M 9.0+ possible for subduction zones along ‘Ring of Fire,’ suggests new study

The magnitude of the 2011 Tohoku quake (M 9.0) caught many seismologists by surprise, prompting some to revisit the question of calculating the maximum magnitude earthquake possible for a particular fault. New research offers an alternate view that uses the concept of probable maximum magnitude events over a given period, providing the magnitude and the recurrence rate of extreme events in subduction zones for that period. Most circum Pacific subduction zones can produce earthquakes of magnitude greater than 9.0, suggests the study.

The idea of identifying the maximum magnitude for a fault isn’t new, and its definition varies based on context. This study, published online by the Bulletin of the Seismological Society of America (BSSA), calculates the “probable maximum earthquake magnitude within a time period of interest,” estimating the probable magnitude of subduction zone earthquakes for various time periods, including 250, 500 and 10,000 years.

“Various professionals use the same terminology – maximum magnitude – to mean different things. The most interesting question for us was what was going to be the biggest magnitude earthquake over a given period of time?” said co-author Yufang Rong, a seismologist at the Center for Property Risk Solutions of FM Global, a commercial and industrial property insurer. “Can we know the exact, absolute maximum magnitude? The answer is no, however, we developed a simple methodology to estimate the probable largest magnitude within a specific time frame.”

The study’s results indicated most of the subduction zones can generate M 8.5 or greater over a 250-return period; M 8.8 or greater over 500 years; and M 9.0 or greater over 10,000 years.

“Just because a subduction zone hasn’t produced a magnitude 8.8 in 499 years, that doesn’t mean one will happen next year,” said Rong. “We are talking about probabilities.”

The instrumental and historical earthquake record is brief, complicating any attempt to confirm recurrence rates and estimate with confidence the maximum magnitude of an earthquake in a given period. The authors validated their methodology by comparing their findings to the seismic history of the Cascadia subduction zone, revealed through deposits of marine sediment along the Pacific Northwest coast. While some subduction zones have experienced large events during recent history, the Cascadia subduction zone has remained quiet. Turbidite and onshore paleoseismic studies have documented a rich seismic history, identifying 40 large events over the past 10,000 years.

“Magnitude limits of subduction zone earthquakes” is co-authored by Rong, David Jackson of UCLA, Harold Magistrale of FM Global, and Chris Goldfinger of Oregon State University. The paper will be published online Sept. 16 by BSSA as well as in its October print edition.

Study of Chilean quake shows potential for future earthquake

Near real-time analysis of the April 1 earthquake in Iquique, Chile, showed that the 8.2 event occurred in a gap on the fault unruptured since 1877 and that the April event was not what the scientists had expected, according to an international team of geologists.

“We assumed that the area of the 1877 earthquake would eventually rupture, but all indications are that this 8.2 event was not the 8.8 event we were looking for,” said Kevin P. Furlong, professor of geophysics, Penn State. “We looked at it to see if this was the big one.”

But according to the researchers, it was not. Seismologists expect that areas of faults will react the same way over and over. However, the April earthquake was about nine times less energetic than the one in 1877 and was incapable of releasing all the stress on the fault, leaving open the possibility of another earthquake.

The Iquique earthquake took place on the northern portion of the subduction zone formed when the Nazca tectonic plate slides under the South American plate. This is one of the longest uninterrupted plate boundaries on the planet and the site of many earthquakes and volcanos. The 8.2 earthquake was foreshadowed by a systematic sequence of foreshocks recorded at 6.0, 6.5, 6.7 and 6.2 with each foreshock triggering the next until the main earthquake occurred.

These earthquakes relieved the stresses on some parts of the fault. Then the 8.2 earthquake relieved more stress, followed by a series of aftershocks in the range of 7.7. While the aftershocks did fill in some of the gaps left by the 8.2 earthquake, the large earthquake and aftershocks could not fill in the entire gap where the fault had not ruptured in a very long time. That area is unruptured and still under stress.

