From U Chicago: “Study suggests tectonic plates began moving half a billion years earlier than thought”

University of Chicago

September 21, 2017
Louise Lerner

While previous studies had argued that Earth’s crust 3.5 billion years ago looked like these Hawaiian lavas, a new study led by UChicago scientists suggests by then much of it had already been transformed into lighter-colored felsic rock by plate tectonics.
Photo by Basil Greber

The Earth’s history is written in its elements, but as the tectonic plates slip and slide over and under each other over time, they muddy that evidence—and with it the secrets of why Earth can sustain life.

A new study led by UChicago geochemists rearranges the picture of the early Earth by tracing the path of metallic element titanium through the Earth’s crust across time. The research, published Sept. 22 in Science, suggests significant tectonic action was already taking place 3.5 billion years ago—about half a billion years earlier than currently thought.

The crust was once made of uniformly dark, magnesium- and iron-rich mafic minerals. But today the crust looks very different between land and ocean: The crust on land is now a lighter-colored felsic, rich in silicon and aluminum. The point at which these two diverged is important, since the composition of minerals affects the flow of nutrients available to the fledgling life struggling to survive on Earth.

“This question has been discussed since geologists first started thinking about rocks,” said lead author Nicolas Dauphas, the Louis Block Professor and head of the Origins Laboratory in the Department of the Geophysical Sciences and the Enrico Fermi Institute. “This result is a surprise and certainly an upheaval in that discussion.”

To reconstruct the crust changing over time, geologists often look at a particular kind of rock called shales, made up of tiny bits of other rocks and minerals that are carried by water into mud deposits and compressed into rock. The only problem is that scientists have to adjust the numbers to account for different rates of weathering and transport. “There are many things that can foul you up,” Dauphas said.

To avoid this issue, Dauphas and his team looked at titanium in the shales over time. This element doesn’t dissolve in water and isn’t taken up by plants in nutrient cycles, so they thought the data would have fewer biases with which to contend.

They crushed samples of shale rocks of different ages from around the world and checked in what form its titanium appeared. The proportions of titanium isotopes present should shift as the rock changes from mafic to felsic. Instead, they saw little change over three and a half billion years, suggesting that the transition must have occurred before then.

These granite peaks are an example of felsic rock, created via plate tectonics. Photo by Basil Greber

This also would mark the beginning of plate tectonics, since that process is believed to be needed to create felsic rock.

“With a null response like that, seeing no change, it’s difficult to imagine an alternate explanation,” said Matouš Ptáček, a UChicago graduate student who co-authored the study.

“Our results can also be used to track the average composition of the continental crust through time, allowing us to investigate the supply of nutrients to the oceans going back 3.5 billion years ago,” said Nicolas Greber, the first author of the paper, then a postdoctoral researcher at UChicago and now with the University of Geneva.

Phosphorous leads to life

The question about nutrients is important for our understanding of the circumstances around a mysterious but crucial turning point called the great oxygenation event. This is when oxygen started to emerge as an important constituent of Earth’s atmosphere, wreaking a massive change on the planet—and making it possible for multi-celled beings to evolve.

The flood of oxygen came from a surge of photosynthetic microorganisms; and in turn their work was fostered by a surge of nutrients to the oceans, particularly phosphorus. “Phosphorus is the most important limiting nutrient in the modern ocean. If you fertilize the ocean with phosphorus, life will bloom,” Dauphas said.

The titanium timeline suggests that the primary trigger of the surge of phosphorus was the change in the makeup of mafic rock over time. As the Earth cooled, the mafic rock coming out of volcanoes and underground melts became richer in phosphorus.

“We’ve known for a long time that mafic rock changed over time, but what we didn’t know was that their contribution to the crust has stayed rather consistent,” Ptáček said.

Other institutions on the study were the University of California-Riverside, University of Oregon-Eugene and the University of Johannesburg.

See the full article here .

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One of the world’s premier academic and research institutions, the University of Chicago has driven new ways of thinking since our 1890 founding. Today, UChicago is an intellectual destination that draws inspired scholars to our Hyde Park and international campuses, keeping UChicago at the nexus of ideas that challenge and change the world.

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From Eos: “Revising an Innovative Way to Study Cascadia Megaquakes”

AGU

Eos

9.12.17
Sarah Witman

Researchers probe natural environments near subduction zones to decrypt underlying mechanisms of major earthquakes.

FEMA

A diagram of the Cascadia Subduction Zone provided by the Oregon Historical Society.

The Cascadia subduction zone is likely to experience a megathrust earthquake in the next 50 years or so, but a revised technique uses heat data to better understand the physical nature of subduction zones. Credit: NASA/ISS

Along the west coast of North America, the Cascadia subduction zone stretches more than 1,000 kilometers from Vancouver Island to Cape Mendocino, Calif. It produced a magnitude 9 megathrust earthquake about 300 years ago, one of the biggest quakes in world history.

Scientists know that Cascadia will produce another earthquake at some point in the future; the question is how soon. The odds of it happening in the next 50 years are 1 in 3. The Federal Emergency Management Agency projects that Cascadia’s next megathrust earthquake will cause thousands of deaths and injuries and leave millions in need of shelter, food, and water.

To better understand subduction zones, scientists often study the thermal environments of material that has been pushed up onto the surface during past earthquakes. This buildup of material, called an accretionary wedge, might consist of rock, soil, sand, shells, or any other kind of debris. These wedges also sport subtly different average temperatures at various depths, compared to material located off the wedge.

In a recent study, Salmi et al. [Journal of Geophysical Research] examined the thermal environment of the Cascadia subduction zone’s accretionary wedge, which stretches for about 97 kilometers along the coast of the state of Washington. Their goal was to find out more about the physical changes of fluids and solids within the wedge in the hopes that the knowledge can help them better anticipate future earthquakes.

Using data collected on a cruise by the R/V Marcus G. Langseth, the researchers found significant variations in temperature within this section of the Cascadia subduction zone, as well as signs of gas hydrates (ice-like deposits that form from natural gas at the bottom of the ocean) throughout the region. They also detected that most fluids from the deep move upward through the accretionary wedge instead of through the crust, which is different than in most other subduction zones. This change in fluid pathway prevents the plate from cooling and reduces the area where an earthquake might rupture along the two plates: completely within the accretionary wedge, rather than under the continental plate.

This is the first study to concentrate on the southern Washington margin alone, rather than the subduction zone as a whole, revealing the influence of fluid distribution on local, small-scale temperature variability. This insight opens the door to further research into how local temperature variability might interact with other factors, like stress or fault roughness, to affect earthquake hazards. Overall, this study provides a revised method for probing the thermal environment of an accretionary wedge, a crucial link to the cause of ruptures in Earth’s crust that can lead to earthquakes and tsunamis.

