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.

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.”

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.”

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.

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.

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.