Earthquakes and Volcanoes

A fault slips deep underground, and that sudden break releases energy as waves that spread out in all directions through rock, the way a dropped stone sends ripples across a pond. The ground hundreds of kilometers away shakes because those waves keep traveling outward, losing strength but not disappearing, until they reach a seismic station's instruments far from the actual slip.

Two wave types race from that same starting point. P waves (primary) push and pull through rock and move fastest, around 6 km/s. S waves (secondary) shear side to side and lag behind, around 3.5 km/s. Both leave the fault at the same instant, but because P waves are faster, they arrive first, and the gap between the two arrivals grows the farther the waves have traveled. Measure that gap at a station, divide it by the difference in speed, and you recover distance: a lag of 40 seconds divided by (1/3.5 - 1/6) km per second gives roughly 336 km to the epicenter.

Magnitude scales don't add distance info, they measure shaking strength, and they do it logarithmically. Each whole step up multiplies ground motion by about ten and energy by even more, so a jump from magnitude 3 to 6 is not double but closer to a thousand times the shaking. Volcanoes connect to this same underlying idea of material behavior: silica-rich lava is thick and sluggish, so it piles up near the vent into steep stratovolcanoes, while silica-poor, runny lava spreads for kilometers into broad, gently sloped shield volcanoes.