Download Shaken Beliefs: Seismic Lessons from Japan’s Tohoku Earthquake

Shaken Beliefs: Seismic Lessons from Japan’s To...
MARCH 11, 2016
Save paper and follow @newyorker on Twitter
http://www.newyorker.com/tech/elements/shaken-...
Shaken Beliefs: Seismic Lessons from
Japan’s Tohoku Earthquake
BY MARCIA BJORNERUD
Five years after the 9.0-magnitude earthquake in Tohoku,
geologists are still learning from the disaster.
ILLUSTRATION BY DANIEL ZENDER
J
apan could well be called the cradle of
seismology. It occupies one of Earth’s most
precarious tectonic settings, at the nexus of
four plates, and its written record of
earthquakes extends back to 599 A.D., during the
reign of Empress Suiko. Japanese seismologists of the early twentieth century made
a number of significant contributions to geophysics, some of which anticipated
plate-tectonic theory by decades. In 1899, several years after a powerful tsunami
(http://www.newyorker.com/tech/elements/a-tsunami-written-in-stone) struck the
Sanriku Coast, a man named Akitsune Imamura correctly inferred the type of fault
slip (now called a megathrust event) that generates such waves. In the nineteentwenties, another Japanese scientist, Kiyoo Wadati, collected seismic data that
became critical to the development of the Richter scale and, eventually, the
discovery of the process of subduction, in which old ocean crust sinks back into
Earth’s mantle. Today, Japan spends more money per capita than any other country
on earthquake research, engineering, and preparedness.
1 of 4
And yet the disaster of five years ago—a magnitude-9.0 megathrust earthquake off
Tohoku (http://www.newyorker.com/magazine/2015/07/20/the-really-big-one),
followed by a major tsunami and nuclear accident (http://www.newyorker.com
/magazine/2011/10/17/the-fallout)—came as a surprise. Until March 11, 2011, the
consensus among seismologists was that a particular stretch of fault would observe
certain rules, rupturing at consistent intervals in events of similar size. These events
are known as characteristic earthquakes. For the subduction zone east of northern
Honshu, where the Tohoku quake originated, the characteristic earthquake was
considered to be in the lower magnitude-eight range. Japan’s civil-defense
strategies, including its tsunami walls, were designed in accordance with that view.
As it turned out, though, the Tohoku earthquake wasn’t characteristic. It was the
fourth-largest seismic event ever recorded, outranked only by the 2004 Indonesian
(http://www.newyorker.com/magazine/2005/02/07/mission-to-sumatra) and 1964
Alaskan earthquakes (both 9.2) and the enormous but less well-known 1960
Chilean earthquake, a 9.5 event whose resulting tsunami devastated Hilo, Hawaii,
03/12/2016 02:49 PM
some six thousand miles away.
To understand
why
Tohoku
defied
geophysical wisdom, consider the
Shaken Beliefs: Seismic
Lessons
from
Japan’s
To...conventionalhttp://www.newyorker.com/tech/elements/shaken-...
paradox inherent in an earthquake. One of the prerequisites is a strong fault
surface—a plate boundary or rock fracture with a high coefficient of friction, like a
skid-resistant pad under an area rug. (A weak fault will slip continuously, never
storing up much elastic energy.) But the strong fault must also be capable of
slipping, all at once and over a wide area, because this is what causes the tremor. In
technical terms, its dynamic friction must be lower than its static friction. This can
happen when opposing rock surfaces scrape against each other, because the
resultant heat and pulverization act as a kind of lubricant, making the fault surface
suddenly, fleetingly slick. The traditional style of cross-country skiing works on the
same principle—the downward kick stroke requires high static friction, while the
glide is possible thanks to low dynamic friction.
