US Geological Survey Seismic Activity

Short answer

• The US Geological Survey (USGS) detects, locates, measures, and publishes earthquakes and other ground shaking through its Earthquake Hazards Program. The core of that work is the Advanced National Seismic System (ANSS), a network of seismometers operated by the USGS and its regional, state, and academic partners. When ground motion is detected, analysts and automated systems work out where it happened, how deep it was, and how large it was, then publish that information on USGS earthquake pages along with maps of shaking, felt reports from the public, and, where warranted, impact estimates. This article explains that monitoring and publication system, not the general physics of earthquakes, and it makes no claim that earthquakes can be predicted.

Who Monitors Seismic Activity in the United States

Earthquake monitoring in the United States sits in the USGS Earthquake Hazards Program. It does not work alone: much of the day to day monitoring is done through partnerships with universities, state geological surveys, and other regional operators, all linked through the Advanced National Seismic System.

The ANSS is best understood as a system of systems. Its parts, as described on the USGS ANSS page, include a national Backbone network, the National Earthquake Information Center (NEIC), the National Strong Motion Project, and 15 regional seismic networks operated by the USGS and its partners. The regional list includes Alaska, northern California, southern California, the California Geological Survey, Hawaii, the Intermountain West, the central and eastern United States, the Pacific Northwest, and Puerto Rico. That structure explains why one USGS earthquake page can draw on stations run by universities and state surveys.

The ANSS mission page describes the mission as providing accurate and timely data and information products for seismic events, including their effects on buildings and structures, using modern monitoring methods and technologies, with special focus on regions of moderate to high hazard and risk.

From Ground Motion to a Published Event

A seismometer records the shaking of the ground directly beneath it. The USGS magnitude page explains that seismometers record vibrations from earthquakes that travel through the Earth, and that sensitive instruments, which greatly magnify these ground motions, can detect strong earthquakes from sources anywhere in the world.

A single station cannot locate an earthquake on its own. Location comes from combining arrival times at many stations: seismic waves reach nearer stations first and farther stations later, and the pattern of arrival times lets analysts work out a source. Depth is harder than horizontal location, because it depends on station geometry and on the model of how fast waves travel through the crust. That is one reason depth estimates are sometimes revised as more data arrive.

Stations send data to regional data centers and to the NEIC. Analysts and automated systems pick arrival times, locate the event, and estimate its magnitude. An initial event page appears, often within minutes. As more stations report and analysts review the waveforms, location, depth, and magnitude may be updated. A magnitude that changes from an early estimate to a later one is not a dishonest first report. It is an estimate improved as the data set grew.

The Instruments: Seismometers, Strong Motion Sensors, and Stations

Not every seismic station does the same job. A standard seismometer is designed to detect ground motion over a wide range of sizes, including very small earthquakes that people cannot feel. A strong motion instrument is designed to record, without overloading, the violent shaking close to a large earthquake, including shaking inside buildings. The National Strong Motion Project, part of the ANSS, focuses on that strong shaking record, which engineers use to understand how structures actually behaved.

Stations also differ in placement. Some sit on firm rock, where shaking is relatively clean. Others sit on soft sediment, where shaking can be amplified. The USGS magnitude page notes that shaking intensity varies from place to place based on distance, type of surface material, and other factors, and that rupture direction and local geology also affect what a given place experiences. Two towns at the same distance from an earthquake can therefore shake differently.

Station density is the quiet variable behind almost every USGS product. Where stations are dense, locations are tighter, magnitudes settle faster, and ShakeMaps can be drawn with confidence. Where stations are sparse, the same steps take longer and carry more uncertainty. The ANSS page is candid about this: in regions with sufficient seismic stations, a ShakeMap can be produced within minutes, and when the system is fully implemented, it will provide such dense coverage for all at risk urban areas. Until then, readers should expect richer products in well instrumented regions.

Magnitude: One Number for the Size of the Earthquake

The USGS magnitude page states the central rule plainly: magnitude is the size of the earthquake, and an earthquake has a single magnitude. The shaking it causes has many values that vary from place to place. Confusing those two ideas is the most common source of confusion in earthquake reporting.

Magnitude is expressed in whole numbers and decimal fractions. The USGS gives magnitude 5.3 as a moderate earthquake and magnitude 6.3 as a strong earthquake. Because the scale is logarithmic, each whole number increase represents a tenfold increase in measured amplitude on a seismogram. Energy rises even faster: the USGS states that each whole number increase in magnitude represents about 32 times more energy release. That is why a single large earthquake releases far more energy than a long run of small ones.

