Earthquake Magnitude Scale – How Earthquakes Are Measured

Earthquake magnitude is one of those numbers everyone recognizes, but not everyone understands. When you hear that a magnitude 7.1 earthquake struck somewhere, that number is not just a label. It tells us something important about the size of the earthquake at its source.

But magnitude does not tell the whole story. Depth, distance from people, local ground conditions, and building quality all affect how strongly an earthquake is felt and how much damage it may cause.

This page explains what earthquake magnitude means, why the Richter scale is not the main scale used for large earthquakes today, and why a one-number jump in magnitude is much bigger than it sounds.

What Is Earthquake Magnitude?

Magnitude measures the size of an earthquake at its source. It is based on seismic waves recorded by instruments called seismometers.

A key point: an earthquake has one magnitude, but it can have many different shaking intensities depending on where you are.

For example, a magnitude 6.5 earthquake deep below the surface may be felt gently over a wide area. A shallower magnitude 6.5 earthquake near a city may cause much stronger local shaking and more damage.

So magnitude answers this question:

How large was the earthquake itself?

It does not directly answer:

How badly did people feel it where they were?

That second question is about intensity, not magnitude.

What Happened to the Richter Scale?

The Richter scale was created in 1935 by Charles F. Richter at Caltech. It was originally designed for measuring earthquakes in Southern California using a specific type of instrument.

That made it useful for its time, but it has limits. The original Richter scale works best for smaller, local earthquakes. It becomes less reliable for very large earthquakes and for earthquakes recorded far away from the source.

That is why modern earthquake agencies usually rely on moment magnitude or another appropriate magnitude type for larger events

You may still hear people say “on the Richter scale” in news reports or casual conversation. Most of the time, they simply mean “earthquake magnitude.”

The Moment Magnitude Scale

For larger earthquakes, the most important modern scale is the moment magnitude scale, often written as Mw.

Moment magnitude is based on the physical size of the earthquake rupture. It considers things like:

how much of the fault moved
how far it slipped
how strong the surrounding rock is

This makes it better for measuring large earthquakes than the original Richter scale.

The U.S. Geological Survey explains that different magnitude types may be reported depending on the earthquake and available data. On this site, earthquake magnitudes come primarily from public USGS earthquake data, and preliminary values may change as more station data are processed

Why the Magnitude Scale Is Logarithmic

The earthquake magnitude scale is logarithmic. That means each whole-number increase is much bigger than the previous one.

A one-step increase in magnitude means approximately:

10 times greater recorded wave amplitude
about 31 times more energy released

A two-step increase is even more dramatic.

For example:

A magnitude 6 earthquake releases about 32 times more energy than a magnitude 5.
A magnitude 7 earthquake releases about 1,000 times more energy than a magnitude 5.
A magnitude 7 earthquake produces about 100 times the recorded wave amplitude of a magnitude 5.

That is why a magnitude 7 is not just “a bit stronger” than a magnitude 5. It is a completely different scale of event.

Common Magnitude Ranges

These categories are useful as general guides, but real-world impacts depend on depth, distance, population, soil conditions, and building construction.

Below Magnitude 2.0 — Micro Earthquakes

Micro earthquakes are usually not felt by people. They are detected by sensitive instruments and happen very frequently around the world.

Magnitude 2.0 to 3.9 — Minor Earthquakes

Minor earthquakes may be felt by people near the epicenter, especially indoors or in quiet conditions. They rarely cause damage.

Magnitude 4.0 to 4.9 — Light Earthquakes

Light earthquakes are often felt across a local area. They may rattle windows, shake objects, or feel like a brief jolt, but serious structural damage is uncommon in well-built buildings.

Magnitude 5.0 to 5.9 — Moderate Earthquakes

Moderate earthquakes can cause damage, especially if they are shallow and close to populated areas. Older, poorly built, or unreinforced structures are usually more vulnerable.

Magnitude 6.0 to 6.9 — Strong Earthquakes

Strong earthquakes can cause serious damage near the epicenter. The 1989 Loma Prieta earthquake in the San Francisco Bay Area and the 1994 Northridge earthquake in Southern California were both in this range.

Magnitude 7.0 to 7.9 — Major Earthquakes

Major earthquakes can cause severe damage over large areas, especially when they are shallow or close to cities. The 2010 Haiti earthquake was a tragic example of how a magnitude 7.0 event can be devastating when combined with shallow depth and vulnerable buildings.

Magnitude 8.0 to 8.9 — Great Earthquakes

Great earthquakes are rare and can affect very large regions. They are often associated with major plate-boundary ruptures and may generate tsunamis if they occur beneath the ocean and move the seafloor vertically.

Magnitude 9.0 and Above — The Largest Earthquakes

Magnitude 9+ earthquakes are among the most powerful natural events on Earth. The 1960 Valdivia earthquake in Chile, estimated at magnitude 9.5, remains the largest instrumentally recorded earthquake. The 1964 Alaska earthquake reached magnitude 9.2 and is the largest recorded earthquake in North America.

The 2011 Tōhoku earthquake in Japan reached about magnitude 9.0 to 9.1 and generated a devastating tsunami.

Magnitude vs. Intensity

Magnitude and intensity are often confused, but they are not the same thing.

Magnitude describes the size of the earthquake at its source. It is one number for the earthquake.

Intensity describes how strongly the earthquake was felt at a specific place. Intensity changes from location to location.

A person close to the epicenter may experience violent shaking, while someone much farther away may only feel gentle movement. Both people experienced the same earthquake, but not the same intensity.

In the United States, intensity is commonly described using the Modified Mercalli Intensity scale, which ranges from I, meaning not felt, to XII, meaning extreme damage.

Why Depth Matters

Depth is one of the most important details in earthquake reporting.

Earthquakes are often grouped as:

Shallow: 0 to 70 km deep.
Intermediate: 70 to 300 km deep.
Deep: below about 300 km, though the deepest earthquakes on Earth can occur much farther down.

Shallow earthquakes usually cause stronger shaking near the surface because the seismic energy has less distance to travel. A shallow magnitude 5.5 near a city may cause more local damage than a much deeper earthquake with a higher magnitude.

This is why you should never judge an earthquake by magnitude alone.

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