How Earthquakes Are Measured
Moment magnitude, seismograph networks, depth classification, and how USGS detects and locates seismic events globally.
Key Takeaway
Modern earthquakes are measured using moment magnitude (Mw), which captures total energy released. Each whole number increase represents 31.6 times more energy. USGS detects earthquakes using a network of 2,000+ seismograph stations — locating most significant events within minutes using P-wave and S-wave arrival time differences. Depth matters as much as magnitude: shallow earthquakes (under 70 km) cause far more surface damage than deep ones.
The Moment Magnitude Scale
The moment magnitude scale (Mw) is the standard measurement USGS and seismologists worldwide use to quantify earthquake size. Unlike the original Richter scale — which was designed for local Southern California earthquakes using specific 1930s-era instruments — moment magnitude works accurately for any earthquake, anywhere on Earth, at any size.
Moment magnitude is calculated from the seismic moment: the product of three physical quantities — the area of fault that ruptured, the average distance the fault slipped, and the rigidity of the surrounding rock. This gives a direct measure of the physical energy released, not just the shaking amplitude recorded at a single station.
The scale is logarithmic. Each whole number step up represents approximately 31.6 times more energy released and about 10 times more ground motion amplitude. The difference between an M6 and an M9 earthquake — just three steps — represents roughly 31,000 times more energy. This is why the 2011 Japan earthquake (M9.1) was so catastrophically powerful despite sounding like "only" a few numbers above an M6.
| Magnitude | Description | Global Frequency (USGS) |
|---|---|---|
| 2.5–3.0 | Minor — often felt near epicenter | ~900,000/year |
| 4.0–4.9 | Light — felt widely, minor damage | ~13,000/year |
| 5.0–5.9 | Moderate — damage to weak structures | ~1,300/year |
| 6.0–6.9 | Strong — destructive in populated areas | ~130/year |
| 7.0–7.9 | Major — serious damage over large areas | ~15/year |
| 8.0+ | Great — catastrophic, regional destruction | ~1/year |
Depth Classification
Depth — how far below the surface the rupture originates — is one of the most important and most overlooked factors in earthquake damage. USGS classifies earthquake depth into three categories:
- Shallow (0–70 km): The most destructive category. Energy reaches the surface with less attenuation. Most major urban earthquakes, including the 1994 Northridge and 1906 San Francisco events, were shallow. Crustal earthquakes in California are typically 5–20 km deep.
- Intermediate (70–300 km): Common in subduction zones where oceanic plates dive beneath continental plates. Can be felt over very wide areas but energy spreads more before reaching the surface. The Cascadia subduction zone produces intermediate-depth earthquakes.
- Deep (300–700 km): Occur only in subducting plates, which are cold enough to remain brittle at these depths. Despite sometimes large magnitudes, deep earthquakes rarely cause significant surface damage. The Mariana Trench region regularly produces earthquakes below 500 km.
You can explore depth data for all events on the significant earthquakes page and browse yearly earthquake statistics to see how depth distributions vary over time.
Seismograph Networks and Detection
A seismograph measures ground motion — the back-and-forth movement of the Earth's surface caused by seismic waves. Modern instruments are sensitive enough to detect ground displacement smaller than the diameter of a hydrogen atom. USGS operates and integrates data from the Advanced National Seismic System (ANSS), which connects over 2,000 seismograph stations across the US and its territories, plus hundreds of partner stations through the Global Seismographic Network.
Earthquakes generate different types of seismic waves. P-waves (primary waves) are compressional — they move rock in the same direction the wave travels, like a slinky being pushed. They travel fastest and arrive at seismographs first. S-waves (secondary waves) are shear — they move rock perpendicular to the wave's direction, like a rope being shaken sideways. They travel slower and arrive second. The gap between P and S wave arrival times grows with distance from the earthquake source.
By measuring these arrival time differences at multiple stations and applying geometry, USGS computers can determine both the epicenter (surface location) and the depth of any earthquake. For major events, locations are published within minutes. Initial magnitude estimates may be revised as more stations report data.
Why Magnitude Thresholds Differ: Global vs. US
PlainQuake includes earthquakes of M4.0+ globally and M2.5+ within the United States. This difference reflects the density of seismograph networks. The US has one of the densest seismograph networks in the world, allowing reliable detection and location of smaller events. In remote oceanic regions or parts of the developing world, global seismograph coverage is sparser, making accurate detection of events below M4 unreliable.
This means direct country-to-country comparisons of earthquake counts can be misleading — a country with few seismograph stations will show artificially low numbers. Browse global country data and US state data to compare seismic activity with these detection limits in mind.
Safety Note
During an earthquake, Drop, Cover, and Hold On. Drop to your hands and knees, take cover under a sturdy table or desk, and hold on until shaking stops. Stay indoors and away from windows.
