Earthquakes cannot be predicted, that is, scientists cannot identify the time, magnitude and location of a future earthquake with a high level of confidence.
But most scientists agree that the probability of a large potentially damaging quake in an area of known historic earthquake activity is not constant, it varies over days, weeks and months with seismic activity in the region. Sometimes there is an increase in seismic activity or other measured changes that raise the likelihood that a large temblor may occur.
For example, on March 9, 2011, two days before the magnitude 9 earthquake and tsunami that claimed the lives of almost 20,000 people and caused a major nuclear accident, a magnitude 7.3 and three magnitude 6 earthquakes occurred in the same zone. There was also movement detected along the subduction zone interface known as “slow slip” that sometimes precedes an earthquake. The magnitude 7.3 Kumamoto quake on April 16, 2016 was preceded by two temblors, a 6.4 on April 15th and 6.5 on April 14 prior to the larger main shock. But, in most cases, large quakes occur without any immediate prior activity.