Earthquake Magnitude Scale
Explained
A magnitude 6 earthquake isn't twice as strong as a magnitude 3 — it's about 1,000 times stronger. Here's how the scale actually works, and why USGS stopped using the Richter scale decades ago.
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
M5.1 — 2014 South Napa, CA
Moderate quake that caused significant damage to older, unreinforced buildings in downtown Napa and injured over 200 people — 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.
Get Earthquake Alerts on Your Phone
StormCast monitors the USGS earthquake feed every 10 minutes for events M2.5 and above. The alert radius scales with magnitude — a M3.0 only notifies people within about 150 miles, while a M7.0+ notifies everyone, regardless of distance.
