Geologists shed light on formation of Alaska Range

Syracuse University Professor Paul Fitzgerald and a group of students have been studying the Alaska Range. -  Syracuse University
Syracuse University Professor Paul Fitzgerald and a group of students have been studying the Alaska Range. – Syracuse University

Geologists in Syracuse University’s College of Arts and Sciences have recently figured out what has caused the Alaska Range to form the way it has and why the range boasts such an enigmatic topographic signature. The narrow mountain range is home to some of the world’s most dramatic topography, including 20,320-foot Mount McKinley, North America’s highest mountain.

Professor Paul Fitzgerald and a team of students and fellow scientists have been studying the Alaska Range along the Denali fault. They think they know why the fault is located where it is and what accounts for the alternating asymmetrical, mountain-scale topography along the fault.

Their findings were the subject of a recent paper in the journal Tectonics (American Geophysical Union, 2014).

In 2002, the Denali fault, which cuts across south-central Alaska, was the site of a magnitude-7.9 earthquake and was felt as far away as Texas and Louisiana. It was the largest earthquake of its kind in more than 150 years.

“Following the earthquake, researchers flocked to the area to examine the effects,” says Fitzgerald, who serves as professor of Earth Sciences and an associate dean for the College. “They were fascinated by how the frozen ground behaved; the many landslides [the earthquake] caused; how bridges responded; and how the Trans-Alaska oil pipeline survived, as it was engineered to do so.”

Geologists were also surprised by how the earthquake began on a previously unknown thrust-fault; then propagated eastward, along the Denali fault, and finally jumped onto another fault, hundreds of kilometers away.

“From our perspective, the earthquake has motivated analyses of why the highest mountains in the central Alaska Range occur south of the Denali fault and the highest mountains in the eastern Alaska Range occur north of the fault–something that has puzzled us for years,” Fitzgerald adds. “It’s been an enigma staring us in the face.”

He attributes the Alaska Range’s alternating topographic signatures to a myriad of factors: contrasting lithospheric strength between large terranes (i.e., distinctly different rock units); the location of the curved Denali fault; the transfer of strain inland from southern Alaska’s active plate margin; and the shape of the controlling former continental margin against weaker suture-zone rocks.

It’s no secret that Alaska is one of the most geologically active areas on the planet. For instance, scientists know that the North American Plate is currently overriding the Pacific Plate at the latter’s southern coast, while the Yakutat microplate is colliding with North America.

As a result of plate tectonics, Alaska is an amalgamation of terranes that have collided with the North American craton and have accreted to become part of North America.

Cratons are pieces of continents that have been largely stable for hundreds of millions of years.

Terranes often originate as volcanic islands (like those of Hawaii) and, after colliding with one another or a continent, are separated by large discrete faults. When terranes collide and accrete, they form a suture, also known as a collision zone, which is made up of weak, crushed rock. During deformation, suture-zone rocks usually deform first, especially if they are adjacent to a strong rock body.

“Technically, the Denali fault is what we’d call an ‘intercontinental right-lateral strike-slip fault system,'” says Fitzgerald, adding that a strike-slip fault occurs when rocks move horizontally past one another, usually on a vertical fault. “This motion includes a component of slip along the fault and a component of normal motion against the fault that creates mountains. Hence, the shape of the fault determines which of the two components is predominant and where mountains form.”

In Alaska, the shape of the accreted terranes generally controls the location of the Denali fault and the mountains that form along it, especially at the bends in the trace of the fault.

Fitzgerald: “Mount McKinley and the central Alaska Range lie within the concave curve of the Denali fault. There, higher topography and greater exhumation [uplift of rock] occur south of the Denali fault, exactly where you’d expect a mountain range to form, given the regional tectonics. In the eastern Alaska Range, higher topography and greater exhumation are found north of the fault, on its convex side–not an expected pattern at all and very puzzling.”

Using mapped surface geology, geophysical data, and thermochronology (i.e., time-temperature history of the rocks), Fitzgerald and colleagues have determined that much of Alaska’s uplift and deformation began some 25 million years ago, when the Yakutat microplate first started colliding with North America. The bold, glacier-clad peaks comprising the Alaska Range actually derive from within the aforementioned “weak suture-zone rocks” between the terranes.

While mountains are high and give the impression of strength, they are built largely from previously fractured rock units. Rock movement along the Denali fault drives the uplift of the mountains, which form at bends in the fault, where previously fractured suture-zone rocks are pinned against the stronger former North American continental margin.