By understanding these mechanisms more fully, scientists can tell us more about how to prepare for the smallest of tremors and the largest of megaquakes. (Journal of Geophysical Research: Solid Earth, https://doi.org/10.1002/2016JB013839, 2017)

See the full article here .

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Eos is the leading source for trustworthy news and perspectives about the Earth and space sciences and their impact. Its namesake is Eos, the Greek goddess of the dawn, who represents the light shed on understanding our planet and its environment in space by the Earth and space sciences.

From COSMOS: “Zircons: How tiny crystals open a window into the early history of Earth”

COSMOS Magazine

11 September 2017
Richard A. Lovett

These microscopic zircons collected from Mount Narryer in Western Australia have been dated at more than 4.1 billion years old. Auscape / Getty

Zirconium is the eighteenth most common element in the Earth’s crust – more common than such well-known substances as zinc, copper, nickel, and chromium. But most people have never heard of it, unless in the form of imitation diamonds known as cubic zirconia.

In nature, zirconium forms another type of crystal called zircons. To geophysicists, these are the true gems, because they provide vital time capsules from the Earth’s deepest past.

Chemically, zircons are nothing fancy. They are tiny lumps of zirconium silicate (ZrSiO4) that are ubiquitous in volcanic rocks. But they’re typically only 0.1 mm to 0.3 mm across, making them hard to spot without a magnifying glass. Not exactly the type of thing most of us would notice, let alone care about.

But they have two important traits.

One is that they are incredibly durable. The rocks in which they initially formed may weather away, but the zircons survive as tiny grains of sand that may later be incorporated into the next generation of rocks.

“We have no rocks that are older than 4 billion years,” says John Valley, a geochemist at the University of Wisconsin, Madison. (The Earth itself is 4.543 billion years old.) “[Zircons] are what we study if we want to analyze things that formed that far back.”

Their other trait is that they aren’t pure zirconium silicate. They contain trace amounts of other elements, most importantly uranium, trapped within them as they crystalize. Over the eons, that uranium slowly decays to lead. By comparing the amounts of uranium and lead, scientists can determine the date at which the crystal formed.

Another element, oxygen (the “O” in ZrSiO4), helps tell the conditions under which each zircon formed. That’s because oxygen has two well-known stable isotopes, 16O and 18O, either of which can be incorporated into the crystal as it grows.

Typically, these come from water (H2O), which can contain either 16O or 18O (or a more rare stable isotope called 17O). All these forms of water are chemically identical, but 18O-containing water is about ten percent heavier than 16O-containing water. That causes the two types of water to (very slightly) separate — and to do so by different amounts under different conditions.

Geologists once thought the early Earth was far too hot for its surface to be anything but an ocean of magma, let alone to have liquid water. In fact, the earliest period in the Earth’s history, from its formation to 4 billion years ago, is called the Hadean because it was widely believed to resemble hell, or Hades.That meant zircons from the Hadean period should have oxygen isotope ratios comparable to that of water molecules in the Earth’s mantle. But geologists studying the Jack Hills region of Western Australia, which has yielded the oldest zircons ever found, have been unearthing zircons from as far back as 4.375 billion years ago whose oxygen isotope ratios show they may have formed from magma that incorporated liquid water.

Other zircon research has suggested that life too may date back a lot further than we once thought. This research involves the ratio of non-radioactive carbon isotopes (12C and 13C) in tiny diamonds incorporated in the zircon structure. These diamonds have carbon isotope signatures suggesting the carbon from which they were formed may have included organic material from living organisms.

“This implies that there was life in the Hadean,” says Craig O’Neill, a geodynamicist at Macquarie University. Though, he notes, there are other explanations involving purely geologic processes. “It’s hard to be sure,” he says.

Still more studies have used hyper-sensitive magnets to look for trace magnetic fields carried by magnetic impurities in ancient zircons, in the hope of determining the strength of the Earth’s magnetic field at the time these zircons formed. “The analysis takes about a week,” says John Tarduno, a geophysicist at the University of Rochester, New York. Such studies, he says, indicate that the Earth’s magnetic field might be as old as its zircons.

And that’s just the beginning. In a 2017 study in Science Advances, geophysicists used zircons in Moon rocks brought back by Apollo astronauts to determine that the Moon’s crust solidified 4.51 billion years ago, only 60 million years after the formation of the first protoplanets. And zircons in meteorites blasted off the surface of Mars are being studied to peer nearly as far back into the Red Planet’s early history.

So who cares if copper, zinc, nickel, and chromium are vastly more valuable to the modern economy? Lowly zirconium may be what helps us unravel the greatest of all mysteries: who we are and where we came from.

See the full article here .

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From Curtin: “New research ‘rocks’ long-held geological theory”

Curtin University

28 February 2017 [Just appeared in social media, better late than never]

Tamara Hunter
Media Consultant (Monday to Wednesday)
Supporting Humanities and Science & Engineering
Tel: +61 8 9266 3353
Mob: +61 401 103 683
tamara.hunter@curtin.edu.au

Hailey Ross
Media Relations Manager, Public Relations
Tel: +61 8 9266 3357
Mob: +61 478 310 708
hailey.ross@curtin.edu.au

New research into ancient rocks in Western Australia contradicts the commonly held belief that Earth’s first stable continents were formed in a plate tectonic setting.

The Curtin University-led paper, Earth’s first stable continents did not form by subduction, was published today in Nature.

Dr Tim Johnson, from The Institute for Geoscience Research (TIGeR) and the Department of Applied Geology at the Curtin WA School of Mines, explained that the geodynamic environment in which Earth’s first stable continents formed remained controversial.

“Uniformitarianism is the precept in geology that the processes we can observe happening today are those that have operated throughout Earth’s history,” Dr Johnson said.

“Many geologists have subscribed to the uniformitarian view that the first continental crust formed in subduction zones via modern-style plate tectonics.

The tectonic plates of the world were mapped in 1996, USGS.

“Some, however, believe that alternative (non-uniformitarian) processes were involved – our research supports the latter.”

Dr Johnson said examples of truly ancient continental crust – rocks formed around 3.5-4 billion years ago, in the early stages of the Archaean Eon – could still be found today, in places like Australia, South Africa, India, North America and Europe.

“Most exposed areas of Archaean crust include a specific type of granitic rocks known as TTGs,” [Tonalite-Trondhjemite-Granodiorite] Dr Johnson said.

“These rocks were formed by partial melting of a low magnesium basaltic source, and have a trace element signature that resemble crust produced in modern subduction settings.

