Magnitude vs Intensity: Why One Quake Feels Different in Each Town

Magnitude measures the size of an earthquake at its source, and each earthquake has one preferred magnitude. Intensity describes how strongly the ground shook at a particular place, so a single earthquake has many intensity values. In the United States, intensity is rated on the Modified Mercalli Intensity (MMI) scale, written in Roman numerals.

A USGS example shows the gap. The October 16, 1999, Hector Mine earthquake in California had a magnitude of 7.1. Shaking intensity reached VII (very strong) in the towns nearest the epicenter and II to III (weak) as far as 300 miles (500 km) away.

What is the difference between magnitude and intensity?

	Magnitude	Intensity
What it describes	The size of the earthquake at the source	The strength of shaking at one location
Values per earthquake	One preferred magnitude	Many, one for each place
How it is written	Decimal number, such as 6.4	Roman numeral, such as VI
What it is based on	Seismic waves recorded on seismographs	Recorded ground motion and observed effects on people and buildings
Where USGS shows it	Event page title and origin details	ShakeMap and “Did You Feel It?” maps

For more on the number itself, see our guide to earthquake magnitude. USGS answers the same question in its FAQ on the difference between magnitude and intensity.

What do the Modified Mercalli Intensity levels mean?

The MMI scale was developed in 1931 by American seismologists Harry Wood and Frank Neumann. USGS notes that it has no mathematical basis. It is a ranking based on observed effects. The lower levels describe how people feel the shaking, and the higher levels are based on damage to structures.

The scale runs from I to XII. The abbreviated USGS table covers I to X, summarized here.

Intensity	Shaking	Typical observed effects (USGS, abbreviated)
I	Not felt	Felt only by a very few people under especially favorable conditions
II	Weak	Felt by a few people at rest, especially on upper floors
III	Weak	Felt quite noticeably indoors, especially on upper floors; many people do not recognize it as an earthquake; parked cars may rock slightly
IV	Light	Felt indoors by many and outdoors by few during the day; some people awakened at night; dishes and windows rattle
V	Moderate	Felt by nearly everyone; many awakened; some dishes and windows broken; unstable objects overturned
VI	Strong	Felt by all, many frightened; some heavy furniture moved; damage slight
VII	Very strong	Damage negligible in well-designed buildings and considerable in poorly built ones; some chimneys broken
VIII	Severe	Slight damage in specially designed structures; partial collapse in ordinary buildings; chimneys, monuments and walls can fall
IX	Violent	Well-designed frame structures thrown out of plumb; partial collapse of substantial buildings; some shifted off foundations
X	Extreme	Most masonry and frame structures destroyed with their foundations

The full wording is on the USGS page for the Modified Mercalli Intensity Scale.

Why does the same earthquake feel different in different places?

USGS says intensity varies from place to place mostly with distance from the fault rupture. Shaking weakens as seismic waves travel, so towns farther away usually report lower intensity.

Depth works the same way. USGS notes that shaking at the surface from an earthquake 500 km (311 miles) deep is considerably weaker than if the same earthquake had happened 20 km (12 miles) deep. Our guide to earthquake depth covers this factor in detail.

Local ground matters too. USGS research shows that shaking on soft sediment can last about 3 times as long as on stable bedrock such as granite. A USGS fact sheet on southern California found that shaking levels double from the edge of the Los Angeles Basin, where sediments are thin, to its deeper center. In San Francisco, damage from both the 1906 earthquake and the 1989 Loma Prieta earthquake was concentrated in areas on soft soil deposits.

Buildings and floors change what people notice. The lower MMI levels mention people on upper floors feeling shaking first, and USGS lists building design among the factors that decide how destructive an earthquake is.

What is a USGS ShakeMap?

A ShakeMap is a near-real-time map of ground motion and shaking intensity after a significant earthquake. USGS produces it with the regional seismic networks, and preliminary maps typically appear within several minutes of the earthquake.

The map starts from peak ground motions recorded on seismic sensors. Where stations are sparse, ShakeMap fills gaps with estimated values and applies corrections for local site amplification. Intensity colors come from relationships between recorded ground motion and MMI.

A ShakeMap describes the shaking, not the earthquake source. USGS says agencies and companies use ShakeMaps for response, recovery, public information and planning. See the USGS ShakeMap page for maps of recent events.

How does “Did You Feel It?” turn reports into intensity?

“Did You Feel It?” (DYFI) is a USGS questionnaire that people fill out after an earthquake. USGS introduced it in 1999, and a 2020 USGS paper counted more than 5 million reports.

USGS groups responses into communities, such as ZIP code areas or cities, or into map boxes 1 km or 10 km across when people give a precise location. For each community, the answers become a Community Decimal Intensity (CDI). USGS calibrated CDI so that, on average, it is similar to MMI, though it says DYFI is not identical to the older MMI survey method.

Reports of not feeling the earthquake count. USGS says not-felt responses keep ZIP code averages from being too high and show where shaking stopped being noticed. Automatic DYFI maps are generally made for magnitude 3.8 and greater, or 2.8 in certain urban areas, and DYFI data also help constrain ShakeMap and the PAGER impact estimate.

To add your own report:

1. Find the earthquake on the USGS Latest Earthquakes map or in our list of recent earthquakes in California and the other state pages.
2. Open the USGS event page and choose the “Did You Feel It?” form.
3. Answer the questions for the place where you were when it happened, even if you felt nothing.
4. Check back later. USGS updates the DYFI page each time it receives new responses.

Which number should you look at after an earthquake?

Use magnitude to compare the size of earthquakes. Use ShakeMap and DYFI intensity to see how strong the shaking was in a given town. For larger events, USGS adds a PAGER alert level that estimates impact, explained in our guide on how to read a USGS earthquake report. You can check recent events in our live list of USGS earthquakes.

Bottom line

Magnitude is one number for the earthquake. Intensity is the shaking at a location, and it depends on distance, depth, local ground and buildings. The USGS figures show why a distant magnitude 7 can register only II to III in one town while a nearby magnitude 5 typically reaches VI to VII close to its epicenter.

FAQ

Is the Mercalli scale the same as the Richter scale?

No. The Richter scale (ML) is a magnitude, one number for the size of the earthquake. The Modified Mercalli scale rates shaking at a place, from I to XII, based on observed effects.

Why is the DYFI map different from the ShakeMap?

DYFI uses reports from people, grouped by ZIP code, city or map box. ShakeMap uses instrument recordings with estimates between stations. USGS uses DYFI data to help constrain ShakeMap, but the two products are made in different ways.

Can I report an earthquake days later?

Yes. USGS says it does not enforce a cutoff date for DYFI contributions. You cannot change a report after you submit it.

At what intensity does damage start?

In the abbreviated USGS table, damage is slight at VI, considerable in poorly built structures at VII and heavier from VIII upward.