“The patterns of deformation help us understand regional tectonics and the formation of the Alaska Range, which is fascinating to geologists and non-geologists alike,” says Fitzgerald. “Being able to determine patterns or how to reveal them, while others see chaos, is often the key to finding the answer to complex problems. … To us scientists, the real significance of this work is that it helps us understand the evolution of our planet, how faults and mountain belts form, and why earthquakes happen. It also provides a number of hypotheses about Alaskan tectonics and rock deformation that we can test, using the Alaska Range as our laboratory.”

In addition to Fitzgerald, the paper was co-authored by Sarah Roeske, a research scientist at the University of California, Davis; Jeff Benowitz, a research scientist at the Geophysical Institute at the University of Alaska Fairbanks; Steven Riccio and Stephanie Perry, graduate students in Earth Sciences at Syracuse; and Phillip Armstrong, professor and chair of geological sciences at California State University, Fullerton.

Housed in Syracuse’s College of Arts and Sciences, the Department of Earth Sciences offers graduate and undergraduate degree opportunities in crustal evolution and tectonics, environmental sciences and climate change, hydrogeology, sedimentology and paleolimnology, geochemistry, and paleobiology.

Study shows tectonic plates not rigid, deform horizontally in cooling process

Corné Kreemer, associate professor in the College of Science at the University of Nevada, Reno, conducts research on plate tectonics and geodetics. His latest research shows that oceanic tectonic plates deform due to cooling, causing shortening of the plates and mid-plate seismicity. -  Photo by Mike Wolterbeek, University of Nevada, Reno.
Corné Kreemer, associate professor in the College of Science at the University of Nevada, Reno, conducts research on plate tectonics and geodetics. His latest research shows that oceanic tectonic plates deform due to cooling, causing shortening of the plates and mid-plate seismicity. – Photo by Mike Wolterbeek, University of Nevada, Reno.

The puzzle pieces of tectonic plates that make up the outer layer of the earth are not rigid and don’t fit together as nicely as we were taught in high school.

A study published in the journal Geology by Corné Kreemer, an associate professor at the University of Nevada, Reno, and his colleague Richard Gordon of Rice University, quantifies deformation of the Pacific plate and challenges the central approximation of the plate tectonic paradigm that plates are rigid.

Using large-scale numerical modeling as well as GPS velocities from the largest GPS data-processing center in the world – the Nevada Geodetic Laboratory at the University of Nevada, Reno – Kreemer and Gordon have showed that cooling of the lithosphere, the outermost layer of Earth, makes some sections of the Pacific plate contract horizontally at faster rates than other sections. This causes the plate to deform.

Gordon’s idea is that the plate cooling, which makes the ocean deeper, also affects horizontal movement and that there is shortening and deformation of the plates due to the cooling. In partnering with Kreemer, the two put their ideas and expertise together to show that the deformation could explain why some parts of the plate tectonic puzzle didn’t fall neatly into place in recent plate motion models, which is based on spreading rates along mid-oceanic ridges. Kreemer and Gordon also showed that there is a positive correlation between where the plate is predicted to deform and where intraplate earthquakes occur. Their work was supported by the National Science Foundation.

Results of the study suggest that plate-scale horizontal thermal contraction is significant, and that it may be partly released seismically. . The pair of researchers are, as the saying goes, rewriting the textbooks.

“This is plate tectonics 2.0, it revolutionizes the concepts of plate rigidity,” Kreemer, who teaches in the University’s College of Science, said. “We have shown that the Pacific plate deforms, that it is pliable. We are refining the plate tectonic theory and have come up with an explanation for mid-plate seismicity.”

The oceanic plates are shortening due to cooling, which causes relative motion inside the plate, Kreemer said. The oceanic crust of the Pacific plate off shore California is moving 2 mm to the south every year relative to the Pacific/Antarctic plate boundary.

“It may not sound like much, but it is significant considering that we can measure crustal motion with GPS within a fraction of a millimeter per year,” he said. “Unfortunately, all existing GPS stations on Pacific islands are in the old part of the plate that is not expected nor shown to deform. New measurements will be needed within the young parts of the plate to confirm this study’s predictions, either on very remote islands or through sensors on the ocean floor.”

This work is complementary to Kreemer’s ongoing effort to quantify the deformation in all of the Earth’s plate boundary zones with GPS velocities – data that are for a large part processed in the Nevada Geodetic Laboratory. The main goal of the global modeling is to convert the strain rates to earthquake forecast maps.

