Earthquake Magnitude Explained: What a 4.0 vs 7.0 Actually Means

Magnitude measures the energy an earthquake releases. The scale is logarithmic: each whole-number increase means about 10 times the measured ground-motion amplitude and roughly 32 times the energy. A magnitude 7.1 is not slightly larger than a 5.0; it releases more than 1,000 times as much energy. Magnitude alone still does not predict the damage at one address because depth, distance, soil, and building design all matter.

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

Aftershocks happen as the crust adjusts around the rupture. They can continue for days, weeks, or months, and a moderate aftershock can bring down a wall already weakened by the main quake. USGS may publish an aftershock forecast on the earthquake's event page when it has enough data. A later quake can also turn the first event into a foreshock, but nobody can identify that sequence in advance. Stay out of damaged buildings and keep your kit ready.

What Magnitude Means for Your Earthquake Prep

Magnitude is energy at the source, not a damage forecast for your address. Distance, depth, soil, building quality, and local intensity determine what you feel. Use the number to check a layered response kit, not to panic or wait for a perfect threshold.

• Information after cell service fails: A Midland ER310 Emergency Radio receives NOAA weather and emergency broadcasts and carries a 72-plus-hour battery. Use it for local instructions; use USGS event pages for aftershock forecasts when internet service is available.
• Water during a utility interruption: Two … 7-gallon jugs give a household a practical starting point for the one-gallon-per-person-per-day baseline. Rotate stored water and follow local advisories before drinking tap water again.
• Power for the phone and radio: The EcoFlow RIVER 3 Portable Power Station stores 245Wh and can recharge phones and small radios from a wall, car, or solar input. It is a battery station, not a gas generator, so size the plan around those loads.

That is the useful translation from a magnitude headline: expect a longer disruption when the event is large, then use local alerts and building conditions to decide whether to shelter, evacuate, or stay out of a damaged structure. The first-aid layer is already listed below; the four pieces work together without turning a science explainer into a gear roundup.

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 a damaging quake, stay clear of weakened structures and check the USGS event page or local alerts for updated guidance.

Magnitude tells you the energy released at the source. Local intensity and building conditions tell you what to do next.