When an earthquake strikes, the first number in the news is its magnitude: 5.2, 6.8, 7.4. Those numbers look close together, but they describe enormous differences. A magnitude 7 earthquake releases far more energy than a magnitude 6, and understanding why helps make sense of the headlines.
Magnitude: the size of the earthquake
Magnitude measures how much energy an earthquake releases at its source. The scale is logarithmic, which means each whole number represents a big jump:
- Each step up the scale means about 10 times larger ground motion on a seismograph recording.
- Each step up means about 32 times more energy released.
So a magnitude 7 releases about 32 times more energy than a magnitude 6, and about 1,000 times more than a magnitude 5.
Richter vs. moment magnitude
The Richter scale, developed by seismologist Charles Richter in 1935, was the first widely used magnitude scale. It works well for small and moderate local earthquakes but underestimates very large ones.
Today, the U.S. Geological Survey and most seismologists use the moment magnitude scale for larger earthquakes. It is based on the physics of the fault rupture itself, including how much rock slipped and over what area, so it stays accurate even for the biggest quakes. News reports often still say “Richter scale” out of habit, but the number usually comes from moment magnitude or a related method.
Intensity: how strong the shaking feels
Magnitude is a single number for an earthquake. Intensity describes the shaking and damage at a particular place, and it varies from location to location. In the United States, it is measured on the Modified Mercalli Intensity scale, which uses Roman numerals from I (not felt) to XII (total destruction).
Three things largely decide how strongly you feel an earthquake:
- Distance from the source: shaking weakens with distance.
- Depth: shallow earthquakes generally cause stronger shaking at the surface than deep ones of the same magnitude.
- Ground conditions: soft soils and filled land can amplify shaking compared with solid rock.
That is why a moderate, shallow earthquake directly under a city can be more damaging than a much larger, deep one far away.
The biggest on record
The largest earthquake ever recorded was a magnitude 9.5 in Chile in 1960. The largest in U.S. history was the magnitude 9.2 Alaska earthquake of 1964. Earthquakes of that size occur at subduction zones, where one tectonic plate dives beneath another.
Small earthquakes are extremely common. Seismic networks around the world record many thousands each year, most of them too small to feel.
Early warning
In California, Oregon and Washington, the USGS operates ShakeAlert, an earthquake early warning system. It detects the first, faster waves from an earthquake and sends alerts to phones and other systems before the stronger shaking arrives, sometimes giving several seconds of warning. That may sound brief, but it can be enough to take cover, slow trains or stop surgery.
What to do when the shaking starts
Emergency officials recommend Drop, Cover and Hold On:
- Drop onto your hands and knees, before the shaking knocks you down.
- Cover your head and neck with your arms, and get under a sturdy table or desk if one is nearby.
- Hold on until the shaking stops.
Don’t run outside during shaking; many injuries come from falling debris near buildings. If you are in bed, stay there and protect your head with a pillow. If you are driving, pull over away from bridges, overpasses and power lines and stay in the car.
What we think
Magnitude is a useful headline number, but it describes the earthquake, not what happens to you. The shaking at your location, set by distance, depth and the ground beneath you, determines the damage.
For anyone in an earthquake-prone area, the most valuable preparation is surprisingly simple: secure heavy furniture, keep a basic emergency kit and practice Drop, Cover and Hold On until it becomes automatic.