“Because we don’t have GPS data in the right places of the Pacific plate, our prediction of how that plate deforms can supplement the strain rates I’ve estimated in parts of the world where we can quantify them with GPS data,” Kreemer said. “Ultimately, we hope to have a good estimate of strain rates everywhere so that the models not only forecast earthquakes for places like Reno and San Francisco, but also for places where you may expect them the least.”

Pacific plate shrinking as it cools

A map produced by scientists at the University of Nevada, Reno, and Rice University shows predicted velocities for sectors of the Pacific tectonic plate relative to points near the Pacific-Antarctic ridge, which lies in the South Pacific ocean. The researchers show the Pacific plate is contracting as younger sections of the lithosphere cool. -  Corné Kreemer and Richard Gordon
A map produced by scientists at the University of Nevada, Reno, and Rice University shows predicted velocities for sectors of the Pacific tectonic plate relative to points near the Pacific-Antarctic ridge, which lies in the South Pacific ocean. The researchers show the Pacific plate is contracting as younger sections of the lithosphere cool. – Corné Kreemer and Richard Gordon

The tectonic plate that dominates the Pacific “Ring of Fire” is not as rigid as many scientists assume, according to researchers at Rice University and the University of Nevada.

Rice geophysicist Richard Gordon and his colleague, Corné Kreemer, an associate professor at the University of Nevada, Reno, have determined that cooling of the lithosphere — the outermost layer of Earth — makes some sections of the Pacific plate contract horizontally at faster rates than others and cause the plate to deform.

Gordon said the effect detailed this month in Geology is most pronounced in the youngest parts of the lithosphere — about 2 million years old or less — that make up some the Pacific Ocean’s floor. They predict the rate of contraction to be 10 times faster than older parts of the plate that were created about 20 million years ago and 80 times faster than very old parts of the plate that were created about 160 million years ago.

The tectonic plates that cover Earth’s surface, including both land and seafloor, are in constant motion; they imperceptibly surf the viscous mantle below. Over time, the plates scrape against and collide into each other, forming mountains, trenches and other geological features.

On the local scale, these movements cover only inches per year and are hard to see. The same goes for deformations of the type described in the new paper, but when summed over an area the size of the Pacific plate, they become statistically significant, Gordon said.

The new calculations showed the Pacific plate is pulling away from the North American plate a little more — approximately 2 millimeters a year — than the rigid-plate theory would account for, he said. Overall, the plate is moving northwest about 50 millimeters a year.

“The central assumption in plate tectonics is that the plates are rigid, but the studies that my colleagues and I have been doing for the past few decades show that this central assumption is merely an approximation — that is, the plates are not rigid,” Gordon said. “Our latest contribution is to specify or predict the nature and rate of deformation over the entire Pacific plate.”

The researchers already suspected cooling had a role from their observation that the 25 large and small plates that make up Earth’s shell do not fit together as well as the “rigid model” assumption would have it. They also knew that lithosphere as young as 2 million years was more malleable than hardened lithosphere as old as 170 million years.

“We first showed five years ago that the rate of horizontal contraction is inversely proportional to the age of the seafloor,” he said. “So it’s in the youngest lithosphere (toward the east side of the Pacific plate) where you get the biggest effects.”

The researchers saw hints of deformation in a metric called plate circuit closure, which describes the relative motions where at least three plates meet. If the plates were rigid, their angular velocities at the triple junction would have a sum of zero. But where the Pacific, Nazca and Cocos plates meet west of the Galápagos Islands, the nonclosure velocity is 14 millimeters a year, enough to suggest that all three plates are deforming.

“When we did our first global model in 1990, we said to ourselves that maybe when we get new data, this issue will go away,” Gordon said. “But when we updated our model a few years ago, all the places that didn’t have plate circuit closure 20 years ago still didn’t have it.”

There had to be a reason, and it began to become clear when Gordon and his colleagues looked beneath the seafloor. “It’s long been understood that the ocean floor increases in depth with age due to cooling and thermal contraction. But if something cools, it doesn’t just cool in one direction. It’s going to be at least approximately isotropic. It should shrink the same in all directions, not just vertically,” he said.

A previous study by Gordon and former Rice graduate student Ravi Kumar calculated the effect of thermal contraction on vertical columns of oceanic lithosphere and determined its impact on the horizontal plane, but viewing the plate as a whole demanded a different approach. “We thought about the vertically integrated properties of the lithosphere, but once we did that, we realized Earth’s surface is still a two-dimensional problem,” he said.

