Analog Science Fiction and Fact - June 2013

ANALOG
top seventeen, in fact, if you include the Indian
Ocean side of Indonesia, which may not
border the Pacific but is part of the same tectonic
region.
Another high-danger part of that region lies
offshore from Lima, Peru. “[Since] the Spanish
conquest in 1543, Lima has been destroyed
three times by a major earthquake with
tsunamis,” says Emile Okal, a geophysicist
from Northwestern University, Evanston, Illinois.
“How large, we are not sure, but enough
to completely wreck the city as it was built [at
the time]. Now construction would be a bit
better, but the population has increased tenfold
or something, so I would put Lima as a
city very much at risk.”
Also at risk is Indonesia (and the same farflung
regions of the Indian Ocean that were
hard hit in 2004). At one level, that’s surprising,
because classic tectonic theory says that
earthquakes release long-accumulating strain.
This means that a fault should not rupture
again in the same place until enough time has
passed for strain to re-accumulate.
But faults appear to come in segments, and
an earthquake in one segment can transfer
stress to adjacent ones. And, while Indonesia
has seen more than its share of giant earthquakes,
one segment of the Sunda Subduction
Zone—the fault that generated both the 2004
Indian Ocean tsunami and a subsequent magnitude
8.6 earthquake nearby—hasn’t seen a
major earthquake since 1833 . . . and even that
earthquake may have released only a portion
of the accumulating strain.
That segment lies offshore of the densely
populated city of Padang, which is several
times larger than Banda Aceh, the largest Indonesian
city hit by the 2004 tsunami. “There
are places where we know of very large historical
earthquakes that are more or less ripe
for [another],” says Okal. “I would put Padang
as [such] a bad place.”
Other “bad” places circle much of the Ring
of Fire. For example, Okal says, there may still
be strain to be released in northern Chile. “We
know there was one earthquake [there] in
1877, and a smaller one in 1922,” he says.
“But there is the possibility [of another].”
Moving north, he notes that Mazatlán, Mex-
ico, could be at risk, though probably not of a
mega-quake (magnitude 9). Similarly, southern
California doesn’t seem likely to have a giant
subduction-zone earthquake, though, of
course, it’s always vulnerable to waves coming
from far afield. And, as we’ll discuss later,
subduction zone temblors aren’t the only
source of tsunamis.
Continuing to move north, we hit Cascadia,
then Alaska, where the giant “Good Friday”
earthquake of 1964 (magnitude 9.2) unleashed
a tsunami that killed people as far
away as Crescent City, California. But that was
less than fifty years ago and it’s unlikely that
enough stress for a repeat will reaccumulate
anytime soon.
Better primed for big earthquakes are the
Aleutian Islands, parts of which, Okal says,
haven’t seen large earthquakes since 1788.
“But they are very lightly populated.” Still, he
notes, a big earthquake here could beam
tsunami waves toward Los Angeles and other
parts of California. “This is a scenario which is
considered by some people [to be] a major
priority in terms of hazard assessment on the
U.S. West Coast,” he says.
Other parts of the Aleutians, he adds, could
produce a tsunami that “would strike Hawaii
pretty badly.”
Continuing around the Ring of Fire, Kamchatka,
Russia, had a huge earthquake in 1952
(variously reported as magnitude 8.2 to 9.0)
that produced a tsunami whose effects the Soviet
Union long kept secret. “It killed upward
of seventeen thousand people,” Okal says. But
here, too, the strain has probably been fully
released, making a near-future repeat unlikely.
Moving to Japan, the best thing that can be
said is that the fault zone that produced the
2011 earthquake is probably safe for hundreds
of years. But there’s always the threat of a big
tsunami from other areas hitting Tokyo, as
happened in 1923, when one hundred thousand
died, with another forty thousand missing.
8 Farther south, Okal adds, the big
question is a fault zone known as the Nankai
Trough. “Some parts are ripe for rupture,” he
says.
Continuing south, the Ryukyu Islands
(which run from Japan to Taiwan) appear to
8 Not all, or even most, of these people died from the tsunami. The earthquake produced fires, which swept
through the city, killing tens of thousands. A subsequent typhoon added to the misery, and the death toll.
20 RICHARD A. LOVETT