Seismic waves are measured using instruments called seismometers, which detect ground motion and convert it into a visual record called a seismogram. From that record, scientists extract two key pieces of information: how strong the earthquake was and where it originated. The process involves specialized sensors, precise timing, and a global network of about 150 stations that monitor the Earth around the clock.
How Seismometers Detect Ground Motion
A seismometer works on a simple principle: a heavy mass suspended inside the instrument stays relatively still while the ground (and the instrument’s frame) moves around it. The difference between the motion of the frame and the stillness of the mass gets converted into an electrical signal, which is recorded as a seismogram. That wavy line you’ve seen in photos or news reports is a direct translation of how the ground moved over time.
Not all seismometers are built for the same job. Broadband seismometers are extremely sensitive and can pick up faint vibrations from earthquakes happening thousands of kilometers away. They’re the workhorses of global monitoring. Strong-motion accelerometers, on the other hand, are designed to measure violent shaking close to the source. They detect accelerations ranging from 0.001 to 2 times the force of gravity at frequencies up to 100 Hz or more. They’re less sensitive than broadband instruments, but they stay on scale during the most intense shaking, when a broadband sensor would max out and produce useless data. Most modern seismic stations run both types side by side.
Reading a Seismogram
A seismogram records two main types of seismic waves, and learning to tell them apart is central to earthquake measurement. Compressional waves, called P-waves (for “primary”), always arrive first. They push and pull the ground in the same direction the wave travels, similar to how a slinky compresses and stretches. Shear waves, called S-waves (for “secondary”), arrive next and move the ground perpendicular to their travel direction, more like a rope being flicked side to side.
The time gap between the P-wave and S-wave arrivals tells you how far away the earthquake occurred. A seismogram from the 2008 Alamo, California earthquake recorded at UC Berkeley showed a P-S gap of just 3.5 seconds, indicating a relatively close source. A distant earthquake might show a gap of several minutes. This time difference is the foundation for locating where an earthquake happened.
Locating the Epicenter
Finding an earthquake’s epicenter requires readings from at least three seismometer stations, using a method called triangulation. Each station measures the time difference between the P-wave and S-wave arrivals. Because the two wave types travel at known, different speeds through the Earth, that time gap translates directly into a distance. A short gap means the earthquake was nearby; a long gap means it was far away.
Each station’s distance estimate defines a circle on a map, centered on that station. With one station, you know the earthquake was somewhere on that circle. With two, you narrow it down to two possible points where the circles intersect. With three stations, the circles intersect at a single point: the epicenter. In practice, modern networks use data from dozens or even hundreds of stations and run computer algorithms that refine the location to within a couple of kilometers.
Measuring Earthquake Size: Magnitude
Magnitude quantifies the energy an earthquake releases at its source. The scale most widely used today is the moment magnitude scale (Mw), which replaced the older Richter scale for all but the smallest local earthquakes. Moment magnitude is tied directly to the physical properties of the fault: how large an area ruptured, how far the two sides of the fault slipped past each other, and how rigid the surrounding rock is. An earthquake that ruptures a longer fault over a greater distance produces a proportionally larger seismic moment, and therefore a higher magnitude.
The scale is logarithmic. Each whole number increase represents roughly 32 times more energy released. A magnitude 5.3 earthquake is classified as moderate, while a 6.3 is considered strong. “Great” earthquakes start at magnitude 8.0 and involve fault ruptures that can stretch hundreds of kilometers. The thousands of small earthquakes that happen daily around the world, combined, still release far less energy than a single great earthquake.
Measuring Earthquake Effects: Intensity
Magnitude tells you how much energy was released. Intensity tells you how strongly the shaking was felt at a specific location, and it varies depending on distance from the epicenter, soil type, and building construction. The Modified Mercalli Intensity (MMI) scale runs from I to XII and is based entirely on observed effects rather than instrument readings.
At the low end, MMI I means the earthquake wasn’t felt at all and was only detected by instruments. MMI VI is where things start getting interesting: objects fall from shelves, plaster cracks, and people have trouble standing. At the extreme end, MMI XII describes total destruction, with the ground itself shearing apart, massive landslides, and river banks collapsing. The same earthquake can register MMI VIII near the fault and MMI III a few hundred kilometers away, which is why intensity maps often look like concentric rings radiating outward from the epicenter.
The Global Monitoring Network
Individual seismometers are useful, but the real power of seismic measurement comes from networks. The Global Seismographic Network (GSN), a partnership among the U.S. Geological Survey, the National Science Foundation, and EarthScope, operates approximately 150 broadband stations distributed across every continent and many ocean islands. This near-uniform global coverage ensures that no significant earthquake goes unrecorded, no matter where it occurs.
These stations do more than track earthquakes. After very large events, researchers use GSN data to study the Earth’s free oscillations, the way the entire planet rings like a bell at extremely low frequencies (roughly 0.3 to 10 millihertz). These oscillations reveal details about Earth’s deep interior structure that can’t be observed any other way.
Newer Sensing Technologies
A growing alternative to traditional seismometers is distributed acoustic sensing, or DAS. This technique sends laser pulses through ordinary fiber optic cables, the same cables already buried underground for internet and telecommunications. When the ground vibrates, it slightly deforms the fiber, which changes the properties of the returning laser signal. By analyzing those changes, scientists can turn kilometers of existing cable into a dense array of seismic sensors.
DAS fills a gap where installing hundreds of conventional seismometers would be impractical or impossible. Researchers have used it to study glaciers and ice sheets, monitor offshore fault zones using undersea telecom cables, and create ultra-dense urban sensor networks without installing a single new instrument. The resolution is remarkable: a single fiber optic cable can provide measurements every few meters along its entire length.
Earthquake Early Warning Systems
One of the most consequential applications of seismic measurement is earthquake early warning. These systems exploit a basic fact: electronic signals travel at the speed of light, while seismic waves travel at a few kilometers per second. When the first P-waves hit the nearest sensors, algorithms estimate the earthquake’s location and magnitude within seconds and broadcast an alert before the more damaging S-waves and surface waves arrive at populated areas.
Speed is everything in these systems. Recent research on a cloud-based detection system in Italy demonstrated location accuracy of about 1.8 kilometers after just 251 milliseconds of processing, with a final error under 2 kilometers in just over 6 seconds. That system achieved a 3.39-second speed advantage over Italy’s standard regional early warning approach. Those few extra seconds can be enough for trains to brake, surgical teams to pause, and people to get under sturdy furniture before strong shaking arrives.