“My previous research came to a similar conclusion, that plate tectonic processes are not required for the formation of the earliest continents, and that other mechanisms are plausible. We wanted to explore the hypothesis further, leading to our current findings.”

Samples were selected from the low magnesium basalts of the Coucal Formation at the base of the Pilbara Supergroup in the East Pilbara Terrane, Western Australia. These rocks, amongst the oldest basaltic lavas on Earth, have previously been shown to contain a trace element composition consistent with source rocks for TTGs .

“Through phase equilibria modelling of the Coucal basalts, we confirmed their suitability as TTG parents, suggesting they were produced by melting in a high geothermal gradient environment. By contrast, many researchers maintain that TTGs were formed in subduction zones, which are characterised by very low geothermal gradients,” Dr Johnson said.

“Additionally, the trace element signature of the Coucal basalts indicates that they were derived from an earlier generation of mafic rocks, suggesting this signature was inherited from an ancestral lineage.

“This leads us to believe that a protracted, multistage process, in combination with high geothermal gradients, was required for the production and stabilisation of the first continents. These results are not consistent with formation of TTGs in subduction zones, but rather favour their production near the base of thick basaltic plateaux in the early Archaean.”

See the full article here .

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Curtin is ranked in the top one per cent of universities worldwide in the prestigious Academic Ranking of World Universities 2017.

We are WA’s most preferred university and are globally recognised for our strong connections with industry, high-impact research and wide range of innovative courses. We are also WA’s largest and most multicultural university, welcoming more than 52,000 students, around a third of whom come from a country other than Australia.

From Eos: “Quakes Pack More Punch in Eastern Than in Central United States”

AGU

Eos

8.18.17
Kimberly M. S. Cartier

A new finding rests on the recognition that fault types differ between the two regions. It helps explain prior evidence that human-induced quakes and natural ones behave the same in the nation’s center.

A broken angel statue lies among other damage on the roof of the Washington National Cathedral, Washington, D. C., after a magnitude 5.8 earthquake that impacted the eastern United States and Canada on 23 August 2011. Credit: AP Photo/J. Scott Applewhite

Earthquakes in the eastern United States and Canada are many times more severe than central U.S. earthquakes of human or natural origin, earthquake scientists have found, highlighting a crucial need to separate the two regions when designing future earthquake hazard maps. The study separated the regions from the Mississippi-Alabama border up to the base of Lake Michigan, approximately 87°W.

“People have never really compared these two regions very carefully,” said Yihe Huang, assistant professor of Earth and environmental sciences at the University of Michigan, Ann Arbor, and lead author of a study published in Science Advances on 2 August.

Because earthquakes have occurred rarely in the central and eastern United States until recently, seismologists have not studied those areas as closely as they have more high-risk ones like the U.S. West Coast. “They are always taken as one region in the hazard models, but…if you look closely, they actually [are] very different,” she said. “We didn’t really think about this before.”

Huang’s research shows that there is a fundamental and important difference in the stress released, and therefore in the hazard level, of central U.S. quakes compared with those in the eastern United States and Canada, said Gail Atkinson, professor of Earth sciences and Industrial Research Chair in Hazards from Induced Seismicity at Western University in London, Ontario, Canada.

Different Triggers, Same Shaking

Huang and her coauthors began their investigation questioning whether seismologists can use existing earthquake hazard models—developed using data from naturally occurring tectonic earthquakes—to accurately predict the severity of quakes induced by human activity.

They expected the trigger mechanism to be a major source of uncertainty in hazard prediction models, but they found instead that the biggest difference was geography. Earthquakes they analyzed from the eastern United States and Canada along the Appalachians released 5–6 times more energy than their central counterparts. Consequently, Huang argued that “we should treat the central and eastern U.S. tectonic earthquakes differently in our hazard prediction.”

Their study confirmed that earthquakes in the central United States released similar amounts of energy and shook the ground the same way whether they were induced or natural. So seismologists can use the same models to study them all, report Huang and her colleagues.

“Within the central U.S., all of the earthquakes appear to be the same, and we’re really comparing apples and apples,” said William Ellsworth, professor of geophysics at Stanford University in Stanford, Calif., and a coauthor on the paper.

“We don’t need to discriminate why the earthquake occurred to describe its shaking,” he said.

Different Types of Stress Relief

Why do the two regions produce earthquakes of such different severity? The reason, the researchers explained, is that the central and eastern regions release underground stress using different mechanisms. The way that ground layers shift and slide against each other to dissipate energy determines the violence of the stress release and strength of high-frequency motion aboveground, the shaking most relevant for engineering safety and seismic hazard assessment.

Huang explained that in the central United States, seven of nine earthquakes they examined happened when chunks of Earth’s crust slid horizontally against each other along strike-slip faults. All eight of the eastern earthquakes they analyzed occurred at reverse faults, where the ground shifts vertically against the pull of gravity. Separating by region, Huang said, equates to separating by fault type.

A comparison of earthquake magnitudes in eastern and central regions underscores the greater power of eastern temblors, according to Huang. The team’s list of natural events, reaching back more than 15 years, contains only one earthquake stronger than magnitude 5 in the central United States but three from the eastern United States. The strongest, an M5.8 quake in Mineral, Va., on 23 August 2011, caused significant property damage but only minor injuries.

Ellsworth explained that industrial processes in the central and eastern United States, like the disposal of wastewater from oil production and hydraulic fracturing, may simply be speeding up the normal geologic processes nearby by releasing underground pressure that builds up naturally. “We might be speeding up the processes by hundreds of thousands of years,” he said.

The researchers noted in their paper that wastewater injection is likely acting as a trigger for stress release but that subsequent shaking follows natural tectonic physics. Because the shaking is similar, Huang said, existing ground motion prediction equations can actually be used to predict the severity of induced earthquakes as long as they first account for the fault type at work.

Improving Hazard Predictions Nationwide

Now that this new work has revealed a significant difference in the types of earthquake-producing faults prevalent in the central and eastern regions, Huang said that she wants to conduct a broader investigation into seismic events nationwide to see if there are other overlooked patterns related to earthquake strength.

In the meantime, the new recognition of an eastern versus central difference in typical fault type should help improve future hazard prediction maps and guide the construction of earthquake-safe structures, Ellsworth said.

“The more accurate we can make that forecast,” he said, “the more it actually reduces the cost of ensuring seismic safety.”

See the full article here .

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Eos is the leading source for trustworthy news and perspectives about the Earth and space sciences and their impact. Its namesake is Eos, the Greek goddess of the dawn, who represents the light shed on understanding our planet and its environment in space by the Earth and space sciences.

