tsunami

Tsunami produces giant Antarctic icebergs, video

icebergsSeveral icebergs the size of Manhattan broke off the Sulzberger Ice Shelf in West Antarctica last March as a result of the March 11, 2011 Japanese earthquake and tsunami.  It had long been theorized that tsunamis could cause glacial calving.  Now NASA has proof that the Japanese tsunami caused formation of icebergs. See NASA photos and video here.

The Sulzberger Ice Shelf was especially susceptible to waves because there was no nearby sea ice, landfast ice or pack ice in the area.

NASA expects another large chunk, 340 square miles, to break off the Pine Island Glacier later this year or in early 2012.  Already there is a split 200 ft deep, 800 feet wide, running 20 miles so far along the glacier.  Pine Island Glacier last calved a significant iceberg in 2001, and some scientists have speculated recently that it was primed to calve again.  See NASA slide show and video here.

“While Pine Island has scientists’ attention because it is both big and unstable – scientists call it the largest source of uncertainty in global sea level rise projections – the calving underway now is part of a natural process for a glacier that terminates in open water. Gravity pulls the ice in the glacier westward along Antarctica’s Hudson Mountains toward the Amundsen Sea. A floating tongue of ice reaches out 30 miles into the Amundsen sea beyond the grounding line, the below-sea-level point where the ice shelf locks onto the continental bedrock. As ice pushes toward the sea from the interior, inevitably the ice shelf will crack and send a large iceberg free.” – NASA

NASA is keeping close watch. This may be the first time such a large calving is seen as it  happens.

See also:

Where the Next Big American Earthquake and Tsunami Might Occur

Ice Ages and Glacial Epochs

Earth’s Magnetic Poles, Reversing or Not

Where the Next Big American Earthquake and Tsunami Might Occur

When you think of earthquakes in the U.S., you usually think of Southern California and the San Andreas fault. In 1906, such an earthquake destroyed San Francisco. The next “big one” could certainly hit along the San Andreas, but there is another, lesser known possibility: off the coast of Oregon, Washington and British Columbia along the Cascadia subduction zone, the site of many historic earthquakes.

Natural Resources Canada sets the scene:

cas11

“At 9PM on January 26, 1700 one of the world’s largest earthquakes occurred along the west coast of North America. The undersea Cascadia thrust fault ruptured along a 1000 km length, from mid Vancouver Island to northern California in a great earthquake, producing tremendous shaking and a huge tsunami that swept across the Pacific. The Cascadia fault is the boundary between two of the Earth’s tectonic plates: the smaller offshore Juan de Fuca plate that is sliding under the much larger North American plate.” That earthquake is estimated to have been magnitude 9.0. You can read accounts of it here and a paper recounting American Indian stories about an M7.3 earthquake along the related Seattle fault that happened about A.D. 900.

Rob Witter, coastal geologist with the Oregon Department of Geology and Mineral Industries, says “there’s a 10 to 14 percent chance a powerful earthquake and tsunami will strike the Oregon coast in the next 50 years.” That prediction is based on several independent studies. (See here and here.)

According to the U.S. Geological Survey, “Analysis of geologic deposits indicates that a number of earthquakes, possibly of magnitude 8-9, have occurred in the past,” along the Cascadia subduction zone. See here for USGS map and cross sections.

According to Natural Resources Canada, evidence for multiple large earthquakes in the region include:

Buried tidal marsh or coastal forest soils point to sudden land subsidence of about 1 meter occurring at the same time from Vancouver Island to Northern California.

 Changes in tree ring growth from coastal old-growth also suggest a sudden, widespread subsidence and drowning of roots.

Sand layers on top of the buried coastal marshes, driven in from offshore bars by the wave of the large tsunami that rushed into the subsided coastal region.

 Silt turbidite (landslide) layers on the deep sea floor far off the coast from underwater landslides, likely caused by strong seismic shaking.

 Tsunami evidence from:

local sources – marine organisms swept into and preserved in the bottom muds of coastal lakes that are separated from the ocean by land elevations of some 5 m high distant sources – large tsunami in Japan with no local Japanese earthquake.

We really don’t know where or when the “big one” will hit, but geologic evidence shows that the northwest coast of the U.S. has all the makings.

See also: The Measure of an Earthquake.

The Measure of an Earthquake

We heard that the recent Japanese earthquake measured 8.9. What does that number mean? The number used to refer to the Richter Scale, but now refers to the moment magnitude scale, but the numbers are calculated so that they are the same in both scales. The moment scale measures the size of an earthquake in terms of the rigidity of the earth, the amount of movement and the size of the area affected. In other words, the amount of wiggle on a seismograph. Both the Richter and moment scales are logarithmic, meaning that an earthquake of size 7 is 10 times stronger than an earthquake of size 6. But the amount of energy released is another matter.

Lee Alison, Arizona State Geologist, explains on his blog:

How does the Japan earthquake of magnitude 8.9 compare to other recent large quakes?

The news media do a better job than they used to of noting that each magnitude number is 10 times that of the lower number. But most everyone assumes that refers to the relative amount of energy released by the quake – comparable to measuring the power of atomic bombs for instance.

Not true.

The magnitude is a measure of the amplitude of the seismic waves. But each 1.0 magnitude increase is equal to approximately a 32 times increase in energy release. Each increase of magnitude by 2.0 equals 32 x 32 or (about) 1,000 times increase in energy released.

The M8.9 Japan quake released the equivalent of 336 megatons of TNT. In comparison, last month’s Christchurch, New Zealand M6.3 quake was equal to 43 kilotons, and last year’s M7.0 Haiti quake was equal to 474 kilotons.

The Japan quake was about 7814 times bigger than the Christchurch quake and 709 times larger than the Haiti quake.

I’ve simplified this in regards to Richter magnitude vs moment magnitude but my intent is to emphasize the power of the Japan quake.

 The strongest recorded earthquake was in Valdivia, Chile, May 1960. It measured 9.5. The second largest, measuring 9.2, was in Alaska in 1964. The 1906 earthquake in San Francisco had a moment magnitude of 7.9. The U.S. Geological Survey says that an earthquake of about 8.0 or more occurs on average of once per year.

The Alaskan earthquake is interesting because it demonstrated a certain property of some clays that contributed to the extensive damage in Anchorage. Some clays are thixotropic, meaning that when subject to shear stress, that is, you shake them, they turn to liquid. Thixotropic substances are normally thick and viscous, but turn very liquid under shear stress. You have experienced thixotropy with a ketchup bottle.

Besides shaking and breaking, seismic sea waves, tsunamis, are the greatest danger. Tsunamis are long-wavelength ocean waves with energy extending from the sea surface to the ocean floor. When the wave reaches shallow water near the coast, all that energy is concentrated into a smaller and smaller space, hence its destructive force. In mid-ocean, a tsunami is barely noticeable.

You can see a list of the largest recorded earthquakes here.

Most earthquakes occur near the edges of tectonic plates, but there are some intra-plate quakes as well. For instance, on December, 16, 1811, a large earthquake, estimated strength 7.2-8.1, occurred near New Madrid, Missouri. New York is not immune to earthquakes either. See here for earthquake information by state.

 And some earthquakes are caused by human intervention. I experienced the Denver earthquakes of 1967-1968. The Rocky Mountain Arsenal near Denver was disposing of waste material by pumping it down more than 12,000 feet beneath the surface. That lubricated a deep range-front fault and caused is to slip.

 Here is a map from the US Geological Survey showing locations of major earthquakes since 1900. The pattern describes the boundaries of major tectonic plates and the volcanoes of Hawaii.

worldearthquakes-USGS