Magnitude vs. intensity
Magnitude describes the size of an earthquake at its source. Intensity describes how strongly the ground shook at a particular place. They answer different questions.
Key concepts
- One earthquake has one preferred magnitude
- Intensity varies from place to place
- The Modified Mercalli Intensity (MMI) scale
- Why a bigger magnitude does not always mean worse shaking where you are
Two different questions
When an earthquake happens, people usually want to know two things: how big was it, and how strong was the shaking where I live? These sound similar, but scientists answer them with two different measurements. Magnitude answers the first question. Intensity answers the second.
Mixing them up causes a lot of confusion. A news headline might say a magnitude 6 earthquake struck, while people a few kilometres apart describe very different experiences. Both can be true at the same time, because magnitude belongs to the earthquake while intensity belongs to a location.
Magnitude: the size of the earthquake
Magnitude is a single number that characterises the relative size of an earthquake at its source. Seismologists calculate it from the ground motion recorded by instruments called seismographs, corrected for the distance to the earthquake. Several magnitude types exist, such as local magnitude (ML) and moment magnitude (Mw). Moment magnitude is based on the area of the fault that slipped, how far it slipped and the stiffness of the rock, so it works well for large earthquakes.
Magnitude scales are logarithmic. Each whole-number step corresponds to roughly 32 times more energy released. A magnitude 7 earthquake therefore releases about 32 times more energy than a magnitude 6, and about 1,000 times more than a magnitude 5. Early magnitudes are often revised as more data arrive, so the number you see in the first minutes may change.
Intensity: what the shaking was like at a place
Intensity describes the effects of shaking at a specific location: how people felt it, whether objects fell and whether buildings were damaged. In the United States intensity is usually reported on the Modified Mercalli Intensity scale, written with Roman numerals from I (not felt) to X and above (extreme).
One earthquake produces many different intensities. Shaking generally weakens with distance from the rupture, but distance is not the only factor. The depth of the earthquake, the direction the rupture travelled, and local ground conditions all matter. Soft, water-saturated soils can amplify shaking compared with nearby solid rock, and the type and condition of buildings affect the damage people observe.
A worked example
Imagine one magnitude 6.5 earthquake. Town A sits 15 km from the epicentre on firm rock. Town B is 60 km away. Town C is also 60 km away but is built on soft river sediments. Town A might report strong shaking (around MMI VII), Town B moderate shaking (around MMI V), and Town C stronger shaking than Town B (perhaps MMI VI) despite being the same distance away. These numbers are illustrative only; real intensities come from instruments, felt reports and models such as ShakeMap.
The magnitude did not change between the three towns. Only the experience of shaking did.
How EQRadar shows each one
EQRadar shows magnitude in the event lists, the magnitude badges and the map colours, and labels the map legend as magnitude rather than damage. When USGS publishes a ShakeMap for an event, the Shaking Maps page shows the estimated intensity across the region, and you can look up the estimate for a specific place.
Remember that neither number alone tells you what happened to a particular building. Intensity estimates are not damage assessments, and local reports from officials are the authority after a damaging earthquake.
Three takeaways
- Magnitude describes the earthquake's size at its source; an earthquake has one preferred magnitude, although early values can be revised.
- Intensity describes shaking at a place and changes with distance, depth, rupture direction and local ground conditions.
- Each whole step in magnitude means about 32 times more energy, but a larger magnitude does not automatically mean stronger shaking where you are.
Sources
- Earthquake Magnitude, Energy Release, and Shaking Intensity — U.S. Geological Survey
- The Modified Mercalli Intensity Scale — U.S. Geological Survey
- Magnitude Types — U.S. Geological Survey