How can an earthquake have a negative magnitude?
Magnitude calculations are based on a logarithmic scale, so a ten-fold drop in amplitude decreases the magnitude by 1.
If an amplitude of 20 millimetres as measured on a seismic signal corresponds to a magnitude 2 earthquake, then:
- 10 times less (2 millimetres) corresponds to a magnitude of 1;
- 100 times less (0.2 millimetres) corresponds to magnitude 0;
- 1000 times less (0.02 millimetres) corresponds to magnitude -1.
An earthquake of negative magnitude is a very small earthquake that is not felt by humans.
Related Content
What is a Geoid? Why do we use it and where does its shape come from?
Contrast of the Geoid model with an Ellipsoid and cross-section of the Earth's surface. (Public domain.) A geoid is the irregular-shaped “ball” that scientists use to more accurately calculate depths of earthquakes, or any other deep object beneath the earth’s surface...
Seismometers, seismographs, seismograms - what's the difference? How do they work?
How can I make my own seismometer?
What was the first instrument that actually recorded an earthquake?
What was the duration of the earthquake? Why don't you report the duration of each earthquake? How does the duration affect the magnitude?
What does it mean that the earthquake occurred at a depth of 0 km? How can an earthquake have a negative depth; that would mean it’s in the air. What is the geoid, and what does it have to do with earthquake depth?
How do seismologists locate an earthquake?
How are earthquakes recorded? How are earthquakes measured? How is the magnitude of an earthquake determined?
Moment magnitude, Richter scale - what are the different magnitude scales, and why are there so many?
What is the difference between earthquake magnitude and earthquake intensity? What is the Modified Mercalli Intensity Scale?
How do you determine the magnitude for an earthquake that occurred prior to the creation of the magnitude scale?
United States Geological Survey (USGS) Natural Hazards Response
The primary goal of U.S. Geological Survey (USGS) Natural Hazards Response is to ensure that the disaster response community has access to timely, accurate, and relevant geospatial products, imagery, and services during and after an emergency event. To accomplish this goal, products and services provided by the National Geospatial Program (NGP)...
Lamb, Rynn M.; Jones, Brenda K.
The Global Seismographic Network
Gee, Lind S.; Leith, William S.
Earthquake hazards: a national threat
Earthquakes are one of the most costly natural hazards faced by the Nation, posing a significant risk to 75 million Americans in 39 States. The risks that earthquakes pose to society, including death, injury, and economic loss, can be greatly reduced by (1) better planning, construction, and mitigation practices before earthquakes happen, and (2)...
The USGS Earthquake Hazards Program - investing in a safer future
Filson, John R.; McCarthy, Jill; Ellsworth, William L.; Zoback, Mary Lou; Stauffer, Peter H.; Hendley, James W.
ANSS-Advanced National Seismic System
Benz, Harley M.; Filson, John; Arabasz, Walter; Gee, Lind; Wald, Lisa
Monitoring earthquakes across the United States
Buland, Ray P.; Benz, Harley Mitchell; Brown, William M.
Taking the Earth's pulse
During the past 35 years, scientists have developed a vast network of seismometers that record earthquakes, volcanic eruptions, and nuclear explosions throughout the world. Seismographic data support disaster response, scientific research, and global security. With this network, the United States maintains world leadership in monitoring the...
Woodward, Robert L.; Benz, Harly M.; Brown, William M.
Updated USGS Earthquake Monitoring Strategy Released
The USGS Earthquake Hazards Program recently released a new strategic plan for earthquake monitoring entitled the “Advanced National Seismic System – Current Status, Development Opportunities, Priorities, 2017-2027.”
A 100-year-long History of Earthquakes and Seismic Monitoring in Hawaii
The Hawaiian Volcano Observatory’s 1912–2012 Centennial—100 Years of Tracking Eruptions and Earthquakes
HAWAI‘I ISLAND, Hawaii —The history of earthquakes and seismic monitoring in Hawai‘i during the past century will be the topic of a presentation at the University of Hawai‘i at Hilo on Thursday, January 26, at 7:00 p.m.
Recovery Act Funds Will Upgrade Earthquake Monitoring
USGS will Grant Universities $5 Million to Beef Up Public Safety Grants totaling $5 million under the American Recovery and Reinvestment Act are being awarded to 13 universities nationwide to upgrade critical earthquake monitoring networks and increase public safety.
USGS Forecast for Ground Shaking Intensity from Earthquakes in 2017
USGS map displaying intensity of potential ground shaking from natural and human-induced earthquakes. There is a small chance (one percent) that ground shaking intensity will occur at this level or higher. There is a greater chance (99 percent) that ground shaking will be lower than what is displayed in these maps.
Earthquakes Greater Than or Equal to Magnitude 2.7 Since 1980
USGS charts showing the number of earthquakes greater than or equal to magnitude 2.7 since 1980 in the five focus areas identified as having especially high ground-shaking hazard in the central and eastern U.S. in 2017.
Seismographs at the U.S. Geological Survey
Seismographs at the U.S. Geological Survey record (1) north-south horizontal, (2) east-west horizontal, and (3) vertical components of the earthquake.
Eastern Earthquakes
This map shows earthquakes above magnitude 4.0 in the eastern United States since 1973, the first year with a complete catalog. There are 184 earthquakes recorded. An earthquake of magnitude 4.0 or greater can cause minor or more significant damage. The circle sizes correspond to earthquake magnitude, ranging from 4.0 to 5.9 (the largest was in the Gulf of Mexico).
Scenario Earthquake Map
Interactive map showing earthquake scenario data