Is there an ocean beneath our feet?

Scientists at the University of Liverpool have shown that deep sea fault zones could transport much larger amounts of water from the Earth’s oceans to the upper mantle than previously thought.

Seismologists at Liverpool have estimated that over the age of the Earth, the Japan subduction zone alone could transport the equivalent of up to three and a half times the water of all the Earth’s oceans to its mantle.

Water is carried to the mantle by deep sea fault zones which penetrate the oceanic plate as it bends into the subduction zone. Subduction, where an oceanic tectonic plate is forced beneath another plate, causes large earthquakes such as the recent Tohoku earthquake, as well as many earthquakes that occur hundreds of kilometers below the Earth’s surface.

Using seismic modelling techniques the researchers analysed earthquakes which occurred more than 100 km below the Earth’s surface in the Wadati-Benioff zone, a plane of Earthquakes that occur in the oceanic plate as it sinks deep into the mantle.

Analysis of the seismic waves from these earthquakes shows that they occurred on 1 – 2 km wide fault zones with low seismic velocities. Seismic waves travel slower in these fault zones than in the rest of the subducting plate because the sea water that percolated through the faults reacted with the oceanic rocks to form serpentinite – a mineral that contains water.

Some of the water carried to the mantle by these hydrated fault zones is released as the tectonic plate heats up. This water causes the mantle material to melt, causing volcanoes above the subduction zone such as those that form the Pacific ‘ring of fire’. Some water is transported deeper into the mantle, and is stored in the deep Earth.

“It has been known for a long time that subducting plates carry oceanic water to the mantle,” said Tom Garth, a PhD student in the Earthquake Seismology research group led by Professor Rietbrock. “This water causes melting in the mantle, which leads to arc releasing some of the water back into the atmosphere. Part of the subducted water however is carried deeper into the mantle and may be stored there.

“We found that fault zones that form in the deep oceanic trench offshore Northern Japan persist to depths of up to 150 km. These hydrated fault zones can carry large amounts of water, suggesting that subduction zones carry much more water from the ocean down to the mantle than has previously been suggested.This supports the theory that there are large amounts of water stored deep in the Earth.

Understanding how much water is delivered to the mantle contributes to our knowledge of how the mantle convects and how it melts. This is important to understanding how plate tectonics began and how the continental crust was formed.

Is there an ocean beneath our feet?

Scientists at the University of Liverpool have shown that deep sea fault zones could transport much larger amounts of water from the Earth’s oceans to the upper mantle than previously thought.

Seismologists at Liverpool have estimated that over the age of the Earth, the Japan subduction zone alone could transport the equivalent of up to three and a half times the water of all the Earth’s oceans to its mantle.

Water is carried to the mantle by deep sea fault zones which penetrate the oceanic plate as it bends into the subduction zone. Subduction, where an oceanic tectonic plate is forced beneath another plate, causes large earthquakes such as the recent Tohoku earthquake, as well as many earthquakes that occur hundreds of kilometers below the Earth’s surface.

Using seismic modelling techniques the researchers analysed earthquakes which occurred more than 100 km below the Earth’s surface in the Wadati-Benioff zone, a plane of Earthquakes that occur in the oceanic plate as it sinks deep into the mantle.

Analysis of the seismic waves from these earthquakes shows that they occurred on 1 – 2 km wide fault zones with low seismic velocities. Seismic waves travel slower in these fault zones than in the rest of the subducting plate because the sea water that percolated through the faults reacted with the oceanic rocks to form serpentinite – a mineral that contains water.

Some of the water carried to the mantle by these hydrated fault zones is released as the tectonic plate heats up. This water causes the mantle material to melt, causing volcanoes above the subduction zone such as those that form the Pacific ‘ring of fire’. Some water is transported deeper into the mantle, and is stored in the deep Earth.

“It has been known for a long time that subducting plates carry oceanic water to the mantle,” said Tom Garth, a PhD student in the Earthquake Seismology research group led by Professor Rietbrock. “This water causes melting in the mantle, which leads to arc releasing some of the water back into the atmosphere. Part of the subducted water however is carried deeper into the mantle and may be stored there.

