Earthquake Magnitude Explained: What a 4.0 vs 7.0 Actually Means
What does earthquake magnitude mean? Learn the logarithmic scale, magnitude vs intensity, USGS ratings, damage at each size, and how to prep for the next quake.
The July 28 Kyushu earthquake — a 7.1 that killed 34 people, collapsed a shopping mall, and triggered days of aftershocks — sent millions to Google with one question: what does a 7.1 actually mean? A 5.0 rattled the Texas Panhandle on July 23, and small quakes buzzed through Sonoma County on August 1, each sparking the same search. Here's the answer: magnitude measures the energy an earthquake releases, on a logarithmic scale where each whole number means about 10x the ground motion and roughly 32x the energy. A 7.1 is not a slightly bigger 5.0 — it releases more than 1,000 times the energy.
Earthquake Magnitude Explained: What a 4.0 vs a 7.0 Really Means
Think of magnitude as a ladder where every rung multiplies, not adds. 4.0 to 5.0 = 32x the energy; 4.0 to 6.0 = about 1,000x; 4.0 to 7.0 = roughly 32,000x — the difference between a truck hitting a pothole and a small nuclear blast.
| Magnitude | Relative energy (vs a 4.0) | Typical effects |
|---|---|---|
| 2.0–2.9 | 1/1,000x | Rarely felt; recorded by instruments |
| 3.0–3.9 | 1/32x | Felt by some indoors; rattling, no real damage |
| 4.0–4.9 | 1x (baseline) | Felt by many; dishes rattle, minor damage to weak structures |
| 5.0–5.9 | 32x | Felt by all; cracked walls and windows, minor-to-moderate damage |
| 6.0–6.9 | 1,000x | Destructive in populated areas; building damage, many aftershocks |
| 7.0–7.9 | 32,000x | Major quake; widespread collapse and damage — the Kyushu scenario |
| 8.0+ | 1,000,000x | Great quake; catastrophic damage across entire regions |
That's why the Kyushu 7.1 flattened a mall while a 4.0 feels like a bad truck driving by. The number matters — and so does your distance from the source.
Magnitude vs Intensity: Two Different Numbers
People mix these up constantly. Magnitude (what the USGS reports first) is one number for the size of the earthquake at its source — the energy released, measured with the moment magnitude scale. Intensity is how strongly shaking is felt at a specific location, ranked on the Modified Mercalli scale from I (barely noticeable) to XII (total destruction).
A 7.1 in Kyushu felt like a gentle sway hundreds of miles away in Tokyo and like the end of the world at the epicenter. A shallow 5.0 under a city can shake harder than a deep 7.0 far away. When you see "felt reports," that's intensity — always check both.
Why Small Quakes Dominate the News
Here's the counterintuitive part: small quakes often make headlines. A 3.8 in Sonoma County on August 1 and a flurry of 2.5s in SoCal get clicks because they're under populated areas — people feel them and share them. The USGS logs thousands of tiny quakes daily; a 2.5 is roughly the threshold where someone nearby notices.
Meanwhile a 7.1 on the other side of the Pacific is the real story — 34 dead, a collapsed mall, days of aftershocks. When a small magnitude makes the news, ask: how close is it to people? Proximity makes headlines; magnitude determines destruction.
How the USGS Ranks Earthquakes — and What to Expect
The USGS uses moment magnitude (Mw), a modern replacement for the Richter scale that measures total energy release and doesn't top out at large quakes. Every earthquake gets a single magnitude from the global seismometer network, and anything at or above 4.5 gets a quick "significant" alert with damage estimates.
What to expect from each range:
- 4.0–4.9 — Light shaking, scattered minor damage. Usually no injuries.
- 5.0–5.9 — Moderate damage to older buildings; cracked walls, fallen chimneys, broken glass.
- 6.0–6.9 — Destructive. Buildings damaged, people injured; can kill in dense areas.
- 7.0+ — Major. Collapsed structures, fires, and serious casualties — even with good building codes, as Kyushu showed.
The Aftershock Reality
Kyushu's quake didn't end when the shaking stopped — thousands of aftershocks followed, some strong enough to rock already-damaged buildings. Aftershocks are the fault relaxing after the main rupture; a 7.1 can produce months of them, and roughly 5–20% of major quakes are followed by something larger. Stay out of damaged buildings, keep your kit ready, and check for new alerts for weeks afterward.
Quick Recommendations
- Learn your local risk — check USGS quake maps for nearby faults and set up earthquake alerts on your phone.
- Prep your home now — strap bookshelves and the water heater, secure heavy furniture, and know your shut-off valve in our guide to preparing your home for an earthquake.
- Know the drill — drop, cover, and hold on until shaking stops; don't run outside. Get the full steps in what to do during and after an earthquake.
- Keep a first-aid kit handy — cuts and falls are the most common quake injuries, so a First Aid Only 298-Piece All-Purpose Kit within arm's reach pays off the moment the shaking stops.
- Plan for aftershocks — after any quake above 5.0, expect weeks of smaller ones and stay clear of damaged structures.
Magnitude tells you how much energy hit the ground. Preparedness tells you what happens next.