Study of Chile earthquake finds new rock structure that affects earthquake rupture

Scientists used computer models to track the path of seismic waves through the Earth and generate 3-D images,  These images revealed a new and previously jknknown rock structure in the Chile fault line. -  Stephen Hicks, University of Liverpool
Scientists used computer models to track the path of seismic waves through the Earth and generate 3-D images, These images revealed a new and previously jknknown rock structure in the Chile fault line. – Stephen Hicks, University of Liverpool

Researchers from the University of Liverpool have found an unusual mass of rock deep in the active fault line beneath Chile which influenced the rupture size of a massive earthquake that struck the region in 2010.

The geological structure, which was not previously known about, is unusually dense and large for this depth in the Earth’s crust. The body was revealed using 3-D seismic images of Earth’s interior based on the monitoring of vibrations on the Pacific seafloor caused by aftershocks from the magnitude 8.8 Chile earthquake. This imaging works in a similar way to CT scans that are used in hospitals.

Analysis of the 2010 earthquake also revealed that this structure played a key role in the movement of the fault, causing the rupture to suddenly slow down.

Seismologists think that the block of rock was once part of Earth’s mantle and may have formed around 220 million years ago, during the period of time known as the Triassic.

Liverpool Seismologist, Stephen Hicks from the School of Environmental Sciences, who led the research, said: “It was previously thought that dense geological bodies in an active fault zone may cause more movement of the fault during an earthquake.”

“However, our research suggests that these blocks of rock may in fact cause the earthquake rupture to suddenly slow down. But this slowing down can generate stronger shaking at the surface, which is more damaging to man-made structures.”

“It is now clear that ancient geology plays a big role in the generation of future earthquakes and their subsequent aftershocks.”

Professor Andreas Rietbrock, head of the Earthquake Seismology and Geodynamics research group added: “This work has clearly shown the potential of 3D ‘seismic’ images to further our understanding of the earthquake rupture process.

We are currently establishing the Liverpool Earth Observatory (LEO), which will allow us together with our international partners, to carry out similar studies in other tectonically active regions such as northern Chile, Indonesia, New Zealand and the northwest coast United States. This work is vital for understanding risk exposure in these countries from both ground shaking and tsunamis.”

Chile is located on the Pacific Ring of Fire, where the sinking of tectonic plates generates many of the world’s largest earthquakes.

The 2010 magnitude 8.8 earthquake in Chile is one of the best-recorded earthquakes, giving seismologists the best insight to date into the ruptures of mega-quakes.

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The research, funded by the Natural Environment Research Council, is published in the journal Earth and Planetary Science Letters.

Mysterious Midcontinent Rift is a geological hybrid

The volcanic rocks of the 1.1 billion-year-old Midcontinent Rift play a prominent role in the natural beauty of Isle Royale National Park in Lake Superior. -  Seth Stein, Northwestern University
The volcanic rocks of the 1.1 billion-year-old Midcontinent Rift play a prominent role in the natural beauty of Isle Royale National Park in Lake Superior. – Seth Stein, Northwestern University

An international team of geologists has a new explanation for how the Midwest’s biggest geological feature — an ancient and giant 2,000-mile-long underground crack that starts in Lake Superior and runs south to Oklahoma and to Alabama — evolved.

Scientists from Northwestern University, the University of Illinois at Chicago (UIC), the University of Gottingen in Germany and the University of Oklahoma report that the 1.1 billion-year-old Midcontinent Rift is a geological hybrid, having formed in three stages: it started as an enormous narrow crack in the Earth’s crust; that space then filled with an unusually large amount of volcanic rock; and, finally, the igneous rocks were forced to the surface, forming the beautiful scenery seen today in the Lake Superior area of the Upper Midwest.

The rift produced some of the Midwest’s most interesting geology and scenery, but there has never been a good explanation for what caused it. Inspired by vacations to Lake Superior, Seth and Carol A. Stein, a husband-and-wife team from Northwestern and UIC, have been determined to learn more in recent years.