The foreshocks eased some of the built up stress on 60 to 100 miles of fault, and the main shock released stress on about 155 miles, but about 155 miles of fault remain unchanged, the researchers report today (Aug. 13) in Nature.

“There can still be a big earthquake there,” said Furlong. “It didn’t release the total hazard, but it told us something about this large earthquake area. That an 8.8 rupture doesn’t always happen.”

The researchers were able to do this analysis in near real time because of the availability of large computing power and previously laid groundwork.

The computing power allowed researchers to model the fault more accurately. In the past, subduction zones were modeled as if they were on a plane, but the plate that is subducting curves underneath the other plate creating a 3-dimensional fault line. The researchers used a model that accounted for this curving and so more accurately recreated the stresses on the real geology at the fault.

“One of the things the U.S. Geological Survey and we have been doing is characterizing the major tectonic settings,” said Furlong. “So when an earthquake is imminent, we don’t need a lot of time for the background.”

In essence, they are creating a library of information about earthquake faults and have completed the first level, a general set of information on areas such as Japan, South America and the Caribbean. Now they are creating the levels of north and south Japan or Chile, Peru and Ecuador.

Knowing where the old earthquake occurred, how large it was and how long ago it happened, the researchers could look at the foreshocks, see how much stress they relieved and anticipate, at least in a small way, what would happen.

“This is what we need to do in the future in near real time for decision makers,” said Furl.

Studies show movements of continents speeding up after slow ‘middle age’

Two studies show that the movement rate of plates carrying the Earth’s crust may not be constant over time. This could provide a new explanation for the patterns observed in the speed of evolution and has implications for the interpretation of climate models. The work is presented today at Goldschmidt 2014, the premier geochemistry conference taking place in Sacramento, California, USA.

The Earth’s continental crust can be thought of as an archive of Earth’s history, containing information on rock formation, the atmosphere and the fossil record. However, it is not clear when and how regularly crust formed since the beginning of Earth history, 4.5 billion years ago.

Researchers led by Professor Peter Cawood, from the University of St. Andrews, UK, examined several measures of continental movement and geologic processes from a number of previous studies. They found that, from 1.7 to 0.75 billion years ago (termed Earth’s middle age), Earth appears to have been very stable in terms of its environment, with little in the way of crust building activity, no major fluctuations in atmospheric composition and few major developments seen in the fossil record. This contrasts markedly with the time periods either side of this, which contained major ice ages and changes in oxygen levels. Earth’s middle age also coincides with the formation of a supercontinent called Rodinia, which appears to have been stable throughout this time.

Professor Cawood suggests this stability may have been due to the gradual cooling of the earth’s crust over time. “Before 1.7 billion years ago, the Earth’s crust would have been substantially hotter, meaning that continental plate movement may have been governed by different rules to those that operate today,” said Professor Cawood. “0.75 billion years ago, the crust reached a point where it had cooled sufficiently to allow modern day plate tectonics to start working, in particular allowing subduction zones to form (where one plate of the crust moves under another). This increase in activity could have kick-started a myriad of changes including the break-up of Rodinia and changes to levels of key elements in the atmosphere and seas, which in turn may have induced evolutionary changes in the life forms present.”

This view is backed up by work from Professor Kent Condie from New Mexico Tech, USA, which suggests the movement rate of the Earth’s crust is not constant but may be speeding up over time. Professor Condie examined how supercontinents assemble and break up. “Our results challenge the view that the rate of plate movement is stable over time,” said Professor Condie. “The interpretation of data from many other disciplines such as stable isotope geochemistry, palaeontology and paleoclimatology in part rely on the assumption that the movement rate of the Earth’s crust is constant.”

Results from these fields may now need to be re-examined in light of Condie’s findings. “We now urgently need to collect further data on critical time periods to understand more about the constraints on plate speeds and the frequency of collision between continental blocks,” concluded Professor Condie.

Computer models solve geologic riddle millions of years in the making

An international team of scientists that included USC’s Meghan Miller used computer modeling to reveal, for the first time, how giant swirls form during the collision of tectonic plates – with subduction zones stuttering and recovering after continental fragments slam into them.

