What is the Ring of Fire?

The Ring of Fire is a roughly 25,000-mile arc tracing the edges of the Pacific Ocean, running up the western coast of South and North America, across the Aleutians, and down through Japan, the Philippines, Indonesia, and New Zealand. It exists because the Pacific Plate and several smaller plates meet surrounding plates along these margins, and at most of those boundaries one plate is being forced beneath another in a process called subduction. Water carried down with the descending plate lowers the melting point of the mantle above it, generating magma that rises to form volcanic arcs, while the grinding contact between plates generates earthquakes.

The concentration of hazard is extreme. Roughly three quarters of the world's active and dormant volcanoes lie along this belt, and about 90 percent of earthquakes occur there, including nearly all of the largest ever recorded. The great subduction zone earthquakes are the ones capable of generating ocean-crossing tsunamis, as in the 1960 Chile earthquake, the largest instrumentally recorded, the 2004 Indian Ocean event on a related boundary, and the 2011 Tohoku earthquake off Japan.

Two implications follow. Hundreds of millions of people live within the belt, including in some of the world's largest cities, which makes building codes, early warning systems, and evacuation planning consequential public policy rather than technical footnotes. Japan and Chile in particular have invested heavily in seismic engineering, and the difference in outcomes between similar magnitude events in well-prepared and poorly prepared regions is measured in orders of magnitude of deaths. The Cascadia subduction zone off the Pacific Northwest of North America is a section that has been comparatively quiet in recorded history, with geological evidence of a very large earthquake in 1700 confirmed by tsunami records in Japan, and it is regarded as capable of producing a similar event again.