Richter Scale and Moment Magnitude

Most news reports still say Richter scale, but the USGS is clear that the original Richter scale, known as local magnitude or ML, is not commonly used anymore except for small earthquakes recorded locally. Charles F. Richter introduced the concept of earthquake magnitude in 1935, working at the California Institute of Technology, and his original definition held only for California earthquakes within 600 km of a particular type of seismograph, the Woods-Anderson torsion instrument. The scale compared the maximum signal amplitude recorded on a seismograph, with corrections for distance.

The problem with the original scale is that it underestimates very large earthquakes, because very large earthquakes involve a long length of fault rupturing, and the long period energy from that rupture is not fully captured by the Richter calculation. For all but small local earthquakes, the USGS therefore uses moment magnitude, written Mw, as a more accurate measure of earthquake size.

Moment magnitude is based on the seismic moment, which the USGS defines with a simple formula: moment equals rigidity times area times slip. Rigidity is the strength of the rock along the fault, area is the area of the fault that slipped, and slip is the distance the fault moved. Stronger rock, a larger slipped area, or more movement all produce a larger magnitude. The seismic moment is then converted to a magnitude designed to be roughly equal to the Richter scale in the range where the two overlap.

The USGS also notes an energy magnitude, Me, based on radiated energy rather than seismic moment. Because the two measure different properties, their values are not the same, even for the same earthquake. Readers who see two different magnitude numbers for one event may be seeing different magnitude types, or an early estimate later revised, rather than a contradiction.

Intensity: Many Values for How Strongly Each Place Shook

If magnitude answers how big the earthquake was, intensity answers how strongly a particular place shook. The USGS states that intensity is the measure of shaking at each location, that there are many intensity values for each earthquake distributed across the area around the epicenter, and that intensity varies mostly with distance from the fault rupture area, but also with rupture direction and local surface geology.

The intensity scale used in the United States is the Modified Mercalli Intensity Scale, expressed in Roman numerals such as VI or X. Traditionally, intensity was a subjective measure derived from human observations and reports of felt shaking and damage. The USGS notes that data used to be gathered from postal questionnaires, but are now collected using a web based form, and that instrumental data at each station location can also be used to calculate an estimated intensity. Modern intensity maps therefore blend human reports and instrument readings.

The USGS magnitude page gives a useful worked comparison. The shaking from the magnitude 6.7 Northridge, California earthquake was more intense and covered a wider area than the slightly larger magnitude 6.8 Nisqually, Washington earthquake, because the Northridge earthquake occurred closer to the surface, at 3 to 11 miles deep, while the Nisqually earthquake had a deeper hypocenter, at 30 to 36 miles. A slightly smaller but shallower earthquake can therefore shake a wider area more strongly than a slightly larger but deeper one. Magnitude alone never tells the full story.

ShakeMap: A Map of Shaking, Not a Map of the Earthquake

ShakeMap is a map of ground shaking and shaking intensity, not a map of the earthquake source itself. The ANSS page explains that in regions with sufficient seismic stations, a ShakeMap showing the distribution of potentially damaging ground shaking can be produced within minutes, and that this information is used to target post earthquake response efforts.

A ShakeMap blends recorded ground motion at stations, models that fill the gaps between stations using distance, magnitude, depth, and site conditions, and felt reports from the public. Near stations, the colors track recorded motion closely. Between stations, the map is an estimate, and its confidence falls as distance from the nearest station grows. Later versions can therefore differ from earlier ones as more stations and felt reports are incorporated.

ShakeMap colors are usually read as intensity, on the Modified Mercalli scale, rather than as raw acceleration. Warm colors mean stronger shaking and, in general, greater potential for damage, but damage also depends on building type and construction quality. A ShakeMap is therefore a guide for response and a record of shaking, not a damage survey. Actual damage must be confirmed on the ground.

ShakeMaps also feed other products. The USGS ShakeMap documentation describes PAGER, the Prompt Assessment of Global Earthquakes for Response, as an automated system that uses ShakeMap output as its primary data source and compares the population exposed to each level of shaking intensity with models of economic and fatality losses based on past earthquakes in each country or region. PAGER alerts are therefore estimates based on shaking and population exposure, not counts of actual casualties or damage, and they are revised as ShakeMap and population data improve.