From Rice: “Hot spot at Hawaii? Not so fast”

Rice University

August 18, 2017
Mike Williams

Rice University scientists’ model shows global mantle plumes don’t move as quickly as thought

Through analysis of volcanic tracks, Rice University geophysicists have concluded that hot spots like those that formed the Hawaiian Islands aren’t moving as fast as recently thought.

Hot spots are areas where magma pushes up from deep Earth to form volcanoes. New results from geophysicist Richard Gordon and his team confirm that groups of hot spots around the globe can be used to determine how fast tectonic plates move.

Rice University geophysicists have developed a method that uses the average motion of hot-spot groups by plate to determine that the spots aren’t moving as fast as geologists thought. For example, the Juan Fernandez Chain (outlined by the white rectangle) on the Nazca Plate west of Chile was formed by a hot spot now at the western end of the chain as the Nazca moved east-northeast relative to the hotspot forming the chain that includes Alejandro Selkirk and Robinson Crusoe islands. The white arrow shows the direction of motion of the Nazca Plate relative to the hot spot, and it is nearly indistinguishable from the direction predicted from global plate motions relative to all the hot spots on the planet (green arrow). The similarity in direction indicates that very little motion of the Juan Fernandez hot spot relative to other hot spots is needed to explain its trend. Illustration by Chengzu Wang.

Gordon, lead author Chengzu Wang and co-author Tuo Zhang developed a method to analyze the relative motion of 56 hot spots grouped by tectonic plates. They concluded that the hot-spot groups move slowly enough to be used as a global reference frame for how plates move relative to the deep mantle. This confirmed the method is useful for viewing not only current plate motion but also plate motion in the geologic past.

The study appears in Geophysical Research Letters.

Hot spots offer a window into the depths of Earth, as they mark the tops of mantle plumes that carry hot, buoyant rock from deep Earth to near the surface and produce volcanoes. These mantle plumes were once thought to be straight and stationary, but recent results suggested they can also shift laterally in the convective mantle over geological time.

The primary evidence of plate movement relative to the deep mantle comes from volcanic activity that forms mountains on land, islands in the ocean or seamounts, mountain-like features on the ocean floor. A volcano forms on a tectonic plate above a mantle plume. As the plate moves, the plume gives birth to a series of volcanoes. One such series is the Hawaiian Islands and the Emperor Seamount Chain; the youngest volcanoes become islands while the older ones submerge. The series stretches for thousands of miles and was formed as the Pacific Plate moved over a mantle plume for 80 million years.

The Rice researchers compared the observed hot-spot tracks with their calculated global hot-spot trends and determined the motions of hot spots that would account for the differences they saw. Their method demonstrated that most hot-spot groups appear to be fixed and the remainder appear to move slower than expected.

“Averaging the motions of hot-spot groups for individual plates avoids misfits in data due to noise,” Gordon said. “The results allowed us to say that these hot-spot groups, relative to other hot-spot groups, are moving at about 4 millimeters or less a year.

“We used a method of analysis that’s new for hot-spot tracks,” he said. “Fortunately, we now have a data set of hot-spot tracks that is large enough for us to apply it.”

For seven of the 10 plates they analyzed with the new method, average hot-spot motion measured was essentially zero, which countered findings from other studies that spots move as much as 33 millimeters a year. Top speed for the remaining hot-spot groups — those beneath the Eurasia, Nubia and North America plates — was between 4 and 6 millimeters a year but could be as small as 1 millimeter per year. That’s much slower than most plates move relative to the hot spots. For example, the Pacific Plate moves relative to the hot spots at about 100 millimeters per year.

Gordon said those interested in paleogeography should be able to make use of the model. “If hot spots don’t move much, they can use them to study prehistorical geography. People who are interested in circum-Pacific tectonics, like how western North America was assembled, need to know that history of plate motion.

“Others who will be interested are geodynamicists,” he said. “The motions of hot spots reflect the behavior of mantle. If the hot spots move slowly, it may indicate that the viscosity of mantle is higher than models that predict fast movement.”

“Modelers, especially those who study mantle convection, need to have something on the surface of Earth to constrain their models, or to check if their models are correct,” Wang said. “Then they can use their models to predict something. Hot-spot motion is one of the things that can be used to test their models.”

Gordon is the W.M. Keck Professor of Earth Science. Wang and Zhang are Rice graduate students. The National Science Foundation supported the research.

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In his 1912 inaugural address, Rice University president Edgar Odell Lovett set forth an ambitious vision for a great research university in Houston, Texas; one dedicated to excellence across the range of human endeavor. With this bold beginning in mind, and with Rice’s centennial approaching, it is time to ask again what we aspire to in a dynamic and shrinking world in which education and the production of knowledge will play an even greater role. What shall our vision be for Rice as we prepare for its second century, and how ought we to advance over the next decade?

This was the fundamental question posed in the Call to Conversation, a document released to the Rice community in summer 2005. The Call to Conversation asked us to reexamine many aspects of our enterprise, from our fundamental mission and aspirations to the manner in which we define and achieve excellence. It identified the pressures of a constantly changing and increasingly competitive landscape; it asked us to assess honestly Rice’s comparative strengths and weaknesses; and it called on us to define strategic priorities for the future, an effort that will be a focus of the next phase of this process.

From Eos: “A Closer Look at an Undersea Source of Alaskan Earthquakes”

AGU

Eos

15 August 2017
Daniel S. Brothers
Peter Haeussler
Amy East
Uri ten Brink
Brian Andrews
Peter Dartnell
Nathan Miller
Jared Kluesner

All is calm in southern Alaska’s Lisianski Inlet in this 2015 view from the deck of the R/V Solstice. A systematic survey of the nearby Queen Charlotte–Fairweather Fault, the source of several major earthquakes, has produced valuable information on the fault’s structure and slip mechanisms. Credit: Daniel S. Brothers

During the past century, movement along the Queen Charlotte–Fairweather fault, which lies for most of its length beneath the waters off southeastern Alaska and British Columbia, has generated at least seven earthquakes of magnitude 7 or greater. This includes a magnitude 8.1 earthquake in 1949, the largest ever recorded in Canada.

Other events include a magnitude 7.8 earthquake in 1958 that dislodged a massive landslide above Lituya Bay, Alaska. The earthquake generated a tsunami that sent water 525 meters up the mountainside, a world record run-up [Miller, 1960]. The 2012 magnitude 7.8 Haida Gwaii earthquake, centered on Moresby Island, British Columbia, and the 2013 magnitude 7.5 earthquake near Craig, Alaska [Walton et al., 2015], increased awareness of the potential geologic hazards posed to residents of southeastern Alaska and western British Columbia.

Together, these events highlight the need for a greater understanding of the Queen Charlotte–Fairweather fault and its history.