Their study, which utilized cutting-edge geologic software and seismic images of rock located below the Earth’s surface in areas of the rift, will be presented Oct. 20 at the Geological Society of America annual meeting in Vancouver.

“The Midcontinent Rift is a very strange beast,” said the study’s lead author, Carol Stein, professor of Earth and Environmental Sciences at UIC. “Rifts are long, narrow cracks splitting the Earth’s crust, with some volcanic rocks in them that rise to fill the cracks. Large igneous provinces, or LIPs, are huge pools of volcanic rocks poured out at the Earth’s surface. The Midcontinent Rift is both of these — like a hybrid animal.”

“Geologists call it a rift because it’s long and narrow,” explained Seth Stein, a co-author of the study, “but it’s got much more volcanic rock inside it than any other rift on a continent, so it’s also a LIP. We’ve been wondering for a long time how this could have happened.” He is the William Deering Professor of Geological Sciences at the Weinberg College of Arts and Sciences.

This question is one of those that EarthScope, a major National Science Foundation program involving geologists from across the U.S., seeks to answer. In this case, the team used images of the Earth at depth from seismic experiments across Lake Superior and EarthScope surveys of other parts of the Midcontinent Rift. The images show the rock layers at depth, much as X-ray photos show the bones in people’s bodies.

In reviewing the images, the researchers found the Midcontinent Rift appeared to evolve in three stages.

“First, the rocks were pulled apart, forming a rift valley,” Carol Stein said. “As the rift was pulling apart, magma flowed into the developing crack. After about 10 million years, the crack stopped growing, but more magma kept pouring out on top. Older magma layers sunk under the weight of new magma, so the hole kept deepening. Eventually the magma ran out, leaving a large igneous province — a 20-mile-thick pile of volcanic rocks. Millions of years later, the rift got squeezed as a new supercontinent reassembled, which made the Earth’s crust under the rift thicker.”

To test this idea, the Steins turned to Jonas Kley, professor of geology at Germany’s Gottingen University, their host during a research year in Germany sponsored by the Alexander von Humboldt Foundation.

Kley used software that allows geologic time to run backwards. “We start with the rocks as they are today,” Kley explained, “and then undo movement on faults and vertical movements. It’s like reconstructing a car crash. When we’re done we have a picture of what happened and when. This lets us test ideas and see if they work.”

Kley’s analysis showed that the three-stage history made sense — the Midcontinent Rift started as a rift and then evolved into a large igneous province. The last stage brought rocks in the Lake Superior area to the surface.

Other parts of the picture fit together nicely, the Steins said. David Hindle, also from Gottingen University, used a computer model to show that the rift’s shape seen in the seismic images results from the crust bending under weight of magma.

Randy Keller, a professor and director of the Oklahoma Geological Survey, found that the weight of the dense magma filling the rift explains the stronger pull of gravity measured above the rift. He points out that these variations in the gravity field are the major evidence used to map the extent of the rift.

“It’s funny,” Seth Stein mused. “Carol and I have been living in Chicago for more than 30 years. We often have gone up to Lake Superior for vacations but didn’t think much about the geology. It’s only in the past few years that we realized there’s a great story there and started working on it. There are many studies going on today, which will give more results in the next few years.”

The Steins now are working with other geologists to help park rangers and teachers tell this story to the public. For example, a good way to think about how rifts work is to observe what happens if you pull both ends of a Mars candy bar: the top chocolate layer breaks, and the inside stretches.

“Sometimes people think that exciting geology only happens in places like California,” Seth Stein said. “We hope results like this will encourage young Midwesterners to study geology and make even further advances.”