Did You Feel It: How Public Reports Become a Map

Did You Feel It, often shortened to DYFI, is the USGS system that turns public experience into science. People who felt shaking go to the USGS earthquake site, select the event they felt, and answer questions about what they experienced: whether they were woken, whether objects fell from shelves, and how strong the shaking felt, tied to their ZIP code. Those answers are converted into intensity values and plotted as a Community Internet Intensity Map, or CIIM.

The USGS fact sheet on Did You Feel It explains the logic: since the early 1990s, the magnitude and location of an earthquake have been available within minutes on the internet, and with the cooperation of regional seismic networks, people who experience an earthquake can now share information about its effects to help create a map of shaking intensities and damage. Such maps contribute greatly to quickly assessing the scope of an earthquake emergency, even in areas lacking seismic instruments.

That last point is the key to DYFI. Felt reports fill geographic gaps where stations are sparse, and they capture effects that instruments alone cannot describe. The USGS felt earthquake page adds that CIIMs show average intensity values for each ZIP code region from which a report is received, and that the more felt reports received for a ZIP code, the more reliable the average intensity assigned to that ZIP code will be. ShakeMaps also incorporate DYFI reports alongside instrument measurements, so a submitted report can sharpen the map for responders and researchers.

There are limits. Reports are self selected: people who felt shaking strongly are more likely to report than people who felt nothing, and a ZIP code with few reports can show a noisy average. Reports describe experience, not engineering damage, so a high DYFI intensity does not by itself prove structural damage. And DYFI is a reporting tool for earthquakes already felt, not a warning system for earthquakes yet to come.

ShakeAlert: Early Warning After an Earthquake Has Started

ShakeAlert is the USGS earthquake early warning system for the West Coast, and its name causes the most persistent misunderstanding about USGS seismic activity. ShakeAlert is not earthquake prediction. The USGS ShakeAlert fact sheet states this directly: ShakeAlert is not earthquake prediction. Rather, it detects an earthquake that has already started and estimates its magnitude and intensity, so that alerts can be delivered potentially seconds before shaking arrives at a location farther from the source.

The principle is speed. Seismic waves travel through the Earth more slowly than the electronic signals that carry station data. When an earthquake begins, nearby sensors detect the first, faster waves and send data into the ShakeAlert system. Computers rapidly determine the location and magnitude and estimate where significant shaking is expected. If that shaking crosses alerting thresholds, warnings can reach people and automated systems before the stronger, slower waves arrive.

The USGS ShakeAlert infographic describes the sequence as detect, deliver, protect. Detection comes from the ANSS seismic network. Delivery goes through technical partners, including government alerting systems and app providers. Protection includes personal actions, such as taking cover, and automated actions, such as slowing trains, opening firehouse doors so they do not jam shut, and closing valves to protect water systems.

Warning time depends above all on distance from the source. People close to the epicenter may get little or no warning. People farther away may get seconds to tens of seconds. The USGS infographic notes that over 50 million people in Washington, Oregon, and California can take protective actions before feeling shaking from an earthquake. That reflects the population covered in those three states, not a promise of a useful warning for every earthquake. ShakeAlert warns when strong shaking is expected, rather than for every detected earthquake.

The National Seismic Hazard Model: Long Term Hazard, Not a Forecast

Separate from the real time products is the long term hazard work, and confusing the two is another common error. Real time products describe an earthquake that has happened. Hazard models describe the likelihood of shaking over decades, for planning and building codes.

The USGS National Seismic Hazard Model, updated in 2023 for all 50 states, is the current standard for that long term picture. The USGS data release for the 2023 model describes it as a time independent probabilistic seismic hazard model, built from earthquake seismicity, fault ruptures, ground motions, and hazard estimation techniques, and intended as a standard of practice for public policy applications. It draws on new earthquake catalogs, fault based models, multi fault rupture forecasts, ground motion models, and site amplification models conditioned on the upper 30 meters of soil and deeper sedimentary structure.

The USGS hazard maps page explains what the maps are for: they integrate what scientists have learned about earthquake sources, crustal deformation, active faulting, and ground shaking, and translate that information into a form that can reduce risk and improve public safety. The USGS maps are the basis for seismic provisions in building codes and for risk models used in insurance rate structures. An integral part of the project is a database describing Quaternary faults and digital maps of those faults for the United States and its territories.