Yet despite the dramatic effects of this fault’s activity, a near absence of high-resolution marine geophysical and geological data limits scientific understanding of its slip rate, earthquake recurrence interval, paleoseismic history, and rupture dynamics.

The U.S. Geological Survey (USGS) has now completed a systematic examination of the tectonic geomorphology along a 500-kilometer-long undersea section of the Queen Charlotte–Fairweather fault that offers new insights into activity at this strike-slip boundary, where the North American and Pacific plates slide horizontally past each other.

Fig. 1. Recent geophysical surveys provided high-resolution seafloor depth data for the northernmost undersea portion of the Queen Charlotte–Fairweather fault (area outlined in red). The colored seafloor relief represents multibeam echo sounder data acquired along the continental shelf and slope in 2015 and 2016; the gray seafloor relief in deeper water west of the fault was acquired by the University of New Hampshire in 2005. Black boxes are locations of depth imagery shown in Figures 2a–2d. Purple lines represent high-resolution seismic reflection profiles that were acquired in 2016 aboard the R/V Norseman. One such profile (green line) is shown in Figure 3. AMT represents the Alaska-Aleutian megathrust, and ME indicates Mount Edgecumbe.

A Complicated Boundary

The Queen Charlotte–Fairweather fault system and its better known counterpart, the San Andreas fault (which is highly visible on land in California), form the boundary between the North American and Pacific tectonic plates. The Queen Charlotte–Fairweather fault system defines this plate boundary for a distance of more than 1,200 kilometers, from Yakutat, Alaska, to the Queen Charlotte Triple Junction, a confluence of three faults west of British Columbia (Figure 1). Within this system, the Queen Charlotte fault represents the underwater section and is widely recognized as one of the world’s most seismically active continent-ocean transform faults [Plafker et al., 1978; Bruns and Carlson, 1987; Nishenko and Jacob, 1990; Walton et al., 2015].

The northern part of the boundary between the North American and Pacific plates is complicated by the collision of the Yakutat terrane, a block of crustal material surrounded by faults, with southern Alaska. In this region, the Pacific Plate begins to subduct, or plunge beneath, the North American Plate along a boundary known as the Alaska-Aleutian megathrust.

The Fairweather fault is the only stretch of the fault system accessible by land. To the south of Icy Point, the Fairweather fault runs offshore, becoming the Queen Charlotte fault, which extends about 900 kilometers southward along the continental slope.

Earlier studies estimated a slip rate of 41 to 58 millimeters per year on the Fairweather fault [Plafker et al., 1978; Bruns and Carlson, 1987; Elliot et al., 2010], but few direct observations of horizontal seafloor displacement existed [Bruns and Carlson, 1987] because of the absence of high-resolution seabed data.

Geophysical Surveys

In 2015, our team conducted two marine geophysical surveys, one aboard the research vessel R/V Solstice and a second on R/V Alaskan Gyre. We collected high-resolution seafloor depth data using multibeam sonar along the northernmost section of the fault. We also used a chirp subbottom profiler, which returns detailed images down to 50 meters beneath the seafloor.

The Queen Charlotte–Fairweather fault lies off the coast of southeastern Alaska. New imagery of a 400-kilometer-long undersea section of this transform fault provides a striking view of its structure and offers insights into activity at the boundary between the North American and Pacific tectonic plates. This perspective view of depth data acquired during recent surveys of the area shows the fault as it emerges from the Alaskan coast and stretches as a distinct line across the ocean floor. The color spectrum from red to purple represents increasing water depth.

In 2016, two additional cruises (aboard R/V Medeia and R/V Norseman) extended data coverage of the Queen Charlotte–Fairweather fault an additional 325 kilometers southward. We again used multibeam sonar to map the ocean floor and multichannel seismic reflection to image deeper layers of sediment. Most recently, seismic reflection and chirp surveys were completed in July 2017 aboard the R/V Ocean Starr.

In total, during 95 days of seagoing operations, we collected more than 5,000 square kilometers of high-resolution depth data, 9,400 kilometers of high-resolution multichannel seismic reflection profiles, and 500 kilometers of subbottom chirp data.

A Clearer View of the Fault System

Imagery from the surveys shows the fault in pristine detail, cutting straight across the seafloor, with offsetting seabed channels and submerged glacial valleys (Figure 2). The continuous knife-edge character of the fault is evident over the entire 500-kilometer-long survey area. At the same time, we can see several previously unknown features, including a series of subtle bends and steps in the fault that appear to form basins within the fault zone.

Fig. 2. High-resolution depth images at four locations along the Queen Charlotte fault show the morphological features of the fault and the continental slope. Red arrows indicate the relative sense of motion (see Figure 1 for locations).

Because the surveys spanned four sections of the fault that ruptured in significant historical earthquakes, the results provide a unique catalog of geomorphic features commonly associated with active strike-slip faults.

The Fairweather fault bends 20° as it extends southward across the shoreline near Icy Point (Figures 1 and 2a) and then continues southward at a 340° strike along the shelf edge as a single fault trace for another 150 kilometers.

Numerous submarine canyons, gullies, and ridges have been displaced or warped along the fault. Fault valleys parallel to the margin locally separate geomorphically distinct upper and lower sections of the continental slope (Figures 2b and 3). A Pleistocene basaltic-andesitic volcanic edifice exposed at the seabed extends from Mount Edgecumbe to the shelf edge (Figure 2b).

West of southern Baranof Island, the fault takes a series of subtle 3° to 5° right steps and bends that form en echelon pull-apart basins along the shelf edge (Figure 2c). The fault continues southward as a single lineament but exhibits a subtle warp and series of westward steps displacing submarine canyon valleys (Figure 2d) before crossing Noyes Canyon and extending southward into Canadian waters [see, e.g., Barrie et al., 2013].

Fig. 3. A seismic reflection profile acquired in August 2016 highlights the structure and stratigraphy of the continental slope.

Fault Slip Rates

The offset features along the seabed provide important information for reconstructing past fault motion. From the ages of these features we can calculate the average rate of motion along the fault, then estimate the typical recurrence interval for large earthquakes.

For example, the southern margin of the Yakobi Sea Valley has been sliced and translated about 925 meters by the linear, knife-edge fault trace (Figure 2a). Ice likely retreated from the valley about 17,000 years ago. Thus, the slip rate of the Queen Charlotte–Fairweather fault across the Yakobi Sea Valley exceeds 50 millimeters per year: one of the fastest-slipping continent-ocean transform faults in the world [Brothers et al., 2015].

Furthermore, we observe coincidence between the pull-apart basins shown in Figure 2c and the northernmost extent of the 2013 Craig earthquake, implying that changes in fault geometry likely influenced the length of rupture propagation [e.g., Walton et al., 2015].