“We found that fault zones that form in the deep oceanic trench offshore Northern Japan persist to depths of up to 150 km. These hydrated fault zones can carry large amounts of water, suggesting that subduction zones carry much more water from the ocean down to the mantle than has previously been suggested.This supports the theory that there are large amounts of water stored deep in the Earth.

Understanding how much water is delivered to the mantle contributes to our knowledge of how the mantle convects and how it melts. This is important to understanding how plate tectonics began and how the continental crust was formed.

World’s first magma-enhanced geothermal system created in Iceland

This image shows a flow test of the IDDP-1 well at Krafla. Note the transparent superheated steam at the top of the rock muffler. -  Kristján Einarsson.
This image shows a flow test of the IDDP-1 well at Krafla. Note the transparent superheated steam at the top of the rock muffler. – Kristján Einarsson.

In 2009, a borehole drilled at Krafla, northeast Iceland, as part of the Icelandic Deep Drilling Project (IDDP), unexpectedly penetrated into magma (molten rock) at only 2100 meters depth, with a temperature of 900-1000 C. The borehole, IDDP-1, was the first in a series of wells being drilled by the IDDP in Iceland in the search for high-temperature geothermal resources.

The January 2014 issue of the international journal Geothermics is dedicated to scientific and engineering results arising from that unusual occurrence. This issue is edited by Wilfred Elders, a professor emeritus of geology at the University of California, Riverside, who also co-authored three of the research papers in the special issue with Icelandic colleagues.

“Drilling into magma is a very rare occurrence anywhere in the world and this is only the second known instance, the first one, in 2007, being in Hawaii,” Elders said. “The IDDP, in cooperation with Iceland’s National Power Company, the operator of the Krafla geothermal power plant, decided to investigate the hole further and bear part of the substantial costs involved.”

Accordingly, a steel casing, perforated in the bottom section closest to the magma, was cemented into the well. The hole was then allowed to heat slowly and eventually allowed to flow superheated steam for the next two years, until July 2012, when it was closed down in order to replace some of the surface equipment.

“In the future, the success of this drilling and research project could lead to a revolution in the energy efficiency of high-temperature geothermal areas worldwide,” Elders said.

He added that several important milestones were achieved in this project: despite some difficulties, the project was able to drill down into the molten magma and control it; it was possible to set steel casing in the bottom of the hole; allowing the hole to blow superheated, high-pressure steam for months at temperatures exceeding 450 C, created a world record for geothermal heat (this well was the hottest in the world and one of the most powerful); steam from the IDDP-1 well could be fed directly into the existing power plant at Krafla; and the IDDP-1 demonstrated that a high-enthalpy geothermal system could be successfully utilized.

“Essentially, the IDDP-1 created the world’s first magma-enhanced geothermal system,” Elders said. “This unique engineered geothermal system is the world’s first to supply heat directly from a molten magma.”

Elders explained that in various parts of the world so-called enhanced or engineered geothermal systems are being created by pumping cold water into hot dry rocks at 4-5 kilometers depths. The heated water is pumped up again as hot water or steam from production wells. In recent decades, considerable effort has been invested in Europe, Australia, the United States, and Japan, with uneven, and typically poor, results.

“Although the IDDP-1 hole had to be shut in, the aim now is to repair the well or to drill a new similar hole,” Elders said. “The experiment at Krafla suffered various setbacks that tried personnel and equipment throughout. However, the process itself was very instructive, and, apart from scientific articles published in Geothermics, comprehensive reports on practical lessons learned are nearing completion.”

The IDDP is a collaboration of three energy companies – HS Energy Ltd., National Power Company and Reykjavik Energy – and a government agency, the National Energy Authority of Iceland. It will drill the next borehole, IDDP-2, in southwest Iceland at Reykjanes in 2014-2015. From the onset, international collaboration has been important to the project, and in particular a consortium of U.S. scientists, coordinated by Elders, has been very active, authoring several research papers in the special issue of Geothermics.

World’s first magma-enhanced geothermal system created in Iceland

This image shows a flow test of the IDDP-1 well at Krafla. Note the transparent superheated steam at the top of the rock muffler. -  Kristján Einarsson.
This image shows a flow test of the IDDP-1 well at Krafla. Note the transparent superheated steam at the top of the rock muffler. – Kristján Einarsson.