Frequently Asked Questions
What is moment magnitude (Mw) and how is it different from the Richter scale?
Moment magnitude (Mw) measures the total energy released by an earthquake, calculated from the seismic moment — the product of the fault area, average slip, and rock rigidity. The Richter scale (local magnitude, ML) was designed for small to moderate earthquakes in Southern California using a specific type of seismograph. Moment magnitude replaced the Richter scale because it works accurately for all earthquake sizes and anywhere on Earth. Most modern USGS magnitude values are moment magnitudes or closely related measurements.
How much stronger is an M7 earthquake than an M6?
Each whole number increase on the magnitude scale represents about 31.6 times more energy released and roughly 10 times more ground shaking amplitude. An M7 earthquake releases about 31.6 times more energy than an M6, and an M8 releases about 1,000 times more energy than an M6. This logarithmic scale means the jump from M8 to M9 is enormous — the 2011 Tohoku earthquake (M9.1) released more energy than all M8 earthquakes worldwide over the prior decade combined.
What does earthquake depth tell us?
Depth is how far below the surface the earthquake originates. Shallow earthquakes (0–70 km) cause the most surface damage because the energy has less distance to travel and spread out. Intermediate earthquakes (70–300 km) can still be felt widely but usually cause less surface damage. Deep earthquakes (300–700 km) occur in subducting tectonic plates and rarely cause major surface damage despite potentially large magnitudes. The Marianas Trench region regularly produces deep earthquakes below 500 km depth.
How does USGS locate an earthquake?
USGS locates earthquakes using arrival time differences between P-waves (compressional) and S-waves (shear) recorded at multiple seismograph stations. P-waves travel faster and arrive first; S-waves follow. The further a station is from the earthquake, the greater the time gap between P and S wave arrivals. By comparing these timing differences across at least 3–4 stations, computers can triangulate the earthquake epicenter and calculate depth. Modern algorithms process hundreds of station readings simultaneously for improved accuracy.
What is a seismograph network and how large is the USGS network?
A seismograph network is a coordinated system of seismometers spread across a region, connected to a central data processing center. USGS operates the Advanced National Seismic System (ANSS), which integrates data from over 2,000 seismograph stations across the US and its territories, plus hundreds of partner stations worldwide through the Global Seismographic Network (GSN). This coverage allows USGS to detect and locate earthquakes globally within minutes of occurrence.
What is the minimum magnitude earthquake that people can feel?
Most people can feel earthquakes of about M2.5 or larger when they are close to the epicenter. M3 earthquakes are felt by many people nearby but rarely cause damage. M4 earthquakes are felt over a wider area and may cause minor damage like cracked plaster. The USGS ComCat data on PlainQuake includes M2.5+ events for the US and M4+ globally — the M4 global threshold reflects both the smaller scale of local shaking and the practical detection limits of global seismograph networks in remote areas.
Sources
- USGS Earthquake Hazards Program — ComCat Catalog, 2005–2025
- USGS — Earthquake Magnitude, Energy, and Shaking Intensity
- USGS Advanced National Seismic System (ANSS)
- USGS — How We Measure Earthquake Size
This content is for educational purposes only. For official earthquake information and warnings, visit the USGS Earthquake Hazards Program at earthquake.usgs.gov.
Understanding the Data
The information presented throughout this guide is informed by publicly available public records published by federal and state government agencies. Our database aggregates and standardizes these records to make them more accessible and easier to interpret for general audiences. When we reference specific statistics or trends, they are drawn directly from these authoritative sources unless explicitly noted otherwise.
It is important to understand the limitations of any large-scale data dataset. Records may contain errors from the original data collection process, some fields may be incomplete for older entries, and classification systems may have changed over time. Our analysis accounts for these factors by clearly labeling data vintage, flagging records with missing critical fields, and noting when temporal comparisons span methodology changes in the source data.
For readers who want to conduct their own research, we recommend going directly to the source whenever possible. federal and state government agencies provides detailed documentation on collection methodology, sampling frames, and known data quality issues. Our goal is not to replace primary sources but to make them more approachable and to highlight patterns that may not be immediately obvious when browsing raw records.
How We Analyze Data Records
Our analytical approach involves several steps designed to surface meaningful insights from large datasets. First, we clean and standardize the raw data, handling variations in naming conventions, date formats, and categorical labels. Then we compute summary statistics, distributions, and comparative benchmarks across relevant dimensions such as geography, time period, and category type.
Key metrics we examine include statistical records, geographic distributions, temporal trends. These indicators provide a multi-dimensional view of each entity in our database, allowing users to understand not just individual records but how they compare to peers, regional averages, and national benchmarks. We believe this contextual approach is far more valuable than presenting raw numbers in isolation.
