Global Seismicity

What an earthquake is

Tectonic plates drift slowly across the Earth's surface and snag at faults, building strain in the rock for decades or centuries until a sudden slip releases it as seismic waves. That snap-back is the earthquake.

• Hypocentre: the point underground where the fault slip begins.
• Epicentre: the point on the surface directly above the hypocentre.
• Fault rupture: large quakes do not break at a single point; the slip unzips along the fault, sometimes for hundreds of kilometres.

How they are measured

Seismometers around the world feed agencies that locate each quake within minutes, with ShakeMap modelling the ground shaking and crowd-sourced reports capturing what people actually felt.

• Moment magnitude (M): one number for the whole quake. The scale is logarithmic, so each step of one is about thirty-two times more energy.
• Depth: shallow quakes put their energy close to the surface, so they shake far harder than deep ones.
• Intensity (MMI): the shaking felt at a place, graded I to XII, with many values for a single quake.

Who collects this

The USGS National Earthquake Information Center locates every significant earthquake worldwide within minutes and runs ComCat, ShakeMap and the PAGER impact-alert system. This series is built on the USGS Comprehensive Catalog.

The European-Mediterranean Seismological Centre and the International Seismological Centre provide the other primary global records.