Saturday 8 December 2012

Earthquake predictions


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
54.039°S, 1.964°W
18.7 km (11.6 miles)
BOUVET ISLAND REGION
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
horizontal +/- 18.2 km (11.3 miles); depth +/- 6.1 km (3.8 miles)
NST= 40, Nph= 40, Dmin=>999 km, Rmss=1.58 sec, Gp= 58°,
M-type=body wave magnitude (Mb), Version=4
  • Magnitude: USGS NEIC (WDCS-D)
    Location: USGS NEIC (WDCS-D)
usc000e587

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