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Tidal Forces Trigger Tremors on San Andreas Fault
Saturn’s Moons Are Cuter Than Sugar Plum Fairies
Female Ducks’ Twisty Tracts Defend Against Screwy Males
7 Tipping Points That Could Transform Earth

Tidal Forces Trigger Tremors on San Andreas Fault

Posted: 24 Dec 2009 03:30 AM PST

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The subtle changes in stress caused by tides in Earth's crust can trigger small, deep quakes along a seismically active portion of California's San Andreas fault, a new analysis suggests.

The same forces of attraction that cause ocean tides also cause tides in Earth's rocks, says Amanda Thomas, a seismologist at the University of California, Berkeley. And although the continual variations in rock stress associated with those tides are minuscule, they apparently are big enough to trigger small tremors along some faults, Thomas and Berkeley colleagues Robert M. Nadeau and Roland Bürgmann report in the Dec. 24/31 Nature.

Evidence for the claim comes from earthquake data collected near Cholame, Calif., a tiny town near the southeastern end of a portion of the San Andreas fault where small, deep tremors are common. The study focused on a 110-kilometer-square area around the town where more than 1,700 minor quakes occurred between July 2001 and May 2008, Thomas says.

When analyzing these quakes, she and her colleagues found that the mini-temblors were much more likely to occur at times when tidal stresses tended to shear the fault in the direction that it normally breaks — that is, when the Pacific tectonic plate is being pulled to the north-northwest relative to the North American tectonic plate, which lies to the east of the fault. In a sense, the added stress on a fault poised to slip acts like the straw that breaks the camel's back.

When tidal stresses act in the other direction and therefore tend to relieve stress on the fault, the frequency of small quakes drops substantially.

The fault's extreme sensitivity to small changes in stress suggests that the deep portions of the fault where these small quakes occur is well lubricated, probably by water that is under exceedingly high pressure, the researchers suggest.

Previous studies bolster the notion that small changes in stress along faults can set off earthquakes. In December 2004, seismic waves spreading from the tsunami-spawning temblor that occurred off the west coast of Indonesia triggered a flurry of small quakes near Alaska's Mount Wrangell, a quarter of the way around the globe (SN: 8/27/05, p. 136).

Image: USGS

Posted: 24 Dec 2009 02:00 AM PST

We love Cassini more with every amazing image the intrepid spacecraft sends back from its travels, and this latest treat is no exception.

The Cassini imaging team has assembled images of some of Saturn's moons in action into this sequence, bringing another corner of the solar system to life for us. It's still unclear to us whether setting the video to Tchaikovsky's Nutcracker Suite adds to or subtracts from its appeal, but that's what the mute button is for.

Our only complaint is that Enceladus doesn't make an appearance, but we have a separate tribute to our favorite Saturnian moon planned, so stay tuned.

Video: NASA/JPL

Posted: 23 Dec 2009 10:55 AM PST

muskovy_duck

The first intimate duck videos show that it's mate and checkmate in the battle of the sexes.

High-speed cameras document that a male Muscovy duck can fully extend his 20-centimeter penis in a third of second, says evolutionary biologist Patricia Brennan of Yale University (Watch the researcher's video). That may be about all the time he has with a resisting female trying to escape him. Male Muscovy ducks rank among the waterfowl that often fail to take no for an answer.

Also on camera, duck handlers coaxed males to deploy into glass tubes instead of into females. The tubes featured bends and wrong-way spirals that kept the males from extending very far, Brennan and her colleagues report online Dec. 22 in Proceedings of the Royal Society B.

Real female ducks have such shapes, even spirals that squiggle in the direction opposite to male anatomy. And Brennan has proposed that a female duck refusing to relax the tension in all her complicated bits presents a challenge to an unwelcome male. For a mate she approves, though, a female Muscovy can ease the constrictions.

Even within the same species, what serves females well as far as number and frequency of mates may not be the best for males. In the case of Muscovy ducks, the males, like competitors in an arms race, have developed explosive extension, but females may be countering with resistant geometry.

The new videos support this idea of female resistance, Brennan says. The obstructive powers of internal tracts fit with the scenario that corkscrewy genitals in certain waterfowl evolved through sexual conflict, Brennan and her colleagues report.

This evidence for competing genitals could be a first for vertebrates. As far as he knows, says evolutionary biologist Locke Rowe of the University of Toronto, elaborate reproductive organs in waterfowl are the only evidence so far of sexual conflict driving coevolution of traits in vertebrates.

Biologists pay so much attention to reproductive organs not out of personal weirdness, Rowe says, but because "they're some of the most diverse traits known." Myriad forms suggest the organs evolve fast under the pressures of sexual selection. For example, baroque reproductive tracts might benefit female ducks by allowing them to choose only high-quality or high-compatibility sires for their young.

Females obviously have some way of resisting unwanted males, Brennan says. Studies of mallards, for example, show that, despite plenty of trying, unwelcome males end up siring only a few percent of a female's offspring.