Future Plans

The USGS, the Geological Survey of Canada, the Sitka Sound Science Center, and the University of Calgary will jointly lead a research cruise in September 2017 to collect sediment cores along the Queen Charlotte–Fairweather fault in Canadian and U.S. territories to constrain the sedimentation history along the margin and date features offset by fault motion.

Overall, this project has shown that the Queen Charlotte–Fairweather fault is an ideal laboratory to examine the tectonic geomorphology of a major strike-slip fault and the associated processes responsible for generating offshore hazards.

Acknowledgments

We thank J. Currie, G. Hatcher, R. Wyland, A. Balster-Gee, P. Hart, J. Conrad, T. O’Brien, A. Nichols, M. Walton, R. Marcuson, and E. Moore of the U.S. Geological Survey (USGS); K. Green of the Alaska Department of Fish and Game; G. Greene of Moss Landing Marine Laboratories; V. Barrie and K. Conway of the Geological Survey of Canada; and the crews of the R/V Solstice, R/V Medeia, R/V Norseman, R/V Ocean Starr, and R/V Alaskan Gyre. We also thank J. Warrick, R. von Huene, J. Watt, and an anonymous reader for helpful reviews. The USGS Coastal and Marine Geology Program funded this study. Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. government.

References

Barrie, J. V., K. W. Conway, and P. T. Harris (2013), The Queen Charlotte fault, British Columbia: Seafloor anatomy of a transform fault and its influence on sediment processes, Geo Mar. Lett., 33, 311–318, https://doi.org/10.1007/s00367-013-0333-3.

Brothers, D. S., et al. (2015), High-resolution geophysical constraints on late Pleistocene–Present deformation history, seabed morphology, and slip-rate along the Queen Charlotte-Fairweather fault, offshore southeastern Alaska, Abstract NH23B-1882 presented at 2015 Fall Meeting, AGU, San Francisco, Calif.

Bruns, T. R., and P. R. Carlson (1987), Geology and petroleum potential of the southeast Alaska continental margin, in Geology and Petroleum Potential of the Continental Margin of Western North America and Adjacent Ocean Basins, Beaufort Sea to Baja California, Earth Sci. Ser., vol. 9, edited by D. W. Scholl, A. Grantz, and J. G. Vedder, pp. 269–282, Circum-Pac. Counc. for Energy and Miner. Resour., Houston, Texas.

Elliot, J. L., et al. (2010), Tectonic block motion and glacial isostatic adjustment in southeast Alaska and adjacent Canada constrained by GPS measurements, J. Geophys. Res., 115, B09407, https://doi.org/10.1029/2009JB007139.

Miller, D. J. (1960), Giant waves in Lituya Bay, Alaska, U.S. Geol. Surv. Prof. Pap., 354-C, 51–86, scale 1:50,000.

Nishenko, S. P., and K. H. Jacob (1990), Seismic potential of the Queen Charlotte-Alaska-Aleutian seismic zone, J. Geophys. Res., 95(B3), 2511–2532, https://doi.org/10.1029/JB095iB03p02511.

Plafker, G., et al. (1978), Late Quaternary offsets along the Fairweather fault and crustal plate interactions in southern Alaska, Can. J. Earth Sci., 15(5), 805–816, https://doi.org/10.1139/e78-085.

Walton, M. A. L., et al. (2015), Basement and regional structure along strike of the Queen Charlotte fault in the context of modern and historical earthquake ruptures, Bull. Seismol. Soc. Am., 105, 1090–1105, https://doi.org/10.1785/0120140174.

Author Information

Daniel S. Brothers (email: dbrothers@usgs.gov; @DBrothersSC), Pacific Coastal and Marine Science Center, U.S. Geological Survey (USGS), Santa Cruz, Calif.; Peter Haeussler, Alaska Science Center, USGS, Anchorage; Amy East, Pacific Coastal and Marine Science Center, USGS, Santa Cruz, Calif.; Uri ten Brink and Brian Andrews, Woods Hole Science Center, USGS, Mass.; Peter Dartnell, Pacific Coastal and Marine Science Center, USGS, Santa Cruz, Calif.; Nathan Miller, Woods Hole Science Center, USGS, Mass.; and Jared Kluesner, Pacific Coastal and Marine Science Center, USGS, Santa Cruz, Calif.
Citation: Brothers, D. S., P. Haeussler, A. East, U. ten Brink, B. Andrews, P. Dartnell, N. Miller, and J. Kluesner (2017), A closer look at an undersea source of Alaskan earthquakes, Eos, 98, https://doi.org/10.1029/2017EO079019. Published on 15 August 2017.

© 2017. The authors. CC BY-NC-ND 3.0

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From Carnegie: “Big data points humanity to new minerals, new deposits”

Carnegie Institution for Science

August 01, 2017
Reference to Person:
Robert Hazen

Applying big data analysis to mineralogy offers a way to predict minerals missing from those known to science, as well as where to find new deposits, according to a groundbreaking study.

In a paper published by American Mineralogist, scientists report the first application to mineralogy of network theory (best known for analysis of e.g. the spread of disease, terrorist networks, or Facebook connections).

The results, they say, pioneer a potential way to reveal mineral diversity and distribution worldwide, their evolution through deep time, new trends, and new deposits of valuable minerals such as gold or copper.

Led by Shaunna Morrison of the Deep Carbon Observatory and DCO Executive Director Robert Hazen (both at the Carnegie Institution for Science in Washington, D.C.), the paper’s 12 authors include DCO colleagues Peter Fox and Ahmed Eleish at the Keck Foundation-sponsored Deep-Time Data Infrastructure Data Science Teams at Rensselaer Polytechnic Institute, in Troy NY.

“The quest for new mineral deposits is incessant, but until recently, mineral discovery has been more a matter of luck than scientific prediction,” says Morrison. “All that may change thanks to big data.”

Humans have collected a vast amount of information on Earth’s more than 5,200 known mineral species (each of which has a unique combination of chemical composition and atomic structure).

Millions of mineral specimens from hundreds of thousands of localities around the world have been described and catalogued. Databases containing details of where each mineral was discovered, all of its known occurrences, and the ages of those deposits are large and growing by the week.

Databases also record essential information on chemical compositions and a host of physical properties, including hardness, color, atomic structure, and more.

Coupled with data on the surrounding geography, the geological setting, and coexisting minerals, Earth scientists now have access to “big data” resources ripe for analysis.

Until recently, Earth scientists didn’t have the necessary modelling and visualization tools to capitalize on these giant stockpiles of information.

Network analysis offers new insight into minerals, just as complex data sets offer important understanding of social media connections, city traffic patterns, and metabolic pathways, to name a few examples.