The team’s 3D models suggest a likely answer to a question that has long plagued geologists: why do long, curving mountain chains form along some subduction zones – where two tectonic plates collide, pushing one down into the mantle?

Based on the models, the researchers found that parts of the slab that is being subducted sweep around behind the collision, pushing continental material into the mountain belt.

With predictions confirmed by field observations, the 3D models show a characteristic pattern of intense localized heating, volcanic activity and fresh sediments that remained enigmatic until now.

“The new model explains why we see curved mountains near colliding plates, where material that has been scraped off of one plate and accreted on another is dragged into a curved path on the continent,” Miller said.

Miller collaborated with lead author Louis Moresi from Monash University and his colleagues Peter Betts (also from Monash) and R. A. Cayley from the Geological Survey of Victoria in Australia. Their research was published online by Nature on March 23.

Their research specifically looked at the ancient geologic record of Eastern Australia, but is also applicable to the Pacific Northwest of the United States, the Mediterranean, and southeast Asia. Coastal mountain ranges from Northern California up to Alaska were formed by the scraping off of fragment of the ancient Farallon plate as it subducted beneath the North American continent. The geology of the Western Cordillera (wide mountain belts that extend along all of North America) fits the predictions of the computer model.

“The amazing thing about this research is that we can now interpret arcuate-shaped geological structures on the continents in a whole new way,” Miller said. “We no longer need to envision complex motions and geometries to explain the origins of ancient or modern curved mountain belts.”

The new results from this research will help geologists interpret the formation of ancient mountain belts and may prove most useful as a template to interpret regions where preservation of evidence for past collisions is incomplete – a common, and often frustrating, challenge for geologists working in fragmented ancient terrains.

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.

Global map to predict giant earthquakes

A team of international researchers, led by Monash University’s Associate Professor Wouter Schellart, have developed a new global map of subduction zones, illustrating which ones are predicted to be capable of generating giant earthquakes and which ones are not.

The new research, published in the journal Physics of the Earth and Planetary Interiors, comes nine years after the giant earthquake and tsunami in Sumatra in December 2004, which devastated the region and many other areas surrounding the Indian Ocean, and killed more than 200,000 people.

Since then two other giant earthquakes have occurred at subduction zones, one in Chile in February 2010 and one in Japan in March 2011, which both caused massive destruction, killed many thousands of people and resulted in billions of dollars of damage.

Most earthquakes occur at the boundaries between tectonic plates that cover the Earth’s surface. The largest earthquakes on Earth only occur at subduction zones, plate boundaries where one plate sinks (subducts) below the other into the Earth’s interior. So far, seismologists have recorded giant earthquakes for only a limited number of subduction zone segments. But accurate seismological records go back to only ~1900, and the recurrence time of giant earthquakes can be many hundreds of years.

“The main question is, are all subduction segments capable of generating giant earthquakes, or only some of them? And if only a limited number of them, then how can we identify these,” Dr Schellart said.

Dr Schellart, of the School of Geosciences, and Professor Nick Rawlinson from the University of Aberdeen in Scotland used earthquake data going back to 1900 and data from subduction zones to map the main characteristics of all active subduction zones on Earth. They investigated if those subduction segments that have experienced a giant earthquake share commonalities in their physical, geometrical and geological properties.

They found that the main indicators include the style of deformation in the plate overlying the subduction zone, the level of stress at the subduction zone, the dip angle of the subduction zone, as well as the curvature of the subduction zone plate boundary and the rate at which it moves.

Through these findings Dr Schellart has identified several subduction zone regions capable of generating giant earthquakes, including the Lesser Antilles, Mexico-Central America, Greece, the Makran, Sunda, North Sulawesi and Hikurangi.

“For the Australian region subduction zones of particular significance are the Sunda subduction zone, running from the Andaman Islands along Sumatra and Java to Sumba, and the Hikurangi subduction segment offshore the east coast of the North Island of New Zealand. Our research predicts that these zones are capable of producing giant earthquakes,” Dr Schellart said.

