Recently in Geophysics Category

Our historical record of seismic activity is very short, by geological time scales. So extrapolating that record to predict future earthquakes can lead to nasty surprises, such as 2008's devastating earthquake in Sichuan, China, which occurred on a fault that had seen little recent activity. Large earthquakes are typically followed by aftershocks whose frequency decays to some background level of seismicity, following an empirical relation known as Omori's law. But determining the time scale of the decay and the baseline activity can be difficult. A new model by Seth Stein of Northwestern University and Mian Liu of the University of Missouri–Columbia posits an inverse relationship between the aftershock-sequence durations and the slip rates along faults. Large earthquakes are most common along the boundaries of tectonic plates, and the occurrences of aftershocks tend to decay quickly—within a decade or so—to a relatively high background. The relative plate motion at such boundaries can be rapid, faster than 10 mm/yr. Continental interiors, far away from plate boundaries, deform much more slowly, typically less than 1 mm/yr. And thanks to that slower rate of fault loading, aftershocks can last hundreds of years or longer, as shown in the figure. Thus, warn the researchers, interpreting continental earthquakes as steady-state seismicity can overestimate the hazard in presently active areas and underestimate it elsewhere. (S. Stein, M. Liu, Nature 462, 87, 2009.)—Richard J. Fitzgerald

With political sentiment growing in favor of greenhouse gas (GHG) restrictions, biofuels from plant cellulose are being considered among the alternatives to fossil fuels: Plants are renewable and biodegradable, and they sequester carbon. Yet a new report validates concerns that a global biofuels program could put intense pressure on land supply and distribution. To predict the impact of a biofuels-based economy on climate change, an international team of researchers from the US, Brazil, and China linked an economic model of land use with a terrestrial biogeochemical model of global GHG levels. The team considered two cases for cellulosic biofuel crop growth: The primary focus of case1 is on converting unfarmed areas such as forests, as shown in the image; of case 2, on exploiting existing farmland to the extent possible. In both cases, biofuel feedstock becomes a dominant global crop by year 2100, but in the process, total forest area is cut—by 56% in case 1 and by 24% in case 2. The loss of carbon-sequestering trees in case 1 results in a net release of carbon. In case 2, the gains from biofuel production ultimately lead to increased carbon sequestration in the farmed soil from the addition of nitrogen fertilizer, which paradoxically releases N₂O, another potent GHG. The research suggests that stabilizing GHG levels will require a limited and more efficient use of forests and fertilizers for biofuel crop production. (J. M. Melillo et al., Science Express, 22 October 2009, doi:10.1126/science.1180251. Image courtesy of Chris Neill, Marine Biological Laboratory, Woods Hole, MA.)—Jermey N. A. Matthews

Combining the powerful notion that energy is conserved with observational data on surface temperature, ocean heat content, and radiative fluxes, researchers have determined our planet's energy budget for the past half century—without recourse to any climate models. Energy in the form of heat is added to Earth by the Sun's direct radiation and by the well-characterized radiative contributions from greenhouse gases. Those positive contributions (so-called forcings) are balanced, as shown in the figure, by stratospheric aerosols of volcanic origins that reflect incoming sunlight, increased outgoing IR radiation from a warming Earth, long-term heating of Earth—almost entirely of its oceans, which have far higher heat capacities than the atmosphere, land, or ice—and a residual term that mainly represents direct and indirect cooling effects of anthropogenic aerosols throughout the atmosphere. The analysis shows that the anthropogenic aerosols contribute a net cooling of 1.1 Wm⁻², in agreement with the 2007 assessment by the Intergovernmental Panel on Climate Change but with tighter error bars. The researchers, led by Daniel Murphy of the National Oceanic and Atmospheric Administration in Boulder, Colorado, say that the agreement gives confidence both in their technique and in the global circulation models used by the IPCC. They also found that the aerosols’ effects over time reflected the stabilized amount of atmospheric sulfates that resulted from the adoption of emission controls in North America in the 1970s. (D. M. Murphy et al., J. Geophys. Res. 114, D17107, 2009, doi: 10.1029/2009JD012105.) —Stephen G. Benka