New view of Rainier’s volcanic plumbing

This image was made by measuring how the ground conducts or resists electricity in a study co-authored by geophysicist Phil Wannamaker of the University of Utah Energy & Geoscience Institute. It  shows the underground plumbing system that provides molten and partly molten rock to the magma chamber beneath the Mount Rainier volcano in Washington state. The scale at left is miles depth. The scale at bottom is miles from the Pacific Coast. The Juan de Fuca plate of Earth's Pacific seafloor crust and upper mantle is shown in blue on the left half of the image as it dives or 
'subducts' eastward beneath Washington state. The reddish orange and yellow colors represent molten and partly molten rock forming atop the Juan de Fuca plate or 'slab.' The image shows the rock begins to melt about 50 miles beneath Mount Rainier (the red triangle at top). Some is pulled downward and eastward as the slab keeps diving, but other melts move upward to the orange magma chamber shown under but west of Mount Rainier. The line of sensors used to make this image were placed north of the 14,410-foot peak, so the image may be showing a lobe of the magma chamber that extends northwest of the mountain. Red ovals on the left half of the page are the hypocenters of earthquakes. -  R Shane McGary, Woods Hole Oceanographic Institution.
This image was made by measuring how the ground conducts or resists electricity in a study co-authored by geophysicist Phil Wannamaker of the University of Utah Energy & Geoscience Institute. It shows the underground plumbing system that provides molten and partly molten rock to the magma chamber beneath the Mount Rainier volcano in Washington state. The scale at left is miles depth. The scale at bottom is miles from the Pacific Coast. The Juan de Fuca plate of Earth’s Pacific seafloor crust and upper mantle is shown in blue on the left half of the image as it dives or
‘subducts’ eastward beneath Washington state. The reddish orange and yellow colors represent molten and partly molten rock forming atop the Juan de Fuca plate or ‘slab.’ The image shows the rock begins to melt about 50 miles beneath Mount Rainier (the red triangle at top). Some is pulled downward and eastward as the slab keeps diving, but other melts move upward to the orange magma chamber shown under but west of Mount Rainier. The line of sensors used to make this image were placed north of the 14,410-foot peak, so the image may be showing a lobe of the magma chamber that extends northwest of the mountain. Red ovals on the left half of the page are the hypocenters of earthquakes. – R Shane McGary, Woods Hole Oceanographic Institution.

By measuring how fast Earth conducts electricity and seismic waves, a University of Utah researcher and colleagues made a detailed picture of Mount Rainier’s deep volcanic plumbing and partly molten rock that will erupt again someday.

“This is the most direct image yet capturing the melting process that feeds magma into a crustal reservoir that eventually is tapped for eruptions,” says geophysicist Phil Wannamaker, of the university’s Energy & Geoscience Institute and Department of Civil and Environmental Engineering. “But it does not provide any information on the timing of future eruptions from Mount Rainier or other Cascade Range volcanoes.”

The study was published today in the journal Nature by Wannamaker and geophysicists from the Woods Hole Oceanographic Institution in Massachusetts, the College of New Jersey and the University of Bergen, Norway.

In an odd twist, the image appears to show that at least part of Mount Rainier’s partly molten magma reservoir is located about 6 to 10 miles northwest of the 14,410-foot volcano, which is 30 to 45 miles southeast of the Seattle-Tacoma area.

But that could be because the 80 electrical sensors used for the experiment were placed in a 190-mile-long, west-to-east line about 12 miles north of Rainier. So the main part of the magma chamber could be directly under the peak, but with a lobe extending northwest under the line of detectors, Wannamaker says.

The top of the magma reservoir in the image is 5 miles underground and “appears to be 5 to 10 miles thick, and 5 to 10 miles wide in east-west extent,” he says. “We can’t really describe the north-south extent because it’s a slice view.”

Wannamaker estimates the reservoir is roughly 30 percent molten. Magma chambers are like a sponge of hot, soft rock containing pockets of molten rock.

