ARI SHAPIRO, HOST:
Tsunamis, the giant waves generated by undersea earthquakes or landslides, have hit U.S. shorelines before. Now geologists say portions of the sea floor along the Aleutian Islands in Southwestern Alaska could produce tsunamis more devastating than anything seen in the last century. As NPR's Christopher Joyce reports, they say California and Hawaii are directly in the line of fire.
CHRISTOPHER JOYCE, BYLINE: The Aleutian Island chain curves in an arc across the North Pacific. Right underneath, there's a trench where two pieces of the Earth's crust are colliding. The edge of the Pacific plate is shoving itself underneath the North American plate. Occasionally, a segment of the trench gives way with ferocious results - big earthquake causing tsunamis. Several over the past century have inundated Hawaii, Alaska and parts of California. Geophysicist John Miller has been studying one particular segment that worries him. It's quiet - too quiet.
JOHN MILLER: The stress isn't being relieved by small seismic events. It's suggested it's building up a tremendous amount of tension.
JOYCE: Too much tension, and the segment will unzip and cause a quake. Miller says this segment, called the Semidi, poses a special risk.
MILLER: The Semidi segment perpendicular to that section of the trench aims right at California.
JOYCE: So you get a particularly large tsunami hitting California.
MILLER: That's correct.
JOYCE: Miller and a team at the U.S. Geological Survey have found evidence that this segment ruptures about once every 180 to 270 years. The last time it erupted was 1788.
MILLER: That last great earthquake - it was 227 years ago, so there's a possibility that we're going to have another big one at any time.
JOYCE: Miller's research appears in the journal Geochemistry, Geophysics, Geosystems. Coincidentally, another USGS team says there's another part of the Aleutian chain that poses a previously unrecognized tsunami threat. Geologist Robert Witter led that team.
ROBERT WITTER: In the area of - that we were focusing, which I kind of call the Fox Island section - that area points things straight towards Hawaii.
JOYCE: Witter says this segment of the trench was not considered a threat because it's creeping. The plates are actually moving there but very slowly. That theoretically relieves stress, making a quake unlikely. But not so fast, says Witter. It seems this creeping segment has caused quakes and tsunamis. His team found evidence of at least six of them over the past 1,700 years. They probably emanated from this Fox Island section. The evidence for this includes sheets of sand and debris that were pushed onto hills on an Alaskan island. The debris was 60 feet above sea level. That's a big tsunami. Hawaii and California have warning systems that would give people at these four hours' notice of a tsunami from here, but Witter says people have to pay attention to those warning.
WITTER: I think the take-home message here is be aware, and practice your evacuation plan. A tsunami along the coastlines could happen. It could happen tomorrow.
JOYCE: Witter's research appears in the journal Geophysical Research Letters. Christopher Joyce, NPR News.