Hidden movements of Greenland Ice Sheet, runoff revealed

For years NASA has tracked changes in the massive Greenland Ice Sheet. This week scientists using NASA data released the most detailed picture ever of how the ice sheet moves toward the sea and new insights into the hidden plumbing of melt water flowing under the snowy surface.

The results of these studies are expected to improve predictions of the future of the entire Greenland ice sheet and its contribution to sea level rise as researchers revamp their computer models of how the ice sheet reacts to a warming climate.

“With the help of NASA satellite and airborne remote sensing instruments, the Greenland Ice Sheet is finally yielding its secrets,” said Tom Wagner, program scientist for NASA’s cryosphere program in Washington. “These studies represent new leaps in our knowledge of how the ice sheet is losing ice. It turns out the ice sheet is a lot more complex than we ever thought.”

University at Buffalo geophysicist Beata Csatho led an international team that produced the first comprehensive study of how the ice sheet is losing mass based on NASA satellite and airborne data at nearly 100,000 locations across Greenland. The study found that the ice sheet shed about 243 gigatons of ice per year from 2003-09, which agrees with other studies using different techniques. The study was published today in the Proceedings of the National Academy of Sciences.

The study suggests that current ice sheet modeling is too simplistic to accurately predict the future contribution of the Greenland ice sheet to sea level rise, and that current models may underestimate ice loss in the near future.

The project was a massive undertaking, using satellite and aerial data from NASA’s ICESat spacecraft, which measured the elevation of the ice sheet starting in 2003, and the Operation IceBridge field campaign that has flown annually since 2009. Additional airborne data from 1993-2008, collected by NASA’s Program for Arctic Regional Climate Assessment, were also included to extend the timeline of the study.

Current computer simulations of the Greenland Ice Sheet use the activity of four well-studied glaciers — Jakobshavn, Helheim, Kangerlussuaq and Petermann — to forecast how the entire ice sheet will dump ice into the oceans. The new research shows that activity at these four locations may not be representative of what is happening with glaciers across the ice sheet. In fact, glaciers undergo patterns of thinning and thickening that current climate change simulations fail to address, Csatho says.

As a step toward building better models of sea level rise, the research team divided Greenland’s 242 glaciers into 7 major groups based on their behavior from 2003-09.

“Understanding the groupings will help us pick out examples of glaciers that are representative of the whole,” Csatho says. “We can then use data from these representative glaciers in models to provide a more complete picture of what is happening.”

The team also identified areas of rapid shrinkage in southeast Greenland that today’s models don’t acknowledge. This leads Csatho to believe that the ice sheet could lose ice faster in the future than today’s simulations would suggest.

In separate studies presented today at the American Geophysical Union annual meeting in San Francisco, scientists using data from Operation IceBridge found permanent bodies of liquid water in the porous, partially compacted firn layer just below the surface of the ice sheet. Lora Koenig at the National Snow and Ice Data Center in Boulder, Colorado, and Rick Forster at the University of Utah in Salt Lake City, found signatures of near-surface liquid water using ice-penetrating radar.

Across wide areas of Greenland, water can remain liquid, hiding in layers of snow just below the surface, even through cold, harsh winters, researchers are finding. The discoveries by the teams led by Koenig and Forster mean that scientists seeking to understand the future of the Greenland ice sheet need to account for relatively warm liquid water retained in the ice.

Although the total volume of water is small compared to overall melting in Greenland, the presence of liquid water throughout the year could help kick off melt in the spring and summer. “More year-round water means more heat is available to warm the ice,” Koenig said.

Koenig and her colleagues found that sub-surface liquid water are common on the western edges of the Greenland Ice Sheet. At roughly the same time, Forster used similar ground-based radars to find a large aquifer in southeastern Greenland. These studies show that liquid water can persist near the surface around the perimeter of the ice sheet year round.

Another researcher participating in the briefing found that near-surface layers can also contain masses of solid ice that can lead to flooding events. Michael MacFerrin, a scientist at the Cooperative Institute for Research in Environmental Sciences at the University of Colorado Boulder, and colleagues studying radar data from IceBridge and surface based instruments found near surface patches of ice known as ice lenses more than 25 miles farther inland than previously recorded.