Prior to the Tohoku event, the prevailing assumption was that a subduction zone
could generate megathrust earthquakes at only limited depths—specifically,
between about five and twenty-five miles down. On the deep end of this range, the
temperature is high enough that the rock, softened by heat from Earth’s interior, is
malleable. On the shallow end, the rock is relatively weak and watery, because it is
made up of seafloor sediment. In either case, the fault surface is all glide and no
kick: it ought to be too slippery to generate a big earthquake. But Tohoku ignored
that idea. The greatest lurching occurred along the shallowest part of the fault. The
coastal area near Sendai, where the subduction zone is about fifteen miles down,
was shoved an astonishing fourteen feet eastward. At the uppermost part of the
plate boundary, though, the figure was closer to a hundred feet. This, in turn,
displaced an immense volume of overlying seawater, triggering the tsunami that
caused most of the disaster’s sixteen thousand deaths.
Most seismologists now agree that Tohoku should be considered a composite
event. What started as a large but otherwise unremarkable earthquake unleashed a
chain reaction of secondary effects, and it morphed into a monster. One hypothesis
is that the initial jolt was strong enough to dislodge a particularly high-friction
patch of the subduction zone, which then activated the otherwise stable, water-rich
upper section. Once this began slipping, a process called thermal pressurization
—heating and expansion of fluids on the fault—may have led to runaway failure at
shallow depths. Like the thin film of water beneath the blade of a skate, the heated
fluids would have kept the rock surfaces out of contact with each other, reducing
the friction between them virtually to zero. Tragically, because the early stages of
the fault slip looked like an ordinary earthquake, Japan’s sophisticated automatic
early-warning systems initially underestimated the magnitude of the event. As a
result, some people ignored the first alerts that were issued. The arcane details of
rock friction, then, are of not only geophysical but also humanitarian concern.
2 of 4
Whatever the cause of its anomalously large and shallow slip, the Tohoku
earthquake has forced the seismological community to abandon the characteristicearthquake concept, at least for the biggest events, and to confront the unsettling
03/12/2016 02:49 PM
reality that even an incipient earthquake doesn’t know at the outset whether it will
become aLessons
magnitude
nine
or not.
Rather,
Shaken Beliefs: Seismic
from
Japan’s
To...
its propagation
and eventual size will
http://www.newyorker.com/tech/elements/shaken-...
depend on a cascade of effects that play out over seconds or minutes. Earlier this
month, a group of four scientists published a paper in Nature
(http://www.nature.com/nature/journal/v531/n7592
/full/nature16945.html) addressing the question of why the Japanese earthquake
did not in fact become even larger. In particular, they focus on why the slip
apparently stopped abruptly at a line that runs across the overriding plate—the
upper slice in the subduction sandwich, which carries Japan itself. Combining
geologic maps, high-resolution depth and gravity readings, and historical patterns
of earthquakes in the region, the authors argue that this sudden halt was due to a
geologic discontinuity in the plate, where Japan’s oldest rocks (to the north) are
juxtaposed against more recent, mainly volcanic rocks. They suggest that the
contrasting rock types, with their distinct geologic histories, created differences in
friction along the subduction zone. They behaved like a patch of grass in the snow,
catching the ski as it whizzed over.
For seismologists, the Tohoku earthquake was a humbling reminder that our
geophysical records offer only a peephole view of Earth’s behavior over time, and
that our most advanced models for geological phenomena are cartoonish
oversimplifications of nature. A hundred years of cutting-edge seismology can be
undermined in a hundred seconds.
Watch: Kathryn Schulz, a staff writer for The New Yorker, moderates a panel on
the earthquake that is expected to devastate the Pacific Northwest.
3 of 4
Marcia Bjornerud is a professor of geology at Lawrence University, in Appleton,03/12/2016
Wisconsin,02:49
and PM
the author
of Lessons
“Readingfrom
the Japan’s
Rocks: The
of the Earth.”
Shaken Beliefs:
Seismic
To... Autobiography
http://www.newyorker.com/tech/elements/shaken-...
4 of 4
03/12/2016 02:49 PM