For the new study, Gordon and Kreemer started by determining how much the contractions would, on average, strain the horizontal surface. They divided the Pacific plate into a grid and calculated the strain on each of the nearly 198,000 squares based on their age, as determined by the seafloor age model published by the National Geophysical Data Center.

“That we could calculate on a laptop,” Gordon said. “If we tried to do it in three dimensions, it would take a high-powered computer cluster.”

The surface calculations were enough to show likely strain fields across the Pacific plate that, when summed, accounted for the deformation. As further proof, the distribution of recent earthquakes in the Pacific plate, which also relieve the strain, showed a greater number occurring in the plate’s younger lithosphere. “In the Earth, those strains are either accommodated by elastic deformation or by little earthquakes that adjust it,” he said.

“The central assumption of plate tectonics assumes the plates are rigid, and this is what we make predictions from,” said Gordon, who was recently honored by the American Geophysical Union for writing two papers about plate movements that are among the top 40 papers ever to appear in one of the organization’s top journals. “Up until now, it’s worked really well.”

“The big picture is that we now have, subject to experimental and observational tests, the first realistic, quantitative estimate of how the biggest oceanic plate departs from that rigid-plate assumption.”

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 Stéphane 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.

Extinct undersea volcanoes squashed under Earth’s crust cause tsunami earthquakes, according to new

New research has revealed the causes and warning signs of rare tsunami earthquakes, which may lead to improved detection measures.

Tsunami earthquakes happen at relatively shallow depths in the ocean and are small in terms of their magnitude. However, they create very large tsunamis, with some earthquakes that only measure 5.6 on the Richter scale generating waves that reach up to ten metres when they hit the shore.

A global network of seismometers enables researchers to detect even the smallest earthquakes. However, the challenge has been to determine which small magnitude events are likely to cause large tsunamis.

In 1992, a magnitude 7.2 tsunami earthquake occurred off the coast of Nicaragua in Central America causing the deaths of 170 people. Six hundred and thirty seven people died and 164 people were reported missing following a tsunami earthquake off the coast of Java, Indonesia, in 2006, which measured 7.2 on the Richter scale.

The new study, published in the journal Earth and Planetary Science Letters, reveals that tsunami earthquakes may be caused by extinct undersea volcanoes causing a “sticking point” between two sections of the Earth’s crust called tectonic plates, where one plate slides under another.

The researchers from Imperial College London and GNS Science in New Zealand used geophysical data collected for oil and gas exploration and historical accounts from eye witnesses relating to two tsunami earthquakes, which happened off the coast of New Zealand’s north island in 1947. Tsunami earthquakes were only identified by geologists around 35 years ago, so detailed studies of these events are rare.

The team located two extinct volcanoes off the coast of Poverty Bay and Tolaga Bay that have been squashed and sunk beneath the crust off the coast of New Zealand, in a process called subduction.

The researchers suggest that the volcanoes provided a “sticking point” between a part of the Earth’s crust called the Pacific plate, which was trying to slide underneath the New Zealand plate. This caused a build-up of energy, which was released in 1947, causing the plates to “unstick” and the Pacific plate to move and the volcanoes to become subsumed under New Zealand. This release of the energy from both plates was unusually slow and close to the seabed, causing large movements of the sea floor, which led to the formation of very large tsunami waves.

All these factors combined, say the researchers, are factors that contribute to tsunami earthquakes. The researchers say that the 1947 New Zealand tsunami earthquakes provide valuable insights into what geological factors cause these events. They believe the information they’ve gathered on these events could be used to locate similar zones around the world that could be at risk from tsunami earthquakes. Eyewitnesses from these tsunami earthquakes also describe the type of ground movement that occurred and this provides valuable clues about possible early warning signals for communities.

Dr Rebecca Bell, from the Department of Earth Science and Engineering at Imperial College London, says: “Tsunami earthquakes don’t create massive tremors like more conventional earthquakes such as the one that hit Japan in 2011, so residents and authorities in the past haven’t had the same warning signals to evacuate. These types of earthquakes were only identified a few decades ago, so little information has been collected on them. Thanks to oil exploration data and eyewitness accounts from two tsunami earthquakes that happened in New Zealand more than 70 years ago, we are beginning to understand for first time the factors that cause these events. This could ultimately save lives.”

