Every earthquake, the moment it happens.

Earth’s outer shell is broken into about 15 major plates of rigid lithosphere, each up to ~100 km thick, gliding over the hotter, weaker asthenosphere at 2–10 cm a year — about as fast as fingernails grow. Almost every earthquake happens where plates meet.

A fault is a fracture where blocks of rock have moved past each other. How they move depends on whether the crust is being stretched, squeezed or sheared.

When a fault ruptures, energy spreads out as seismic waves. Seismometers record them in a fixed order, and the gaps between arrivals tell us where and how big the earthquake was.

Magnitude measures the size of an earthquake at its source — one number per event. Intensity measures the shaking at a particular place, and changes with distance, depth and ground type.

Earthquakes happen from the surface down to about 700 km. Depth changes how strongly shaking reaches us, and tells us about the plates beneath.

A tsunami is a series of waves caused by the sudden displacement of a huge volume of water — most often when a megathrust earthquake lifts or drops the sea floor.

No one can predict earthquakes, but preparation saves lives. Most injuries come from falling objects and debris, not collapsing buildings.

The Ring of Fire is a horseshoe of ocean trenches, volcanoes and faults about 40,000 km long around the edge of the Pacific Ocean: from New Zealand, Tonga and Indonesia through the Philippines, Japan and Kamchatka, across the Aleutians and Alaska, and down the Americas to the tip of Chile. About 90% of the world’s earthquakes happen along it, including most of the largest.