Ice lenses form when firn collects surface meltwater like a sponge. When this shallow ice melts, as was seen during July 2012, they can release large amounts of water that can lead to flooding. Warm summers and resulting increased surface melt in recent years have likely caused ice lenses to grow thicker and spread farther inland. “This represents a rapid feedback mechanism. If current trends continue, the flooding will get worse,” MacFerrin said.




Video
Click on this image to view the .mp4 video
This animation (from March 2014) portrays the changes occurring in the surface elevation of the Greenland Ice Sheet since 2003 in three drainage areas: the southeast, the northeast and the Jakobshavn regions. In each region, the time advances to show the accumulated change in elevation, 2003-2012.

Downloadable video: http://svs.gsfc.nasa.gov/cgi-bin/details.cgi?aid=4022 – NASA SVS NASA’s Goddard Space Flight Center

First harvest of research based on the final GOCE gravity model

This image, based on the final GOCE gravity model, charts current velocities in the Gulf Stream in meters per second. -  TUM IAPG
This image, based on the final GOCE gravity model, charts current velocities in the Gulf Stream in meters per second. – TUM IAPG

Just four months after the final data package from the GOCE satellite mission was delivered, researchers are laying out a rich harvest of scientific results, with the promise of more to come. A mission of the European Space Agency (ESA), the Gravity Field and Steady-State Ocean Circulation Explorer (GOCE) provided the most accurate measurements yet of Earth’s gravitational field. The GOCE Gravity Consortium, coordinated by the Technische Universität München (TUM), produced all of the mission’s data products including the fifth and final GOCE gravity model. On this basis, studies in geophysics, geology, ocean circulation, climate change, and civil engineering are sharpening the picture of our dynamic planet – as can be seen in the program of the 5th International GOCE User Workshop, taking place Nov. 25-28 in Paris.

The GOCE satellite made 27,000 orbits between its launch in March 2009 and re-entry in November 2013, measuring tiny variations in the gravitational field that correspond to uneven distributions of mass in Earth’s oceans, continents, and deep interior. Some 800 million observations went into the computation of the final model, which is composed of more than 75,000 parameters representing the global gravitational field with a spatial resolution of around 70 kilometers. The precision of the model improved over time, as each release incorporated more data. Centimeter accuracy has now been achieved for variations of the geoid – a gravity-derived figure of Earth’s surface that serves as a global reference for sea level and heights – in a model based solely on GOCE data.

The fifth and last data release benefited from two special phases of observation. After its first three years of operation, the satellite’s orbit was lowered from 255 to 225 kilometers, increasing the sensitivity of gravity measurements to reveal even more detailed structures of the gravity field. And through most of the satellite’s final plunge through the atmosphere, some instruments continued to report measurements that have sparked intense interest far beyond the “gravity community” – for example, among researchers concerned with aerospace engineering, atmospheric sciences, and space debris.

Moving on: new science, future missions


Through the lens of Earth’s gravitational field, scientists can image our planet in a way that is complementary to approaches that rely on light, magnetism, or seismic waves. They can determine the speed of ocean currents from space, monitor rising sea level and melting ice sheets, uncover hidden features of continental geology, even peer into the convection machine that drives plate tectonics. Topics like these dominate the more than 100 talks scheduled for the 5th GOCE User Workshop, with technical talks on measurements and models playing a smaller role. “I see this as a sign of success, that the emphasis has shifted decisively to the user community,” says Prof. Roland Pail, director of the Institute for Astronomical and Physical Geodesy at TUM.

This shift can be seen as well among the topics covered by TUM researchers, such as estimates of the elastic thickness of the continents from GOCE gravity models, mass trends in Antarctica from global gravity fields, and a scientific roadmap toward worldwide unification of height systems. For his part Pail – who was responsible for delivery of the data products – chose to speak about consolidating science requirements for a next-generation gravity field mission.


TUM has organized a public symposium on “Seeing Earth in the ‘light’ of gravity” for the 2015 Annual Meeting of the American Association for the Advancement of Science in San Jose, California. This session, featuring speakers from Australia, Canada, Denmark, France, Germany and Italy, takes place on Feb. 14, 2015. (See http://meetings.aaas.org/.)

This research was supported in part by the European Space Agency.

