Earthquake magnitude vs. intensity, and why “felt” differs from “recorded”

After an earthquake, two kinds of number turn up in the news and in apps. One looks like M6.4. The other looks like intensity VI, or in Japan, shindo 5-. They sound alike, but they answer different questions. Magnitude says how big the earthquake was. Intensity says how hard the ground shook in one particular place.

The difference explains why a "small" quake can rattle your kitchen while a bigger one far away goes unnoticed. It also explains why the number sometimes changes an hour later, and why an earthquake list is a record rather than a warning.

Magnitude: the size of the earthquake

Magnitude is a single number for the whole earthquake. Seismologists work it out from instrument recordings. For most moderate and large quakes, the US Geological Survey (USGS) reports moment magnitude, written Mw. It is tied to the physical size of the event: how large an area of the fault slipped, and by how much.

The scale is logarithmic. Going up one whole number means about ten times more ground motion on a seismogram and roughly 32 times more energy released, according to USGS. Two steps compound: an M7 releases about a thousand times the energy of an M5 (32 × 32).

Magnitude says nothing directly about your street. An M7 deep under the ocean far offshore can be felt only lightly on land. A shallow M5 under a town can break windows.

Why the magnitude changes after the first report

The first number comes from automatic processing of the stations that have reported so far. Some seismic waves take over an hour to travel around the planet, and some network data arrives late, so USGS often updates a magnitude within a few hours and again during later catalog review (USGS FAQ). As a seismologist reviews the event, the location can change too. A quake first listed as M5.9 can become M6.1, or the other way round. The USGS event page shows whether an event is still automatic or has been reviewed. That's why two apps, or one app at two moments, can show slightly different numbers for the same earthquake.

Different agencies can also report different magnitudes for one quake. They use different methods and station networks. Japan's JMA, for example, has its own magnitude scale. A difference of a few tenths between agencies is normal.

Intensity: the shaking where you are

Intensity describes the effects of shaking at a specific place: on people, on objects, on buildings. One earthquake has a single magnitude but many intensities. Shaking is usually strongest near the fault and weaker farther away.

In the United States, intensity is reported on the Modified Mercalli Intensity (MMI) scale. It runs from I to XII in Roman numerals. A paraphrase of the lower and middle levels:

The USGS page on the Mercalli scale has the full descriptions.

USGS publishes intensity in two main ways. ShakeMap combines instrument readings with models of how shaking spreads and maps the estimated intensity across a region. Did You Feel It? collects reports from the public, turns each one into an intensity, and maps the results by area. Anyone can submit a report from the earthquake's USGS event page.

Why "felt" differs from "recorded"

Seismic networks record far more earthquakes than people notice. Many small quakes happen every day and are logged by instruments without anyone feeling them. And when people do feel a quake, the shaking at two addresses a few kilometres apart can differ a lot. The main reasons:

• Distance. Shaking generally weakens with distance from the part of the fault that slipped. For a large quake that can be tens or hundreds of kilometres long, not just the single epicenter point shown on a map.
• Depth. A shallow quake concentrates strong shaking near the epicenter. A deep one spreads gentler shaking over a wider area.
• Ground. Soft soil, river and lake sediment, and filled land can amplify shaking compared with nearby bedrock. Sediment-filled basins can also make shaking last longer.
• Buildings. Upper floors sway more than ground floors. Tall buildings can respond to slow, rolling motion from a large, distant quake that people at street level barely notice.
• What you were doing. People who are still, indoors or upstairs notice weak shaking far more often than people who are walking, driving or asleep.

A USGS listing that says an M4.2 happened 30 km away can be perfectly accurate even if nobody in your building noticed it.

Japan's seismic intensity scale (shindo)

Japan uses its own intensity scale, run by the Japan Meteorological Agency (JMA): seismic intensity, or 震度 (shindo). It has ten levels:

0, 1, 2, 3, 4, 5 lower (5弱), 5 upper (5強), 6 lower (6弱), 6 upper (6強) and 7.

Unlike Mercalli, shindo is measured by instruments: seismic intensity meters at observation sites across Japan, rather than surveys of damage or people's reports. It's also the number Japanese news leads with. After a quake you'll see a list of places with their shindo, plus one magnitude for the event. Shindo 5 lower near the epicenter and shindo 1 a long way off is a normal pattern for a single earthquake.

JMA publishes tables in English describing what people and buildings typically experience at each level. Shindo and MMI are separate scales built in different ways. A rough comparison can help, but there is no exact one-to-one conversion.

A record is not a warning

Earthquake catalogues such as the USGS latest earthquakes map report earthquakes after they happen. Any notification built on them describes an event that has already occurred. It cannot warn you that shaking is on its way.

Earthquake early warning is a separate system with a different purpose. It detects an earthquake that has already begun and estimates who will feel strong shaking, so alerts can go out to places the waves haven't reached yet. Near the epicenter there may be no time at all. Two examples:

• ShakeAlert, operated by USGS with state and university partners, covers California, Oregon and Washington. Its alerts reach phones through Wireless Emergency Alerts, the earthquake alerts built into Android and recent versions of iOS, and apps such as MyShake (delivery thresholds).
• JMA's Earthquake Early Warning (緊急地震速報) covers Japan, through TV, radio and mobile phones.

No agency can predict earthquakes. If you feel shaking or get an early warning, the advice from emergency officials is the same: drop, cover and hold on, and follow local authorities afterwards.

Reading an earthquake listing

A typical USGS entry looks like M5.8 – 165 km SSE of Vilyuchinsk, Russia. That line holds:

• the magnitude (one number for the event, which may still be revised);
• a place description, as a distance and direction from a reference town or landmark, not from you;
• the time, which USGS gives in UTC in its data, while many apps convert it to local time;
• on the event page, the depth, whether it has been reviewed, and links to ShakeMap and Did You Feel It? if they exist.

In Herebeat, earthquakes come from the USGS feed and show the magnitude, USGS's place description, the time and the distance from your place. Like the USGS list itself, they are records of quakes that happened, not early warnings.

If you want to know how strong the shaking was at your address, skip the magnitude and open the ShakeMap or Did You Feel It? map for the event, or check the shindo list in Japan.