A journey to Earth's center would take us through the crust, the mantle, and then two regions of the core. Innermost of those is an iron crystalline solid; surrounding that is a freely flowing, conducting fluid, capped by a thin boundary layer that abuts the rocky mantle. The presence of the fluid core is deduced from seismic studies, and its dynamics can be extracted—with a few assumptions—from time-dependent observations of Earth's surface magnetic field (see Physics Today, February 2008, page 31). A new study does just that, by using nearly 150 years' worth of surface magnetic measurements to determine the magnetic flux in the liquid core. Motions there are modeled with a set of 20 nested cylinders coaxial with Earth's rotation axis and the key assumption that fluid waves in the core must balance Lorentz, Coriolis, buoyancy, and pressure forces. Jean Dickey (Jet Propulsion Laboratory) and Olivier de Viron (Institute of Earth Physics, Paris, and University Paris Diderot) found four robust modes of angular-momentum oscillations corresponding to waves—with periods of 85, 50, 35, and 28 years and diminishing amplitudes—that propagate inward from the core–mantle boundary. There was some previous observational evidence for two of the modes, and theorists had predicted all four modes having similar periods. Now, the strong concurrence of all the results lends credibility to the new modes' existence. (J. O. Dickey, O. de Viron, Geophys. Res. Lett. 36, L15302, 2009.) —Stephen G. Benka

The Arctic Ocean’s floating sea-ice cover naturally waxes and wanes with the seasons. But the decline in area covered by perennial ice—that which survives the summer melt—has been accelerating in recent years. The nonlinear trend, no doubt, reflects the Arctic’s response to a warming climate: Thanks to the ice albedo–ocean feedback, a drop in ice cover increases the absorption of solar radiation, which warms the ocean, prolongs the melting, and reduces ice cover yet further. For quantitative details on the ice pack’s changing mass and heat capacity, though, researchers need more than areal measurements. Ronald Kwok and colleagues at NASA and the University of Washington have now published what may be the most comprehensive thickness maps of the entire Arctic basin. The lidar system they use aboard NASA’s ICESat can precisely distinguish height differences between the sea surface and ice floes. After measuring that “freeboard,” Kwok and company use Archimedes’ principle to calculate the portion that’s underwater. Because much of the brine drains from sea ice as it ages, making it more reflective to microwaves, the researchers are also able to distinguish between young, seasonal ice and older, perennial ice. The surveys taken by ICESat over five years reveal that the Arctic has thinned by about 0.7 m and lost 1.5 × 10⁶ km² of perennial ice—more than twice the area of Texas. Moreover, the volume of perennial ice shrank by 57%—so much that seasonal ice has become dominant for the first time on record. (R. Kwok et al., J. Geophys. Res. 114, C07005, 2009, doi:10.1029/2009JC005312.)—R. Mark Wilson

Atmospheric aerosols affect climate: The particles scatter, absorb, and emit radiation, and they also induce cloud formation. (See Physics Today, May 2004, page 24.) Much of the aerosol mass is produced by oxidation of organic compounds emitted into the atmosphere through human activity and from the biosphere. But many aspects of aerosol formation are poorly understood. It’s thought that biogenic aerosols might be formed from isoprene, a light hydrocarbon given off in large quantities by certain trees. Isoprene itself is volatile—at sea level it boils at 34 °C—so it must undergo a series of chemical reactions before it can form long-lived aerosol particles. Now Caltech’s Paul Wennberg, graduate student Fabien Paulot, and their colleagues offer some insight into what those reactions are. In laboratory experiments designed to replicate atmospheric conditions far from any human pollution, they found that isoprene reacts several times with OH radicals to ultimately form large amounts of dihydroxyepoxide, a newly identified airborne molecule and a likely aerosol precursor. Its hydroxyl groups make it hydrophilic, so it should be readily taken up by existing aqueous aerosol particles. And epoxides under acidic conditions can form low-volatility polymers; a similar reaction is used in epoxy adhesive. Knowing the reactants and their mechanisms will help researchers improve atmospheric models to better predict the consequences of human activity such as deforestation and pollution. (F. Paulot et al., Science 325, 730, 2009.) —Johanna Miller

Iceland, located on top of the Mid-Atlantic Ridge, is one of the most geologically active places on Earth. Its Katla volcano—buried deep under a huge glacier—is particularly large and active: It tends to explosively erupt every 50 or 60 years, although its most recent eruption was back in 1918. One type of nearby seismic activity is so-called long-period (lp) earthquakes, which have shallow origins and magnitudes of less than 3.3. Swarms of those quakes are sometimes thought to indicate an imminent eruption; after more than 900 lp events took place in October 2002, local authorities developed evacuation plans. But Kristín Jónsdóttir (shown here making field measurements) and her colleagues at Uppsala University in Sweden conclude that the glacier, not the volcano, is the culprit. As the glacier gradually flows down a long valley, part of it reaches a cliff where gargantuan 80-meter-thick ice sheets break off and drop 100 meters, carrying more than enough energy to account for the seismicity. The researchers analyzed data from more than 14 000 lp events in 1991–2007. The events' locations, waveforms, seasonal variations, and other variables all pointed to the calving outlet glacier as the seismic source. The authors note that continued global warming could increase such seismicity. For more on glaciers and earthquakes, see Physics Today, September 2008, page 17. (K. Jónsdóttir et al., Geophys. Res. Lett. 36, L11402, 2009, doi:10.1029/2009GL038234.) —Stephen G. Benka