By studying the data and reports, the researchers have built up a picture of what happened in New Zealand in 1947 when the tsunami earthquakes hit. In the March earthquake, eyewitnesses around Poverty Bay on the east coast of the country, close to the town of Gisborne, said that they didn’t feel violent tremors, which are characteristic of typical earthquakes. Instead, they felt the ground rolling, which lasted for minutes, and brought on a sense of sea sickness. Approximately 30 minutes later the bay was inundated by a ten metre high tsunami that was generated by a 5.9 magnitude offshore earthquake. In May, an earthquake measuring 5.6 on the Richter scale happened off the coast of Tolaga Bay, causing an approximate six metre high tsunami to hit the coast. No lives were lost in the New Zealand earthquakes as the areas were sparsely populated in 1947. However, more recent tsunami earthquakes elsewhere have devastated coastal communities.

The researchers are already working with colleagues in New Zealand to develop a better warning system for residents. In particular, new signage is being installed along coastal regions to alert people to the early warning signs that indicate a possible tsunami earthquake. In the future, the team hope to conduct new cutting-edge geophysical surveys over the sites of other sinking volcanoes to better understand their characteristics and the role they play in generating this unusual type of earthquake.

New study reveals insights on plate tectonics, the forces behind earthquakes, volcanoes

The Earth's outer layer is broken into moving, interacting plates whose motion at the surface generates most earthquakes, creates volcanoes and builds mountains. In this image, the orange layer represents the deformable, warm asthenosphere in which there is active mantle flow. The green layer is the lithospheric plate, which forms at the mid ocean ridge, then cools down and thickness as it moves away from the ridge. The cooling of the plate overprints a compositional boundary that forms at the ridge by dehydration melting and is preserved as the plate ages. The more easily deformable, hydrated rocks align with mantle flow. The directions of past and present-day mantle flow can be detected by seismic waves, and changes in the alignment of the rocks inside and at the bottom of the plate can be used to identify layering. -  Nicholas Schmerr/University of Maryland
The Earth’s outer layer is broken into moving, interacting plates whose motion at the surface generates most earthquakes, creates volcanoes and builds mountains. In this image, the orange layer represents the deformable, warm asthenosphere in which there is active mantle flow. The green layer is the lithospheric plate, which forms at the mid ocean ridge, then cools down and thickness as it moves away from the ridge. The cooling of the plate overprints a compositional boundary that forms at the ridge by dehydration melting and is preserved as the plate ages. The more easily deformable, hydrated rocks align with mantle flow. The directions of past and present-day mantle flow can be detected by seismic waves, and changes in the alignment of the rocks inside and at the bottom of the plate can be used to identify layering. – Nicholas Schmerr/University of Maryland

The Earth’s outer layer is made up of a series of moving, interacting plates whose motion at the surface generates earthquakes, creates volcanoes and builds mountains. Geoscientists have long sought to understand the plates’ fundamental properties and the mechanisms that cause them to move and drift, and the questions have become the subjects of lively debate.

A study published online Feb. 27 by the journal Science is a significant step toward answering those questions.

Researchers led by Caroline Beghein, assistant professor of earth, planetary and space sciences in UCLA’s College of Letters and Science, used a technique called seismic tomography to study the structure of the Pacific Plate – one of eight to 12 major plates at the surface of the Earth. The technique enabled them to determine the plate’s thickness, and to image the interior of the plate and the underlying mantle (the layer between the Earth’s crust and outer core), which they were able to relate to the direction of flow of rocks in the mantle.

“Rocks deform and flow slowly inside the Earth’s mantle, which makes the plates move at the surface,” said Beghein, the paper’s lead author. “Our research enables us to image the interior of the plate and helps us figure out how it formed and evolved.” The findings might apply to other oceanic plates as well.

Even with the new findings, Beghein said, the fundamental properties of plates “are still somewhat enigmatic.”

Seismic tomography is similar to commonly used medical imaging techniques like computed tomography, or CT, scans. But instead of using X-rays, seismic tomography employs recordings of the seismic waves generated by earthquakes, allowing scientists to detect variations in the speed of seismic waves inside the Earth. Those variations can reveal different layers within the mantle, and can help scientists determine the temperature and chemistry of the mantle rocks by comparing observed variations in wave speed with predictions from other types of geophysical data.

Seismologists often use other types of seismic data to identify this layering: They detect seismic waves that bounce off the interface that separates two layers. In their study, Beghein and co-authors compared the layering they observed using seismic tomography with the layers revealed by these other types of data. Comparing results from the different methods is a continuing challenge for geoscientists, but it is an important part of helping them understand the Earth’s structure.

