ATLAS · TECTONICS

Reading Earth’s Crust

Earthquakes don’t happen just anywhere — they trace the same lines, year after year, where slabs of the Earth’s surface grind against each other. This overlay draws those lines over any Atlas map: the 13,696 active fault traces of the GEM database, coloured the way a geology textbook colours them — red where the crust collides, blue where it pulls apart, amber where it slides past — laid over the great tectonic plate boundaries that form the structure beneath.

Open the Atlas map & toggle Tectonics on →

On the Atlas canvas, the Tectonics switch sits just under the overlays panel. The faults and plate boundaries draw over whatever metric (or the plain political map) you have up.

The three ways the ground moves

What makes one boundary build a mountain range and another tear open the seafloor is simply the direction the two sides move relative to each other. There are three cases — and the map colours every fault by which one it is.

COMPRESSION · plates collidingSubduction zones and thrust mountain fronts, where the crust is forced together — the highest-energy margins on Earth, and the source of the greatest earthquakes and tsunamis. The Andes, Cascadia and the Japan trench all read red.

EXTENSION · crust pulling apartRifts and spreading ridges, where the crust is stretching and new ground is being made — the Mid-Atlantic Ridge on the seafloor, the East African Rift tearing a continent open on land.

STRIKE-SLIP · crust sliding pastTransform faults, where two plates grind horizontally past each other, making and destroying no crust — the San Andreas Fault in California is the classic example.

A small number of faults sit in neutral slate — the database records no sense of motion for them, so we mark them honestly as unclassified rather than guess. Beneath the faults, the heavier structural lines are the plate boundaries themselves, with the great subduction zones(the Pacific’s trench systems) drawn heaviest of all.

The Ring of Fire

Put the collisions together and a pattern jumps out: a horseshoe of subduction zones almost completely encircling the Pacific — up the Andes and the North American coast, across to Japan and Kamchatka, down through Indonesia and New Zealand. Because so much of the world’s subduction is crowded along it, roughly nine in ten of the planet’s earthquakes and about three-quarters of its active volcanoes happen there. On the map it reads as the near-unbroken chain of heavy convergent boundaries and red faults wrapping the Pacific rim.

Reading Earth's Crust — Fault Lines and Plate Boundaries Explained →

About this overlay & its data

The fault network is the GEM Global Active Faults Database— an open, peer-reviewed compilation maintained by the Global Earthquake Model Foundation, the same kind of data used in professional seismic-hazard analysis. It records about 13,700 active fault traces with their sense of motion; we colour them by that motion and show the raw classification in the popup. About 97% carry a recorded slip type; the small remainder we show as motion-unclassified. The plate boundaries are the widely used PB2002model published by Peter Bird in 2003. Both are real digitised geometry, dated and attributed — not artistic impressions of where the lines roughly go. There are no per-place pages, because a fault line isn’t a place.

Active faults: GEM Global Active Faults (CC BY-SA 4.0). Plate boundaries: Peter Bird (2003) / Hugo Ahlenius, Nordpil (ODC-By). Basemap © CARTO.

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