Publication:


“EGM_TIM_RL05: An Independent Geoid with Centimeter Accuracy Purely Based on the GOCE Mission,” Jan Martin Brockmann, Norbert Zehentner, Eduard Höck, Roland Pail, Ina Loth, Torsten Mayer-Gürr, and Wolf-Dieter Shuh. Geophysical Research Letters 2014, doi:10.1002/2014GL061904.

Fountain of youth underlies Antarctic Mountains

Images of the ice-covered Gamburtsev Mountains revealed water-filled valleys, as seen by the cluster of vertical lines in this image. -  Tim Creyts
Images of the ice-covered Gamburtsev Mountains revealed water-filled valleys, as seen by the cluster of vertical lines in this image. – Tim Creyts

Time ravages mountains, as it does people. Sharp features soften, and bodies grow shorter and rounder. But under the right conditions, some mountains refuse to age. In a new study, scientists explain why the ice-covered Gamburtsev Mountains in the middle of Antarctica looks as young as they do.

The Gamburtsevs were discovered in the 1950s, but remained unexplored until scientists flew ice-penetrating instruments over the mountains 60 years later. As this ancient hidden landscape came into focus, scientists were stunned to see the saw-toothed and towering crags of much younger mountains. Though the Gamburtsevs are contemporaries of the largely worn-down Appalachians, they looked more like the Rockies, which are nearly 200 million years younger.

More surprising still, the scientists discovered a vast network of lakes and rivers at the mountains’ base. Though water usually speeds erosion, here it seems to have kept erosion at bay. The reason, researchers now say, has to do with the thick ice that has entombed the Gamburtsevs since Antarctica went into a deep freeze 35 million years ago.

“The ice sheet acts like an anti-aging cream,” said the study’s lead author, Timothy Creyts, a geophysicist at Columbia University’s Lamont-Doherty Earth Observatory. “It triggers a series of thermodynamic processes that have almost perfectly preserved the Gamburtsevs since ice began spreading across the continent.”

The study, which appears in the latest issue of the journal Geophysical Research Letters, explains how the blanket of ice covering the Gamburtsevs has preserved its rugged ridgelines.

Snow falling at the surface of the ice sheet draws colder temperatures down, closer to protruding peaks in a process called divergent cooling. At the same time, heat radiating from bedrock beneath the ice sheet melts ice in the deep valleys to form rivers and lakes. As rivers course along the base of the ice sheet, high pressures from the overlying ice sheet push water up valleys in reverse. This uphill flow refreezes as it meets colder temperature from above. Thus, ridgelines are cryogenically preserved.

The oldest rocks in the Gamburtsevs formed more than a billion years ago, in the collision of several continents. Though these prototype mountains eroded away, a lingering crustal root became reactivated when the supercontinent Gondwana ripped apart, starting about 200 million years ago. Tectonic forces pushed the land up again to form the modern Gamburtsevs, which range across an area the size of the Alps. Erosion again chewed away at the mountains until earth entered a cooling phase 35 million years ago. Expanding outward from the Gamburtsevs, a growing layer of ice joined several other nucleation points to cover the entire continent in ice.

The researchers say that the mechanism that stalled aging of the Gamburtsevs at higher elevations may explain why some ridgelines in the Torngat Mountains on Canada’s Labrador Peninsula and the Scandinavian Mountains running through Norway, Sweden and Finland appear strikingly untouched. Massive ice sheets covered both landscapes during the last ice age, which peaked about 20,000 years ago, but many high-altitude features bear little trace of this event.

“The authors identify a mechanism whereby larger parts of mountains ranges in glaciated regions–not just Antarctica–could be spared from erosion,” said Stewart Jamieson, a glaciologist at Durham University who was not involved in the study. “This is important because these uplands are nucleation centers for ice sheets. If they were to gradually erode during glacial cycles, they would become less effective as nucleation points during later ice ages.”

Ice sheet behavior, then, may influence climate change in ways that scientists and computer models have yet to appreciate. As study coauthor Fausto Ferraccioli, head of the British Antarctic Survey’s airborne geophysics group, put it: “If these mountains in interior East Antarctica had been more significantly eroded then the ice sheet itself
may have had a different history.”

