Earthquake magnitude is a single number that represents the overall size of an earthquake, specifically how much energy it released at its source. Every earthquake gets one magnitude value, no matter where you are when you feel it. A magnitude 5.0 earthquake is a magnitude 5.0 whether you’re standing a mile from the fault or a hundred miles away. What changes with distance is the shaking you experience, which is a separate measurement called intensity.
How Magnitude Is Measured
Most people have heard of the Richter scale, but seismologists no longer use it for moderate to large earthquakes. Charles Richter developed his scale in the 1930s for Southern California, and as seismograph stations spread around the world, scientists found his method was only accurate within certain distance and frequency ranges. It worked well for small, local earthquakes but became unreliable for the large events that matter most.
The replacement is the moment magnitude scale, abbreviated Mw. It measures the total energy released by an earthquake based on three physical properties: the area of the fault that slipped, how far it slipped, and how rigid the surrounding rock is. Because it’s tied to the actual physics of the rupture rather than instrument readings at a specific distance, moment magnitude gives reliable size estimates across the full range of earthquakes, from tiny tremors to the most catastrophic events on record. When you see a magnitude reported in the news today, it’s almost always moment magnitude.
Why the Numbers Are Deceptive
Magnitude uses a logarithmic scale, which means the gap between numbers is far larger than it looks. Each whole number increase represents roughly 10 times more ground shaking and about 31.6 times more energy released. A magnitude 7.0 earthquake doesn’t release twice the energy of a 6.0. It releases over 31 times more. Jump two whole numbers and the difference is staggering: a magnitude 8.0 releases about 1,000 times more energy than a 6.0.
This is why the difference between a 6.5 and a 7.5 earthquake isn’t just “one point worse.” That single digit represents a fundamentally different event in terms of destructive potential. It also explains why truly massive earthquakes are so rare. The amount of energy required for each step up the scale is enormous, and only the longest, most active fault systems on Earth can accumulate and release that much strain.
What Different Magnitudes Feel Like
Earthquakes below about 2.5 happen constantly around the world, millions of times per year, but they’re too small for people to notice. You’d need instruments to detect them. Between 2.5 and 4.0, people near the epicenter often feel shaking, but damage is rare. These get reported to local seismological agencies but rarely make the news.
At magnitudes 4.0 to 5.0, the shaking becomes obvious indoors. Objects rattle on shelves, hanging lights swing, and the event feels unmistakable. Damage is still uncommon at this range, though older or poorly constructed buildings can develop cracks. Earthquakes in the 5.0 to 6.0 range cause moderate damage near the epicenter: broken windows, fallen plaster, and in vulnerable structures, partial collapses.
From 6.0 to 7.0, the potential for serious destruction increases sharply, particularly in populated areas without earthquake-resistant construction. Buildings can collapse, roads buckle, and landslides are common in hilly terrain. Above 7.0, events are classified as major earthquakes. These can devastate entire regions, and if they occur under the ocean, they can generate tsunamis. Above 8.0, the scale of destruction extends across hundreds of miles.
The largest earthquake ever recorded by instruments struck off the coast of southern Chile on May 22, 1960, at magnitude 9.5. The rupture extended along roughly 600 miles of fault, triggered tsunamis that crossed the Pacific Ocean, and led directly to the creation of the International Tsunami Warning System.
How Often Large Earthquakes Happen
The relationship between magnitude and frequency is predictable in a statistical sense. Millions of tiny earthquakes (below magnitude 2.0) occur every year worldwide, though they’re only detected by sensitive instruments. Earthquakes large enough to feel happen thousands of times per year. Major earthquakes, magnitude 7.0 and above, occur on average about 15 times per year globally. A “great” earthquake of magnitude 8.0 or higher happens roughly once a year on average, though there’s significant variation. Some years see several; others see none.
Magnitude vs. Intensity
One of the most common sources of confusion is the difference between magnitude and intensity. Magnitude is a property of the earthquake itself. It’s fixed. Intensity describes the shaking at a specific location and varies depending on how far you are from the fault, what type of soil or rock you’re standing on, and how deep the earthquake was. A single earthquake produces one magnitude but many different intensity values spread across the surrounding area.
In the United States, intensity is measured on the Modified Mercalli Intensity Scale, which runs from I (not felt at all) to XII (total destruction). Other countries use different intensity scales. Intensity has traditionally been a subjective measure, based on human reports of what people felt and what damage occurred, though modern instruments now also produce automated shaking maps. Two earthquakes of identical magnitude can produce very different intensity patterns depending on depth, geology, and distance to populated areas. This is why a magnitude 6.0 earthquake can be barely newsworthy in a remote area but catastrophic beneath a city.
Why Depth Matters
The depth of an earthquake’s origin, called the hypocenter, dramatically affects how dangerous it is at the surface. A shallow earthquake (less than about 20 kilometers deep) concentrates its energy close to the surface, producing intense shaking over a smaller area. A deeper earthquake spreads its energy more broadly, so the shaking is weaker but affects a wider region. Two earthquakes of the same magnitude can feel completely different depending on whether the rupture happened 5 kilometers down or 100 kilometers down. When you see a magnitude reported in the news, checking the depth gives you a much better sense of the potential impact than the magnitude number alone.