Earthquake Scale Explained: Richter vs. Magnitude
The Richter scale is history. Here is how earthquakes are actually measured today, what every magnitude range means in practice, and how to prepare based on where you live.
The Richter Scale Is Retired — Here Is What Replaced It
Every time an earthquake makes news, reporters reach for the same word: Richter. There is only one problem — seismologists stopped using the Richter scale decades ago. Understanding how earthquakes are actually measured, what the numbers mean for your home, and where the next major event is likely to strike is practical knowledge, not trivia.
The Richter Scale: How It Worked and Why It Was Replaced
Charles Richter developed his magnitude scale in 1935 at Caltech — a tool for ranking Southern California earthquakes on a specific seismograph. The scale is logarithmic, with each whole number representing roughly 31 times more energy released. It worked well within its original scope, but had hard limits: it only worked accurately within about 600 kilometers of the epicenter, it saturated (very large earthquakes all clustered near magnitude 8 no matter their actual size), and it required a specific instrument no longer in standard use. By the 1970s, the field needed a unified replacement.
The Moment Magnitude Scale (Mw): What Scientists Use Today
The Moment Magnitude Scale (Mw), developed in 1979 by Thomas Hanks and Hiroo Kanamori, calculates magnitude from seismic moment: the area of fault that ruptured, the average slip distance, and the rigidity of the rock. It works for all earthquake sizes without saturating — meaning it accurately distinguishes between a magnitude 8.5 and a magnitude 9.5, something the Richter scale could not do. It applies globally regardless of distance or instrument type.
When USGS reports an earthquake magnitude today, it is Mw. The numbers are close to the old Richter scale for mid-size quakes — which is why the public never noticed the switch — but they diverge at the high end, where accuracy matters most.
Earthquake Magnitude Chart: What Each Level Means in Practice
The scale is logarithmic — a magnitude 7.0 releases roughly 1,000 times more seismic energy than a magnitude 5.0. Here is what each range means on the ground:
| Magnitude | Frequency (global, per year) | What You Feel | Typical Damage |
|---|---|---|---|
| Below 2.0 | Millions | Not felt | None |
| 2.0 to 2.9 | About 1 million | Rarely felt | None |
| 3.0 to 3.9 | About 100,000 | Often felt, no damage | None |
| 4.0 to 4.9 | About 13,000 | Widely felt; rattles dishes, objects fall | Minor to some in vulnerable structures |
| 5.0 to 5.9 | About 1,300 | Strong shaking; felt by all | Moderate to older or poorly built structures |
| 6.0 to 6.9 | About 130 | Destructive in populated areas | Significant damage to well-built structures |
| 7.0 to 7.9 | About 15 | Major — serious damage over wide area | Collapses likely; major infrastructure damage |
| 8.0 to 8.9 | About 1 | Great — catastrophic damage | Widespread collapse; devastation over large region |
| 9.0 and above | Rare (about 1 per decade) | Mega-quake | Near-total destruction near epicenter; tsunamis |
Practical threshold: Magnitude 5.0 is where unreinforced masonry and older construction starts to fail. At magnitude 6.0, wood-frame homes can sustain structural damage. At magnitude 7.0 and above, collapses become routine outside modern seismic code construction.
The Modified Mercalli Intensity Scale: Shaking Where You Stand
Mw measures total energy at the source. The Modified Mercalli Intensity (MMI) Scale measures what happens at a specific location — how hard the ground shook at your address. One earthquake produces very different MMI readings across a region:
| MMI Level | Description | What You Experience |
|---|---|---|
| I | Not felt | No perception |
| II to III | Weak | Felt by some indoors; hanging objects swing |
| IV | Light | Felt indoors by many; rattling dishes and windows |
| V | Moderate | Felt by nearly all; some dishes broken; small objects shifted |
| VI | Strong | Felt by all; heavy furniture moves; slight damage to poorly built structures |
| VII | Very Strong | Difficult to stand; moderate damage to ordinary structures |
| VIII | Severe | Partial collapse of ordinary buildings; chimneys fall |
| IX | Violent | Many buildings destroyed; ground cracks |
| X to XII | Extreme to Catastrophic | Most structures destroyed; landslides; railroad rails bent |
USGS publishes “Did You Feel It?” MMI maps within minutes of any significant earthquake, crowdsourced from public reports.
Why both scales matter: A magnitude 6.5 centered directly under a city produces MMI VIII or IX — catastrophic. The same magnitude at 50 kilometers depth produces MMI V or VI — strongly felt but manageable in modern buildings. Depth and distance matter as much as raw magnitude.
Historical Earthquakes as Reference Points
1906 San Francisco Earthquake (Magnitude 7.9)
The 1906 San Francisco earthquake struck at 5:12 AM on April 18, rupturing roughly 477 kilometers of the San Andreas Fault. The shaking collapsed thousands of buildings, but fires ignited from broken gas lines — burning for three days after water mains ruptured — killed an estimated 3,000 people and destroyed 28,000 buildings. The event reshaped California building codes and introduced the now well-understood secondary hazard: post-earthquake fire.