One widely quoted hazard map shows peak ground accelerations having a 2 percent probability of being exceeded in 50 years, for a firm rock site, based on the USGS models for the conterminous United States (2023), Hawaii (2021), and Alaska (2023). The USGS adds an important qualification: locally, the hazard may be greater than shown, because site geology may amplify ground motions. A hazard map is therefore a planning tool at regional scale, not a site specific engineering assessment.

Keep three distinctions straight. A hazard map is not a prediction for a date. A ShakeMap is not a hazard map. And neither says when the next earthquake will occur. Hazard is a long term probability. Real time monitoring is a record of what has already happened. Early warning is a seconds scale forecast of shaking from an earthquake already under way.

How to Use USGS Seismic Information Well

Start with the event page, not with a screenshot. Screenshots freeze a preliminary magnitude and strip away the revision history, the magnitude type, and the time stamp. The live page carries all three.

Read magnitude and intensity together. Magnitude tells you the size of the source. Intensity, on a ShakeMap or a DYFI map, tells you how strongly each place shook. A story that quotes only magnitude is missing half the picture.

Check depth and local geology before judging shaking. A shallow earthquake shakes more strongly at the surface than a deep earthquake of the same magnitude, and a site on soft sediment shakes more than a site on rock at the same distance.

Submit a DYFI report if you felt shaking, answering the questions as literally as you can. A single report in a ZIP code with few reports still adds information, and a cluster of reports can sharpen the intensity map for responders.

Treat early warnings as shaking forecasts, not as guarantees, and use hazard maps for planning, not for daily decisions. A hazard map says damaging shaking is more or less likely in a region over decades. It does not say shaking is likely today. Finally, respect revision: revised estimates mean new data were incorporated.

Quick Summary

• US Geological Survey seismic activity is the earthquakes and other ground shaking detected, located, measured, and published by the USGS Earthquake Hazards Program, principally through the Advanced National Seismic System.
• The ANSS links a national Backbone network, the National Earthquake Information Center, the National Strong Motion Project, and 15 regional seismic networks operated by the USGS and its partners.
• A seismometer records ground motion beneath one station. Location, depth, and magnitude come from combining many stations, and early estimates are revised as more data arrive.
• Magnitude is a single number for earthquake size. Moment magnitude, Mw, is the standard for most earthquakes, while the original Richter scale, ML, is now used mainly for small local earthquakes. Each whole magnitude step means about ten times the wave amplitude and about 32 times the energy release.
• Intensity is a set of values, one per place, describing how strongly that place shook, on the Modified Mercalli Intensity Scale, varying with distance, depth, rupture direction, and local geology.
• ShakeMap maps shaking intensity from stations, models, and felt reports. Did You Feel It turns public reports into Community Internet Intensity Maps that fill gaps between stations and sharpen ShakeMap. PAGER uses ShakeMap output and population exposure to estimate potential impact, not to count actual damage.
• ShakeAlert is early warning after an earthquake has started, giving seconds of warning where distance and station density allow it. It is not earthquake prediction.
• The 2023 National Seismic Hazard Model gives long term hazard probabilities for planning and building codes. It is not a forecast for a date, and local site geology can make hazard greater than a regional map shows.

Frequently Asked Questions

What is US Geological Survey seismic activity?

It is the earthquakes and other ground shaking that the USGS detects through its seismic networks, locates and measures, and publishes as event pages, maps, and catalogs. The phrase describes a monitoring and publication system, not a single live measurement.

How does the USGS detect an earthquake?

Seismometers at stations across the ANSS record ground motion. Regional data centers and the National Earthquake Information Center combine arrival times and waveforms from many stations to locate the event, estimate its depth, and calculate its magnitude, then publish the result on a USGS event page.

What is the Advanced National Seismic System?

The ANSS is the USGS system that organizes, modernizes, and standardizes seismic monitoring in the United States. It includes a national Backbone network, the National Earthquake Information Center, the National Strong Motion Project, and 15 regional seismic networks operated by the USGS and its partners.

What is the difference between magnitude and intensity?

Magnitude is a single number for the size of the earthquake. Intensity is a set of values describing how strongly each place shook, on the Modified Mercalli Intensity Scale, varying with distance, depth, rupture direction, and local geology.

Why do news reports still say Richter scale?

