Why do USGS earthquake magnitudes differ from those published by other agencies?

Summary

Magnitude estimates for a given earthquake can vary between reporting agencies due to differences in methodology, data availability, and inherent uncertainties in seismic data. Individual agencies use magnitude estimation procedures designed to meet the agency's specific needs and monitoring capabilities. Even for well recorded events, differences in magnitude of 0.2 or 0.3 units are common and representative of the inherent uncertainty of the magnitude estimation process.

Details

Obtaining an accurate measure of an earthquake's size is difficult. Earthquakes are complex processes that occur below the Earth’s surface away from direct observation and measurement. Determining a single number to represent an earthquake’s size has inherent uncertainties due to our assumptions about the material in which they occur and our inability to fully reconstruct the hidden process.

Multiple methods are used to estimate magnitude. Fundamentally different calculation methods are specified by magnitude type. The USGS National Earthquake Information Center (NEIC) routinely calculates as many as a dozen different magnitude types. Multiple magnitude types are necessary because no single method is capable of accurately estimating the size of all earthquakes. Some magnitude types are calculated to provide a consistent comparison to past earthquakes, for which modern techniques cannot be applied. Attempts have been made to calibrate these magnitudes to the original Richter Scale but since different techniques often measure different physical processes there can be large discrepancies between different magnitude types calculated for the same earthquake. These differences can be as large as 0.5 even in magnitude ranges where both techniques are considered valid. Some magnitudes types are fast to calculate and can be completely automated, while others require manual processing by a trained seismologist. Changes in the preferred USGS magnitude in the minutes and hours following significant earthquakes are often a result of a change in magnitude type as well as the inclusion of more data. This iterative procedure addresses the desire for rapid information and allows for timely improvement to derived USGS products such as ShakeMap and PAGER.

Even for a given magnitude type, there can be differences in magnitude estimates from different agencies. These differences generally arise due to the use of different Earth models, data availability and data processing. The USGS National Earthquake Information Center reports on earthquakes worldwide. We strive for global consistency in the methods used for calculating earthquake parameters. This approach requires a systematic process as well as a more generic model of Earth's physical properties. A seismic network that is focused on a specific region will often tune their underlying models to their area of interest and monitoring history.

To obtain the most consistent and meaningful information from magnitude estimates, researchers and policy makers should consider and specify both the magnitude type and source of that information. For example, local networks are generally the best source for consistent quality information in that region whereas global monitoring agencies such as the USGS National Earthquake Information Center can provide valuable information when comparing earthquakes across the globe.

Learn More:

Earthquake Information by Region

Global Seismographic Network

Centennial Earthquake Catalog

Related Content

Filter Total Items: 12

Why don’t you report earthquakes in the local time where the earthquake occurred?

Since we and other seismic network agencies record earthquakes around the globe in all the various time zones, using a single standard time reference is best for record-keeping and exchange of data. Also, we tried converting UTC to the local time zones for epicenters and reporting them that way for a while on our website, and it confused our web...

Why do so many earthquakes occur at a depth of 10km?

10 km is a "fixed depth". Sometimes data are too poor to compute a reliable depth for an earthquake. In such a case, the depth is assigned to be 10 km. In many areas around the world, reliable depths tend to average 10 km or close to it. For example, if we made a histogram of the reliable depths in such an area, we'd expect to see a peak around 10...

How fast does the earthquake information get posted to the web site, get sent out via the Earthquake Notification Service (ENS), ATOM feeds, etc?

First of all, USGS earthquake information mechanisms are all triggered by the same system, so they all receive the information at the same time. The time it takes for the system to receive the information primarily depends on where the earthquake is and how large it is. An Earthquake in California will get processed and posted to the system in 2-1...

Where can I see current or past seismograms?

The Earthquake Hazards Program has helicorders (seismogram displays) available for several areas in the United States and the World.

When are tsunami information and links put on the event page?

1. If we get information (alerts) from any of NOAA's Tsunami Warning Centers or 2. For any earthquake, magnitude 5.0 or greater, that occurred in a. the Pacific Ocean, or b. Indonesia, or c. Papua New Guinea, or d. the Caribbean Sea, or e. Hawaii (criteria from David Applegate, 07/20/2005) For More Information, See NOAA's Tsunami Warning Centers:...