The new image doesn’t reveal the plumbing tying Mount Rainier to the magma chamber 5 miles below it. Instead, it shows water and partly molten and molten rock are generated 50 miles underground where one of Earth’s seafloor crustal plates or slabs is “subducting” or diving eastward and downward beneath the North America plate, and how and where those melts rise to Rainier’s magma chamber.

The study was funded largely by the National Science Foundation’s Earthscope program, which also has made underground images of the United States using seismic or sound-wave tomography, much like CT scans show the body’s interior using X-rays.

The new study used both seismic imaging and magnetotelluric measurements, which make images by showing how electrical and magnetic fields in the ground vary due to differences in how much underground rock and fluids conduct or resist electricity.

Wannamaker says it is the most detailed cross-section view yet under a Cascades volcanic system using electrical and seismic imaging. Earlier seismic images indicated water and partly molten rock atop the diving slab. The new image shows melting “from the surface of the slab to the upper crust, where partly molten magma accumulates before erupting,” he adds.

Wannamaker and Rob L. Evans, of the Woods Hole Oceanographic Institution, conceived the study. First author R Shane McGary – then at Woods Hole and now at the College of New Jersey – did the data analysis. Other co-authors were Jimmy Elsenbeck of Woods Hole and St├ęphane Rondenay of the University of Bergen.

Mount Rainier: Hazardous Backdrop to Metropolitan Seattle-Tacoma

Mount Rainier, the tallest peak in the Cascades, “is an active volcano that will erupt again,” says the U.S. Geological Survey. Rainier sits atop volcanic flows up to 36 million years old. An ancestral Rainier existed 2 million to 1 million years ago. Frequent eruptions built the mountain’s modern edifice during the past 500,000 years. During the past 11,000 years, Rainier erupted explosively dozens of times, spewing ash and pumice.

Rainier once was taller until it collapsed during an eruption 5,600 years ago to form a large crater open to the northeast, much like the crater formed by Mount St. Helens’ 1980 eruption. The 5,600-year-old eruption sent a huge mudflow west to Puget Sound, covering parts or all of the present sites of the Port of Tacoma, Seattle suburbs Kent and Auburn, and the towns Puyallup, Orting, Buckley, Sumner and Enumclaw.

Rainier’s last lava flows were 2,200 years ago, the last flows of hot rock and ash were 1,100 years ago and the last big mudflow 500 years ago. There are disputed reports of steam eruptions in the 1800s.

Subduction Made Simple – and a Peek beneath a Peak

The “ring of fire” is a zone of active volcanoes and frequent earthquake activity surrounding the Pacific Ocean. It exists where Earth’s tectonic plates collide – specifically, plates that make up the seafloor converge with plates that carry continents.

From Cape Mendocino in northern California and north past Oregon, Washington state and into British Columbia, an oceanic plate is being pushed eastward and downward – a process called subduction – beneath the North American plate. This relatively small Juan de Fuca plate is located between the huge Pacific plate and the Pacific Northwest.

New seafloor rock – rich with water in cracks and minerals – emerges from an undersea volcanic ridge some 250 miles off the coast, from northern California into British Columbia. That seafloor adds to the western edge of the Juan de Fuca plate and pushes it east-northeast under the Pacific Northwest, as far as Idaho.

The part of the plate diving eastward and downward is called the slab, which ranges from 30 to 60 miles thick as it is jammed under the North American plate. The part of the North American plate above the diving slab is shaped like a wedge.

When the leading, eastern edge of the diving slab descends deep enough, where pressures and temperatures are high, water-bearing minerals such as chlorite and amphibole release water from the slab, and the slab and surrounding mantle rock begin to melt. That is why the Cascade Range of active volcanoes extends north-to-south – above the slab and parallel but about 120 miles inland from the coast – from British Columbia south to Mount Shasta and Lassen Peak in northern California.

In the new image, yellow-orange-red areas correspond to higher electrical conductivity (or lower resistivity) in places where fluids and melts are located.