Other Authors


Hugh Carr and Tom Jordan of the British Antarctic Survey; Robin Bell, Michael Wolovick and Nicholas Frearson of Lamont-Doherty; Kathryn Rose of University of Bristol; Detlef Damaske of Germany’s Federal Institute for Geosciences and Natural Resources; David Braaten of Kansas University; and Carol Finn of the U.S. Geological Survey.

Copies of the paper, “Freezing of ridges and water networks preserves the Gamburtsev Subglacial Mountains for millions of years,” are available from the authors.

Scientist Contact


Tim Creyts

845-365-8368

tcreyts@ldeo.columbia.edu

Worldwide retreat of glaciers confirmed in unprecedented detail

The worldwide retreat of glaciers is confirmed in unprecedented detail. This new book presents an overview and detailed assessment of changes in the world's glaciers by using satellite imagery -  Springer
The worldwide retreat of glaciers is confirmed in unprecedented detail. This new book presents an overview and detailed assessment of changes in the world’s glaciers by using satellite imagery – Springer

Taking their name from the old Scottish term glim, meaning a passing look or glance, in 1994 a team of scientists began developing a world-wide initiative to study glaciers using satellite data. Now 20 years later, the international GLIMS (Global Land Ice Measurements from Space) initiative observes the world’s glaciers primarily using data from optical satellite instruments such as ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer) and Landsat.

More than 150 scientists from all over the world have contributed to the new book Global Land Ice Measurements from Space, the most comprehensive report to date on global glacier changes. While the shrinking of glaciers on all continents is already known from ground observations of individual glaciers, by using repeated satellite observations GLIMS has firmly established that glaciers are shrinking globally. Although some glaciers are maintaining their size, most glaciers are dwindling. The foremost cause of the worldwide reductions in glaciers is global warming, the team writes.

Full color throughout, the book has 25 regional chapters that illustrate glacier changes from the Arctic to the Antarctic. Other chapters provide a thorough theoretical background on glacier monitoring and mapping, remote sensing techniques, uncertainties, and interpretation of the observations in a climatic context. The book highlights many other glacier research applications of satellite data, including measurement of glacier thinning from repeated satellite-based digital elevation models (DEMs) and calculation of surface flow velocities from repeated satellite images.

These tools are key to understanding local and regional variations in glacier behavior, the team writes. The high sensitivity of glaciers to climate change has substantially decreased their volume and changed the landscape over the past decades, affecting both regional water availability and the hazard potential of glaciers. The growing GLIMS database about glaciers also contributed to the Intergovernmental Panel on Climate Change (IPCC)’s Fifth Assessment Report issued in 2013. The IPCC report concluded that most of the world’s glaciers have been losing ice at an increasing rate in recent decades.

More than 60 institutions across the globe are involved in GLIMS. Jeffrey S. Kargel of the Department of Hydrology and Water Resources at the University of Arizona coordinates the project. The GLIMS glacier database and GLIMS web site are developed and maintained by the National Snow and Ice Data Center (NSIDC) at the University of Colorado in Boulder.

Global Land Ice Measurements from Space</em?

Hardcover $279.00; £180.00; € 199,99

Springer and Praxis Publishing (2014) ISBN 978-3-540-79817-0

Also available as an eBook

The breathing sand

An Eddy Correlation Lander analyzes the strength of the oxygen fluxes at the bottom of the North Sea. -  Photo: ROV-Team, GEOMAR
An Eddy Correlation Lander analyzes the strength of the oxygen fluxes at the bottom of the North Sea. – Photo: ROV-Team, GEOMAR

A desert at the bottom of the sea? Although the waters of the North Sea exchange about every two to three years, there is evidence of decreasing oxygen content. If lower amounts of this gas are dissolved in seawater, organisms on and in the seabed produce less energy – with implications for larger creatures and the biogeochemical cycling in the marine ecosystem. Since nutrients, carbon and oxygen circulate very well and are processed quickly in the permeable, sandy sediments that make up two-thirds of the North Sea, measurements of metabolic rates are especially difficult here. Using the new Aquatic Eddy Correlation technique, scientists from GEOMAR Helmholtz Centre for Ocean Research Kiel, Leibniz Institute of Freshwater Ecology and Inland Fisheries, the University of Southern Denmark, the University of Koblenz-Landau, the Scottish Marine Institute and Aarhus University were able to demonstrate how oxygen flows at the ground of the North Sea. Their methods and results are presented in the Journal of Geophysical Research: Oceans.

