Likely Cause For Recent Southeast US Earthquakes: Underside Of The North American Plate Peeling Off

The southeastern United States should, by all means, be relatively quiet in terms of seismic activity. It’s located in the interior of the North American Plate, far away from plate boundaries where earthquakes usually occur. But the area has seen some notable seismic events — most recently, the 2011 magnitude-5.8 earthquake near Mineral, Virginia that shook the nation’s capital.

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Now scientists report in a new study a likely explanation for this unusual activity: pieces of the mantle under this region have been periodically breaking off and sinking down into the Earth. This thins and weakens the remaining plate, making it more prone to slipping that causes earthquakes. The study authors conclude this process is ongoing and likely to produce more earthquakes in the future.

“Our idea supports the view that this seismicity will continue due to unbalanced stresses in the plate,” said Berk Biryol, a seismologist at the University of North Carolina at Chapel Hill and lead author of the new study. “The [seismic] zones that are active will continue to be active for some time.” The study was published today in the Journal of Geophysical Research — Solid Earth, a journal of the American Geophysical Union.

Compared to earthquakes near plate boundaries, earthquakes in the middle of plates are not well understood and the hazards they pose are difficult to quantify. The new findings could help scientists better understand the dangers these earthquakes present.

Old plates and earthquakes

Tectonic plates are composed of Earth’s crust and the uppermost portion of the mantle. Below is the asthenosphere: the warm, viscous conveyor belt of rock on which tectonic plates ride.

Earthquakes typically occur at the boundaries of tectonic plates, where one plate dips below another, thrusts another upward, or where plate edges scrape alongside each other. Earthquakes rarely occur in the middle of plates, but they can happen when ancient faults or rifts far below the surface reactivate. These areas are relatively weak compared to the surrounding plate, and can easily slip and cause an earthquake.

Today, the southeastern U.S. is more than 1,056 miles from the nearest edge of the North American Plate, which covers all of North America, Greenland and parts of the Atlantic and Arctic oceans. But the region was built over the past billion years by periods of accretion, when new material is added to a plate, and rifting, when plates split apart. Biryol and colleagues suspected ancient fault lines or pieces of old plates extending deep in the mantle following episodes of accretion and rifting could be responsible for earthquakes in the area.

“This region has not been active for a long time,” Biryol said. “We were intrigued by what was going on and how we can link these activities to structures in deeper parts of the Earth.”

A CAT scan of the Earth

To find out what was happening deep below the surface, the researchers created 3D images of the mantle portion of the North American Plate. Just as doctors image internal organs by tracing the paths of x-rays through human bodies, seismologists image the interior of the Earth by tracing the paths of seismic waves created by earthquakes as they move through the ground. These waves travel faster through colder, stiffer, denser rocks and slower through warmer, more elastic rocks. Rocks cool and harden as they age, so the faster seismic waves travel, the older the rocks.

The researchers used tremors caused by earthquakes more than 2,200 miles away to create a 3D map of the mantle underlying the U.S. east of the Mississippi River and south of the Ohio River. They found plate thickness in the southeast U.S. to be fairly uneven — they saw thick areas of dense, older rock stretching downward and thin areas of less dense, younger rock.

“This was an interesting finding because everybody thought that this is a stable region, and we would expect regular plate thickness,” Biryol said.

At first, they thought the thick, old rocks could be remnants of ancient tectonic plates. But the shapes and locations of the thick and thin regions suggested a different explanation: through past rifting and accretion, areas of the North American Plate have become more dense and were pulled downward into the mantle through gravity. At certain times, the densest parts broke off from the plate and sank into the warm asthenosphere below. The asthenosphere, being lighter and more buoyant, surged in to fill the void created by the missing pieces of mantle, eventually cooling to become the thin, young rock in the images.

The researchers concluded this process is likely what causes earthquakes in this otherwise stable region: when the pieces of the mantle break off, the plate above them becomes thinner and more prone to slip along ancient fault lines. Typically, the thicker the plate, the stronger it is, and the less likely to produce earthquakes.