In 2009, a borehole drilled at Krafla, northeast Iceland, as part of the Icelandic Deep Drilling Project (IDDP), unexpectedly penetrated into magma (molten rock) at only 2100 meters depth, with a temperature of 900-1000 C. The borehole, IDDP-1, was the first in a series of wells being drilled by the IDDP in Iceland in the search for high-temperature geothermal resources.

The January 2014 issue of the international journal Geothermics is dedicated to scientific and engineering results arising from that unusual occurrence. This issue is edited by Wilfred Elders, a professor emeritus of geology at the University of California, Riverside, who also co-authored three of the research papers in the special issue with Icelandic colleagues.

“Drilling into magma is a very rare occurrence anywhere in the world and this is only the second known instance, the first one, in 2007, being in Hawaii,” Elders said. “The IDDP, in cooperation with Iceland’s National Power Company, the operator of the Krafla geothermal power plant, decided to investigate the hole further and bear part of the substantial costs involved.”

Accordingly, a steel casing, perforated in the bottom section closest to the magma, was cemented into the well. The hole was then allowed to heat slowly and eventually allowed to flow superheated steam for the next two years, until July 2012, when it was closed down in order to replace some of the surface equipment.

“In the future, the success of this drilling and research project could lead to a revolution in the energy efficiency of high-temperature geothermal areas worldwide,” Elders said.

He added that several important milestones were achieved in this project: despite some difficulties, the project was able to drill down into the molten magma and control it; it was possible to set steel casing in the bottom of the hole; allowing the hole to blow superheated, high-pressure steam for months at temperatures exceeding 450 C, created a world record for geothermal heat (this well was the hottest in the world and one of the most powerful); steam from the IDDP-1 well could be fed directly into the existing power plant at Krafla; and the IDDP-1 demonstrated that a high-enthalpy geothermal system could be successfully utilized.

“Essentially, the IDDP-1 created the world’s first magma-enhanced geothermal system,” Elders said. “This unique engineered geothermal system is the world’s first to supply heat directly from a molten magma.”

Elders explained that in various parts of the world so-called enhanced or engineered geothermal systems are being created by pumping cold water into hot dry rocks at 4-5 kilometers depths. The heated water is pumped up again as hot water or steam from production wells. In recent decades, considerable effort has been invested in Europe, Australia, the United States, and Japan, with uneven, and typically poor, results.

“Although the IDDP-1 hole had to be shut in, the aim now is to repair the well or to drill a new similar hole,” Elders said. “The experiment at Krafla suffered various setbacks that tried personnel and equipment throughout. However, the process itself was very instructive, and, apart from scientific articles published in Geothermics, comprehensive reports on practical lessons learned are nearing completion.”

The IDDP is a collaboration of three energy companies – HS Energy Ltd., National Power Company and Reykjavik Energy – and a government agency, the National Energy Authority of Iceland. It will drill the next borehole, IDDP-2, in southwest Iceland at Reykjanes in 2014-2015. From the onset, international collaboration has been important to the project, and in particular a consortium of U.S. scientists, coordinated by Elders, has been very active, authoring several research papers in the special issue of Geothermics.

Source of Galapagos eruptions is not where models place it

Birds flock not far from a volcano on Isabella Island, where two still active volcanoes are located. The location of the mantle plume, to the southeast of where computer modeling had put it, may explain the continued activity of volcanoes on the various Galapagos Islands. -  Douglas Toomey
Birds flock not far from a volcano on Isabella Island, where two still active volcanoes are located. The location of the mantle plume, to the southeast of where computer modeling had put it, may explain the continued activity of volcanoes on the various Galapagos Islands. – Douglas Toomey

Images gathered by University of Oregon scientists using seismic waves penetrating to a depth of 300 kilometers (almost 200 miles) report the discovery of an anomaly that likely is the volcanic mantle plume of the Galapagos Islands. It’s not where geologists and computer modeling had assumed.

The team’s experiments put the suspected plume at a depth of 250 kilometers (155 miles), at a location about 150 kilometers (about 100 miles) southeast of Fernandina Island, the westernmost island of the chain, and where generations of geologists and computer-generated mantle convection models have placed the plume.

The plume anomaly is consistent with partial melting, melt extraction, and remixing of hot rocks and is spreading north toward the mid-ocean ridge instead of, as projected, eastward with the migrating Nazca plate on which the island chain sits, says co-author Douglas R. Toomey, a professor in the UO’s Department of Geological Sciences.