“The so-called ‘Eddy Correlation’ technique detects the flow of oxygen through these small turbulences over an area of several square meters. It considers both the mixing of sediments by organisms living in it and the hydrodynamics of the water above the rough sea floor”, Dr. Peter Linke, a marine biologist at GEOMAR, explains. “Previous methods overlooked only short periods or disregarded important parameters. Now we can create a more realistic picture.” The new method also takes into account the fact that even small objects such as shells or ripples shaped by wave action or currents are able to impact the oxygen exchange in permeable sediments.

On the expedition CE0913 with the Irish research vessel CELTIC EXPLORER, scientists used the underwater robot ROV KIEL 6000 to place three different instruments within the “Tommeliten” area belonging to Norway: Two “Eddy Correlation Landers” recorded the strength of oxygen fluxes over three tidal cycles. Information about the distribution of oxygen in the sediment was collected with a “Profiler Lander”, a seafloor observatory with oxygen sensors and flow meters. A “Benthic chamber” isolated 314 square centimetres of sediment and took samples from the overlying water over a period of 24 hours to determine the oxygen consumption of the sediment.

“The combination of traditional tools with the ‘Eddy Correlation’ technique has given us new insights into the dynamics of the exchange of substances between the sea water and the underlying sediment. A variety of factors determine the timing and amount of oxygen available. Currents that provide the sandy sediment with oxygen, but also the small-scale morphology of the seafloor, ensure that small benthic organisms are able to process carbon or other nutrients. The dependencies are so complex that they can be decrypted only by using special methods”, Dr. Linke summarizes. Therefore, detailed measurements in the water column and at the boundary to the seafloor as well as model calculations are absolutely necessary to understand basic functions and better estimate future changes in the cycle of materials. “With conventional methods, for example, we would never have been able to find that the loose sandy sediment stores oxygen brought in by the currents for periods of less water movement and less oxygen introduction.”

Original publication:
McGinnis, D. F., S. Sommer, A. Lorke, R. N. Glud, P. Linke (2014): Quantifying tidally driven benthic oxygen exchange across permeable sediments: An aquatic eddy correlation study. Journal of Geophysical Research: Oceans, doi:10.1002/2014JC010303.

Links:

GEOMAR Helmholtz Centre for Ocean Research Kiel

Eddy correlation information page

Leibniz Institute of Freshwater Ecology and Inland Fisheries, IGB

University of Southern Denmark

University of Koblenz-Landau

Scottish Marine Institute

Aarhus University

Images:
High resolution images can be downloaded at http://www.geomar.de/n2110-e.

Video footage is available on request.

Contact:
Dr. Peter Linke (GEOMAR FB2-MG), Tel. 0431 600-2115, plinke@geomar.de

Maike Nicolai (GEOMAR, Kommunikation & Medien), Tel. 0431 600-2807, mnicolai@geomar.de

Researcher receives $1.2 million to create real-time seismic imaging system

This is Dr. WenZhan Song. -  Georgia State University
This is Dr. WenZhan Song. – Georgia State University

Dr. WenZhan Song, a professor in the Department of Computer Science at Georgia State University, has received a four-year, $1.2 million grant from the National Science Foundation to create a real-time seismic imaging system using ambient noise.

This imaging system for shallow earth structures could be used to study and monitor the sustainability of the subsurface, or area below the surface, and potential hazards of geological structures. Song and his collaborators, Yao Xie of the Georgia Institute of Technology and Fan-Chi Lin of the University of Utah, will use ambient noise to image the subsurface of geysers in Yellowstone National Park.

“This project is basically imaging what’s underground in a situation where there’s no active source, like an earthquake. We’re using background noise,” Song said. “At Yellowstone, for instance, people visit there and cars drive by. All that could generate signals that are penetrating through the ground. We essentially use that type of information to tap into a very weak signal to infer the image of underground. This is very frontier technology today.”

The system will be made up of a large network of wireless sensors that can perform in-network computing of 3-D images of the shallow earth structure that are based solely on ambient noise.

Real-time ambient noise seismic imaging technology could also inform homeowners if the subsurface below their home, which can change over time, is stable or will sink beneath them.