According to Biryol, pieces of the mantle have most likely been breaking off from underneath the plate since at least 65 million years ago. Because the researchers found fragments of hard rocks at shallow depths, this process is still ongoing and likely to continue into the future, potentially leading to more earthquakes in the region, he said.

Although Boiling, Water Does Shape Martian Terrain

At present, liquid water on Mars only exists in small quantities as a boiling liquid, and only during the warmest time of day in summer. Its role has therefore been considered insignificant until now. However, an international team including scientists from the CNRS, Université de Nantes and Université Paris-Sud and headed by Marion Massé, from the Laboratoire de Planétologie et Géodynamique de Nantes (CNRS/Université de Nantes)has now shown that even though water that emerges onto the surface of Mars immediately begins to boil, it creates an unstable, turbulent flow that can eject sediment and cause dry avalanches. The flow of small amounts of a boiling liquid therefore significantly alters the surface. The discovery of this exotic process, unknown on our planet, radically changes our interpretation of the Martian surface, making it difficult to undertake a direct comparison of flows on the Earth and on Mars. These findings are published on 2 May 2016 in the journal Nature Geoscience.

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It is well known that water boils at 100 °C. But this is only true at sea level, since boiling point depends on atmospheric pressure: the higher the altitude, the thinner the atmosphere, and the lower the boiling point. For instance, at the top of Mount Everest, water boils at 60 °C. But on Mars, where the atmosphere is much thinner than on Earth, it can boil at temperatures as low as 0 °C. During the Martian summer, when the subsurface water ice begins to melt and emerge at the surface, where the mean temperature reaches 20 °C, it immediately starts to boil. This is also the case for the flows of saline water discovered last year. So could an evaporating liquid alter the Martian landscape?

To find out, a team of researchers from the Open University (UK) used a former diving decompression chamber to reproduce the low pressure of the Martian atmosphere. At the same time, another team from the GEOPS laboratory (CNRS/Université Paris-Sud) carried out the same experiment, but this time in a cold chamber at Earth’s atmospheric pressure. In both chambers, a block of pure water ice, followed by one of saline water ice, were melted at a temperature of 20 °C (as on Mars in summer) on a sand-covered slope.

The experiments showed that, in the flows produced under terrestrial conditions, the water gradually seeped into the sand, leaving no trace on the surface after drying. However, what was observed in the Martian chamber was very different. The water produced by the melting ice started to boil as soon as it reached the surface, and the gas released caused the ejection of sand grains . These gradually formed small ridges at the front of the flow, which, as they grew larger, became unstable and actually produced avalanches of dry sand. The process was even more violent at lower pressures. Contrary to what is observed on Earth, the surface, once dry, therefore exhibited a series of ridges.

This process is not as efficient in the case of saline water since it is more stable than pure water under Martian conditions. However, since saline water is more viscous, it can carry along sand grains and form small channels, a process that can sometimes become explosive under low pressure.

These findings, analyzed along with other laboratories worldwide, including the Institut d’Astrophysique Spatiale (CNRS/Université Paris Sud), provide new insight into the effect of the flow of water — whether saline or not — on the surface of Mars. Far from making its action insignificant, the water’s instability considerably increases its impact on surface morphology. This broadens the potential range of processes that could explain the activity on the Martian surface, such as that observed in spring on the planet’s slopes during the melt of winter frost made up of CO2 and water ice, as well as the dark flows (Recurring Slope Lineae) seen in summer.

mars

The possible presence of liquid water on the surface of Mars is a key question in the search for environments potentially favorable to life. Until now, detecting liquid water depended on identifying morphologies similar to those produced on Earth by the flow of liquid water, such as channels, gullies, or simply the seasonal appearance of dark traces caused by dampening of the surface. However the flows produced in the laboratory show that morphologies produced under either Martian or terrestrial conditions are very different. Direct comparison between landforms produced on Earth and on Mars does not therefore appear to be appropriate for detecting the appearance of a liquid on Mars, thus altering our interpretation of the Martian surface.

