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?