“We overcame this challenge by trying to push the observational science to the highest resolutions, allowing us to more readily compare observations across datasets,” said Nicholas Schmerr, the study’s co-author and an assistant research scientist in geology at the University of Maryland.

The researchers were the first to discover that the Pacific Plate is formed by a combination of mechanisms: The plate thickens as the rocks of the mantle cool, the chemical makeup of the rocks that form the plate changes with depth, and the mechanical behavior of the rocks change with depth and their proximity to where the plate is being formed at the mid-ocean ridge.

“By modeling the behavior of seismic waves in Earth’s mantle, we discovered a transition inside the plate from the top, where the rocks didn’t deform or flow very much, to the bottom of the plate, where they are more strongly deformed by tectonic forces,” Beghein said. “This transition corresponds to a boundary between the layers that we can image with seismology and that we attribute to changes in rock composition.”

Oceanic plates form at ocean ridges and disappear into the Earth’s mantle, a process known as subduction. Among geoscientists, there is still considerable debate about what drives this evolution. Beghein and her research team advanced our understanding of how oceanic plates form and evolve as they age by using and comparing two sets of seismic data; the study revealed the presence of a compositional boundary inside the plate that appears to be linked to the formation of the plate itself.

Slippery fault unleashed destructive Tohoku-Oki earthquake and tsunami

UCSC researcher Patrick Fulton is shown with core samples from the drilling operation. -  IODP
UCSC researcher Patrick Fulton is shown with core samples from the drilling operation. – IODP

For the first time, scientists have measured the frictional heat produced by the fault slip during an earthquake. Their results, published December 5 in Science, show that friction on the fault was remarkably low during the magnitude 9.0 Tohoku-Oki earthquake that struck off the coast of Japan in March 2011 and triggered a devastating tsunami.

“The Tohoku fault is more slippery than anyone expected,” said Emily Brodsky, a geophysicist at the University of California, Santa Cruz, and coauthor of three papers on the Tohoku-Oki earthquake published together in Science. All three papers are based on results from the international Japan Trench Fast Drilling Project (JFAST), which Brodsky helped organize.

Because friction generates heat (like rubbing your hands together), taking the temperature of a fault after an earthquake can provide a measure of the fault’s frictional resistance to slip. But that hasn’t been easy to do. “It’s been difficult to get this measurement because the signal is weak and it dissipates over time, so we needed a big earthquake and a rapid response,” said Brodsky, a professor of Earth and planetary sciences at UCSC.

The JFAST expedition drilled across the Tohoku fault in 2012 and installed a temperature observatory in one of three boreholes nearly 7 kilometers below the ocean surface. The logistically and technically challenging operation successfully recovered temperature measurements and other data as well as core samples from across the fault.

The low resistance to slip on the fault may help explain the large amount of slip–an unprecedented 50 meters of displacement–that occurred during the earthquake, according to UC Santa Cruz researcher Patrick Fulton, who is first author of the paper focusing on the temperature measurements. An abundance of weak, slippery clay material in the fault zone–described in the two companion papers–may account for the low friction during the earthquake, he said.

The Tohoku-Oki earthquake occurred in a “subduction zone,” a boundary between two tectonic plates where one plate is diving beneath another–in this case, the Pacific plate dives beneath the Eurasian plate just east of Japan. Fulton explained that the epicenter, where the earthquake started, was much deeper than the shallow portion of the fault examined by JFAST. One of the surprising things about the earthquake, in addition to the 50 meters of slip, was that the fault ruptured all the way to the surface of the seafloor.

“The large slip at shallow depths contributed to the tsumani that caused so much damage in Japan. Usually, these earthquakes don’t rupture all the way to the surface,” Fulton said.

The strain that is released in a subduction zone earthquake is thought to build up in the deep portion of the fault where the two plates are “locked.” The shallow portion of the fault was not expected to accumulate a large amount of stress and was considered unlikely to produce a large amount of slip. The JFAST results show that the frictional stress on the shallow portion of the fault was very low during the earthquake, which means that either the stress was low to begin with or all of the stress was released during the earthquake.

“It’s probably a combination of both–the fault was pretty slippery to begin with, and whatever stress was on the fault at that shallow depth was all released during the earthquake,” Fulton said.

An earlier paper by JFAST researchers, published in Science in February 2013 (Lin et al.), also suggested a nearly total stress drop during the earthquake based on an analysis of geophysical data collected during drilling.

“We now have four lines of evidence that frictional stress was low during the earthquake,” Brodsky said. “The key measure is temperature, but those results are totally consistent with the other papers.”