The underground image produced by the new study shows where water and molten rock accumulate atop the descending slab, and the route they take to the magma chamber that feeds eruptions of Mount Rainier:

– The rock begins to melt atop the slab about 50 miles beneath Mount Rainier. Wannamaker says it is best described as partly molten rock that contains about 2 percent water and “is a mush of crystals within an interlacing a network of molten rock.”

– Some water and partly molten rock actually gets dragged downward atop the descending slab, to depths of 70 miles or more.

– Other partly molten rock rises up through the upper mantle wedge, crosses into the crust at a depth of about 25 miles, and then rises into Rainier’s magma chamber – or at least the lobe of the chamber that crosses under the line of sensors used in the study. Evidence suggests the magma moves upward at least 0.4 inches per year.

– The new magnetotelluric image also shows a shallower zone of fluid perhaps 60 miles west of Rainier and 25 miles deep at the crust-mantle boundary. Wannamaker says it is largely water released from minerals as the slab is squeezed and heated as it dives.

The seismic data were collected during 2008-2009 for other studies. The magnetotelluric data were gathered during 2009-2010 by authors of the new study.

Wannamaker and colleagues placed an east-west line of magnetotelluric sensors: 60 that made one-day measurements and looked as deep as 30 miles into the Earth, and 20 that made measurements for a month and looked at even greater depths.

Innovative technology provides insight into what’s below the Earth’s surface





New technology which provides images from below the Earth's surface has been unveiled
New technology which provides images from below the Earth’s surface has been unveiled

From oil fields and meteorite impact craters, to potential tsunami triggering submarine landslides, innovative new technology which provides images from below the Earth’s surface has been unveiled.



The Virtual Seismic Atlas (VSA) website allows scientists access to geological pictures and information, to enhance research into understanding the Earth’s structure and natural resources.



The seismic images available on the website reveal astounding geological insights, in particular of the seabed, that include mud volcanoes and massive submarine landslides that could possibly trigger tsunami waves.



The website, which is free to use, is the world’s first to link data with interpretation, making crucial seismic images available to experts and non-specialists alike.



The development of the VSA website is a collaborative project between the University of Aberdeen and the University Leeds funded by the Natural Environment Research Council in partnership with a consortium of energy organisations. Crucial input to the website was provided by key multinational oil and gas companies, seismic companies and geological surveys.



From North Sea oil field structures to submarine canyons in offshore Nigeria and mud volcanoes from the Caspian Sea, the technology provides key seismic data for research and training purposes.


The technology will open up access to this crucial information to a worldwide audience as Rob Butler, Professor of Tectonics at the University of Aberdeen explains: “Traditionally this important geological images and information was extremely exclusive, only accessible by a limited group of scientists. The VSA website will enable students, professionals and members of the public to view these exciting images which illustrate what lies below the Earth’s surface.



“This is extremely significant not only in terms of academic and research developments, but also in enhancing the understanding of earth science within the wider community.”



Seismic images are used within the oil and gas industry to help organisations detect oil reservoirs. This data is generated through controlled explosions which reflect energy back from rocks, allowing areas where natural resources can be found to be identified.



Professor Butler continues: “Seismic data is integral to the oil and gas industry and the information which is collected by the sector is humanity’s greatest resource of geophysical data in terms of both volume and financial investment.



“The VSA website will make access to this information simple in a way which has never been possible before. Digitally collating seismic data and bringing the information together in this way will allow for improved research and simpler cross-referencing of information. The design of the website also involves an on-line community aspect which means the bank of available images will grow organically over time.”



Professor Bill McCaffrey, University of Leeds School of Earth and Environment says: “The sort of data we are releasing to the public is usually very difficult to obtain for scientific research. Yet it is our primary source of geological information about the top 10-20 km of the Earth’s crust.”



Visit the Virtual Seismic Atlas (VSA) at www.seismicatlas.org