Three Potentially Habitable Worlds Found Around Nearby Ultracool Dwarf Star

Astronomers using the TRAPPIST telescope at ESO’s La Silla Observatory have discovered three planets orbiting an ultracool dwarf star just 40 light-years from Earth. These worlds have sizes and temperatures similar to those of Venus and Earth and are the best targets found so far for the search for life outside the Solar System. They are the first planets ever discovered around such a tiny and dim star. The new results will be published in the journal Nature on 2 May 2016.

habitable planets

A team of astronomers led by Michaël Gillon, of the Institut d’Astrophysique et Géophysique at the University of Liège in Belgium, have used the Belgian TRAPPIST telescope to observe the star 2MASS J23062928-0502285 now also known as TRAPPIST-1. They found that this dim and cool star faded slightly at regular intervals, indicating that several objects were passing between the star and the Earth. Detailed analysis showed that three planets with similar sizes to the Earth were present.

TRAPPIST-1 is an ultracool dwarf star — it is much cooler and redder than the Sun and barely larger than Jupiter. Such stars are both very common in the Milky Way and very long-lived, but this is the first time that planets have been found around one of them. Despite being so close to the Earth, this star is too dim and too red to be seen with the naked eye or even visually with a large amateur telescope. It lies in the constellation of Aquarius (The Water Carrier).

Emmanuël Jehin, a co-author of the new study, is excited: “This really is a paradigm shift with regards to the planet population and the path towards finding life in the Universe. So far, the existence of such ‘red worlds’ orbiting ultra-cool dwarf stars was purely theoretical, butnow we have not just one lonely planet around such a faint red star but a complete system of three planets!”

Michaël Gillon, lead author of the paper presenting the discovery, explains the significance of the new findings: “Why are we trying to detect Earth-like planets around the smallest and coolest stars in the solar neighbourhood? The reason is simple: systems around these tiny stars are the only places where we can detect life on an Earth-sized exoplanet with our current technology. So if we want to find life elsewhere in the Universe, this is where we should start to look.”

Astronomers will search for signs of life by studying the effect that the atmosphere of a transiting planet has on the light reaching Earth. For Earth-sized planets orbiting most stars this tiny effect is swamped by the brilliance of the starlight. Only for the case of faint red ultra-cool dwarf stars — like TRAPPIST-1 — is this effect big enough to be detected.

Follow-up observations with larger telescopes, including the HAWK-I instrument on ESO’s 8-metre Very Large Telescope in Chile, have shown that the planets orbiting TRAPPIST-1 have sizes very similar to that of Earth. Two of the planets have orbital periods of about 1.5 days and 2.4 days respectively, and the third planet has a less well determined period in the range 4.5 to 73 days.

“With such short orbital periods, the planets are between 20 and 100 times closer to their star than the Earth to the Sun. The structure of this planetary system is much more similar in scale to the system of Jupiter’s moons than to that of the Solar System,” explains Michaël Gillon.

Although they orbit very close to their host dwarf star, the inner two planets only receive four times and twice, respectively, the amount of radiation received by the Earth, because their star is much fainter than the Sun. That puts them closer to the star than the habitable zone for this system, although it is still possible that they possess habitable regions on their surfaces. The third, outer, planet’s orbit is not yet well known, but it probably receives less radiation than the Earth does, but maybe still enough to lie within the habitable zone.

“Thanks to several giant telescopes currently under construction, including ESO’s E-ELT and the NASA/ESA/CSA James Webb Space Telescope due to launch for 2018, we will soon be able to study the atmospheric composition of these planets and to explore them first for water, then for traces of biological activity. That’s a giant step in the search for life in the Universe,” concludes Julien de Wit, a co-author from the Massachusetts Institute of Technology (MIT) in the USA.

This work opens up a new direction for exoplanet hunting, as around 15% of the stars near to the Sun are ultra-cool dwarf stars, and it also serves to highlight that the search for exoplanets has now entered the realm of potentially habitable cousins of the Earth. The TRAPPIST survey is a prototype for a more ambitious project called SPECULOOS that will be installed at ESO’s Paranal Observatory.