“Big data is a big thing,” says Hazen. “You hear about it in all kinds of fields—medicine, commerce, even the U.S. National Security Agency to analyze phone records—but until recently no one had applied big data methods to mineralogy and petrology.”

“I think this is going to expand the rate of mineral discovery in ways that we can’t even imagine now.”

The network analysis technique enables Earth scientists to represent data from multiple variables on thousands of minerals sampled from hundreds of thousands of locations within a single graph.

These visualizations can reveal patterns of occurrence and distribution that might otherwise be hidden within a spreadsheet.

In other words, big data provides an intimate picture of which minerals coexist with each other, as well as what geological, physical, chemical, and (perhaps most surprising) biological characteristics are necessary for their appearance.

From those insights it’s a relatively simple step to predict what minerals are missing from scientific lists, as well as where to go to find new deposits.

Says Hazen: “Network analysis can provide visual clues to mineralogists regarding where to go and what to look for. This is a brand new idea in the paper and I think it will open up an entirely new direction in mineralogy.”

Already the technique has been used to predict 145 missing carbon-bearing minerals and where to find them, leading to creation of the Deep Carbon Observatory’s Carbon Mineral Challenge. Ten have been found so far.

The estimate came from a statistical analysis of carbon-bearing minerals known today, then extrapolating how many scientists should be looking for.

Abellaite and parisite-(La) (photos below) are examples of new-to-science carbon-bearing minerals predicted before they were found, thanks in part to big data analysis.

“We have used the same kinds of techniques to predict that at least 1,500 minerals of all kinds are ‘missing,’ to predict what some of them are, and where to find them,” Hazen says.

Says Morrison: “These new approaches to data-driven discovery allow us to predict both minerals unknown to science today and the location of new deposits. Additionally, understanding how minerals have changed through geologic time, coupled with our knowledge of biology, is leading to new insights regarding the co-evolution of the geosphere and biosphere. ”

In a test case, the researchers explored minerals containing copper, which plays critical roles in modern society (e.g., pipes, wires), as well as essential roles in biological evolution. The element is extremely sensitive to oxygen, so the nature of copper in a mineral offers a clue to the level of oxygen in the atmosphere at the time the mineral formed.

The investigators also performed an analysis of common minerals in igneous rocks—those formed from a hot molten state. The mineral networks of igneous rocks revealed through big data recreated “Bowen’s reaction series” (based on Norman L. Bowen’s painstaking lab experiments in the early 1900s), which shows how a sequence of characteristic minerals appears as the magma cools.

The analysis showed the exact same sequence of minerals embedded in the mineral networks.

The researchers hope that these techniques will lead to an understanding and appreciation of previously unrecognized mineral relationships in varied mineral deposits.

Mineral networks will also serve as effective visual tools for learning about mineralogy and petrology — the branches of science concerned with the origin, composition, structure, properties, and classification of rocks and minerals.

Network analysis has numerous potential applications in geology, both for research and mineral exploration.

Mining companies could use the technology to predict the locations of unknown mineral deposits based on existing data.

Researchers could use these tools to explain how Earth’s minerals have changed over time and incorporate data from biomarker molecules to show how cells and minerals interact.

And ore geologists hope to use mineral network analysis to lead to valuable new deposits.

Dr. Morrison also hopes to use network analysis to reveal the geologic history of other planets. She is a member of the NASA Mars Curiosity Rover team identifying Martian minerals through X-ray diffraction data sent back to Earth. By applying these tools to analyze sedimentary environments on Earth, she believes scientists may also start answering similar questions about Mars.

“Minerals provide the basis for all our material wealth,” she notes, “not just precious gold and brilliant gemstones, but in the brick and steel of every home and office, in cars and planes, in bottles and cans, and in every high-tech gadget from laptops to iPhones.”

“Minerals form the soils in which we grow our crops, they provide the gravel with which we pave our roads, and they filter the water we drink.”

“This new tool for understanding minerals represents an important advance in a scientific field of vital interest.”

Caption: A network diagram for 403 carbon minerals reveals previously hidden patterns in their diversity and distribution. Each colored circle represents a different carbon mineral. The size and color of the circles indicates how common or rare each mineral is on Earth.

Four examples illustrated are: (1) calcite, the commonest carbon-bearing mineral, which occurs at tens of thousands of localities; (2) malachite, a beautiful green ornamental copper carbonate mineral that is known from thousands of localities; (3) lanthanite, a carbonate of rare earth elements reported from only 14 localities around the world; and (4) the exceedingly rare calcium-zinc carbonate mineral skorpionite, which is known from only one locality in Namibia.

The black circles represent more than 300 different regional localities at which these minerals are found. The sizes of the circles indicate how many carbon-bearing minerals are found at each locality, and the lines link mineral species and their localities.

The distribution of minerals and localities follows a distinctive pattern with a few very common minerals and many more rare species—a distribution that has led to the prediction that more than 1500 mineral species occur on Earth but have yet to be discovered and described. The hunt is now on for these “missing” minerals.

Image credit: Keck DTDI Project.

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From Rutgers: “Sea Cave Preserves 5,000-Year Snapshot of Tsunamis”

Rutgers University

July 19, 2017
Ken Branson

Record tells us we don’t know much about predicting earthquakes that cause tsunamis.

An international team of scientists digging in a sea cave in Indonesia has discovered the world’s most pristine record of tsunamis, a 5,000-year-old sedimentary snapshot that reveals for the first time how little is known about when earthquakes trigger massive waves.

“The devastating 2004 Indian Ocean tsunami caught millions of coastal residents and the scientific community off-guard,” says co-author Benjamin Horton, a professor in the Department of Marine and Coastal Sciences at Rutgers University-New Brunswick.“Our geological record from a cave illustrates that we still cannot predict when the next earthquake will happen.”

“Tsunamis are not evenly spaced through time,” says Charles Rubin, the study’s lead author and a professor at the Earth Observatory of Singapore, part of Nanyang Technological University. “Our geological record from a cave illustrates that we still cannot predict when the next earthquake will happen.” There can be long periods between tsunamis, but you can also get major tsunamis that are separated by just a few decades.”

The discovery, reported in the current issue of Nature Communications, logs a number of firsts: the first record of ancient tsunami activity found in a sea cave; the first record for such a long time period in the Indian Ocean; and the most pristine record of tsunamis anywhere in the world.

The stratigraphy of the sea cave in Sumatra excavated by scientists from the Earth Observatory of Singapore, Rutgers and other institutions. The lighter bands are sand deposited by tsunamis over a period of 5,000 years; the darker bands are organic material. Photo: Earth Observatory of Singapore.