“Our work also predicts that several other subduction segments that surround eastern Australia (New Britain, San Cristobal, New Hebrides, Tonga, Puysegur), are not capable of producing giant earthquakes.”

Going deep to study long-term climate evolution

A Rice University-based team of geoscientists is going to great lengths -- from Earth's core to its atmosphere -- to investigate the role that deep-Earth processes play in climate evolution over million-year timescales. -  Rice University
A Rice University-based team of geoscientists is going to great lengths — from Earth’s core to its atmosphere — to investigate the role that deep-Earth processes play in climate evolution over million-year timescales. – Rice University

A Rice University-based team of geoscientists is going to great lengths — from Earth’s core to its atmosphere — to get to the bottom of a long-standing mystery about the planet’s climate.

“We want to know what controls long-term climate change on Earth, the oscillations between greenhouse and icehouse cycles that can last as long as tens of million years,” said Cin-Ty Lee, professor of Earth science at Rice and the principal investigator (PI) on a new $4.3 million, five-year federal grant from the National Science Foundation’s Frontiers in Earth-System Dynamics (FESD) Program.

“There are long periods where Earth is relatively cool, like today, where you have ice caps on the North and South poles, and there are also long periods where there are no ice caps,” Lee said. “Earth’s climate has oscillated between these two patterns for at least half a billion years. We want to understand what controls these oscillations, and we have people at universities across the country who are going to attack this problem from many angles.”

For starters, Lee distinguished between the type of climate change that he and his co-investigators are studying and the anthropogenic climate change that often makes headlines.

“We’re working on much longer timescales than what’s involved in anthropogenic climate change,” Lee said. “We’re interested in explaining processes that cycle over tens of millions of years.”

Lee described the research team as “a patchwork of free spirits” that includes bikers, birdwatchers and skateboarders who are drawn together by a common interest in studying the whole Earth dynamics of carbon exchange. The group has specialists in oceanography, petrology, geodynamics, biogeochemistry and other fields, and it includes more than a dozen faculty and students from the U.S., Europe and Asia. Rice co-PIs include Rajdeep Dasgupta, Gerald Dickens and Adrian Lenardic.

The team will focus on how carbon moves between Earth’s external and internal systems. On the external side, carbon is known to cycle between oceans, atmosphere, biosphere and soils on timescales ranging from a few days to a few hundred thousand years. On million-year to billion-year timescales, carbon in these external reservoirs interacts with reservoirs inside Earth, ranging from crustal carbon stored in ancient sediments preserved on the continents to carbon deep in Earth’s mantle.

“Because of these differences in timescales, carbon cycling at Earth’s surface is typically modeled independently from deep-Earth cycling,” Lee said. “We need to bring the two together if we are to understand long-term greenhouse-icehouse cycling.”

From the fossil record, scientists know that atmospheric carbon dioxide plays a vital role in determining Earth’s surface temperatures. Many studies have focused on how carbon moves between the atmosphere, oceans and biosphere. Lee said the FESD team will examine how carbon is removed from the surface and cycled back into the deep Earth, and it will also examine how volcanic eruptions bring carbon from the deep Earth to the surface. In addition, the team will examine the role that volcanic activity and plate tectonics may play in periodically releasing enormous volumes of carbon dioxide into the atmosphere. One of several hypotheses that will be tested is whether Earth’s subduction zones may at times be dominated by continental arcs, and if so, whether the passage of magmas through ancient carbonates stored in the continental upper plate can amplify the volcanic flux of carbon.

“Long-term climate variability is intimately linked to whole-Earth carbon cycling,” Lee said. “Our task is to build up a clearer picture of how the inputs and outputs change through time.”

In addition to the Rice team, the project’s primary investigators include Jaime Barnes of the University of Texas at Austin, Jade Star Lackey of Pomona College, Michael Tice of Texas A&M University and Richard Zeebe of the University of Hawaii. Research affiliates include Steve Bergman of Shell, Mark Jellinek of the University of British Columbia, Tapio Schneider of the Swiss Federal Institute of Technology and Yusuke Yokoyama of the University of Tokyo.