One of the new papers (Ujiie et al.) presents the results of laboratory experiments on the material recovered from the fault zone. Tests showed very low shear stress (resistance to slip) attributable to the abundance of weak, slippery clay material. The other paper (Chester et al.) focuses on the geology and structure of the fault zone. In addition to the high clay content, the researchers found that the fault zone was surprisingly thin (less than 5 meters thick).

J. Casey Moore, a research professor of Earth sciences at UCSC and coauthor of the Chester et al. paper, said he suspects the clay layer observed in the Tohoku fault zone may play an important role in other fault zones. “Looking for something like that clay may give us a tool to understand the locations of earthquakes that cause tsunamis. It’s potentially a predictive tool,” Moore said.

According to Brodsky, measuring the frictional forces on the fault is the key to a fundamental understanding of earthquake mechanics. “We’ve been hamstrung without in situ measurements of frictional stress, and we now have that from the temperature data,” she said. “It’s hard to say how generalizable these results are until we look at other faults, but this lays the foundation for a better understanding of earthquakes and, ultimately, a better ability to identify earthquake hazards.”

Shaking the Earth: Just add water

Phil Wannamaker, a geophysicist at the University of Utah's Energy and Geoscience Institute, on New Zealand's South Island with the Inland Kaikoura Range in the background. New Zealand sits atop one of the youngest 'subduction zones' on Earth, where the Pacific Plate of Earth's crust is diving beneath New Zealand, which sits on the Australian Plate. Wannamaker led a study that showed how and where water is released in a young subduction zone so that it helps crack the ground and otherwise pave the way for earthquakes. -  George Jiracek, San Diego State University.
Phil Wannamaker, a geophysicist at the University of Utah’s Energy and Geoscience Institute, on New Zealand’s South Island with the Inland Kaikoura Range in the background. New Zealand sits atop one of the youngest ‘subduction zones’ on Earth, where the Pacific Plate of Earth’s crust is diving beneath New Zealand, which sits on the Australian Plate. Wannamaker led a study that showed how and where water is released in a young subduction zone so that it helps crack the ground and otherwise pave the way for earthquakes. – George Jiracek, San Diego State University.

New Zealand is the site of one of the world’s youngest subduction zones, where the Pacific Plate of Earth’s crust dives beneath the Australian Plate. Now, a University of Utah study shows how water deep underground helps the subduction zone mature and paves the way for it to generate powerful earthquakes.

The study in the Aug. 6 issue of the journal Nature “expands our understanding of the sources of earthquake failure,” says Phil Wannamaker, the study’s main author and a geophysicist at the University of Utah’s Energy and Geoscience Institute.

“It hasn’t been on people’s minds that fluid-generating processes way out of sight reach up and cause damage right under our feet,” he adds.

Understanding how one of Earth’s moving tectonic plates can dive or subduct beneath another to create earthquake-generating faults is important because subduction and faulting “are major processes all over the world,” especially in the “Ring of Fire” around the Pacific Ocean, Wannamaker says.

Wannamaker conducted the study with University of Utah geophysics student Virginie Maris; geophysicists George Jiracek of San Diego State University and Yasuo Ogawa of the Tokyo Institute of Technology; and five coauthors from New Zealand’s government geology institute, GNS Science: geophysicists T. Grant Caldwell, Hugh Bibby and Wiebke Heise; student Graham Hill; and field technician Stewart Bennie.

Wannamaker says the study was financed by a $395,000 grant from the National Science Foundation. With additional funding from Japan and from the New Zealand Foundation for Research, Wannamaker estimates the total study cost at $600,000.

Subducting the Kiwis

New Zealand includes two major islands – the North Island and less populated South Island – that extend roughly northeast to southwest in the Pacific Ocean southeast of Australia. Like other nations along the Pacific “Ring of Fire” – including the western coast of North America – New Zealand sits atop a boundary between two of Earth’s slowly moving tectonic plates, and thus has earthquakes and volcanism.

The plates, which can be 100 miles thick, include Earth’s crust and the upper part of the mantle, the rock layer beneath the crust. New tectonic plates are born as volcanic eruptions at mid-ocean ridges add new rock to the plates on both sides of the ridges, like twin conveyor belts moving away from a ridge.

At the other end of these conveyor belts, where an oceanic plate collides with a continental plate, the seafloor plate subducts – or dives downward at a roughly 45-degree angle – beneath the continental plate, a process that generates earthquakes and volcanism.

