New idea tackles Earth core puzzle

Lying 5,000km beneath our feet, the core is beyond the reach of direct investigation

Scientists have proposed a radical new model for the make-up of the Earth's core.

The study may explain a longstanding puzzle about the most inaccessible part of our planet.

It suggests that differences between the east and west hemispheres of the core are explained by the way iron atoms pack together.

Details appear in the journal Scientific Reports.

Lying more than 5,000km beneath our feet, at the centre of the Earth, the core is beyond the reach of direct investigation. Broadly speaking, it consists of a solid sphere of metal sitting within a liquid outer core.

The inner core started to solidify more than a billion years ago. It has a radius of about 1,220km, but is growing by about 0.5mm each year.

But the material that the core is made from remains a longstanding unresolved problem.

Clues come from the speeds that seismic waves generated by earthquakes pass through the core.

These tell us its density and elasticity, but the precise arrangement of iron atoms forming the crystalline core controls these numbers.

How those atoms are arranged remains unclear, since the conditions of extreme pressure and temperature at the core cannot easily be replicated in the laboratory.

Seismic data indicate that the western and eastern hemispheres of Earth's inner core differ, and this has led some to suggest that the core was once subjected to an impulse - presumably from the collision of a space rock or planetoid which shook the whole Earth.

The core, it is suggested, is constantly moving sideways. As it does, the front side is melting and the rear side crystallising, but the core is held centrally by gravity.

Maurizio Mattesini

With all these seismic complexities, the link between the crystal structure and the geophysical observations has yet to be resolved.

In Scientific Reports, Maurizio Mattesini from the Complutense University of Madrid, Spain, and colleagues propose a novel possibility for the structure of the core: that it is composed of mixtures of different iron arrangements distinguished by the way their atoms pack together.

By comparing seismic data from over one thousand earthquakes across the globe with quantum mechanical models for the properties of iron, they suggest that seismic variations directly reflect variations in the iron structure.

They propose that the eastern and western sides of the core differ in the extent of mixing of these distinct structures, and suggest their results account for the dynamic eastward drift of the core through time.

Arwen Deuss

Their complicated picture of the core contrasts with earlier suggestions of a more uniform mineralogy. It has yet to incorporate the effects of minor amounts of other elements in the iron alloy actually thought to be there.

But Dr Arwen Deuss, a seismologist from the University of Cambridge, commented: "This is a step in the right direction, directly comparing seismology with mineral physical properties." She added that it should eventually provide a better understanding of the birth and evolution of our planet.

Tungurahua Volcano, Ecuador, puts on a fiery show

Tungurahua volcano has been very active the past days, with continous ash emissions and occasional large explosions producing ash plumes up to 32,000 ft (ca. 10 km) altitude.

IG scientists on an overflight observed strombolian activity from the inner summit crater which has filled with fresh lava.

At the time of updating, tremor and seismic activity have decreased somewhat.

High-Res Show Crust-Mantle Boundary - Moho, Where Is Earth's Mantle?

This map shows the global Mohorovičić discontinuity, better known as Moho, based on data from the GOCE satellite.

CREDIT: GEMMA project

Beneath the Earth's crust, the outermost hard shell that makes up just 1 percent of the volume of the planet, lies a hot, viscous layer of rock called the mantle.

Together, the crust and upper portion of the mantle. called the lithosphere, are where most important geological processes occur, such as mountain-building, earthquakes and the source of volcanoes.

The slow churning and overturning of the mantle is what drives the movements of Earth's tectonic plates.

New methods of observation using satellites are helping scientists learn more about this important layer of the Earth's inner and outer layers and where it begins under different regions of the planet.

Andrija Mohorovičić
Until just a century ago, we didn’t know Earth has a crust. In 1909, Croatian seismologist Andrija Mohorovičić found that at about 50 km underground there is a sudden change in seismic speed.

Ever since, that boundary between Earth’s crust and underlying mantle has been known as the Mohorovičić discontinuity, or Moho.

Even today, almost all we know about Earth’s deep layers comes from two methods: seismic and gravimetric.

Seismic methods are based on observing changes in the propagation velocity of seismic waves between the crust and mantle.

Gravimetry looks at the gravitational effect due to the density difference caused by the changing composition of crust and mantle.

But the Moho models based on seismic or gravity data are usually limited by poor data coverage or data being only available along single profiles.

GEMMA Project

GEMMA’s Moho map is based on the inversion of homogenous well-distributed gravimetric data.

For the first time, it is possible to estimate the Moho depth worldwide with unprecedented resolution, as well as in areas where ground data are not available.

This will offer new clues for understanding the dynamics of Earth’s interior, unmasking the gravitational signal produced by unknown and irregular subsurface density distribution.

GEMMA is being carried out by Italian scientist Daniele Sampietro and is funded by the Politecnico di Milano and ESA’s Support To Science Element under the Changing Earth Science Network initiative.

