TREMR

Every time a noticeable earthquake hits the news, the same confusion shows up in comments and group chats: "It was only a 5.2 — why did my whole building sway?" Or the opposite: "It was a 7.0 and I barely felt it." Both reactions can be right. Magnitude and intensity measure different things, and mixing them up is one of the most common misunderstandings in earthquake science.

Magnitude is a single number for the earthquake as a whole — roughly, how big the rupture was at the source. Intensity is a map of what that earthquake did to people, buildings, and ground at each place. One earthquake has one magnitude. It can have dozens of intensity values depending on where you stood.

What Magnitude Actually Measures

Modern magnitude scales — including the moment magnitude (Mw) used for most significant earthquakes — estimate the energy released when a fault slips. Seismologists combine the area of the rupture, how far the two sides of the fault moved, and the stiffness of the rock. The result is a single number that describes the event at its source, not what anyone felt on the surface.

That number is logarithmic. Each whole-number step is a huge jump in energy. A magnitude 6 releases about 32 times more energy than a magnitude 5, and a magnitude 7 about 1,000 times more than a magnitude 5. That is why a jump from 6.5 to 7.5 is not "a little bigger" — it is a different class of event.

Magnitude is also mostly independent of where you live. A Mw 6.8 offshore Japan and a Mw 6.8 under Iran are comparable as sources, even if the damage stories diverge completely.

What Intensity Measures

Intensity describes the severity of shaking and damage at a specific location. The scale most people in the Americas encounter is the Modified Mercalli Intensity (MMI) scale, usually written with Roman numerals from I to X+.

• I–II: Instrumental or barely felt by a few people indoors
• III–IV: Felt indoors; hanging objects may swing; parked cars may rock
• V–VI: Felt by most people; dishes break; some plaster cracks
• VII–VIII: Difficult to stand; furniture moves; poorly built structures damaged
• IX–X+: Heavy damage to buildings, ground cracking, landslides

Intensity is not calculated from a single seismometer the way magnitude is. It is compiled from felt reports, instrument recordings of ground motion, and damage surveys. USGS "Did You Feel It?" maps are intensity maps: crowdsourced observations turned into a picture of how shaking varied across a region.

Why the Same Magnitude Can Feel Completely Different

Three factors dominate what you experience:

Distance. Shaking generally decreases as you move away from the rupture. A shallow Mw 5.5 under your city can feel worse than a Mw 7.2 hundreds of kilometres away — even though the distant event released far more energy.

Depth. Shallow earthquakes put energy closer to the surface. A Mw 6.0 at 10 km often produces stronger ground motion than a Mw 6.0 at 120 km. Deep earthquakes can still be widely felt, but the energy has farther to travel and more rock to attenuate through.

Local geology. Soft sediments amplify shaking. Bedrock usually does not. Mexico City's 1985 disaster is the classic example: a large subduction earthquake hundreds of kilometres away produced extreme intensity on the ancient lake-bed under the capital, while harder ground nearby shook less. The magnitude was one number. The intensity map told the real story of where buildings failed.

A Useful Mental Model

Think of magnitude as the wattage of a lightbulb and intensity as how bright the room looks from where you are sitting. The bulb has one wattage. The brightness depends on distance, walls, and whether someone left the curtains open. Earthquake news headlines almost always report the bulb. Your body reports the room.

That is also why two neighbors can disagree after the same quake. One lives on soft fill and felt MMI VI. The other sits on rock a kilometre away and felt MMI IV. Same magnitude. Different intensity.

How Agencies Report Both

Right after a quake, agencies publish magnitude first because it can be estimated quickly from seismic networks. Intensity maps take longer: they need more stations, more felt reports, and often field assessments. On Tremr, the live feed shows USGS magnitude, depth, and location — the source parameters. Felt reports and alert levels, when available, are early clues about intensity near people.

Japan's seismic intensity scale (shindo) works the same conceptual way as MMI, though the numbers differ. A Japanese news report saying "shindo 5 lower in Yokohama" is an intensity statement about Yokohama, not a magnitude for the whole earthquake.

Famous Mismatches

1985 Mexico City: A distant subduction rupture produced catastrophic intensity on soft lake sediments while many closer hard-rock sites fared better. Magnitude explained the source. Intensity explained the deaths.

Deep focus events: Large deep earthquakes under the Pacific or South America sometimes register Mw 7+ yet produce only moderate intensity at the surface because the energy starts so far down.

Small, shallow urban quakes: A Mw 4.5 a few kilometres under a city can rattle nerves, crack plaster, and trend on social media — while a larger remote mid-ocean ridge event passes unnoticed except on maps like Tremr's.

What This Means for Preparedness

Do not let a "small" magnitude talk you out of taking shaking seriously if you are close and the ground is soft. And do not assume a large magnitude automatically means your specific neighborhood is in danger — check distance, depth, and local hazard maps.

Building codes, early warning systems, and home kits are intensity problems as much as magnitude problems. What damages your apartment is the ground motion at your address, not the number on the global headline. Understanding both numbers makes earthquake news clearer — and makes personal risk easier to judge when the next alert lands on your phone.