The hallmark of a conventional crystal such as table salt is translational symmetry. Quasicrystals do not have that symmetry and so can exhibit a wider structural variety than their more constrained brethren. But quasicrystals, like crystals, do have long-range correlations and display sharp, structure-revealing diffraction patterns. To date, more than 100 quasicrystals have been synthesized in the lab. Now Luca Bindi of the Natural History Museum of Florence has teamed up with Paul Steinhardt and colleagues from Princeton and Harvard universities to present evidence for a natural version of one of those quasicrystals: icosahedral Al₆₃Cu₂₄Fe₁₃. The material, a 100-μm grain, is from a mineral assemblage (left figure) excavated from the Koryak Mountains in Russia and now housed in the Florence museum; the very complexity of the sample argues for its natural formation. In consultation with his US-based colleagues, Bindi identified the sample as possibly hosting a quasicrystal. The US team then probed a small piece of it with transmission electron microscopy. Diffraction patterns such as shown in the right figure identified quasicrystal regions; the 10-fold symmetry cannot be generated by crystals. Subsequent analysis of x rays scattered off pure quasicrystal grains determined the material’s chemical formula. Geologists and physicists have much to learn about the conditions under which quasicrystals form. The study of natural materials can help address that question and may turn up new, never-before contemplated structures. (L. Bindi et al., Science 324, 1306, 2009.) —Steven K. Blau

Textbooks depict the Gulf Stream, the Kuroshio, and other great ocean currents as smooth, river-like streams. Reality is messier. Gravitationally bound to the spinning globe, the oceans constitute a complex, turbulent system. How turbulence influences one particular ocean current is revealed in a new field and computational study led by Amy Bower of Woods Hole Oceanographic Institution. In 2003–05 her team released 76 floats off the Newfoundland coast at a rate of about six every three months. Her aim was to trace the southward flow of cold arctic water in the Deep Western Boundary Current (DWBC), which hugs the continental slope of the Eastern Seaboard and eventually meets the Gulf Stream off Cape Hatteras. The floats recorded their courses by triangulating signals from a set of moored sound beacons along the route. Like nuclear submarines, the floats surfaced at the end of their voyage and beamed up their recordings to a satellite. To their surprise, Bower and her colleagues found that only seven floats followed the DWBC's coastal route. Most took a wide, irregular path farther east in the Atlantic interior. The same behavior showed up in the team's simulation of the two-year field study. Emboldened by that resemblance, the researchers simulated a further 13 years of flow and found a richer, more complex pattern than appears in textbooks. Understanding such patterns in the present-day ocean, says Bower, is an essential ingredient for predicting the effects of global warming on Earth's future climate. (A. S. Bower, M. S. Lozier, S. F. Gary, C. W. Böning, Nature 459, 243, 2009.)—Charles Day

In an effort to reduce the pervasive smog in Beijing (see photo), Chinese authorities imposed measures to restrict traffic and close factories around the city during the 2008 Olympics. Were those efforts successful in reducing total atmospheric aerosol? Climate scientists Jan Cermak and Reto Knutti at ETH Zürich in Switzerland attempted to find out. They began by comparing absolute values of aerosol optical thickness—transmittance measurements from the Moderate-Resolution Imaging Spectroradiometer aboard NASA's Terra satellite—for the years 2002–08. They found that within a 150-km radius of Beijing, the average 2008 AOT value was more than 14% lower than the previous years. But what would it have been without the mandated emissions reductions? To answer that question, the researchers used a neural network approach: With data from the preceding six summers, they trained a model to predict AOT as a function of relative humidity, wind velocity, and precipitation. The model then predicted that within a 500-km radius of the city, AOTs in 2008 would have been 10%–14% higher than the actual observed values; the model was less accurate when larger regions were analyzed. Although the magnitude of the reductions was lower than expected, the emissions restrictions did have a statistically significant local impact. (J. Cermak, R. Knutti, Geophys. Res. Lett. 36, L10806, 2009, doi:10.1029/2009GL038572. Photo by Michael Silverman, 6 August 2006.)—Jermey N. A. Matthews