much longer and inject far greater volumes over much larger areas. It’s the disposal wells, not the fracking itself, that are responsible for most induced earthquakes.
Seismology Beyond Earthquakes
One of seismology’s biggest commercial applications is resource exploration. Seismic reflection methods, originally developed for the oil and gas industry, work by sending controlled vibrations into the ground and recording the echoes that bounce back from underground layers. The timing and character of those reflections reveal the depth, shape, and composition of rock formations. The same techniques help locate groundwater, map bedrock surfaces, find underground voids and cavities, and identify mineral deposits. Advances in technology have made these methods increasingly useful for shallow, high-resolution imaging, benefiting environmental studies and engineering projects like tunnel construction or dam safety assessment.
Seismology also extends beyond Earth. NASA’s InSight lander placed the first seismometer on Mars and detected more than 1,300 “marsquakes” before the mission ended. Some were caused by rocks cracking under internal heat and pressure; others came from meteoroids striking the surface. By tracking how seismic waves changed as they traveled through the Martian crust, mantle, and core, scientists built the first detailed picture of Mars’s interior structure. One surprise: impact-generated waves traveled deeper through the planet’s mantle than expected, following what researchers described as a “seismic highway” that carried energy to sensors over 1,600 kilometers away. These findings don’t just tell us about Mars. They help scientists understand how all rocky worlds, including Earth and the Moon, formed and evolved.