IN THE NEWS | AMERICAN GEOPHYSICAL UNION, SAN FRANCISCO, DECEMBER 5–9
Earth
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Weather affects geologic activity
By Devin Powell and Alexandra Witze
If you want to know the chance of
an earthquake in the Himalayas or a
volcanic eruption in Iceland, it may help
to check a calendar. Seasonal patterns
of rainfall and snowfall can affect how
often quakes and volcanoes go off, scientists reported December 8.
In the Himalayas, the weight of water
from monsoon rains helps dampen seis-micity for a few months each year, says
geophysicist Thomas Ader of Caltech.
Seismic records collected from 1998 to
2004 show that the number of quakes
drops in the rainy summer months and
rebounds when things dry out.
Water from monsoon storms runs off
the mountains and into India, where its
weight bends the Earth’s crust slightly.
From season to season, this bending
causes the surface to move back and
forth, as recorded by Global Positioning
System stations, in time with the rise and
fall in earthquake frequency.
Calculations by Ader and his colleagues
suggest that the flexing eases the stress on
the fault where two tectonic plates col-
lide beneath the Himalayas. This change
is small, less than a hundred-thousandth
the stress caused by plate motions. But
it lasts for months, perhaps giving the
earthquakes time to slowly incubate.
Kilauea’s explosive past
the scenic burbling lava fountains of kilauea volcano, on
the Big island of hawaii, belie the mountain’s volatile nature.
New surveys of kilauea’s lava flows show that in the last
2,500 years the volcano has been convulsed by periods
of explosive volcanism, the most recent ending about 200
years ago. one eruption during that last explosive period was
the most lethal ever documented at a u.S. volcano: Several
hundred warriors crossing the island in November 1790 to
do battle passed too close to the summit and perished in a
surge of hot ash and gas. over the last two and a half millennia, explosive periods have prevailed for about 60 percent
of the time, Don Swanson of the u.S. geological Survey’s
hawaiian Volcano observatory reported December 6. “once
these periods start, they could last for centuries,” he said.
on average, 5,000 people, mostly tourists, visit kilauea daily.
—Alexandra Witze
uSgS
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