Earthquake size and its effects are measured in different ways. Magnitude describes how big the earthquake was at its source, while shaking intensity describes what people and places feel in different areas. The U.S. Geological Survey (USGS) also issues rapid alerts that estimate likely impacts to help responders understand the scale of an event.
How magnitude is measured
Magnitude is the size of an earthquake based on signals recorded by a seismometer, which is an instrument that measures ground motion. The USGS explains that each earthquake has one magnitude, but the shaking it causes varies from place to place. Seismometers record vibrations that travel through the Earth, and modern systems amplify and record ground motion as a function of time, according to the USGS Earthquake Hazards Program.
Several magnitude types exist. The Richter scale (ML) is the familiar term, but the USGS says it is mostly used for small, local earthquakes when certain other measures are not available. For most earthquakes, scientists use moment magnitude (Mw) because it is based on physical properties of the fault and is a more accurate measure of size across a wide range of events.
Moment magnitude comes from the seismic moment, which depends on the rock’s rigidity (strength), the area of the fault that slipped, and the amount of slip. The USGS gives the relationship as: Moment (M0) = rigidity × area × slip. It then converts to magnitude using Mw = 2/3 × (log10(M0) − 9.1). In the range where they overlap, Mw is designed to be roughly equal to the Richter scale, according to the USGS.
How magnitude relates to energy
Magnitude and energy are related but not the same. The USGS notes that a one-unit increase in magnitude means a tenfold increase in recorded wave amplitude on a seismogram. It also corresponds to about 32 times more energy released.
Energy can be estimated directly from the shaking across all frequencies, or it can be approximated from the moment magnitude. The USGS provides a conversion for rough estimates: log E = 5.24 + 1.44 × Mw, where E is energy. The agency also describes an “Energy Magnitude” (Me), which measures a different property than Mw, so the two values are not the same.
What shaking intensity means
Intensity measures how strong the shaking is at a specific location. Unlike magnitude, there are many intensity values for a single earthquake because intensity depends mostly on distance from the fault and on local ground conditions, according to the USGS. Other factors include the direction the rupture moved and the type of surface geology beneath a site. Intensities are expressed in Roman numerals using the Modified Mercalli Intensity scale in the United States.
Traditionally, intensity was a subjective measure based on human observations of felt shaking and damage gathered by questionnaires. The USGS says these observations are now often collected through a web form. Instrumental data at stations can also be used to estimate intensity at those locations.
Depth and local geology can change intensity patterns even for earthquakes with similar magnitudes. The USGS illustrates that shallow earthquakes tend to produce stronger and more widespread shaking than deeper ones of similar size, and sites on soft soils can shake more than sites on bedrock at similar distances.
How USGS estimates shaking for alerts
USGS PAGER uses ShakeMap to estimate shaking intensities for each earthquake. A ShakeMap combines observed and estimated shaking values to represent ground motion across a region, the USGS explains. Where there are numerous seismic stations or reported intensities, the maps are better constrained; in regions with sparse data, global ShakeMaps are more predictive and carry more uncertainty, according to the USGS.
ShakeMap estimates depend on the earthquake’s magnitude, fault size and geometry, depth, distance from the fault, local rock and soil conditions, and how seismic waves travel through the crust. The USGS says it then combines these shaking estimates with a global population database to calculate how many people were exposed to each intensity level.
How PAGER alerts work
PAGER stands for Prompt Assessment of Global Earthquakes for Response. The USGS describes it as an automated system that rapidly estimates the scope and impact of significant earthquakes worldwide by modeling shaking, exposure, and likely losses. PAGER is designed to provide early, order-of-magnitude estimates of potential fatalities and economic impacts well before detailed ground reports are available, according to the USGS.
PAGER alerts are color-coded: green, yellow, orange, and red. The USGS says the Earthquake Impact Scale behind these alerts uses two criteria. One is estimated fatalities, with yellow, orange, and red thresholds at 1, 100, and 1,000 fatalities, respectively. The other is estimated economic losses, with yellow, orange, and red thresholds at $1 million, $100 million, and $1 billion (USD), respectively. The two criteria are assessed independently, and the higher alert level becomes the overall alert. The USGS notes that this approach does not assign a value to human life; it is a way to communicate impact and guide response priorities.
According to the USGS, PAGER uses country-specific loss models calibrated with an atlas of about 6,000 ShakeMaps from past earthquakes and associated fatality and damage data. Where fatality data are limited, regional models group countries with similar vulnerability indicators to estimate losses within an average factor of five to 10, with higher accuracy for more fatal events, the USGS says. PAGER presents results as ranges to reflect uncertainty in earthquake parameters, shaking, exposure, and vulnerability.
When alerts are issued and how they are shared
The USGS states that PAGER generates shaking intensity, exposure, and loss products for earthquakes of magnitude 5.5 or greater worldwide, and also for smaller events—greater than about magnitude 3.5 to 4.0—in the contiguous United States, Hawaii, and Alaska. For many events, initial PAGER results are available online within minutes. The USGS explains that estimates for earthquakes larger than magnitude 5.5 are generally available within 20 to 30 minutes and are updated as more data arrive. In some U.S. regions, particularly California, related ShakeMaps and PAGER results can appear within about 10 minutes, according to the USGS. The agency cautions that early information is uncertain and improves as additional sensor data and reports are incorporated.
Alerts go out automatically after review by seismic analysts at the National Earthquake Information Center or regional networks, the USGS says. For initial orange or red alerts, seismologists review the event and PAGER results before releasing loss information, which can add delays on the order of 10 to 20 minutes or longer. Exposure-based information for those events is released immediately.
The USGS notes that PAGER results are not sent directly to the public by email. The public can receive earthquake notifications through the USGS Earthquake Notification Service, which links to event pages where PAGER maps appear under the Maps tab. The USGS also provides a concise “onePAGER” PDF with the main elements of each assessment and can deliver it to critical users.
PAGER addresses losses from direct shaking. The USGS states that tsunami warnings come from the National Oceanic and Atmospheric Administration. When a high likelihood of tsunami is determined, PAGER highlights a link to NOAA’s tsunami pages and may note other secondary hazards, such as landslides or liquefaction, as research procedures become available.