The quake starts before its strongest shaking arrives
An earthquake does not send every kind of shaking outward at once. Its first arrivals are P waves, which squeeze and stretch the ground as they travel. They usually cause gentler motion than the later waves. Slower S waves move the ground from side to side or up and down; surface waves, which travel along the ground, can produce especially strong motion near the surface.
That difference in speed creates a brief interval between the quake’s first detectable movement and its most damaging shaking. The earthquake is already underway during that interval—it has not been predicted.
A small vibration can be an early clue
An animal may notice the initial movement through vibrations or sounds that a person nearby does not register as a clear warning. A shift underfoot that seems negligible to a human can be enough to make an animal startle, flee, or seek cover. Such reactions are not proof that animals can forecast earthquakes; they may simply be responding to the first physical signal.
The useful head start is often only a matter of seconds, and it depends on where the observer is relative to the quake. The first waves reach places closer to the source sooner, leaving less time before stronger shaking follows. Farther away, the gap can be longer.
The same gap can help people
Earthquake early-warning systems use this basic difference too: instruments detect the faster first waves and send alerts before stronger waves arrive at more distant locations. They do not know an earthquake is coming before it begins. They use the brief travel-time gap to warn people that the shaking has started—and that stronger motion may be on its way.