The findings — published online Jan. 19 ahead of print in the February issue of the journal Nature Geoscience — “help explain why so many of the volcanoes in the Galapagos are active,” Toomey said.

The Galapagos chain covers roughly 3,040 square miles of ocean and is centered about 575 miles west of Ecuador, which governs the islands. Galapagos volcanic activity has been difficult to understand, Toomey said, because conventional wisdom and modeling say newer eruptions should be moving ahead of the plate, not unlike the long-migrating Yellowstone hotspot. </p

The separating angles of the two plates in the Galapagos region cloud easy understanding. The leading edge of the Nazca plate is at Fernandina. The Cocos plate, on which the islands’ some 1,000-kilometer-long (620-miles) hotspot chain once sat, is moving to the northeast.

The suspected plume’s location is closer to Isabella and Floreana islands. While a dozen volcanoes remain active in the archipelago, the three most volatile are Fernandina’s and the Cerro Azul and Sierra Negra volcanoes on the southwest and southeast tips, respectively, of Isabella Island, the archipelago’s largest landmass.

The plume’s more southern location, Toomey said, adds fuel to his group’s findings, at three different sites along the globe encircling mid-ocean ridge (where 85 percent of Earth’s volcanic activity occurs), that Earth’s internal convection doesn’t always adhere to modeling efforts and raises new questions about how ocean plates at the Earth’s surface — the lithosphere — interact with the hotter, more fluid asthenosphere that sits atop the mantle.

“Ocean islands have always been enigmatic,” said co-author Dennis J. Geist of the Department of Geological Sciences at the University of Idaho. “Why out in the middle of the ocean basins do you get these big volcanoes? The Galapagos, Hawaii, Tahiti, Iceland — all the world’s great ocean islands – they’re mysterious.”

The Galapagos plume, according to the new paper, extends up into shallower depths and tracks northward and perpendicular to plate motion. Mantle plumes, such as the Galapagos, Yellowstone and Hawaii, generally are believed to bend in the direction of plate migration. In the Galapagos, however, the volcanic plume has decoupled from the plates involved.

“Here’s an archipelago of volcanic islands that are broadly active over a large region, and the plume is almost decoupled from the plate motion itself,” Toomey said. “It is going opposite than expected, and we don’t know why.”

The answer may be in the still unknown rheology of the gooey asthenosphere on which the Earth’s plates ride, Toomey said. In their conclusion, the paper’s five co-authors theorize that the plume material is carried to the mid-ocean ridge by a deep return flow centered in the asthenosphere rather than flowing along the base of the lithosphere as in modeling projections.

“Researchers at the University of Oregon are using tools and technologies to yield critical insights into complex scientific questions,” said Kimberly Andrews Espy, vice president for research and innovation and dean of the UO Graduate School. “This research by Dr. Toomey and his team sheds new light on the volcanic activity of the Galapagos Islands and raises new questions about plate tectonics and the interaction between the zones of the Earth’s mantle.”

Co-authors with Toomey and Geist were: doctoral student Darwin R. Villagomez, now with ID Analytics in San Diego, Calif.; Emilie E.E. Hooft of the UO Department of Geological Sciences; and Sean C. Solomon of the Lamont-Doherty Earth Observatory at Columbia University.

The National Science Foundation (grants OCE-9908695, OCE-0221549 and EAR-0651123 to the UO; OCE-0221634 to the Carnegie Institution of Washington and EAR-11452711 to the University of Idaho) supported the research.

Source of Galapagos eruptions is not where models place it

Birds flock not far from a volcano on Isabella Island, where two still active volcanoes are located. The location of the mantle plume, to the southeast of where computer modeling had put it, may explain the continued activity of volcanoes on the various Galapagos Islands. -  Douglas Toomey
Birds flock not far from a volcano on Isabella Island, where two still active volcanoes are located. The location of the mantle plume, to the southeast of where computer modeling had put it, may explain the continued activity of volcanoes on the various Galapagos Islands. – Douglas Toomey

Images gathered by University of Oregon scientists using seismic waves penetrating to a depth of 300 kilometers (almost 200 miles) report the discovery of an anomaly that likely is the volcanic mantle plume of the Galapagos Islands. It’s not where geologists and computer modeling had assumed.