Molten magma can survive in upper crust for hundreds of millennia

The formations in the Grand Canyon of the Yellowstone, in Yellowstone National Park, are an example of  silica-rich volcanic rock. -  Sarah Gelman/University of Washington
The formations in the Grand Canyon of the Yellowstone, in Yellowstone National Park, are an example of silica-rich volcanic rock. – Sarah Gelman/University of Washington

Reservoirs of silica-rich magma – the kind that causes the most explosive volcanic eruptions – can persist in Earth’s upper crust for hundreds of thousands of years without triggering an eruption, according to new University of Washington modeling research.

That means an area known to have experienced a massive volcanic eruption in the past, such as Yellowstone National Park, could have a large pool of magma festering beneath it and still not be close to going off as it did 600,000 years ago.

“You might expect to see a stewing magma chamber for a long period of time and it doesn’t necessarily mean an eruption is imminent,” said Sarah Gelman, a UW doctoral student in Earth and space sciences.

Recent research models have suggested that reservoirs of silica-rich magma, or molten rock, form on and survive for geologically short time scales – in the tens of thousands of years – in the Earth’s cold upper crust before they solidify. They also suggested that the magma had to be injected into the Earth’s crust at a high rate to reach a large enough volume and pressure to cause an eruption.

But Gelman and her collaborators took the models further, incorporating changes in the crystallization behavior of silica-rich magma in the upper crust and temperature-dependent heat conductivity. They found that the magma could accumulate more slowly and remain molten for a much longer period than the models previously suggested.

Gelman is the lead author of a paper explaining the research published in the July edition of Geology. Co-authors are Francisco Gutiérrez, a former UW doctoral student now with Universidad de Chile in Santiago, and Olivier Bachmann, a former UW faculty member now with the Swiss Federal Institute of Technology in Zurich.

There are two different kinds of magma and their relationship to one another is unclear. Plutonic magma freezes in the Earth’s crust and never erupts, but rather becomes a craggy granite formation like those commonly seen in Yosemite National Park. Volcanic magma is associated with eruptions, whether continuous “oozing” types of eruption such as Hawaii’s Kilauea Volcano or more explosive eruptions such as Mount Pinatubo in the Philippines or Mount St. Helens in Washington state.

Some scientists have suggested that plutonic formations are what remain in the crust after major eruptions eject volcanic material. Gelman believes it is possible that magma chambers in the Earth’s crust could consist of a core of partially molten material feeding volcanoes surrounded by more crystalline regions that ultimately turn into plutonic rock. It is also possible the two rock types develop independently, but those questions remain to be answered, she said.

The new work suggests that molten magma reservoirs in the crust can persist for far longer than some scientists believe. Silica content is a way of judging how the magma has been affected by being in the crust, Gelman said. As the magma is forced up a column from lower in the Earth to the crust, it begins to crystallize. Crystals start to drop out as the magma moves higher, leaving the remaining molten rock with higher silica content.

“These time scales are in the hundreds of thousands, even up to a million, years and these chambers can sit there for that long,” she said.

Even if the molten magma begins to solidify before it erupts, that is a long process, she added. As the magma cools, more crystals form giving the rock a kind of mushy consistency. It is still molten and capable of erupting, but it will behave differently than magma that is much hotter and has fewer crystals.

The implications are significant for volcanic “arcs,” found near subduction zones where one of Earth’s tectonic plates is diving beneath another. Arcs are found in various parts of the world, including the Andes Mountains of South America and the Cascades Range of the Pacific Northwest.

Scientists have developed techniques to detect magma pools beneath these arcs, but they cannot determine how long the reservoirs have been there. Because volcanic magma becomes more silica-rich with time, its explosive potential increases.

“If you see melt in an area, it’s important to know how long that melt has been around to determine whether there is eruptive potential or not,” Gelman said. “If you image it today, does that mean it could not have been there 300,000 years ago? Previous models have said it couldn’t have been. Our model says it could. That doesn’t mean it was there, but it could have been there.”

