Magnitude: one number for the whole earthquake

Editorial explainer · Brown Brothers Media · The Watchers

When an earthquake appears in the headlines, two kinds of numbers often collide: magnitude and intensity. They sound interchangeable. They are not. Magnitude describes the size of the earthquake at its source. Intensity describes how strongly the ground shook at a particular place — and therefore how people and buildings experienced it. Confusing the two is one of the most common ways hazard reporting drifts into either understatement or hype.

This explainer walks through both scales in plain language, shows why the same earthquake can feel different a few tens of kilometres apart, and explains how watchers.LIVE and watchers.news use these terms when we write about seismic events. Publisher Brown Brothers Media holds both sites to the same factual standard; only the format and depth differ.

Magnitude: one number for the whole earthquake

Magnitude is an estimate of the energy released by the earthquake. Modern catalogues usually report moment magnitude (Mw), which is derived from the seismic moment — a product of the fault area that slipped, the average slip distance, and the rigidity of the rock. Older scales such as local magnitude (ML, the original “Richter” scale) and surface-wave magnitude (Ms) still appear in historical records and some regional networks, but they saturate for the largest events. Moment magnitude does not saturate in the same way, which is why Mw 9-class earthquakes can be meaningfully distinguished from Mw 8-class events.

A useful rule of thumb: each whole-number step in magnitude corresponds to roughly 32 times more energy released. An Mw 7.0 earthquake releases about 32 times the energy of an Mw 6.0, and about 1,000 times the energy of an Mw 5.0. That is why a “small” change in the reported magnitude — from 6.8 to 7.1, for example — is scientifically and societally significant. Journalists who treat every tenth of a unit as noise miss the physics.

Magnitude is assigned to the earthquake as a whole. There is not a separate magnitude for each city. Early automatic solutions from global networks can revise as more stations report, as depth is refined, and as moment-tensor analysis improves the estimate of faulting style. When watchers.LIVE posts a significant earthquake, we treat the first authoritative magnitude as provisional and note later revisions when they matter for interpretation. That discipline is part of the thresholds on our reporting criteria page.

Intensity: many numbers, one for each place

Intensity measures shaking severity at a location. The most widely used framework in the United States and many international products is the Modified Mercalli Intensity (MMI) scale, which runs from I (not felt) to X+ (extreme damage). Intensity is inferred from instrumental ground-motion recordings where available, and from felt reports and damage observations where instruments are sparse.

Because intensity depends on distance from the rupture, local geology, and building response, a single earthquake produces a map of intensities, not a single intensity value. Soft sediments can amplify shaking. Hard bedrock can reduce it. Basin effects, topography, and building height all change what people feel. That is why two towns thirty kilometres apart can report very different intensities for the same event.

The USGS “Did You Feel It?” system and ShakeMap products turn these observations into public maps. When we summarise an event, “felt strongly in City A” is an intensity statement. “Magnitude 6.4” is a magnitude statement. Both can be true at once. Readers who grasp that distinction are harder to mislead by a single scary number without place context.

Why depth and distance matter more than the headline number alone

A shallow Mw 5.5 under a city can cause more damage than a deep Mw 6.5 far offshore. Depth controls how much energy reaches the surface nearby. Distance controls how much that energy has attenuated by the time it arrives. Faulting style — strike-slip, reverse, normal — also shapes the radiation pattern. Magnitude without context is incomplete hazard literacy.

Tsunami potential adds another layer. Magnitude and mechanism matter, but so do water depth, rupture length, and whether significant seafloor displacement occurred. A large strike-slip earthquake may produce little tsunami; a smaller thrust event under the ocean may produce more. Intensity on land and tsunami hazard at the coast are related but separate questions. Official tsunami products, not magnitude alone, govern coastal response messaging.

How reporting should use both

Responsible coverage leads with magnitude for scale, then quickly adds depth, location relative to population, and early intensity or felt reports when available. It avoids converting magnitude into casual damage claims. It separates what instruments measured from what is still unknown. Advertising never sets those choices at The Watchers; thresholds are editorial, as explained on About.

For readers, the practical takeaway is simple. Magnitude answers “how big was the earthquake?” Intensity answers “how hard did it shake where I am?” When those two stay distinct in your mind, news about earthquakes becomes clearer, calmer, and more useful. Deeper case studies and scientific context continue on watchers.news; rapid desk updates live here on watchers.LIVE. For life-safety decisions, always follow local emergency authorities — not a news desk.

Further reading on how we source seismic products and handle corrections sits with our masthead materials. Magnitude and intensity are tools. Used carefully, they help the public understand Earth without turning every tremor into a crisis. Used carelessly, they become props for panic or false reassurance. The Watchers’ job is the careful version.