The team’s experiments put the suspected plume at a depth of 250 kilometers (155 miles), at a location about 150 kilometers (about 100 miles) southeast of Fernandina Island, the westernmost island of the chain, and where generations of geologists and computer-generated mantle convection models have placed the plume.

The plume anomaly is consistent with partial melting, melt extraction, and remixing of hot rocks and is spreading north toward the mid-ocean ridge instead of, as projected, eastward with the migrating Nazca plate on which the island chain sits, says co-author Douglas R. Toomey, a professor in the UO’s Department of Geological Sciences.

The findings — published online Jan. 19 ahead of print in the February issue of the journal Nature Geoscience — “help explain why so many of the volcanoes in the Galapagos are active,” Toomey said.

The Galapagos chain covers roughly 3,040 square miles of ocean and is centered about 575 miles west of Ecuador, which governs the islands. Galapagos volcanic activity has been difficult to understand, Toomey said, because conventional wisdom and modeling say newer eruptions should be moving ahead of the plate, not unlike the long-migrating Yellowstone hotspot. </p

The separating angles of the two plates in the Galapagos region cloud easy understanding. The leading edge of the Nazca plate is at Fernandina. The Cocos plate, on which the islands’ some 1,000-kilometer-long (620-miles) hotspot chain once sat, is moving to the northeast.

The suspected plume’s location is closer to Isabella and Floreana islands. While a dozen volcanoes remain active in the archipelago, the three most volatile are Fernandina’s and the Cerro Azul and Sierra Negra volcanoes on the southwest and southeast tips, respectively, of Isabella Island, the archipelago’s largest landmass.

The plume’s more southern location, Toomey said, adds fuel to his group’s findings, at three different sites along the globe encircling mid-ocean ridge (where 85 percent of Earth’s volcanic activity occurs), that Earth’s internal convection doesn’t always adhere to modeling efforts and raises new questions about how ocean plates at the Earth’s surface — the lithosphere — interact with the hotter, more fluid asthenosphere that sits atop the mantle.

“Ocean islands have always been enigmatic,” said co-author Dennis J. Geist of the Department of Geological Sciences at the University of Idaho. “Why out in the middle of the ocean basins do you get these big volcanoes? The Galapagos, Hawaii, Tahiti, Iceland — all the world’s great ocean islands – they’re mysterious.”

The Galapagos plume, according to the new paper, extends up into shallower depths and tracks northward and perpendicular to plate motion. Mantle plumes, such as the Galapagos, Yellowstone and Hawaii, generally are believed to bend in the direction of plate migration. In the Galapagos, however, the volcanic plume has decoupled from the plates involved.

“Here’s an archipelago of volcanic islands that are broadly active over a large region, and the plume is almost decoupled from the plate motion itself,” Toomey said. “It is going opposite than expected, and we don’t know why.”

The answer may be in the still unknown rheology of the gooey asthenosphere on which the Earth’s plates ride, Toomey said. In their conclusion, the paper’s five co-authors theorize that the plume material is carried to the mid-ocean ridge by a deep return flow centered in the asthenosphere rather than flowing along the base of the lithosphere as in modeling projections.

“Researchers at the University of Oregon are using tools and technologies to yield critical insights into complex scientific questions,” said Kimberly Andrews Espy, vice president for research and innovation and dean of the UO Graduate School. “This research by Dr. Toomey and his team sheds new light on the volcanic activity of the Galapagos Islands and raises new questions about plate tectonics and the interaction between the zones of the Earth’s mantle.”

Co-authors with Toomey and Geist were: doctoral student Darwin R. Villagomez, now with ID Analytics in San Diego, Calif.; Emilie E.E. Hooft of the UO Department of Geological Sciences; and Sean C. Solomon of the Lamont-Doherty Earth Observatory at Columbia University.

The National Science Foundation (grants OCE-9908695, OCE-0221549 and EAR-0651123 to the UO; OCE-0221634 to the Carnegie Institution of Washington and EAR-11452711 to the University of Idaho) supported the research.