Why/When does the USGS update the magnitude of an earthquake?

The USGS often updates an earthquake's magnitude in the hours and sometimes days following the event. Updates occur as more data become available for analysis and more time-intensive analysis is performed. Additional updates are possible as a part of the standard procedure of assembling a final earthquake catalog. There are physical and...

Why do some earthquakes disappear from the map/list?

The USGS and networks contributing to the Advance National Seismic System (ANSS) take great effort to provide accurate and timely earthquake information. Occasionally our systems produce erroneous information that is released to the public via our web pages or Earthquake Notification System . These mistakes are generally promptly identified by...

Why isn't the fault on which the earthquake occurred or the distance to the nearest fault provided?

Seismologists evaluate the hypocenter location and the focal mechanism of an earthquake to decide if the earthquake occurs on a named fault. Research shows that many earthquakes occur on small, un-named faults located near well-known faults. For example, most of the aftershocks of the 1989 M6.9 Loma Prieta earthquake occurred on small, subsidiary...

Can I get on a list to receive an email message when there is an earthquake? How do I sign up for earthquake notifications?  Are there any Feeds I can subscribe to?

Yes, please go to Earthquake Notification Services (ENS) to sign up for free emails or text messages to your phone, or check out the many different Feeds .

Did I feel an earthquake? Can I report feeling an earthquake?

Report an earthquake experience or related observation through the Did You Feel It? citizen science webpage. The best way to do this is to click on the earthquake that you think you felt on one of the lists on the Earthquakes webpage, and then select the "Tell Us!" link. If you don't see the earthquake you think you felt, use the green "Report an...

Why is the earthquake that was reported/recorded by network X, or that I felt, not on the map/list?

The maps and lists show events which have been located by the USGS and contributing agencies within the last 30 days. They should not be considered to be complete lists of all events in the US and adjacent areas and especially should not be considered to be complete lists of all events M4.5+ in the world. In most cases, we locate and report on...

Where can I find current earthquake lists and maps for the world or for a specific area?

The Earthquake Hazards Program Latest Earthquakes Map displays earthquakes in near-realtime and up to the past 30 days of earthquakes. The interface includes three panels: a list of earthquakes, a map, and a settings/options panel. You can pan and zoom the map to view specific areas. Click on an event on the list or map for additional information...
Filter Total Items: 4
Date published: May 25, 2017

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.”

Date published: January 23, 2012

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. 

Date published: May 3, 2010

Millions Awarded for Earthquake Monitoring in the United States

More than $7 million in cooperative agreements will be awarded for earthquake monitoring by the U.S Geological Survey in 2010. This funding will contribute to the development and operation of the USGS Advanced National Seismic System (ANSS).

Date published: September 24, 2009

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.

Filter Total Items: 6
Earthquake Catalog Map Results Example
December 31, 2017

Earthquake Catalog Map Results Example

Example of results returned when searching the USGS Earthquake Catalog. The ANSS Comprehensive Earthquake Catalog (ComCat) contains earthquake source parameters (e.g. hypocenters, magnitudes, phase picks and amplitudes) and other products (e.g. moment tensor solutions, macroseismic information, tectonic summaries, maps) produced by contributing seismic

...
Earthquakes greater than or equal to a magnitude 2.7 since 1980 in the five focus areas
February 24, 2017

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.

Magnitude 6.9 Earthquake off Japan on November 21, 2016
November 21, 2016

Magnitude 6.9 Earthquake off Japan on November 21, 2016

USGS map of the magnitude 6.9 earthquake off the coast of Japan on November 21, 2016.

Latest Earthquakes
August 31, 2016

Latest Earthquakes

Image: Seismographs at the U.S. Geological Survey
October 17, 1989

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.

Cumulative number of earthquakes with a magnitude of 3.0 or larger in the central and eastern United States, 1973-2014

Cumulative number of earthquakes with a magnitude of 3.0 or larger in the central and eastern United States, 1973-2014

Cumulative number of earthquakes with a magnitude of 3.0 or larger in the central and eastern United States, 1973-2014. The rate of earthquakes began to increase starting around 2009 and accelerated in 2013-2014.