This initiative supports young scientists at post-doctoral level in ESA Member States to advance our knowledge in Earth system science by exploiting the observational capacity of ESA missions

Read more at the ESA Goce website

VTSO: Central Virginia Seismic Zone

EARTHQUAKES IN THE CENTRAL VIRGINIA SEISMIC ZONE
Since at least 1774, people in central Virginia have felt small earthquakes and suffered damage from infrequent larger ones. 

The largest damaging earthquake (magnitude 4.8) in the seismic zone occurred in 1875. Smaller earthquakes that cause little or no damage are felt each year or two.

Earthquakes in the central and eastern U.S., although less frequent than in the western U.S., are typically felt over a much broader region. East of the Rockies, an earthquake can be felt over an area as much as ten times larger than a similar magnitude earthquake on the west coast.

A magnitude 4.0 eastern U.S. earthquake typically can be felt at many places as far as 100 km (60 mi) from where it occurred, and it infrequently causes damage near its source. A magnitude 5.5 eastern U.S. earthquake usually can be felt as far as 500 km (300 mi) from where it occurred, and sometimes causes damage as far away as 40 km (25 mi).

FAULTS
Earthquakes everywhere occur on faults within bedrock, usually miles deep. Most bedrock beneath central Virginia was assembled as continents collided to form a supercontinent about 500-300 million years ago, raising the Appalachian Mountains.

Most of the rest of the bedrock formed when the supercontinent rifted apart about 200 million years ago to form what are now the northeastern U.S., the Atlantic Ocean, and Europe.

At well-studied plate boundaries like the San Andreas fault system in California, often scientists can determine the name of the specific fault that is responsible for an earthquake.

In contrast, east of the Rocky Mountains this is rarely the case. The Central Virginia seismic zone is far from the nearest plate boundaries, which are in the center of the Atlantic Ocean and in the Caribbean Sea.

The seismic zone is laced with known faults but numerous smaller or deeply buried faults remain undetected. Even the known faults are poorly located at earthquake depths.

Accordingly, few, if any, earthquakes in the seismic zone can be linked to named faults. It is difficult to determine if a known fault is still active and could slip and cause an earthquake.

As in most other areas east of the Rockies, the best guide to earthquake hazards in the seismic zone is the earthquakes themselves.

Source: NEIC/USGS

Two maps showing the geology of central Virginia: Geology Map 1 | Geology Map 2

VTSO: Central Virginia Seismic Zone

NASA/University Japan Quake Study Yields Surprises

An overhead model of the estimated fault slip due to the earthquake.

The fault responsible for this earthquake dips under Japan, starting at the Japan Trench indicated by the barbed line, the point of contact between the subducting Pacific Plate and the overriding Okhotsk Plate.

The magnitude of fault slip is indicated by the colour and the contours, at 8-meter intervals.

The question mark indicates the general region where researchers currently lack information about future seismic potential.

Credit: Mark Simons/Caltech Seismological Laboratory

Sun, moon causing tremors deep in San Andreas fault

The faint tug of the sun and moon on the San Andreas Fault stimulates tremors deep underground, suggesting that the rock 15 miles below is lubricated with highly pressurized water that allows the rock to slip with little effort, according to a new study by University of California, Berkeley, seismologists.

"Tremors seem to be extremely sensitive to minute stress changes," said Roland Burgmann, UC Berkeley professor of earth and planetary science. "Seismic waves from the other side of the planet triggered tremors on the Cascadia subduction zone off the coast of Washington state after the Sumatra earthquake last year, while the Denali earthquake in 2002 triggered tremors on a number of faults in California. Now we also see that tides - the daily lunar and solar tides - very strongly modulate tremors."

In a paper appearing in the Dec. 24 issue of the journal Nature, UC Berkeley graduate student Amanda M. Thomas, seismologist Robert Nadeau of the Berkeley Seismological Laboratory and Burgmann argue that this extreme sensitivity to stress - and specifically to shearing stress along the fault - means that the water deep underground is under extreme pressure.

"The big finding is that there is very high fluid pressure down there, that is, lithostatic pressure, which means pressure equivalent to the load of all rock above it, 15 to 30 kilometers (10 to 20 miles) of rock," Nadeau said. "Water under very high pressure essentially lubricates the rock, making the fault very weak."

Though tides raised in the Earth by the sun and moon are not known to trigger earthquakes directly, they can trigger swarms of deep tremors, which could increase the likelihood of quakes on the fault above the tremor zone, the researchers say. At other fault zones, such as at Cascadia, swarms of tremors in the ductile zone deep underground correlate with slip at depth as well as increased stress on the shallower "seismogenic zone," where earthquakes are generated. The situation on the San Andreas Fault is not so clear, however.