Distant quakes trigger tremors at US waste-injection sites, says study

Large earthquakes from distant parts of the globe are setting off tremors around waste-fluid injection wells in the central United States, says a new study. Furthermore, such triggering of minor quakes by distant events could be precursors to larger events at sites where pressure from waste injection has pushed faults close to failure, say researchers.

Among the sites covered: a set of injection wells near Prague, Okla., where the study says a huge earthquake in Chile on Feb. 27, 2010 triggered a mid-size quake less than a day later, followed by months of smaller tremors. This culminated in probably the largest quake yet associated with waste injection, a magnitude 5.7 event which shook Prague on Nov. 6, 2011. Earthquakes off Japan in 2011, and Sumatra in 2012, similarly set off mid-size tremors around injection wells in western Texas and southern Colorado, says the study. The paper appears this week in the leading journal Science, along with a series of other articles on how humans may be influencing earthquakes.

“The fluids are driving the faults to their tipping point,” said lead author Nicholas van der Elst, a postdoctoral researcher at Columba University’s Lamont-Doherty Earth Observatory. “The remote triggering by big earthquakes is an indication the area is critically stressed.”

Tremors triggered by distant large earthquakes have been identified before, especially in places like Yellowstone National Park and some volcanically active subduction zones offshore, where subsurface water superheated by magma can weaken faults, making them highly vulnerable to seismic waves passing by from somewhere else. The study in Science adds a new twist by linking this natural phenomenon to faults that have been weakened by human activity.

A surge in U.S. energy production in the last decade or so has sparked what appears to be a rise in small to mid-sized earthquakes in the United States. Large amounts of water are used both to crack open rocks to release natural gas through hydrofracking, and to coax oil and gas from underground wells using conventional techniques. After the gas and oil have been extracted, the brine and chemical-laced water must be disposed of, and is often pumped back underground elsewhere, sometimes causing earthquakes.

From a catalog of past earthquake recordings, van der Elst and his colleagues found that faults near wastewater-injection sites in and around Prague, Snyder, Tex., and Trinidad, Colo., were approaching a critical state when big earthquakes far away triggered a rise in local earthquakes. Injection at the three sites had been ongoing for years, and the researchers hypothesize that passing surface waves from the big events caused small pressure changes on faults, triggering smaller earthquakes.

“These passing seismic waves are like a stress test,” said study coauthor Heather Savage, a geophysicist at Lamont-Doherty. “If the number of small earthquakes increases, it could indicate that faults are becoming critically stressed and might soon host a larger earthquake.”

The 2010 magnitude 8.8 Chile quake, which killed more than 500 people, sent surface waves rippling across the planet, triggering a magnitude 4.1 quake near Prague 16 hours later, the study says. The activity near Prague continued until the magnitude 5.7 quake on Nov. 6, 2011 that destroyed 14 homes and injured two people. A study earlier this year led by seismologist Katie Keranen, also a coauthor of the new study, now at Cornell University, found that the first rupture occurred less than 650 feet away from active injection wells. In April 2012, a magnitude 8.6 earthquake off Sumatra triggered another swarm of earthquakes in the same place. The pumping of fluid into the field continues to this day, along with a pattern of small quakes.

The 2010 Chile quake also set off a swarm of earthquakes on the Colorado-New Mexico border, in Trinidad, near wells where wastewater used to extract methane from coal beds had been injected, the study says. The swarm was followed more than a year later, on Aug. 22 2011, by a magnitude 5.3 quake that damaged dozens of buildings. A steady series of earthquakes had already struck Trinidad in the past, including a magnitude 4.6 quake in 2001 that the U.S. Geological Survey (USGS) has investigated for links to wastewater injection.

The new study found also that Japan’s devastating magnitude 9.0 earthquake on March 11, 2011 triggered a swarm of earthquakes in the west Texas town of Snyder, where injection of fluid to extract oil from the nearby Cogdell fields has been setting off earthquakes for years, according to a 1989 study in the Bulletin of the Seismological Society of America. About six months after the Japan quake, a magnitude 4.5 quake struck Snyder.

