Scientists probing under the seafloor off Alaska have mapped a geologic structure that they say signals potential for a major tsunami in an area that normally would be considered benign. They say the feature closely resembles one that produced the 2011 Tohoku tsunami off Japan, killing some 20,000 people and melting down three nuclear reactors. Such structures may lurk unrecognized in other areas of the world, say the scientists. The findings will be published tomorrow in the print edition of the journal Nature Geoscience.