Crustal earthquakes and subduction-zone earthquakes have many aftershocks, and they will cause a lot of alarm. In a large earthquake, aftershocks will continue for months and even years after the main event. Many of these will be felt, and some can cause damage to already weakened buildings. This is one of the reasons you might be asked to leave your house. Though still standing after the main earthquake, it could be so weakened that it might not survive a large aftershock. Warn your family members that there will be aftershocks.
However, there is always the possibility that the earthquake you just experienced is a foreshock to an even larger one. The great 1857 Earthquake on the San Andreas Fault of M 7.9 was preceded by a foreshock of about M 6 at Parkfield. The Denali Earthquake of M 7.9 in central Alaska on November 3, 2002, was preceded eleven days earlier by a foreshock of M 6.7. The Chinese have based their successful earthquake predictions on foreshocks—in some cases many foreshocks. Normal-fault earthquakes, occurring in crustal regions that are being extended or pulled apart, such as the Basin and Range of Nevada, southeast Oregon, and eastern California, are more likely to have foreshocks.