This technology can also be used in circumstances that don’t need to rely on ambient noise but have an active source that produces signals that can be detected by wireless sensors. It could be used for real-time monitoring and developing early warning systems for natural hazards, such as volcanoes, by determining how close magma is to the surface. It could also benefit oil exploration, which uses methods such as hydrofracturing, in which high-pressure water breaks rocks and allows natural gas to flow more freely from underground.

“As they do that, it’s critical to monitor that in real time so you can know what’s going on under the ground and not cause damage,” Song said. “It’s a very promising technology, and we’re helping this industry reduce costs significantly because previously they only knew what was going on under the subsurface many days and even months later. We could reduce this to seconds.”

Until now, data from oil exploration instruments had to be manually retrieved and uploaded into a centralized database, and it could take days or months to process and analyze the data.

The research team plans to have a field demonstration of the system in Yellowstone and image the subsurface of some of the park’s geysers. The results will be shared with Yellowstone management, rangers and staff. Yellowstone, a popular tourist attraction, is a big volcano that has been dormant for a long time, but scientists are concerned it could one day pose potential hazards.

In the past several years, Song has been developing a Real-time In-situ Seismic Imaging (RISI) system using active sources, under the support of another $1.8 million NSF grant. His lab has built a RISI system prototype that is ready for deployment. The RISI system can be implemented as a general field instrumentation platform for various geophysical imaging applications and incorporate new geophysical data processing and imaging algorithms.

The RISI system can be applied to a wide range of geophysical exploration topics, such as hydrothermal circulation, oil exploration, mining safety and mining resource monitoring, to monitor the uncertainty inherent to the exploration and production process, reduce operation costs and mitigate the environmental risks. The business and social impact is broad and significant. Song is seeking business investors and partners to commercialize this technology.

###

For more information about the project, visit http://sensorweb.cs.gsu.edu/?q=ANSI.

Star Trekish, rafting scientists make bold discovery on Fraser River

SFU geographer Jeremy Venditti (orange jacket; black hat) is among several scientists aboard a Fraser River Rafting Expeditions measuring boat passing through a Fraser River canyon. -  SFU PAMR
SFU geographer Jeremy Venditti (orange jacket; black hat) is among several scientists aboard a Fraser River Rafting Expeditions measuring boat passing through a Fraser River canyon. – SFU PAMR

A Simon Fraser University-led team behind a new discovery has “?had the vision to go, like Star Trek, where no one has gone before: to a steep and violent bedrock canyon, with surprising results.”

That comment comes from a reviewer about a truly groundbreaking study just published in the journal Nature.
Scientists studying river flow in bedrock canyons for the first time have discovered that previous conceptions of flow and incision in bedrock-rivers are wrong.

SFU geography professor Jeremy Venditti led the team of SFU, University of Ottawa and University of British Columbia researchers on a scientific expedition on the Fraser River.

“For the first time, we used oceanographic instruments, commonly used to measure three-dimensional river flow velocity in low land rivers, to examine flow through steep bedrock canyons,” says Venditti. “The 3-D instruments capture downstream, cross-stream and vertical flow velocity.”

To carry out their Star Trek-like expedition, the researchers put their lives into the experienced hands of Fraser River Rafting Expeditions, which took them into 42 bedrock canyons. Equipped with acoustic Doppler current profilers to measure velocity fields, they rafted 486 kilometres of the Fraser River from Quesnel to Chilliwack. Their raft navigated turbulent waters normally only accessed by thrill-seeking river rafters.

“Current models of bedrock-rivers assume flow velocity is uniform, without changes in the downstream direction. Our results show this is not the case,” says Colin Rennie, an Ottawa U civil engineering professor.

“We observed a complicated flow field in which high velocity flow plunges down the bottom of the canyon forming a velocity inversion and then rises along the canyon walls. This has important implications for canyon erosion because the plunging flow patterns result in greater flow force applied to the bed.”

The scientists conclude that river flow in bedrock canyons is far more complex than first thought and the way scientists have linked climate, bedrock incision and the uplift of mountains needs to be rethought. They say the complexity of river flow plays an important role in deciding bedrock canyon morphology and river width.