"These tremors represent slip along the fault 25 kilometers (15 miles) underground, and this slip should push the fault zone above in a similar pattern," Burgmann said. "But it seems like it must be very subtle, because we actually don't see a tidal signal in regular earthquakes. Even though the earthquake zone also sees the tidal stress and also feels the added periodic behavior of the tremor below, they don't seem to be very bothered."

Nevertheless, said Nadeau, "It is certainly in the realm of reasonable conjecture that tremors are stressing the fault zone above it. The deep San Andreas Fault is moving faster when tremors are more active, presumably stressing the seismogenic zone, loading the fault a little bit faster. And that may have a relationship to stimulating earthquake activity."

Seismologists were surprised when tremors were first discovered more than seven years ago, since the rock at that depth - for the San Andreas Fault, between 15 and 30 kilometers (10 to 20 miles) underground - is not brittle and subject to fracture, but deformable, like peanut butter. They called them non-volcanic tremors to distinguish them from tremors caused by fluid - water or magma - fracturing and flowing through rock under volcanoes. It was not clear, however, what caused the non-volcanic tremors, which are on the order of a magnitude 1 earthquake.

To learn more about the source of these tremors, UC Berkeley seismologists began looking for tremors five years ago in seismic recordings from the Parkfield segment of the San Andreas Fault obtained from sensitive bore-hole seismometers placed underground as part of the UC Berkeley's High-Resolution Seismic Network. Using eight years of tremor data, Thomas, Burgmann and Nadeau correlated tremor activity with the effects of the sun and moon on the crust and with the effects of ocean tides, which are driven by the moon.

They found the strongest effect when the pull on the Earth from the sun and moon sheared the fault in the direction it normally breaks. Because the San Andreas Fault is a right-lateral strike-slip fault, the west side of the fault tends to break north-northwestward, dragging Los Angeles closer to San Francisco.

Spiderbots Dropped into Super Volcano cauldron: Mount St Helens

(Image: USGS/Dan Dzurisin)
A helicopter drops off an earlier version of a spiderbot in the crater of Mount St Helens in 2006

A squadron of 'spiderbots' inside Mount St Helens is the first network of volcano sensors that can automatically communicate with each other and with satellites, rather than sending data to a base station first.

Since the system can route data around any sensors that break and can simply be dropped into volcanoes, it is more robust and easier to deploy than current sensor systems, which must be carefully set up by hand.

Similar networked robots could one day be used to study geological activity elsewhere in the solar system, say scientists from NASA's Jet Propulsion Laboratory, which helped develop and monitor the robots.


Fifteen spiderbots, so-named because of the three spindly arms protruding from their suitcase-sized steel bodies, were lowered from a helicopter to spots inside the crater and around the rim of Mount St Helens, an active volcano in the US state of Washington, in July.

Each has a seismometer for detecting earthquakes, an infrared sensor to detect heat from volcanic explosions, a sensor to detect ash clouds, and a global positioning system to sense the ground bulging and pinpoint the exact location of seismic activity.


Once in place, the bots reached out to each other to form what is known as a mesh network. "It's similar to the internet," says Steve Chien, the principal scientist for autonomous systems at JPL. "You just lay them out, and they figure out the best way to route the data."

Self-healing
Other robotic volcano-monitoring systems exist, most notably around Mount Erebus in Antarctica. But they require permanent sensors to be buried in the ground or drilled into rock, which can take days of dangerous human labour.


The spiderbots are flexible and inexpensive enough that they can be set down almost anywhere. "You can imagine just dropping these out of a helicopter, and they'll just land like spikes in the ground and do their thing," Chien says.

The spider web's unique networking capabilities also give it a distinct advantage over other monitoring systems. The network is self-healing – if one node dies, the others automatically route data around it.


The scientists added this innovation after several early models were boiled, crushed or knocked over in the volcano's 2004 eruption. They also made the hardware more resilient. "These are much more rugged," says Rick LaHusen of the US Geological Survey. "They can take an impact and keep on working."

Hip! Hip! USArray

Probably the most ambitious seismological project ever conducted is taking place in the heartland of America. Its name is USArray and its aim is to run what amounts to an ultrasound scan over the 48 contiguous states of the US. Through the seismic shudders and murmurs that rack Earth's innards, it will build up an unprecedented 3D picture of what lies beneath North America.

It is a mammoth undertaking, during which USArray's scanner - a set of 400 transportable seismometers - will sweep all the way from the Pacific to the Atlantic. Having started off in California in 2004, it is now just east of the Rockies, covering a north-south swathe stretching from Montana's border with Canada down past El Paso on the Texas-Mexico border. By 2013, it should have reached the north-east coast, and its mission end.

Though not yet at the halfway stage, the project is already bringing the rocky underbelly of the US into unprecedented focus. Geologists are using this rich source of information to gain new understanding of the continent's tumultuous past - and what its future holds.