The discovery was made in a sea cave on the west coast of Sumatra in Indonesia, just south of the city of Banda Aceh, which was devastated by the tsunami of December 2004. The stratigraphic record reveals successive layers of sand, bat droppings and other debris laid down by tsunamis between 7,900 and 2,900 years ago. The stratigraphy since 2,900 years ago was washed away by the 2004 tsunami.

The L-shaped cave had a rim of rocks at the entrance that trapped successive layers of sand inside. The researchers dug six trenches and analyzed the alternating layers of sand and debris using radio carbon dating. The researchers define “pristine” as stratigraphic layers that are distinct and easy to read. “You have a layer of sand and a layer of organic material that includes bat droppings, so simply it is a layer of sand and a layer of bat crap, and so on, going back for 5,000 years,” Horton says.

The record indicates that 11 tsunamis were generated during that period by earthquakes along the Sunda Megathrust, the 3,300-mile-long fault running from Myanmar to Sumatra in the Indian Ocean. The researchers found there were two tsunami-free millennia during the 5,000 years, and one century in which four tsunamis struck the coast. In general, the scientists report, smaller tsunamis occur relatively close together, followed by long dormant periods, followed by great quakes and tsunamis, such as the one that struck in 2004.

Using flourescent lights, Kerry Sieh and Charles Rubin of the Earth Observatory of Singapore look for charcoal and shells for radiocarbon dating. Photo: Earth Observatory of Singapore.

Rubin, Horton and their colleagues were studying the seismic history of the Sunda Megathrust, which was responsible for the 2004 earthquake that triggered the disastrous tsunami. They were looking for places to take core samples that would give them a good stratigraphy. This involves looking for what Horton calls “depositional places” – coastal plains, coastal lake bottoms, any place to plunge a hollow metal cylinder six or seven meters down and produce a readable sample. But for various reasons, there was no site along the southwest coast of Sumatra that would do the job. But Patrick Daly, an archaeologist at EOS who had been working on a dig in the coastal cave, told Rubin and Horton about it and suggested it might be the place they were looking for.

Looking for tsunami records in a sea cave was not something that would have occurred to Horton, and he says Daly’s professional generosity – archaeologists are careful about who gets near their digs – and his own and Rubin’s openness to insights from other disciplines made the research possible. Horton says this paper may be the most important in his career for another reason.

“A lot of (the research) I’ve done is incremental,” he says. “I have a hypothesis, and I do deductive science to test the hypothesis. But this is really original, and original stuff doesn’t happen all that often.”

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From UC Riverside: “Slow Earthquakes Occur Continuously in the Alaska-Aleutian Subduction Zone”

UC Riverside

July 12, 2017
Iqbal Pittalwala

Image shows tremor sources and low frequency earthquake distribution in the study region and historic large earthquakes in the Alaska-Aleutian subduction zone. Each red star represents the location of 1 min tremor signal determined by the beam back projection method, and the black stars show three visually detected low frequency earthquakes located using arrival times of body waves. Image credit: Ghosh lab, UC Riverside.

Seismologists at the University of California, Riverside studying earthquakes in the seismically and volcanically active Alaska-Aleutian subduction zone have found that “slow earthquakes” are occurring continuously, and could encourage damaging earthquakes.

Slow earthquakes are quiet, can be as large as magnitude 7, and last days to years. Taking place mainly at the boundary between tectonic plates, they happen so slowly that people don’t feel them. A large slow earthquake is typically associated with abundant seismic tremor—a continuous weak seismic chatter—and low frequency (small and repeating) earthquakes.

“In the Alaska-Aleutian subduction zone, we found seismic tremor, and visually identified three low frequency earthquakes,” said Abhijit Ghosh, an assistant professor of Earth sciences, who led the research published recently in Geophysical Research Letters. “Using them as templates, we detected nearly 1,300 additional low frequency earthquakes. Slow earthquakes may play an important role in the earthquake cycles in this subduction zone.”

The Alaska-Aleutian subduction zone, which stretches from the Gulf of Alaska to the Kamchatka Peninsula in the Russian Far East, is one of the most active plate boundaries in the world. It is 3,800 km long and forms the plate boundary between the Pacific and North American plates. In the last 80 years, four massive earthquakes (greater than magnitude 8) have occurred here.

Abhijit Ghosh lands in Alaska to do field work. Photo credit: Ghosh lab, UC Riverside.

Ghosh explained that tectonic tremor—which causes a weak vibration of the ground—and low frequency earthquakes are poorly studied in the Alaska-Aleutian subduction zone due to limited data availability, difficult logistics, and rugged terrain.

But using two months of high-quality continuous seismic data recorded from early July-September 2012 at 11 stations in the Akutan Island, Ghosh and his graduate student, Bo Li, detected near-continuous tremor activity with an average of 1.3 hours of tectonic tremor per day using a “beam back projection” method—an innovative array-based method Ghosh developed to automatically detect and locate seismic tremor. Using the seismic arrays the method continuously scans the subsurface for any seismic activity. Just like a radar antenna, it determines from which direction the seismic signal originates and uses that information to locate it. Practically, it can track slow earthquakes minute-by-minute.

Ghosh and Li found that tremor sources were clustered in two patches with a nearly 25 km gap in between them, possibly indicating that frictional properties determining earthquake activities change laterally along this area. Ghosh explained that this gap impacts the region’s overall stress pattern and can affect earthquake activity nearby.

“In addition, slow earthquakes seem to have ‘sweet spots’ along the subduction fault that produces majority of the tremor activity,” he said. “We found that the western patch has a larger depth range and shows higher tremor source propagation velocities. More frequent tremor events and low frequency earthquakes in the western patch may be a result of higher fluid activity in the region and indicate a higher seismic slip rate than the eastern region.”

Ghosh, Li, and their collaborators in multiple institutions in the United States have taken the next step by installing three additional seismic arrays in a nearby island to simultaneously image the subduction fault and volcanic system.

“This ambitious experiment will provide new insights into the seismic activity and subduction processes in this region,” Ghosh said.

The study [Geophysical Reseach Letters] was supported by grants to Ghosh from the National Science Foundation-Division of Earth Sciences, EarthScope, the United States Geological Survey, and the Alaska Volcano Observatory.

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Widely recognized as one of the most ethnically diverse research universities in the nation, UCR’s current enrollment is more than 21,000 students, with a goal of 25,000 students by 2020. The campus is in the midst of a tremendous growth spurt with new and remodeled facilities coming on-line on a regular basis.

We are located approximately 50 miles east of downtown Los Angeles. UCR is also within easy driving distance of dozens of major cultural and recreational sites, as well as desert, mountain and coastal destinations.