In a seismogram, which sequence of seismic waves is observed from earliest to latest?

• A. P-waves, S-waves, surface waves
• B. S-waves, P-waves, surface waves
• C. Surface waves, P-waves, S-waves
• D. P-waves, surface waves, S-waves

Answer: (A)

P-waves are the fastest seismic disturbances, so they arrive first on a seismogram. They move by compressing and stretching the material in the direction of travel and can pass through solids, liquids, and gases, which makes them the earliest detectable signal. Next come S-waves, which shake the ground perpendicular to the direction of travel and travel more slowly than P-waves; they can only move through solids, so their arrival is later than the P-wave’s. The surface waves travel along the Earth’s surface and are guided by the crust, so they are typically the slowest of the three types and arrive after both body-wave types. Because speed governs arrival time, the usual order from earliest to latest is P-waves, then S-waves, then surface waves. In a seismogram this shows up as an initial sharp P-wave onset, a larger, more abrupt S-wave onset, and finally a long-duration, rolling surface-wave signal that often causes the most damage near the epicenter.