Magnitude Is Logarithmic, Not Linear
Earthquake magnitude is measured on a logarithmic scale. Each whole-number increase represents roughly 32 times more energy released, and about a 10x increase in the amplitude of ground shaking recorded on a seismograph.
So a magnitude 5.0 quake releases about 32 times more energy than a magnitude 4.0. A magnitude 7.0 releases roughly 32,000 times more energy than a magnitude 5.0. Small increases in the number represent huge increases in actual force.
Richter Scale vs. Moment Magnitude Scale
Most people still say "Richter scale," but the USGS hasn't used it since the 1970s for anything but small local quakes. Here's the difference:
Richter Scale (ML)
Developed in 1935 by Charles Richter. Measures local seismic wave amplitude. Accurate for small-to-moderate quakes recorded on nearby instruments, but loses accuracy for anything above roughly magnitude 6.5 — it "saturates" and stops distinguishing between large quakes.
Moment Magnitude (Mw)
The modern standard. Calculates the total energy released based on the size of the fault rupture, how far it slipped, and the rigidity of the rock. Accurate at any size, which is why every major quake reported today — including by USGS — uses Mw, even though headlines still call it "Richter."
What Each Magnitude Range Feels Like
M2.5 – M3.9 — Minor
Often not felt, or felt only as a brief tremor by people near the epicenter. Recorded by seismographs but rarely causes damage. Thousands occur worldwide every year.
M4.0 – M4.9 — Light
Felt indoors, rattling windows and dishes. Objects on shelves may shake. Minor damage is rare but possible in poorly built structures.
M5.0 – M5.9 — Moderate
Can cause damage to poorly constructed buildings. Felt widely, sometimes hundreds of miles from the epicenter depending on ground conditions.
M6.0 – M6.9 — Strong
Can cause significant damage in populated areas. Structures built to modern seismic codes generally hold up; older buildings often don't.
M7.0+ — Major to Great
Capable of serious damage across large areas. Magnitude 8.0+ quakes are classified as "great" and can cause destruction across hundreds of miles — the 2011 Tōhoku quake in Japan was M9.1.
Real Quakes at Each Magnitude
M6.0 — 2014 South Napa, CA
The largest San Francisco Bay Area quake since 1989's Loma Prieta earthquake. Caused significant damage to older, unreinforced buildings in downtown Napa and roughly 200 injuries — a reminder that even mid-range magnitudes can be locally destructive.
M6.7 — 1994 Northridge, CA
One of the costliest US earthquakes in history despite its "only" magnitude 6.7, because the epicenter was directly beneath a densely populated area — a clear case of location mattering as much as magnitude.
M9.1–9.2 — 2004 Sumatra & 2011 Tōhoku
Two of the largest earthquakes ever recorded, both triggering catastrophic tsunamis. These sit near the upper end of what's physically possible — no fault system on Earth is believed capable of producing much beyond magnitude 9.5.
Why Distance and Depth Matter Too
Magnitude alone doesn't tell you how much shaking you'll feel. A shallow M5.0 quake close to the surface can cause more damage than a deep M6.0, because energy dissipates less before reaching the surface. Distance from the epicenter matters just as much — which is why USGS reports both magnitude and depth for every event, and why StormCast factors distance into how it decides who gets notified.
How USGS Actually Calculates Moment Magnitude
Moment magnitude isn't read off a single instrument the way a thermometer reads temperature — it's calculated from three physical measurements of the fault rupture: the area of the fault surface that slipped, the average distance the two sides moved relative to each other, and the rigidity (stiffness) of the rock involved. Multiplying those together gives the seismic moment, a physical measurement of energy in newton-meters, which is then converted through a logarithmic formula into the magnitude number you see reported.
This is why magnitude estimates sometimes get revised in the hours after a quake — USGS often publishes a fast, automated estimate within minutes based on limited data, then refines it as more seismic stations report in and the full rupture geometry becomes clear. A quake initially reported as M6.9 might get revised to M7.1 once analysts finish resolving the full extent of the fault slip.
Magnitude vs. Intensity: Two Different Scales
Magnitude is a single, fixed number for the whole earthquake, describing the total energy released at the source. Intensity is a completely separate measurement — the Modified Mercalli Intensity (MMI) scale — that describes how strongly the shaking was actually felt at a specific location, using a scale from I (not felt) to XII (total destruction). A single earthquake has one magnitude but many intensities: people near the epicenter might report MMI VIII ("severe" — damage to ordinary buildings), while people 100 miles away might only report MMI III ("weak" — felt indoors, may not be recognized as an earthquake). USGS collects these reports through its "Did You Feel It?" citizen-science tool to build a shaking intensity map after every notable quake.
Frequently Asked Questions
Is a magnitude 7 earthquake twice as strong as a magnitude 3.5?
No — it's roughly 178,000 times stronger in terms of energy released. Because the scale is logarithmic, a difference of a few magnitude units represents an enormous, not proportional, increase in force.
Why do news reports sometimes give different magnitudes for the same quake?
Different seismic networks (USGS, regional universities, other countries' agencies) can calculate slightly different values depending on which stations and methods they use, especially in the first few minutes. These typically converge to a consistent number within hours.
What's the smallest earthquake a person can actually feel?
Most people can't feel anything below about magnitude 2.5–3.0, and even then, only if they're close to the epicenter and it's shallow.
Can a low-magnitude quake still be dangerous?
Yes, in specific circumstances — a shallow M4.0–M5.0 directly beneath an urban area with older, unreinforced buildings can cause more local damage than a much larger but deeper or more remote quake.
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A note on this guide: Thresholds and criteria described here are general and can vary by NWS forecast office, region, and over time. This content is educational — always check your local National Weather Service office for the specific criteria that apply to your area.