What is the probability that an earthquake is a foreshock to a larger earthquake?

Around the world, the likelihood that an earthquake will be followed by a larger earthquake nearby and within a week is about 5%.

When an earthquake occurs, it can trigger other earthquakes nearby in what seismologists call an earthquake sequence. In most sequences, these later earthquakes will be smaller than the first one. The first, largest earthquake is called the mainshock and the later, smaller earthquakes are called aftershocks.  

Occasionally, a new earthquake will be larger than any of the earlier events. When that happens, we call the new, largest earthquake the mainshock and we call all the earlier earthquakes in the sequence foreshocks. Observations show that the chance of an earthquake being followed by a larger one nearby and within a week is about 5%. 

That 5% foreshock probability varies with the activity level of an aftershock sequence. Some earthquakes will have more or fewer aftershocks than average making them more or less likely to be followed by a larger earthquake, respectively.  The USGS releases aftershock forecasts for all mainshocks with magnitude greater than 5 in the U.S. and its territories. These forecasts provide more accurate estimates of aftershock and foreshock probabilities.  

Learn more: 

• USGS Aftershock Forecasts
• Foreshocks, aftershocks - what's the difference?