How Earthquakes Are Measured

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.

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.

Worked Example: The 2011 Tohoku Earthquake

The March 11, 2011 earthquake off the coast of Japan is the clearest real-world case of how magnitude estimates refine as better data arrives. Within minutes of the rupture, USGS's rapid seismological analysis put the moment magnitude at M7.9. As more waveform data reached seismographs and the source depth estimate was corrected from an initial 84 km down to 24 km, that figure climbed fast:

That five-step climb from M7.9 to M9.1, released across roughly half an hour then finalized five years later, is not USGS getting it wrong. It is exactly the tradeoff described above: a rapid first estimate that is fast enough to trigger a tsunami warning, superseded by a more complete answer once every seismograph on the network has reported in. PlainQuake's own catalog reflects each earthquake's most current USGS-published magnitude, not the first number that appeared.

Why the M4 Catalog Threshold Matters

PlainQuake catalogs earthquakes of M4.0+ worldwide from the USGS Comprehensive Earthquake Catalog (ComCat), and US state figures are aggregated from that same global M4+ record. The M4 floor reflects the practical detection limits of global seismograph networks: the US has one of the densest networks in the world, but in remote oceanic regions or parts of the developing world coverage is sparser, making reliable detection of events below M4 difficult. A uniform M4 threshold keeps comparisons consistent across every region.

Even at M4+, direct country-to-country comparisons of earthquake counts reflect tectonic setting rather than monitoring quality. Browse global country data and US state data to compare seismic activity across regions.

Frequently Asked Questions