It is not currently possible to predict exactly when and where an earthquake will occur, nor how large it will be.
However, seismologists can estimate where earthquakes may be likely to strike by calculating probabilities and forecasts.
What is an earthquake probability and how is it calculated?
Earthquake probabilities describe the chances of an earthquake of a certain magnitude occurring within a region over a span of years.
Probabilities can be calculated based on the average rate of past seismic activity in a region. This technique is particularly useful in regions where earthquakes have been recorded by seismographs, which first came into wide use in the early 1900s.
Scientists can obtain additional, though less precise, information by digging trenches to examine the geological record for earthquake ruptures that occurred in ancient history.
Probabilities also can be derived mathematically. For example, seismologists estimate the number of years it could take to experience an earthquake of a certain magnitude by accounting for two processes:
the buildup of strain onto faults as a result of the continual motion of tectonic plates, and
the relieving of strain as a result of fault slip, which can occur as an earthquake or as slow creep along a fault line without an earthquake.
What is an earthquake forecast?
Earthquake forecasts provide information on the likelihood of earthquakes over a shorter time window of time. Forecasts are used typically to describe aftershocks, which tend to follow a pattern of decreasing frequency and magnitude over time after an earthquake.