New study shows: Large landmasses existed 2.7 billion years ago

A Cologne working group involving Prof. Carsten Münker and Dr. Elis Hoffmann and their student Sebastian Viehmann (working with Prof. Michael Bau from the Jacobs University Bremen) have managed for the first time to determine the isotope composition of the rare trace elements Hafnium and Neodymium in 2,700 million year-old seawater by using high purity chemical sediments from Temagami Banded Iron Formation (Canada) as an archive.

Earlier work has shown that these rocks from Canada only contain chemical elements that directly precipitated from ocean water. The Temagami Banded Iron Formation, which was formed 2,700 million years ago during the Neoarchean period, can be used as an archive because the isotopic composition of many chemical elements such as Hafnium and Neodymium directly mirrors the composition of Neoarchean seawater. These two very rare elements allow many valuable conclusions about weathering processes to be drawn.

During their investigations, the research team came to the surprising result that has been published in the renowned journal Geology: 2,700 million years ago, seawater contained an unusually high abundance of the radioactive isotope Hafnium 176 but a comparably low abundance of the radioactive isotope Neodymium 143, similar to what can be observed in present day seawater.

“In present day seawater, this can be explained by weathering and the erosion of the Earth’s exposed surface,” explains Prof. Münker. “If in the Neoarchean period 97% of the Earth’s surface had been, as estimated from computer models, covered by water, these geochemical signals would not have been found for Neoarchean seawater,” adds Dr. Hoffmann.

According to the scientific team, the new findings show that 2,700 million years ago relatively large landmasses emerged from the oceans that were exposed to weathering and erosion by the sun, wind and rain. Dr. Hoffmann: “The isotope Hafnium 176 in contrast to its counterpart Neodymium 143 was transported by means of weathering into the oceans and became part of iron-rich sediments on the sea floor 2,700 million years ago.”

The examinations were carried out in the joint clean laboratory of the Universities of Cologne and Bonn. Prof. Münker: “We are able to carry out these isotope measurements for very rare elements, the concentrations of which are in the ppb range, i.e. only a few parts per billion.”

New study shows: Large landmasses existed 2.7 billion years ago

A Cologne working group involving Prof. Carsten Münker and Dr. Elis Hoffmann and their student Sebastian Viehmann (working with Prof. Michael Bau from the Jacobs University Bremen) have managed for the first time to determine the isotope composition of the rare trace elements Hafnium and Neodymium in 2,700 million year-old seawater by using high purity chemical sediments from Temagami Banded Iron Formation (Canada) as an archive.

Earlier work has shown that these rocks from Canada only contain chemical elements that directly precipitated from ocean water. The Temagami Banded Iron Formation, which was formed 2,700 million years ago during the Neoarchean period, can be used as an archive because the isotopic composition of many chemical elements such as Hafnium and Neodymium directly mirrors the composition of Neoarchean seawater. These two very rare elements allow many valuable conclusions about weathering processes to be drawn.

During their investigations, the research team came to the surprising result that has been published in the renowned journal Geology: 2,700 million years ago, seawater contained an unusually high abundance of the radioactive isotope Hafnium 176 but a comparably low abundance of the radioactive isotope Neodymium 143, similar to what can be observed in present day seawater.

“In present day seawater, this can be explained by weathering and the erosion of the Earth’s exposed surface,” explains Prof. Münker. “If in the Neoarchean period 97% of the Earth’s surface had been, as estimated from computer models, covered by water, these geochemical signals would not have been found for Neoarchean seawater,” adds Dr. Hoffmann.

According to the scientific team, the new findings show that 2,700 million years ago relatively large landmasses emerged from the oceans that were exposed to weathering and erosion by the sun, wind and rain. Dr. Hoffmann: “The isotope Hafnium 176 in contrast to its counterpart Neodymium 143 was transported by means of weathering into the oceans and became part of iron-rich sediments on the sea floor 2,700 million years ago.”

The examinations were carried out in the joint clean laboratory of the Universities of Cologne and Bonn. Prof. Münker: “We are able to carry out these isotope measurements for very rare elements, the concentrations of which are in the ppb range, i.e. only a few parts per billion.”

Study faults a ‘runaway’ mechanism in intermediate-depth earthquakes

Nearly 25 percent of earthquakes occur more than 50 kilometers below the Earth’s surface, when one tectonic plate slides below another, in a region called the lithosphere. Scientists have thought that these rumblings from the deep arise from a different process than shallower, more destructive quakes. But limited seismic data, and difficulty in reproducing these quakes in the laboratory, have combined to prevent researchers from pinpointing the cause of intermediate and deep earthquakes.