Mallards and some other waterfowl are among the few birds that have any insertable sperm-delivery organ at all. Biologists have marveled at the spiraling extravagance of some waterfowl penises. But the female side of the picture didn't emerge until 2007, when Brennan uncovered comparable female elaboration. She also observed that species with more contentious sex lives, such as the mallards, tend to have more elaborate genitals in both sexes. In contrast, male Canada geese don't typically force copulations and have pretty uncomplicated organs.

To see how waterfowl use their organs, Brennan and her colleagues turned to a commercial poultry operation in California, where male Muscovy ducks have been trained to deposit sperm into tubes.

Brennan had used four sets of tubes representing what she thought would be easy or difficult geometries. The first time she tried to film males in action, she found that her camera wasn't fast enough.

With a camera that recorded 250 frames per second, Brennan got a first-of-its-kind view of male anatomy in action. When not in use, the male penis lies inside out. Lymph, not blood, powers the expansion.

Brennan filmed 56 occasions of male duck extension, either just in air or into one of the tubes. Males easily extended full length into both a straight tube or into one that curled in the same direction as the duck did. However, tubes that spiraled in the opposite direction or had a bend of 135 degrees stopped the male short of full length, Brennan says.

Males do ejaculate even without full extension, she discovered. What the bends and twists do then is prevent the sperm from getting very far into the reproductive tract, Brennan says. Thus a male forcing himself on a female would be less likely to sire young.

Image: me'nthedogs/Flickr

Posted: 23 Dec 2009 09:40 AM PST

When the Intergovernmental Panel on Climate Change issue its last report in 2007, environmental tipping points were a footnote. A troubling footnote, to be sure, but the science was relatively new and unsettled. Straightforward global warming was enough to worry about.

But when the IPCC meets in 2014, tipping points — or tipping elements, in academic vernacular — will get much more attention. Scientists still disagree about which planetary systems are extra-sensitive to climate shifts, but the possibility can't be ignored.

"The problem with tipping elements is that if any of them tips, it will be a real catastrophe. None of them are small," said Anders Levermann, a climate physicist at the Potsdam Institute for Climate Impact Research in Germany.

Levermann's article on potential disruptions of South Asia's monsoon cycles was featured in a series of tipping element research reviews, published December 8 in the Proceedings of the National Academy of Sciences.

Also discussed were ocean circulation, polar icecaps, Amazon rainforests, seafloor methane deposits and a west African dustbowl. Each is stressed by rising planetary temperatures. Some are less likely than others to tip; some might not be able to tip at all. Ambiguities, probabilities a limited grasp of Earth's complex systems are inherent to the science. But if any tip, it will be an epic disaster.

Wired Sciencetakes you on a tour.

Image: Earth's Eastern and Western Hemispheres/NASA.

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Polar Sea Ice

Dwindling Arctic sea ice and crumbling Antarctic ice sheets are now a common sight. Whether they signal an impending tip, with rapid melts causing Earth's seas to inundate heavily-populated coastal plains, is debated.

The process appears to accelerate itself: Warming ice melts, which exposes darker areas, causing local temperatures to rise further. But in the Arctic, another feedback may stabilize the ice, wrote Max Planck Institute meteorologist Dirk Notz in PNAS. Though most of the ice "will disappear during summer," much of it will re-freeze in the winter. Arctic sea ice loss "is likely to be reversible if the climate were to become cooler again."

But Notz is less optimistic about Antarctic sea ice, its undersides heated by eddying Southern Ocean currents. And the West Antarctic and Greenland ice sheets have shrunk suddenly at least twice in the last several million years, a behavior that's backed up by climate models. It's "well possible that a tipping point exists for a possible collapse" for those sheets, wrote Notz. It could "render the loss of ice sheets and the accompanying sea-level rise unstoppable beyond a certain amount of warming."

Image: NASA.

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Amazon Rainforest

As one of Earth's great carbon sinks, the replacement of Amazon jungles with savannah or forest would drastically accelerate global warming.

On their own, rising temperatures and changing weather patterns would not trigger jungle dieback, wrote researchers led by Oxford University ecosystem scientist Yadvinder Malhi in PNAS. But deforestation combined with intensified dry seasons leaves forests vulnerable to fire, producing more weather-altering deforestation.

"The dieback of the forests of East Amazonia in the 21st century is far from inevitable but remains a distinct possiblity," they wrote.

Image: NASA.

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Bodélé Depression, Chad

Winds whipping across the Bodélé, a 10,000 square mile Saharan plain covered by ancient lakebed sediments, carry 700,000 tons of dust into the atmosphere annually. It floats around the world, blocking sunlight and lowering temperatures in some regions, and causing rain and warming in others. Saharan dust influences Atlantic ecosystems, Caribbean coral reefs and the Amazon. Its full effects are unknown.

Small atmospheric changes "could profoundly alter the behavior of this feature," wrote Richard Washington, a specialist in African weather African weather specialist at Oxford University, and colleagues in PNAS.

At one point in the last 10,000 years, dust ceased to flow altogether from the Bodélé. That doesn't seem to be our problem. "Although subject to a great deal of uncertainty, some simulations of the 21st century indicate the potential for a substantial increase in dust production," wrote the researchers.

Image: NASA.

Thursday, 24 December 2009