On March 11, 2011, a magnitude 9 earthquake struck under the seabed
off Japan—the most powerful quake to hit the country in modern times,
and the fourth most powerful in the world since modern record keeping
began. It generated a series of tsunami waves that reached an
extraordinary 125 to 130 feet high in places. The waves devastated much
of Japan's populous coastline, caused three nuclear reactors to melt
down, and killed close to 20,000 people.
The tsunami's obvious cause: the quake occurred in a subduction zone,
where the tectonic plate underlying the Pacific Ocean was trying to
slide under the adjoining
continental plate
holding up Japan and other landmasses. The plates had been largely
stuck against each other for centuries, and pressure built up. Finally,
something gave. Hundreds of square miles of seafloor suddenly lurched
horizontally some 160 feet, and thrust upward by up to 33 feet.
Scientists call this a megathrust. Like a hand waved vigorously
underwater in a bathtub, the lurch propagated to the sea surface and
translated into waves. As they approached shallow coastal waters, their
energy concentrated, and they grew in height. The rest is history.
But scientists soon realized that something did not add up. Tsunami
sizes tend to mirror earthquake magnitudes on a predictable scale; This
one produced waves three or four times bigger than expected. Just months
later, Japanese scientists identified another, highly unusual fault
some 30 miles closer to shore that seemed to have moved in tandem with
the megathrust. This fault, they reasoned, could have magnified the
tsunami. But exactly how it came to develop there, they could not say.
Now, a new study in the journal
Nature Geoscience gives an answer, and possible insight into other areas at risk of outsize tsunamis.
The study's authors, based at Columbia University's Lamont-Doherty
Earth Observatory, examined a wide variety of data collected by other
researchers before the quake and after. This included seafloor
topographic maps, sediments from underwater boreholes, and records of
seismic shocks apart from the megathrust.
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