New Zealand’s subduction zone – the Hikurangi subduction zone – is considered young because only during the past 20 million years did the edge of the Pacific Plate collide with the Australian Plate in New Zealand and begin diving beneath it.

The zone includes two kinds of quake-generating motions because the Pacific Plate is colliding with the Australian Plate at New Zealand at an angle instead of head-on. Thus, the Pacific Plate not only is moving northwest and diving under the Australian Plate, but it also is sliding southwest under the Australian Plate at the same time.

So while the colliding plates create quake forces like those seen in other subduction zones such as the U.S. Pacific Northwest, the oblique movement also generates “strike-slip” pressures like those that created California’s San Andreas fault. The oblique pressure has created four major strike-slip faults extending northeast to southwest along the length of the northern part of the South Island. Major earthquakes to nearly magnitude 8 have occurred along these and related faults over the past 200 years.

Picturing Water within the Earth

Rocks exposed from old, defunct subduction zones as well as current “Ring of Fire” volcanic rocks show water is released during subduction, so the researchers wanted to determine the role of this water in the maturation of the young subduction zone.

They used a method called magnetotelluric sounding, which is similar to using X-rays for CAT scans of patients and seismic waves to search for oil and gas.

Magnetotelluric sounding uses natural electromagnetic waves generated by the sun and by lightning bolts. Most such waves travel through the air, but “a portion penetrate the Earth, scatter off rock structures of interest and return to the surface, where we can measure them” using fancy volt meters, Wannamaker says.

As the electromagnetic waves pass through Earth’s interior, they travel faster or slower depending on the extent to which rock and other material conducts or resists electricity. Water is more conductive, so it can be detected by this technique.

During 2006 and 2007, Wannamaker and colleagues made measurements at 67 sites along a 125-mile line crossing the northern end of New Zealand’s South Island.

Patterns of backscattered electromagnetic radiation from all the stations then were assembled by a computer program to create an image of Earth’s crust and upper mantle along a cross section of New Zealand’s South Island.

The image revealed large amounts of water in different areas and at different depths, which in turn suggested three distinct processes by which the fluid deforms the crust above it and helps pave the way for earthquakes:

  • Beneath the South Island’s eastern coast, where the Pacific Plate begins to dive under the Australian Plate, water is released about 10 miles underground. It comes from seafloor sediments that are squeezed as they are carried underground on the subducting Pacific Plate. Much of the water rises upward into the overlying crust of the Australian Plate, cracking the crustal rock further to create and widen existing cracks.

    This “mesh” of fault fractures “is weakening the crust and promoting the formation of new strike-slip faulting,” Wannamaker says.

  • Farther west, water is released from hydrated rock – rock with chemically bound water – within the subducting Pacific Plate. The water collects within cracks roughly 6 to 20 miles underground in the “ductile” or taffy-like part of the Earth’s crust.

    Such fluids help accommodate the oblique or southwesterly motion of the Pacific Plate under New Zealand – motion that created the strike-slip faults on the South Island.

    “These fluids certainly could burst upward into the strike-slip zone and trigger major earthquakes,” Wannamaker says. “And many smaller quakes have been centered along the edges of these reservoirs.”

  • The largest accumulation of water beneath the subduction zone also is the deepest and farthest west beneath the South Island. Freed by the action of heat and pressure on hydrated minerals, the water forms a huge plume extending upward from depths of 60 miles or more – something also seen in older, more mature subduction zones. It appears these fluids trigger major earthquakes – and did so during magnitude-7 and larger earthquakes in the Murchison area in the early 20th century, Wannamaker says.

    Faults in that area are high-angle thrusts, meaning that during quakes, ground on one side of the fault moves up and over the ground on the other side. Laws of rock mechanics say such faults should not rupture when they are steep, because it is difficult to push one block of ground up and over the other when the fault between them dips at an angle of more than 30 degrees – unless water is present. Yet big quakes near Murchison have occurred on faults that dip at angles of more than 55 degrees, Wannamaker says.

    That suggests subduction zone water makes quakes possible on such steep faults. “The fluid pressure is rising through the upper mantle and deep crust and into the upper crust, and props open the high-angle thrusts, allowing them fail,” Wannamaker says.

In older subduction zones – including the zone off New Zealand’s North Island – the water from great depths moves largely straight upward into the hot upper mantle, lowering the melting temperature of the rock and ultimately leading to the formation volcanoes on the land above. Subduction has not yet injected the upper mantle with enough water to trigger volcanism on the South Island, Wannamaker says.