The Richter scale was the first widely known magnitude scale, introduced in 1935, and the name stuck in public usage. In practice, the USGS now uses moment magnitude, Mw, for most earthquakes, because the original Richter scale, ML, underestimates very large earthquakes and is used mainly for small earthquakes recorded locally.

Why does a USGS magnitude sometimes change after it is first reported?

Early estimates use the first stations to report. As more stations report and analysts review the waveforms, location, depth, and magnitude are refined. A revised magnitude is the improved estimate from a larger data set.

What is a ShakeMap?

A ShakeMap is a USGS map of ground shaking and shaking intensity for an earthquake, blending recorded ground motion, models that estimate shaking between stations, and felt reports from the public. In well instrumented regions it can be produced within minutes. It maps shaking, not damage.

What is Did You Feel It?

Did You Feel It is a USGS questionnaire that lets people report what they experienced during an earthquake. Their answers, grouped by ZIP code, are turned into a Community Internet Intensity Map showing average intensity by area, filling gaps where stations are sparse and sharpening ShakeMap.

What is PAGER?

PAGER, the Prompt Assessment of Global Earthquakes for Response, is an automated USGS system that uses ShakeMap output and population exposure to estimate the potential scope of an earthquake impact, including estimated ranges of fatalities and economic loss based on models from past earthquakes. Its outputs are estimates for response planning, not counts of actual casualties or damage.

Is ShakeAlert earthquake prediction?

No. ShakeAlert detects an earthquake that has already started, estimates its magnitude and expected shaking, and issues alerts that may arrive seconds before strong shaking reaches locations farther from the source. It does not predict earthquakes before they begin, and people near the epicenter may receive little or no warning.

Which states does ShakeAlert cover?

ShakeAlert serves Washington, Oregon, and California, where the USGS notes that over 50 million people can take protective actions before feeling shaking from an earthquake. Warning time depends on station density and distance from the source, and alerts are issued when forecast shaking crosses thresholds, not for every detected earthquake.

What is the National Seismic Hazard Model?

It is the USGS long term model of earthquake hazard for the United States, updated in 2023 for all 50 states. It combines earthquake catalogs, fault models, rupture forecasts, ground motion models, and site conditions to estimate the likelihood of shaking over decades. Its maps underpin seismic provisions in building codes and risk models used in insurance. They are planning tools, not dated forecasts.

Can the USGS predict earthquakes?

No. ShakeAlert is not earthquake prediction, and hazard work gives long term probabilities rather than dates. Any dated earthquake prediction attributed to the USGS should be checked against a USGS page itself, and treated as false if it does not appear there.

Sources & references

• U.S. Geological Survey. "ANSS - Advanced National Seismic System." https://www.usgs.gov/programs/earthquake-hazards/anss-advanced-national-seismic-system?qt-science_support_page_related_con=4&page=1
• U.S. Geological Survey. "Earthquake Magnitude, Energy Release, and Shaking Intensity." https://www.usgs.gov/programs/earthquake-hazards/earthquake-magnitude-energy-release-and-shaking-intensity
• U.S. Geological Survey. "National Seismic Hazard Model." https://www.usgs.gov/programs/earthquake-hazards/science/national-seismic-hazard-maps
• U.S. Geological Survey. "Data Release for the 2023 U.S. 50-State National Seismic Hazard Model - Overview." https://data.usgs.gov/datacatalog/data/USGS:644af897d34e45f6ddcd13b3
• U.S. Geological Survey. "The 2023 US 50-State National Seismic Hazard Model: Overview and implications." https://pubs.usgs.gov/publication/70251823
• U.S. Geological Survey. "ShakeAlert Infographic: Detect, Deliver, Protect." https://www.USGS.gov/media/images/shakealert-infographic-detect-deliver-protect
• U.S. Geological Survey. "Did you feel it? Community-made earthquake shaking maps." https://www.USGS.gov/publications/did-you-feel-it-community-made-earthquake-shaking-maps
• U.S. Geological Survey. "Felt Earthquakes = Ones That People Feel." https://www.usgs.gov/observatories/hvo/science/felt-earthquakes-ones-people-feel
• U.S. Geological Survey. "The Advanced National Seismic System; management and implementation." https://www.usgs.gov/publications/advanced-national-seismic-system-management-and-implementation
• U.S. Geological Survey ShakeMap documentation. "Related Systems - ShakeMap." https://ghsc.code-pages.usgs.gov/esi/shakemap/manual4_0/ug_related_systems.html