The idea that seismic activity can be triggered by separate earthquakes taking place faraway was once controversial. One of the first cases to be documented was the magnitude 7.3 earthquake that shook California’s Mojave Desert in 1992, near the town of Landers, setting off a series of distant events in regions with active hot springs, geysers and volcanic vents. The largest was a magnitude 5.6 quake beneath Little Skull Mountain in southern Nevada, 150 miles away; the farthest, a series of tiny earthquakes north of Yellowstone caldera, according to a 1993 study in Science led by USGS geophysicist David Hill.

In 2002, the magnitude 7.9 Denali earthquake in Alaska triggered a series of earthquakes at Yellowstone, nearly 2,000 miles away, throwing off the schedules of some of its most predictable geysers, according to a 2004 study in Geology led by Stephan Husen, a seismologist at the Swiss Federal Institute of Technology in Zürich. The Denali quake also triggered bursts of slow tremors in and around California’s San Andreas, San Jacinto and Calaveras faults, according to a 2008 study in Science led by USGS geophysicist Joan Gomberg.

“We’ve known for at least 20 years that shaking from large, distant earthquakes can trigger seismicity in places with naturally high fluid pressure, like hydrothermal fields,” said study coauthor Geoffrey Abers, a seismologist at Lamont-Doherty. “We’re now seeing earthquakes in places where humans are raising pore pressure.”

The new study may be the first to find evidence of triggered earthquakes on faults critically stressed by waste injection. If it can be replicated and extended to other sites at risk of manmade earthquakes it could “help us understand where the stresses are,” said William Ellsworth, an expert on human-induced earthquakes with the USGS who was not involved in the study.

In the same issue of Science, Ellsworth reviews the recent upswing in earthquakes in the central United States. The region averaged 21 small to mid-sized earthquakes each year from the late 1960s through 2000. But in 2001, that number began to climb, reaching a high of 188 earthquakes in 2011, he writes. The risk of setting off earthquakes by injecting fluid underground has been known since at least the 1960s, when injection at the Rocky Mountain Arsenal near Denver was suspended after a magnitude 4.8 quake or greater struck nearby-the largest tied to wastewater disposal until the one near Prague, Okla. In a report last year, the National Academy of Sciences called for further research to “understand, limit and respond [to]” seismic events induced by human activity.

New ‘embryonic’ subduction zone found

A new subduction zone forming off the coast of Portugal heralds the beginning of a cycle that will see the Atlantic Ocean close as continental Europe moves closer to America.

Published in Geology, new research led by Monash University geologists has detected the first evidence that a passive margin in the Atlantic ocean is becoming active. Subduction zones, such as the one beginning near Iberia, are areas where one of the tectonic plates that cover the Earth’s surface dives beneath another plate into the mantle – the layer just below the crust.

Lead author Dr João Duarte, from the School of Geosciences said the team mapped the ocean floor and found it was beginning to fracture, indicating tectonic activity around the apparently passive South West Iberia plate margin.

“What we have detected is the very beginnings of an active margin – it’s like an embryonic subduction zone,” Dr Duarte said.

“Significant earthquake activity, including the 1755 quake which devastated Lisbon, indicated that there might be convergent tectonic movement in the area. For the first time, we have been able to provide not only evidences that this is indeed the case, but also a consistent driving mechanism.”

The incipient subduction in the Iberian zone could signal the start of a new phase of the Wilson Cycle – where plate movements break up supercontinents, like Pangaea, and open oceans, stabilise and then form new subduction zones which close the oceans and bring the scattered continents back together.

This break-up and reformation of supercontinents has happened at least three times, over more than four billion years, on Earth. The Iberian subduction will gradually pull Iberia towards the United States over approximately 220 million years.

The findings provide a unique opportunity to observe a passive margin becoming active – a process that will take around 20 million years. Even at this early phase the site will yield data that is crucial to refining the geodynamic models.

“Understanding these processes will certainly provide new insights on how subduction zones may have initiated in the past and how oceans start to close,” Dr Duarte said.