Forecasts, predictions and early warning

Why prediction is not possible

An earthquake prediction would have to state three things: the date and time, the location, and the magnitude. According to the USGS, neither USGS scientists nor anyone else has ever predicted a major earthquake, and scientists do not expect to know how to do so in the foreseeable future.

Claims that circulate online, such as predictions based on animal behaviour, weather or the alignment of planets, do not meet this standard. A claim that is vague enough to be true somewhere, sometime is not a prediction.

Forecasts are probabilities

What scientists can do is calculate the probability that earthquakes of a certain size will occur in a defined area within a defined period. This is a forecast. Long-term hazard maps estimate the chance of strong shaking over decades. Short-term aftershock forecasts, issued after significant earthquakes, estimate the chance of aftershocks above various magnitudes over the next day, week, month or year.

A forecast always has three parts you should read together: the probability, the magnitude threshold and the time window, plus the area it covers. For example, a statement that there is a 10 percent chance of one or more magnitude 5 or larger aftershocks within the next week means that, out of many similar situations, about one in ten would see such an aftershock in that week. It does not say when or where one will happen, and it is not a promise either way.

Early warning detects an earthquake that has begun

Earthquake early warning is different again. Systems such as ShakeAlert, operated by the USGS on the U.S. West Coast, detect an earthquake that has already started, rapidly estimate its location, magnitude and expected shaking, and send alerts if it meets alert thresholds. ShakeAlert is not earthquake prediction.

The warning works because seismic waves travel much more slowly than electronic signals, and the faster, weaker P waves arrive before the slower, more damaging S waves. That can give people seconds to take protective action. Places very close to the epicentre may receive little or no warning, because strong shaking reaches them before an alert can.

Monitoring tells you what already happened

EQRadar is a monitoring tool. It shows earthquakes after they occur and are reported to the USGS catalog, usually within minutes, sometimes longer. Its in-app alerts tell you about earthquakes that have already been reported near your saved places. They are not forecasts and not early warnings.

On EQRadar's Risk Outlook page you will only see official forecast products, such as USGS aftershock forecasts, together with their source, region, time window and uncertainty. When no official forecast covers a region, EQRadar says so instead of inventing a number.

Side by side

The table summarises how the four ideas differ in what they tell you and when.