1964 Great Alaska Earthquake (Magnitude 9.2)
The Good Friday earthquake, March 27, 1964, was the second-largest ever recorded. The fault ruptured along 800 kilometers of the Aleutian Megathrust, shaking lasted roughly four minutes, and tsunamis reached Crescent City, California. The death toll was 139 — low relative to the energy released, primarily because of low coastal population density. The 1964 event confirmed that subduction zone earthquakes could exceed magnitude 9.
2011 Tohoku Earthquake (Magnitude 9.0)
The Tohoku earthquake struck off the Pacific coast of Japan on March 11, 2011, rupturing a fault segment roughly 500 kilometers long. The resulting tsunami reached 40 meters in some locations and inundated 561 square kilometers of coastline. Approximately 19,000 people died, three nuclear reactors melted down at Fukushima Daiichi, and economic damage exceeded 200 billion dollars. Japan had world-class tsunami warning systems and vertical evacuation structures — and the scale still overwhelmed many of them. The lesson: preparation reduces casualties but cannot eliminate them at the upper end of the magnitude scale.
The Cascadia Subduction Zone: The Pacific Northwest’s Overdue Threat
Off the coasts of northern California, Oregon, Washington, and British Columbia lies the Cascadia Subduction Zone — a 1,000-kilometer fault where the Juan de Fuca plate dives beneath North America. Geologic evidence including buried coastal marshes, drowned forests, and Japanese tsunami records shows it ruptures in full-margin events roughly every 200 to 530 years.
The last full rupture was January 26, 1700 — estimated at magnitude 8.7 to 9.2. Current estimates put the probability of a full-margin rupture in the next 50 years at roughly 10 to 15 percent. FEMA modeling projects such an event could kill more than 10,000 people, injure 30,000 more, and displace up to a million, while a resulting tsunami reaches Oregon and Washington coastal areas within 15 to 30 minutes.
If you live in the Pacific Northwest coastal zone, treat a magnitude 9 scenario as a planning baseline, not a worst case.
How Earthquake Early Warning Systems Work
Early warning is not prediction — no one can predict when an earthquake will strike. What early warning systems do is detect the beginning of an earthquake and send an alert faster than the damaging waves can travel. The fast-moving P-wave (primary wave) arrives first but causes little damage; the slower S-wave (secondary wave) causes the side-to-side shaking that collapses buildings. Systems detect the P-wave, estimate magnitude and location, and push an alert before the S-wave arrives.
ShakeAlert: Western U.S. Coverage
ShakeAlert is the USGS early warning system covering California, Oregon, and Washington, built on a network of over 1,675 seismic sensors. It detects the P-wave, calculates preliminary magnitude and location, and sends an alert before the damaging S-wave and surface waves arrive.
How alerts reach you: Wireless Emergency Alerts (WEA) on cell phones, the MyShake app (Android and iOS), NOAA Weather Radio, and Google’s built-in Android earthquake alert system.
Lead time is real but limited. Close to the epicenter you may get only a few seconds. At 100 kilometers out, lead time can reach 30 to 40 seconds — enough to drop and take cover before shaking arrives. ShakeAlert does not yet cover the eastern U.S., though the New Madrid Seismic Zone would benefit from an eastern extension.
What Magnitude Causes Structural Damage to Typical Homes
Wood-frame construction performs better in earthquakes than unreinforced masonry or older concrete, but it is not immune.
- Magnitude 4.5 to 5.4: Most wood-frame homes survive without structural damage. Chimneys may crack; unsecured items fall from shelves.
- Magnitude 5.5 to 6.4: Plaster cracks, older pre-code homes may sustain structural damage, unreinforced masonry suffers significant cracking and partial collapse.
- Magnitude 6.5 to 7.4: Modern homes sustain moderate to significant damage. Soft-story apartment buildings face collapse risk. Infrastructure failures — water mains, gas lines, bridges — become widespread.
- Magnitude 7.5 and above: Catastrophic damage to all but specially engineered structures. Expect no water, gas, or reliable cell service for days to weeks.
The practical threshold for preparing your home to resist damage starts at magnitude 5.0 — and the structural hardening steps (chimney bracing, water heater strapping, cripple wall bolting) are the same regardless of which end of that range you live in.
Aftershocks: Why They Happen and How Long They Last
After any significant earthquake, the surrounding crust adjusts to the new stress distribution created by the mainshock. These secondary earthquakes are aftershocks.
Omori’s Law describes aftershock decay: the rate of aftershocks decreases roughly proportional to the inverse of time elapsed since the mainshock. Aftershocks are frequent immediately after the mainshock, then taper steadily. But “taper” does not mean “stop.”
