Final answer:
The first signal to arrive at a seismograph station after an earthquake is detected is the P-wave. These primary waves travel faster than S-waves and surface waves, thus arriving first and providing a brief warning before more severe shaking begins.
Step-by-step explanation:
When a seismometer detects an earthquake, the first signal to usually arrive is the P-wave. This is because P-waves, or primary waves, are longitudinal compression waves that are faster than the other types of seismic waves. They move through the Earth's layers at speeds of 4 to 7 km/s, which is significantly higher than those of S-waves, which range from 2 to 5 km/s. Since P-waves travel faster, they provide an initial warning before the arrival of the more damaging S-waves and surface waves.
S-waves, or secondary waves, are transverse shear waves that arrive after the P-waves. Because S-waves move slower, there is usually a noticeable time difference between the arrivals of P-waves and S-waves at seismograph stations. This time difference can be used to calculate the distance to the earthquake's epicenter. In addition, seismologists can provide a brief warning to residents that the stronger shaking from S-waves is imminent.