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New
study finds mega-tsunamis pose potential threat to Hawaiian Islands
It's
almost unimaginable: a tsunami more than 1,000 feet (300 meters) high
bearing down on the island of Hawaii
6
December, 2012
But
scientists have new evidence of these monster waves, called
megatsunamis, doing just that. The findings were presented here
yesterday (Dec. 5) at the annual meeting of the American Geophysical
Union.
Unlike
tsunamis from earthquakes, the Hawaiian tsunamis strike when the
island chain's massive volcanoes collapse in humongous landslides.
This happens about every 100,000 years, and is linked to climate
change, said Gary McMurtry, a professor at the University of Hawaii
in Honolulu.
Sitting
about 30 feet (10 m) away from today's Ka Le (South Point) seashore
are boulders the size of cars. Some 250,000 years ago, a tsunami
tossed the enormous rocks 820 feet (250 m) up the island's slopes,
said Fernando Marques, a professor at the University of Lisbon in
Portugal. (The boulders are closer to the shore now because the main
island of Hawaii is one of the world's largest volcanoes, and its
massive weight sends it sinking into the Earth at a rate of about 1
millimeter a year.)
McMurtry's
team found two younger and slightly smaller tsunami deposits at South
Point on the main island of Hawaii, one 50,000 years old and one
13,000 years old. He suggests the tsunami source is the two Ka Le
submarine landslides, from the flanks of the nearby Mauna Loa
volcano. The waves carried corals and 3-foot (1 m) boulders 500 feet
(150 m) inland.
Deadly,
landslide-triggered tsunamis happen at volcanic islands around the
world, and are a potential hazard for the Eastern United States. "We
find them everywhere, but we don't know of any historical cases, so
we have to go back in time," said Anthony Hildenbrand, a
volcanologist at the University of Paris-Sud in France, who helped
identify the ancient tsunami deposit.
The
falling rock acts like a paddle, giving the water a sudden push.
While landslide tsunamis may have a devastating local effect, they
lose their power in the open ocean and don't destroy distant
coastlines like earthquake tsunamis.
The
giant landslides seem to happen during periods of rising sea levels,
when the climate is also warmer and wetter, Hildenbrand told
OurAmazingPlanet. Researchers speculate that the change from lower
sea level to higher may destabilize a volcanic island's flanks, and
heavier rains could soak its steep slopes, helping trigger
landslides.
There
are at least 15 giant landslides that have slid off the Hawaiian
Islands in the past 4 million years, with the most recent happening
only 100,000 years ago, according to the U.S. Geological Survey. One
block of rock that slid off Oahu is the size of Manhattan.
Lake
Tahoe region overdue for 7.0 magnitude earthquake and mega-tsunami
6
December, 2012
A
tsunami-producing fault in Lake Tahoe is overdue for another
earthquake, scientists said here Tuesday at the annual meeting of the
American Geophysical Union.
The
West Tahoe Fault is capable of producing a magnitude-7.3 earthquake
and tsunamis up to 30 feet (10 meters) high in the clear blue lake,
where million-dollar homes line the shore, researchers said.
Earthquakes
strike every 3,000 to 4,000 years on the fault, and the most recent
shaker was 4,500 years ago, indicating the fault is overdue for
another earthquake, said Jillian Maloney, a graduate student at the
Scripps Institution of Oceanography in San Diego.
The
West Tahoe fault defines the west shore of the lake, coming on shore
at Baldwin Beach, passing through the southern third of Fallen Leaf
Lake, and then descending into Christmas Valley near Echo Summit.
To
trace the fault's history, Maloney and her colleagues examined data
from a CHIRP seismic imaging system, which details underwater
sediment layers at very high resolution. (CHIRP stands for compressed
high intensity radar pulse.) The researchers correlated landslide
deposits, which could be related to past earthquakes, throughout
western Lake Tahoe and in small lakes immediately to the south with
radiocarbon dates from the sediments.
The
West Tahoe Fault has a complicated history, the analysis reveals. The
fault appears to alternate between breaking all at once, in a 31-mile
long (50 kilometer) fracture, and in smaller, shorter segments. The
discovery has implications for the Tahoe's seismic hazard, because
the size of an earthquake relates to the length of a fault rupture,
Maloney said. The biggest earthquakes come from the longest fault
fractures.
The
correlations, while still at an early stage, indicate the last time
the fault's entire length ruptured was 7,800 years ago, Maloney told
OurAmazingPlanet. More recent quakes occurred on individual segments,
she said.
Tsunami
risk
Because
the fault crosses the lake, scientists worry a future earthquake will
cause a tsunami in Lake Tahoe. The monster waves could form in two
ways: by the fault displacing ground under the lake, similar to
Japan's Tohoku tsunami, or by causing landslides that displace the
water. A combination of both could also create an even bigger wave.
Layers
of sediment preserved in and around Lake Tahoe record evidence of
past tsunamis, said Graham Kent, director of the Nevada Seismological
Laboratory in Reno.
However,
having smaller earthquakes on the West Tahoe Fault would be better
for the ski town. "If it breaks up into multiple segments, it
might not be as great a tsunami risk," Kent told
OurAmazingPlanet.
The
most recent earthquake in the Tahoe region was about 575 years ago,
on the Incline Fault, which becomes active about every 10,000 to
15,000 years. Scientists estimate its earthquake size potential at
magnitude 7.
At
more than 1,645 feet (501 meters) deep, Lake Tahoe, which straddles
the California and Nevada border in the seismically active Sierra
Nevada region, is one of the world's deepest freshwater lakes.
Magnitude
4.9 - BOUVET ISLAND REGION – South Africa
2012 December 06 15:46:50 UTC
Earthquake Details
This
event has been reviewed by a seismologist.
4.9
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54.039°S,
1.964°W
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18.7 km
(11.6 miles)
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BOUVET
ISLAND REGION
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349 km
(216 miles) W of Bouvet Island, Bouvet Island
2737 km (1700 miles) SSW of Hermanus, South Africa 2744 km (1705 miles) SSW of Claremont, South Africa 2747 km (1706 miles) SSW of Cape Town, South Africa |
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horizontal
+/- 18.2 km (11.3 miles); depth +/- 6.1 km (3.8 miles)
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NST=
40, Nph= 40, Dmin=>999 km, Rmss=1.58 sec, Gp=
58°,
M-type=body wave magnitude (Mb), Version=4 |
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usc000e587
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