General aftershock patterns:
- The largest aftershock is typically about one magnitude unit smaller than the mainshock (Båth’s Law)
- After a magnitude 6.0, expect aftershocks above magnitude 4.0 for days to weeks
- After a magnitude 7.0, significant aftershocks (5.0 and above) can continue for months
- After a magnitude 9.0 like Tohoku, magnitude 7 aftershocks continued for years
Why aftershocks are dangerous: They strike buildings already weakened by the mainshock. A structure that survived a magnitude 7.0 may fail in a magnitude 6.0 aftershock that would have been harmless before. Never re-enter a damaged structure until it has been assessed by a structural engineer or cleared by your local building department.
How to Interpret USGS Earthquake Alerts and Maps
USGS Earthquake Hazards Program (earthquake.usgs.gov) is your primary source after any significant event. Key tools: the Latest Earthquakes Map (near-real-time global display), Did You Feel It? (community shaking reports that build Mercalli intensity maps within minutes), ShakeMap (geographic distribution of ground motion and MMI), and PAGER (rapid casualty and economic loss estimates with color-coded alert levels).
On any event page, check depth (shallow quakes cause more surface damage), fault type (thrust faults generate tsunamis; strike-slip generally do not), and the aftershock forecast.
Prepare Based on Your Local Seismic Risk
Not all zip codes face the same risk. USGS publishes a National Seismic Hazard Map at earthquake.usgs.gov/hazards/hazmaps/ showing probabilistic ground motion across the entire country. The highest hazard zones are along the Pacific Coast, in the New Madrid Seismic Zone (Missouri, Arkansas, Tennessee, Kentucky), and in parts of the Rocky Mountain region.
Know your zone first. The map tells you the probability of experiencing specific ground motion levels over 50 years — the same data that drives building codes in your area.
Harden your home. The most common residential vulnerabilities: unreinforced masonry chimneys (crack or topple in magnitude 5 to 6 range), unbraced cripple walls, unstrapped water heaters, unsecured bookcases and overhead storage, and soft-story construction over open parking. Addressing these before an earthquake is cheaper and more effective than any amount of post-disaster supply.
Build a 7-day supply baseline. After a major earthquake, water mains may be broken, roads impassable, and emergency services overwhelmed. Store at least 1 gallon of water per person per day for 7 days, plus food that requires no cooking, a first aid kit, and a battery-powered or hand-crank NOAA weather radio.
Make a reunification plan. Cell networks overload immediately after major earthquakes. Agree on a meeting point and an out-of-area contact before an event. Designate a backup contact if the primary is unreachable.
Practice Drop, Cover, Hold On. When shaking starts: drop to hands and knees, get under a sturdy table or against an interior wall away from windows, hold on and cover your head and neck. Do not run outside during shaking. The “doorway” myth persists — modern doorframes offer no special protection. A sturdy table is better.
The Bottom Line
The Richter scale is a historical artifact. Mw is what scientists measure; MMI is what you feel at your address. Magnitude determines total energy released — but depth, distance, local soil type, and building construction determine whether that energy collapses your neighborhood or just rattles your dishes.
Build your preparedness around your specific seismic risk. Start with our earthquake preparedness guide for actionable steps, and if you are in a coastal seismic zone, read our tsunami preparedness guide — the two hazards arrive together.
Frequently Asked Questions
Is the Richter scale still used?
Not by scientists. The Richter scale was developed in 1935 for Southern California earthquakes measured on a specific type of seismograph. It breaks down for large or distant quakes. Modern seismologists use the Moment Magnitude Scale (Mw), which works for all earthquake sizes and locations. When you see a magnitude number reported by USGS today, it is almost always Mw — though news outlets still say 'Richter' out of habit.
What magnitude earthquake causes damage?
Magnitude 4.0 to 4.9 quakes are felt widely and can rattle objects off shelves. Structural damage to poorly built or older buildings begins around magnitude 5.0. Significant damage to well-built structures starts near magnitude 6.0. Major damage and collapsed buildings become common at magnitude 7.0 and above. Catastrophic, widespread destruction occurs at magnitude 8.0 and higher.
How long do aftershocks last after a major earthquake?
Aftershocks follow a predictable decay pattern described by Omori's Law: frequency drops roughly proportional to the inverse of time since the mainshock. After a magnitude 7 quake, aftershocks can continue for months, with the largest typically about one magnitude unit smaller than the mainshock. After a magnitude 9 event like the 2011 Tohoku earthquake, aftershocks continued for years and included multiple magnitude 7 events.
What is ShakeAlert and how does it work?
ShakeAlert is the USGS earthquake early warning system covering California, Oregon, and Washington. It detects the fast-moving but less destructive P-wave from an earthquake and sends an alert before the slower, damaging S-wave and surface waves arrive. Lead time is typically a few seconds to tens of seconds depending on your distance from the epicenter. Alerts reach the public through Wireless Emergency Alerts (WEA) on cell phones, NOAA Weather Radio, and apps like MyShake.