Now a team from MIT and Stanford University has identified a mechanism that helps these deeper quakes spread. By analyzing seismic data from a region in Colombia with a high concentration of intermediate-depth earthquakes, the researchers identified a “runaway process” in which the sliding of rocks at great depths causes surrounding temperatures to spike. This influx of heat, in turn, encourages more sliding – a feedback mechanism that propagates through the lithosphere, generating an earthquake.

German Prieto, an assistant professor of geophysics in MIT’s Department of Earth, Atmospheric and Planetary Sciences, says that once thermal runaway starts, the surrounding rocks can heat up and slide more easily, raising the temperature very quickly.

“What we predict is for medium-sized earthquakes, with magnitude 4 to 5, temperature can rise up to 1,000 degrees Centigrade, or about 1,800 degrees Fahrenheit, in a matter of one second,” Prieto says. “It’s a huge amount. You’re basically allowing rupture to run away because of this large temperature increase.”

Prieto says that understanding deeper earthquakes may help local communities anticipate how much shaking they may experience, given the seismic history of their regions.

He and his colleagues have published their results in the journal Geophysical Research Letters.

Water versus heat: two competing theories


The majority of Earth’s seismic activity occurs at relatively shallow depths, and the mechanics of such quakes is well understood: Over time, abutting plates in the crust build up tension as they shift against each other. This tension ultimately reaches a breaking point, creating a sudden rupture that splinters through the crust.

However, scientists have determined that this process is not feasible for quakes that occur far below the surface. Essentially, higher temperatures and pressures at these depths would make rocks behave differently than they would closer to the surface, gliding past rather than breaking against each other.

By way of explanation, Prieto draws an analogy to glass: If you try to bend a glass tube at room temperature, with enough force, it will eventually shatter. But with heating, the tube will become much more malleable, and bend without breaking.

So how do deeper earthquakes occur? Scientists have proposed two theories: The first, called dehydration embrittlement, is based on the small amounts of water in rocks’ mineral composition. At high pressure and heat, rocks release water, which lubricates surrounding faults, creating fractures that ultimately set off a quake.

The second theory is thermal runaway: Increasing temperatures weaken rocks, promoting slippage that spreads through the lithosphere, further increasing temperatures and causing more rocks to slip, resulting in an earthquake.

Probing the nest


Prieto and his colleagues found new evidence in support of the second theory by analyzing seismic data from a region of Colombia that experiences large numbers of intermediate-depth earthquakes – quakes whose epicenters are 50 to 300 kilometers below the surface. This region, known as the Bucaramanga Nest, hosts the highest concentration of intermediate-depth quakes in the world: Since 1993, more than 80,000 earthquakes have been recorded in the area, making it, in Prieto’s view, an “ideal natural laboratory” for studying deeper quakes.

The researchers analyzed seismic waves recorded by nearby surface seismometers and calculated two parameters: stress drop, or the total amount of energy released by an earthquake, and radiated seismic energy, or the amount of that energy that makes it to the surface as seismic waves – energy that is manifested in the shaking of the ground.

The stronger a quake is, the more energy, or heat, it generates. Interestingly, the MIT group found that only 2 percent of a deeper quake’s total energy is felt at the surface. Prieto reasoned that much of the other 98 percent may be released locally as heat, creating an enormous temperature increase that pushes a quake to spread.

Prieto says the study provides strong evidence for thermal runaway as the likely mechanism for intermediate-depth earthquakes. Such knowledge, he says, may be useful for communities around Bucaramanga in predicting the severity of future quakes.

“Usually people in Bucaramanga feel a magnitude 4 quake every month or so, and every year they experience a larger one that can shake significantly,” Prieto says. “If you’re in a region where you have intermediate-depth quakes and you know the size of the region, you can make a prediction of the type of magnitudes of quakes that you can have, and what kind of shaking you would expect.”

Prieto, a native Colombian, plans to deploy seismic stations above the Bucaramanga Nest to better understand the activity of deeper quakes.

