The USGS uses these four terms to refer to four different things.
Early warning is a notification that is issued after an earthquake starts. Probabilities and forecasts are comparable to climate probabilities and weather forecasts, while predictions are more like statements of when, where, and how large, which is not yet possible for earthquakes.
Here are more detailed descriptions of each:
Earthquake early warning systems use earthquake science and the technology of monitoring systems to alert devices and people when shaking waves generated by an earthquake are expected to arrive at their location. The seconds to tens of seconds of advance warning can allow people and systems to take actions to protect life and property from destructive shaking.
Earthquake Forecasts are like probabilities but for shorter time windows, and we generally apply this term to aftershocks. After a large earthquake, there are aftershocks that are typically less frequent and smaller over time. Most aftershock sequences follow the same pattern, so the probability of an aftershock in a time window following an earthquake can be determined. These probabilities might be larger than 1-in-30.
Earthquake Probabilities describe the long-term chances that an earthquake of a certain magnitude will happen during a time window. Most earthquake probabilities are determined from the average rate of historical events. Assuming the annual rate is constant, one can make a probability statement about the likelihood of such an event in the next so-many years. These probabilities might range from 1-in-30 to 1-in-300.
For some faults, historical occurrences are not available, but rate of slip along the fault can be estimated. Assuming a particular magnitude, one can estimate the number of years it would take to accumulate the required amount of slip. This estimate can be used to give an annual rate and used in the same manner as historical rates. These probabilities might range from 1-in-300 to 1–in-3000.
An earthquake prediction must define 3 elements: 1) the date and time, 2) the location, and 3) the magnitude. Yes, some people say they can predict earthquakes, but here are the reasons why their statements are false:
- They are not based on scientific evidence, and earthquakes are part of a scientific process. For example, earthquakes have nothing to do with clouds, bodily aches and pains, or slugs.
- They do not define all three of the elements required for a prediction.
- Their predictions are so general that there will always be an earthquake that fits; such as, (a) There will be a M4 earthquake somewhere in the U.S. in the next 30 days. (b) There will be a M2 earthquake on the west coast of the U.S. today.
For information on Predictions, see the FAQ under Earthquake Myths – Can you predict earthquakes?
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Earthquake Early Warning
Related Content
- FAQ
Can you predict earthquakes?
No. Neither the USGS nor any other scientists have ever predicted a major earthquake. We do not know how, and we do not expect to know how any time in the foreseeable future. USGS scientists can only calculate the probability that a significant earthquake will occur (shown on our hazard mapping) in a specific area within a certain number of years. An earthquake prediction must define 3 elements: 1...
How do I sign up for the ShakeAlert® Earthquake Early Warning System?
The USGS issues ShakeAlert® Messages but alert delivery will come by other public and private means (internet, radio, television, cellular), including Wireless Emergency Alerts (WEA) -- think of an AMBER Alert -- delivered by FEMA’s Integrated Public Alert and Warning System (IPAWS). Alert delivery via the WEA system is available in California, Oregon, and Washington. To ensure that you can...
What is seismic hazard? What is a seismic hazard map? How are they made? How are they used? Why are there different maps, and which one should I use?
Seismic hazard is the hazard associated with potential earthquakes in a particular area, and a seismic hazard map shows the relative hazards in different areas. The maps are made by considering what we currently know about: Past faults and earthquakes The behavior of seismic waves as they travel through different parts of the U.S. crust The near-surface site conditions at specific locations of...
What is a seismic zone, or seismic hazard zone?
Although you may hear the terms “seismic zone” and “seismic hazard zone” used interchangeably, they really describe two slightly different things. A seismic zone is used to describe an area where earthquakes tend to focus; for example, the New Madrid Seismic Zone in the Central United States. A seismic hazard zone describes an area with a particular level of hazard due to earthquakes. Typically, a...
Can the National Seismic Hazard Maps be used as an earthquake forecast tool for the near future?
Not really. The USGS Seismic Hazard Maps website and the Unified Hazard Tool (for building custom maps) are designed to display the probability of different events that might occur in a several-year to several-decade period. The model bases all probability estimates on the average rate of earthquakes over long periods of time. As you shorten the time window, you should expect greater and greater...
What are the earthquake hazards/risks where I live?
Determining your risk with regard to earthquakes, or more precisely shaking from earthquakes, isn't as simple as finding the nearest fault. The chances of experiencing shaking from an earthquake and/or having property damage is dependent on many different factors. The National Hazard Maps use all available data to estimate the chances of shaking (of different strengths and frequencies) across the...
How will my house hold up in an earthquake? Can the USGS send someone out to evaluate my property?
Published maps will only provide generalized, uninterpreted information about specific areas. Every property consists of a unique combination of geologic and structural factors that must be considered to determine what might happen to a house during an earthquake. Therefore, an individual site study is necessary. Geologic factors include: type of underlying material, depth to bedrock, depth of...
How do I decide whether or not to get earthquake insurance?
You should consider the following factors when deciding whether or not to get earthquake insurance:proximity to active earthquake faultsseismic history of the region (frequency of earthquakes)time since last earthquakebuilding construction (type of building and foundation)architectural layoutmaterials usedquality of workmanshipextent to which earthquake resistance was considered by the...
How do earthquakes affect buildings?
Ground shaking is the primary cause of earthquake damage to man-made structures. Many factors influence the strength of earthquake shaking at a site including the earthquake's magnitude, the site's proximity to the fault, the local geology, and the soil type. More than 250 structures throughout the United States have been outfitted with seismic sensors by the USGS National Strong Motion Project...
What is the likelihood of a large earthquake at location X? Is it safe to go to X since they've been having a lot of earthquakes lately?
The National Seismic Hazards Mapping project provides an online Web tool for determining the probability of a large earthquake within 50 kilometers (~31 miles) of a specific location in the United States over a certain time period. The calculation is based on the latest available information from seismic hazard data. However, asking if it's safe to travel somewhere because of recent earthquakes is...
What is the probability that an earthquake is a foreshock to a larger earthquake?
Worldwide the probability that an earthquake will be followed within 3 days by a large earthquake nearby is somewhere just over 6%. In California, that probability is about 6%. This means that there is about a 94% chance that any earthquake will NOT be a foreshock. In California, about half of the biggest earthquakes were preceded by foreshocks; the other half were not. At this time, we cannot...
What is the probability that an earthquake will occur in the Los Angeles Area? In the San Francisco Bay area?
Los Angeles area:Within the next 30 years the probability is:60% that an earthquake measuring magnitude 6.746% that an earthquake measuring magnitude 731% that an earthquake measuring magnitude 7.5will occur in the Los Angeles region.San Francisco Bay area:Within the next 30 years the probability is:72% that an earthquake measuring magnitude 6.751% that an earthquake measuring magnitude 720% that...
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Earthquake Early Warning
Earthquake early warning (EEW) systems use earthquake science and the technology of monitoring systems to alert devices and people when shaking waves generated by an earthquake are expected to arrive at their location. The seconds to minutes of advance warning can allow people and systems to take actions to protect life and property from destructive shaking. - Multimedia
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