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Largest earthquakes since 1900
Ecuador, M 8. 8
Chile, M 8. 5
Kamchatka, M 8. 5
Indonesia, M 8. 5
Tibet, M 8.6
Kamchatka, M 9.0
Chile, M 9. 5
Alaska, M 8.6
Kuril Islands, M 8. 5
Alaska, M 9. 2
Alaska, M 8. 7
Indonesia, M 8.6
Indonesia, M 9. 1
Indonesia, M 8. 5
Chile, M 8. 8
Japan, M 9.0
the chance of another magnitude 9 or
greater quake occurring somewhere in
the world is 63 percent — compared with
24 percent if the planet were not experiencing a cluster. “We think we’re in an
increased hazard situation from these
very large quakes,” he says.
Other researchers say Bufe has gone
too far out on a statistical limb. Andrew
Michael, a USGS researcher in Menlo
Park, Calif., has done a separate analysis.
“We don’t see clustering,” he says.
Rather than study only the highest
magnitude quakes, Michael probed dif-
ferent magnitudes over different time
periods. Among other tests, he looked at
whether there was a statistically mean-
ingful increase in seismicity worldwide
after big earthquakes, and whether the
time between big quakes followed a
nonrandom pattern. He found that the
statistical tests could produce what
looked like earthquake clusters, but that
the clusters disappeared when differ-
ent magnitudes and time periods were
included. “If we use a range of magnitude
cutoffs, we find that the data are very well
explained by the random model,” he says.
Bufe counters that because clustering
may happen only for very large quakes,
the pattern wouldn’t be seen when looking at lower magnitudes. But Michael
argues that the number of magnitude 9
earthquakes is too small to draw any
Since 1900, most earthquakes of
magnitude 8. 5 or greater have occurred
in two clusters, one in the 1950s and
’60s and the second starting in 2004.
1910 1900 1920 1930 1940 1950 1960 1970 1980 1990 2000 2010 2020
Back Story | JAPAN QUAKE BY THE NUMBERS
In recent weeks seismologists have collected
more details about how Japan’s destructive
March 11 earthquake unfolded.
9.0 Magnitude
Aftershocks equal or greater
to magnitude 6.0
54
300
kilometers
Total length of rupture
2 Zones of rupture (one near the Japan trench, one near the coast)
24
meters
Biggest measured slip (on the
ocean bottom off Japan’s east
coast)
60
meters
Biggest estimated slip
220
seconds
Duration of quake
statistically significant conclusions. “At 9
you just don’t have enough data,” he says.
Richard Aster, a geophysicist at the New
Mexico Institute of Mining and Technology in Socorro, agrees with Michael. He
notes that the list of earthquakes since
1900 — which everyone must use for their
studies — includes around 1,700 quakes
of magnitude 7 or more, about 70 equal
to or greater than magnitude 8 and only
5 magnitude 9 or above.
Aster has done a separate analysis
looking, in part, at the total amount of
energy released by all quakes since 1900.
Big earthquakes dominate; the Japan
quake accounts for about 5 percent of
all global cumulative seismic energy
released since 1900, he reported at the
meeting.
Over the past two decades, the
number of earthquakes greater than
magnitude 7. 5 has been increasing
worldwide, Aster’s team found. But that
increase could be due to natural random
fluctuations as opposed to any actual
trend in tremors worldwide.
Like Michael, Aster does not find
quake groupings beyond the known
effects of aftershocks and local quake
triggering. “We have found that there’s
no evidence for clustering at long scales,
say trans-Pacific scales,” Aster says.
But the evidence so far is not enough
to end the debate. “The only way out
of this will be unfortunately waiting a
long time until we see more large earthquakes,” says Michael. “That is the
problem we face in seismology.” s