Study faults a ‘runaway’ mechanism in intermediate-depth earthquakes

Nearly 25 percent of earthquakes occur more than 50 kilometers below the Earth’s surface, when one tectonic plate slides below another, in a region called the lithosphere. Scientists have thought that these rumblings from the deep arise from a different process than shallower, more destructive quakes. But limited seismic data, and difficulty in reproducing these quakes in the laboratory, have combined to prevent researchers from pinpointing the cause of intermediate and deep earthquakes.

Now a team from MIT and Stanford University has identified a mechanism that helps these deeper quakes spread. By analyzing seismic data from a region in Colombia with a high concentration of intermediate-depth earthquakes, the researchers identified a “runaway process” in which the sliding of rocks at great depths causes surrounding temperatures to spike. This influx of heat, in turn, encourages more sliding – a feedback mechanism that propagates through the lithosphere, generating an earthquake.

German Prieto, an assistant professor of geophysics in MIT’s Department of Earth, Atmospheric and Planetary Sciences, says that once thermal runaway starts, the surrounding rocks can heat up and slide more easily, raising the temperature very quickly.

“What we predict is for medium-sized earthquakes, with magnitude 4 to 5, temperature can rise up to 1,000 degrees Centigrade, or about 1,800 degrees Fahrenheit, in a matter of one second,” Prieto says. “It’s a huge amount. You’re basically allowing rupture to run away because of this large temperature increase.”

Prieto says that understanding deeper earthquakes may help local communities anticipate how much shaking they may experience, given the seismic history of their regions.

He and his colleagues have published their results in the journal Geophysical Research Letters.

Water versus heat: two competing theories


The majority of Earth’s seismic activity occurs at relatively shallow depths, and the mechanics of such quakes is well understood: Over time, abutting plates in the crust build up tension as they shift against each other. This tension ultimately reaches a breaking point, creating a sudden rupture that splinters through the crust.

However, scientists have determined that this process is not feasible for quakes that occur far below the surface. Essentially, higher temperatures and pressures at these depths would make rocks behave differently than they would closer to the surface, gliding past rather than breaking against each other.

By way of explanation, Prieto draws an analogy to glass: If you try to bend a glass tube at room temperature, with enough force, it will eventually shatter. But with heating, the tube will become much more malleable, and bend without breaking.

So how do deeper earthquakes occur? Scientists have proposed two theories: The first, called dehydration embrittlement, is based on the small amounts of water in rocks’ mineral composition. At high pressure and heat, rocks release water, which lubricates surrounding faults, creating fractures that ultimately set off a quake.

The second theory is thermal runaway: Increasing temperatures weaken rocks, promoting slippage that spreads through the lithosphere, further increasing temperatures and causing more rocks to slip, resulting in an earthquake.

Probing the nest


Prieto and his colleagues found new evidence in support of the second theory by analyzing seismic data from a region of Colombia that experiences large numbers of intermediate-depth earthquakes – quakes whose epicenters are 50 to 300 kilometers below the surface. This region, known as the Bucaramanga Nest, hosts the highest concentration of intermediate-depth quakes in the world: Since 1993, more than 80,000 earthquakes have been recorded in the area, making it, in Prieto’s view, an “ideal natural laboratory” for studying deeper quakes.

The researchers analyzed seismic waves recorded by nearby surface seismometers and calculated two parameters: stress drop, or the total amount of energy released by an earthquake, and radiated seismic energy, or the amount of that energy that makes it to the surface as seismic waves – energy that is manifested in the shaking of the ground.

The stronger a quake is, the more energy, or heat, it generates. Interestingly, the MIT group found that only 2 percent of a deeper quake’s total energy is felt at the surface. Prieto reasoned that much of the other 98 percent may be released locally as heat, creating an enormous temperature increase that pushes a quake to spread.

Prieto says the study provides strong evidence for thermal runaway as the likely mechanism for intermediate-depth earthquakes. Such knowledge, he says, may be useful for communities around Bucaramanga in predicting the severity of future quakes.

“Usually people in Bucaramanga feel a magnitude 4 quake every month or so, and every year they experience a larger one that can shake significantly,” Prieto says. “If you’re in a region where you have intermediate-depth quakes and you know the size of the region, you can make a prediction of the type of magnitudes of quakes that you can have, and what kind of shaking you would expect.”

Prieto, a native Colombian, plans to deploy seismic stations above the Bucaramanga Nest to better understand the activity of deeper quakes.