How to play Earthquake P and S
- Choose a test and read the distance, the speeds, or the times on the card.
- Use t = d ÷ v for each wave, the gap Δt = d ÷ v_S − d ÷ v_P, and think about which way each wave moves the ground.
- Type your prediction and press Start. The waves spread through the rock, sped up.
- Compare the orange lines with the P and S bursts on the seismogram, or the orange marker and line with the station and the ground's motion.
P waves, S waves, and finding a quake
An earthquake releases its energy as two kinds of body wave. The P wave, or primary wave, is longitudinal: like sound, it squeezes and stretches the rock along the way it travels, and it is the fastest, about 6 km/s in the crust. The S wave, or secondary wave, is transverse: it shakes the rock up and down or side to side across its path, and it is slower, about 3.5 km/s. So every seismometer records a P burst first and then a stronger S burst. The gap between them grows with distance, about 12 s for every 100 km here: from one gap a station knows how far away the quake was, and three stations together pin down where it happened. The two waves also revealed Earth's insides. S waves need a solid that springs back when it is sheared; a liquid does not, so S waves vanish in Earth's liquid outer core, leaving an S-wave shadow on the far side of the planet.
A focused learning model
The rock is uniform, so both waves travel in straight lines at constant speeds and lose no energy; real waves bend and slow through layers. Each wave is a 3 s burst; the grains, the ground motion, and the circles are drawn larger than real. The seismogram adds the push along the path to the shake across it. Time runs faster than real time by the factor shown, so each run lasts 5 s and ends 4 s after the S wave arrives. Answers within 2% are right.