What Tectonic Plate Am I On?

How the answer is worked out

Your point is checked against the 52 plate outlines of PB2002, the plate model published by geophysicist Peter Bird. The speed and direction come from each plate’s rotation in that model, measured against the average motion of all the plates together, so they describe the whole plate moving across the globe rather than motion relative to one neighbor. A plate turns around a pole, so its speed differs from place to place: the figure shown is for your point.

The nearest boundary is measured in a straight line across Earth’s surface to the closest part of a plate edge. Its type and relative speed come from the same model, which classifies every short stretch of boundary by how the two plates meet there.

Convergent, divergent and transform boundaries

At a convergent boundary two plates move toward each other. Where ocean floor meets another plate it usually sinks beneath it in a subduction zone, as the Juan de Fuca Plate does under the Pacific Northwest; where two continents meet, as India meets Asia, the crust crumples into mountains. At a divergent boundary plates move apart and new crust forms, mostly along mid-ocean ridges such as the Mid-Atlantic Ridge, which rises above the sea in Iceland. At a transform boundary plates slide past each other sideways, as the Pacific and North American plates do along California’s San Andreas Fault.

Some places sit right on the line

Los Angeles and most of coastal Southern California lie on the Pacific Plate, west of the San Andreas Fault, while San Francisco’s downtown sits on the North American Plate just east of it. Iceland is split by the Mid-Atlantic Ridge: Reykjavík is on the North American Plate and the east of the island on the Eurasian Plate. Japan sits where four plates meet. In places like these the answer can change within a few miles, and a boundary on land is usually a belt of faults rather than a single crack.

Active faults and volcanoes

For US points, the nearest active fault comes from the USGS Quaternary Fault and Fold Database. Here “active” means the fault moved within the past 15,000 years; those that moved in the past 150 years are marked as such, with the slip rate where USGS gives one. Outside the United States this database has no coverage, so no fault is shown. The nearest volcano comes from the Smithsonian Global Volcanism Program’s list of volcanoes active in roughly the past 12,000 years, anywhere in the world. Both distances are straight lines from your point.

What this cannot tell you

Distance to a plate boundary or a fault is not an earthquake hazard rating: shaking depends on the size of an earthquake, the ground under a building and how it was built. For hazard at a site, use the USGS earthquake hazard maps and your local building and emergency authorities. For volcanic activity, follow the official volcano observatory for that area. Plate speeds are averaged over geological time and over years of measurement; they are not a forecast for any single day.