Are the big quakes in Ecuador and Japan related?
Originally
published April 17, 2016 at 3:51 pm Updated
April 18, 2016 at 7:57 am
Henry
Fountain
Q:
Earthquakes of magnitudes exceeding 7.0 struck Japan and Ecuador just hours
apart Saturday. Are the two somehow related?
A: No.
The two quakes occurred about 9,000 miles apart. That’s far too distant for
there to be any connection between them.
Large
earthquakes can, and usually do, lead to more quakes — but only in the same
region, along or near the same fault. These are called aftershocks. Sometimes a
large quake can be linked to a smaller quake that occurred earlier, called a
foreshock.
In the
case of the Japanese quake, seismologists believe that several magnitude-6
quakes in the same region the previous day were foreshocks to the Saturday
event.
:
But the two earthquakes are similar in some ways, aren’t they?
A: Not
really. The magnitude 7.8 quake in Ecuador was
what would be considered a classic megathrust event, a type first identified
through the work of George Plafker, a U.S. Geological Survey geologist, on the
great Alaska earthquake of 1964. A megathrust quake occurs in the boundary zone
where one of the planet’s tectonic plates is sliding under another, a process
called subduction.
In the
Ecuadorean quake, the Nazca, a heavy oceanic plate, is sliding under the South
American, a lighter continental plate, at a rate of about 2 inches a year.
Strain builds up at the boundary, which is then released suddenly in the form
of an earthquake.
Because
the boundary area is usually large, megathrust quakes are the most powerful and
include the two strongest quakes ever measured by instruments: the magnitude
9.2 Alaska quake in 1964 and one in coastal Chile in 1960 of magnitude 9.5.
Although
there have been plenty of megathrust earthquakes in Japan — including the 2011
Tohoku quake, which led to the Fukushima nuclear disaster — the earthquake
Saturday on the island of Kyushu in southwest Japan was not that type. Rather,
according to the geological survey, the earthquake occurred at shallow depth
along a different kind of fault — a strike-slip — in the top of the Eurasia plate,
above any subduction zone.
Q:
OK, but two 7.0-plus quakes in the same day — does that mean earthquake
activity is increasing?
A: No.
The geological survey, which monitors earthquakes around the world, says the
average number of quakes per year is remarkably consistent. For earthquakes
between magnitude 7.0 and 7.9, there have been some years with more than 20 and
others with fewer than 10, but the average is about 15. That means there is
more than one per month, on average, and by chance, sometimes two quakes occur
on the same day. (Also by chance, the world sometimes goes a month or longer
without a 7.0-plus quake, as it did between July 27 and Sept. 16 last year.)
Sometimes
it seems earthquakes are increasing in frequency because, as instrumentation improves
and more people occupy more parts of the world, more quakes make the news.
The two
earthquakes Saturday both occurred in heavily populated areas with media and
communication networks, so word got out quickly. If one had occurred in the
middle of the ocean, few people would have noticed.
http://www.seattletimes.com/nation-world/are-the-two-big-earthquakes-in-ecuador-and-japan-related/
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