Earthquake Magnitude and Intensity Explained
What the numbers mean, how earthquakes are measured, and what different magnitudes actually feel like.
Key Takeaway
Earthquake magnitude is logarithmic — each whole number increase represents about 31.6 times more energy. A magnitude 5.0 is not "a little worse" than a 4.0; it releases 31.6 times more energy. Depth, distance, and local soil conditions determine how much shaking you actually feel.
Magnitude — Measuring Energy at the Source
Magnitude quantifies the total energy an earthquake releases at its point of origin (the hypocenter). It is a single number assigned to each earthquake, regardless of where you are when you feel it. The scale used today by seismologists worldwide is the Moment Magnitude Scale (Mw), which replaced the original Richter scale for most purposes.
The Moment Magnitude Scale is logarithmic. Each whole-number increase corresponds to a tenfold increase in measured ground displacement and approximately 31.6 times more energy released. This means a magnitude 7.0 earthquake doesn't release "a bit more" energy than a 6.0 — it releases over 31 times more. A magnitude 8.0 releases roughly 1,000 times the energy of a 6.0.
Magnitude Ranges and Their Effects
Here's what different magnitude ranges typically mean in practice:
| Magnitude | Classification | Typical Effects | Annual Count |
|---|---|---|---|
| 2.0–2.9 | Minor | Generally not felt, recorded by instruments | ~1,000,000 |
| 3.0–3.9 | Minor | Felt by many nearby, rarely causes damage | ~100,000 |
| 4.0–4.9 | Light | Noticeable shaking, rattling objects, minimal damage | ~10,000 |
| 5.0–5.9 | Moderate | Can cause damage to poorly constructed buildings | ~1,000 |
| 6.0–6.9 | Strong | Damaging over populated areas up to 100 miles | ~100 |
| 7.0–7.9 | Major | Serious damage over large areas | ~15 |
| 8.0+ | Great | Devastating over hundreds of miles | ~1 |
Areazine tracks earthquakes of magnitude 4.0 and above globally and 2.5+ within the United States, using data from the USGS. Check our earthquakes page for the latest seismic activity.
Intensity — Measuring Shaking Where You Are
While magnitude describes the earthquake itself, intensity describes the effects at a specific location. The Modified Mercalli Intensity (MMI) scale runs from I (not felt) to XII (total destruction) and is based on observed damage and human reports, not instruments.
A single earthquake has one magnitude but many intensities. The shaking you feel depends on:
- Distance from the epicenter: Shaking weakens with distance, though it doesn't diminish uniformly.
- Earthquake depth: Shallow earthquakes (under 20 km) produce stronger surface shaking than deep ones. A shallow magnitude 5.5 can feel far more violent than a deep magnitude 6.5.
- Local geology: Soft soil and sediment amplify shaking dramatically compared to bedrock. This is why San Francisco's Marina District (built on fill) suffered far worse damage in 1989 than neighborhoods on solid rock just a few miles away.
- Building construction: Engineered buildings with seismic design can withstand shaking that destroys unreinforced masonry structures of the same age.
Depth Matters More Than You Think
Earthquake depth is one of the most underappreciated factors in determining how dangerous an earthquake is. The USGS classifies earthquakes as shallow (0–70 km), intermediate (70–300 km), or deep (300–700 km).
Most destructive earthquakes are shallow. The devastating 2010 Haiti earthquake (magnitude 7.0) had a depth of only 13 km. The 2023 Turkey-Syria earthquake (magnitude 7.8) was at 17.9 km. By contrast, a magnitude 7.0 earthquake at 500 km depth might produce only gentle rolling motion at the surface.
When Areazine reports an earthquake, we always include depth alongside magnitude because both numbers are essential for understanding the true hazard.
Early Warning Systems
The USGS operates ShakeAlert, an earthquake early warning system for the US West Coast. It detects earthquakes that have already begun and sends alerts to phones and infrastructure systems before the strongest shaking arrives. Because seismic waves travel through the earth, there is a brief window — seconds to tens of seconds — between when sensors detect a large earthquake and when damaging shaking reaches populated areas.
ShakeAlert is integrated into the Wireless Emergency Alert (WEA) system on cell phones. If you live in California, Oregon, or Washington, your phone will automatically receive alerts for earthquakes expected to produce MMI IV or stronger shaking at your location. No app installation is required.
Frequently Asked Questions
What is the difference between magnitude and intensity?
Magnitude measures the energy released at the earthquake's source — it's a single number for each event. Intensity measures the shaking felt at a specific location — it varies depending on distance from the epicenter, depth, and local geology. A magnitude 6.0 earthquake has one magnitude but may produce intensity VIII near the epicenter and intensity III 100 miles away.
Is the Richter scale still used?
The original Richter scale (local magnitude, ML) is largely replaced by the Moment Magnitude Scale (Mw) for earthquakes above magnitude 3.5. Mw is more accurate for large earthquakes because it doesn't saturate at high magnitudes the way the Richter scale does. When news reports say "magnitude 7.0," they almost always mean Mw, even if they mention "Richter."
How much stronger is a magnitude 7 than a magnitude 6?
A magnitude 7.0 earthquake releases approximately 31.6 times more energy than a magnitude 6.0. The scale is logarithmic: each whole number increase represents about 31.6 times more energy. This means a magnitude 8.0 releases roughly 1,000 times more energy than a magnitude 6.0 (31.6 × 31.6).
Can earthquakes be predicted?
No. Despite decades of research, scientists cannot predict when, where, or how large a future earthquake will be. Seismologists can identify areas with higher long-term risk based on fault geology and historical activity, but they cannot forecast specific events. Early warning systems (like ShakeAlert in the US) detect earthquakes that have already started and provide seconds to minutes of warning before shaking arrives.
What magnitude earthquake can people feel?
Most people can feel earthquakes starting at about magnitude 3.0 if they are near the epicenter. Below 2.5, earthquakes are rarely felt by anyone. Above 5.0, earthquakes cause varying degrees of damage depending on depth, building construction, and distance. A shallow magnitude 5.0 can cause significant damage in poorly built areas.
What is the largest earthquake ever recorded?
The largest instrumentally recorded earthquake was the 1960 Great Chilean Earthquake at magnitude 9.5. It generated a tsunami that crossed the Pacific Ocean and caused damage as far away as Japan and the Philippines. The 2011 Tōhoku earthquake (Japan) at magnitude 9.1 and the 2004 Indian Ocean earthquake at magnitude 9.1 are among the largest in modern records.
Sources
- U.S. Geological Survey (USGS) — earthquake.usgs.gov
- USGS Earthquake Hazards Program — Magnitude vs. Intensity comparison
- USGS ShakeAlert — shakealert.org
This content is for informational purposes only. During an earthquake, follow official guidance: Drop, Cover, and Hold On. If you are near the coast after a strong earthquake, move to higher ground immediately — do not wait for an official tsunami warning.