“The links between the uplift of mountain ranges, bedrock incision by rivers and climate is one of the most important open questions in science,” notes Venditti. “The incision that occurs in bedrock canyons is driven by climate because the climate system controls precipitation and the amount of water carried in rivers. River flow drives the erosional mechanisms that cut valleys and allow the uplift of majestic mountain peaks.”

Venditti adds that river flow velocity in bedrock canyons also influences the delivery of sediment from mountain-rivers to lowland rivers.

“Sediment delivery controls water levels and stability of lowland rivers, which has important implications for lowland river management, flooding impacts to infrastructure, availability of fish habitat and more.

“Lowland river floodplains and deltas are the most densely populated places on earth, so understanding what is happening in mountain rivers is important because our continued development of these areas is significantly affected by what is happening upstream.”

Seismic gap may be filled by an earthquake near Istanbul

When a segment of a major fault line goes quiet, it can mean one of two things: The “seismic gap” may simply be inactive – the result of two tectonic plates placidly gliding past each other – or the segment may be a source of potential earthquakes, quietly building tension over decades until an inevitable seismic release.

Researchers from MIT and Turkey have found evidence for both types of behavior on different segments of the North Anatolian Fault – one of the most energetic earthquake zones in the world. The fault, similar in scale to California’s San Andreas Fault, stretches for about 745 miles across northern Turkey and into the Aegean Sea.

The researchers analyzed 20 years of GPS data along the fault, and determined that the next large earthquake to strike the region will likely occur along a seismic gap beneath the Sea of Marmara, some five miles west of Istanbul. In contrast, the western segment of the seismic gap appears to be moving without producing large earthquakes.

“Istanbul is a large city, and many of the buildings are very old and not built to the highest modern standards compared to, say, southern California,” says Michael Floyd, a research scientist in MIT’s Department of Earth, Atmospheric and Planetary Sciences. “From an earthquake scientist’s perspective, this is a hotspot for potential seismic hazards.”

Although it’s impossible to pinpoint when such a quake might occur, Floyd says this one could be powerful – on the order of a magnitude 7 temblor, or stronger.

“When people talk about when the next quake will be, what they’re really asking is, ‘When will it be, to within a few hours, so that I can evacuate?’ But earthquakes can’t be predicted that way,” Floyd says. “Ultimately, for people’s safety, we encourage them to be prepared. To be prepared, they need to know what to prepare for – that’s where our work can contribute”

Floyd and his colleagues, including Semih Ergintav of the Kandilli Observatory and Earthquake Research Institute in Istanbul and MIT research scientist Robert Reilinger, have published their seismic analysis in the journal Geophysical Research Letters.

In recent decades, major earthquakes have occurred along the North Anatolian Fault in a roughly domino-like fashion, breaking sequentially from east to west. The most recent quake occurred in 1999 in the city of Izmit, just east of Istanbul. The initial shock, which lasted less than a minute, killed thousands. As Istanbul sits at the fault’s western end, many scientists have thought the city will be near the epicenter of the next major quake.

To get an idea of exactly where the fault may fracture next, the MIT and Turkish researchers used GPS data to measure the region’s ground movement over the last 20 years. The group took data along the fault from about 100 GPS locations, including stations where data are collected continuously and sites where instruments are episodically set up over small markers on the ground, the positions of which can be recorded over time as the Earth slowly shifts.

“By continuously tracking, we can tell which parts of the Earth’s crust are moving relative to other parts, and we can see that this fault has relative motion across it at about the rate at which your fingernail grows,” Floyd says.

From their ground data, the researchers estimate that, for the most part, the North Anatolian Fault must move at about 25 millimeters – or one inch – per year, sliding quietly or slipping in a series of earthquakes.

As there’s currently no way to track the Earth’s movement offshore, the group also used fault models to estimate the motion off the Turkish coast. The team identified a segment of the fault under the Sea of Marmara, west of Istanbul, that is essentially stuck, with the “missing” slip accumulating at 10 to 15 millimeters per year. This section – called the Princes’ Island segment, for a nearby tourist destination – last experienced an earthquake 250 years ago.

Floyd and colleagues calculate that the Princes’ Island segment should have slipped about 8 to 11 feet – but it hasn’t. Instead, strain has likely been building along the segment for the last 250 years. If this tension were to break the fault in one cataclysmic earthquake, the Earth could shift by as much as 11 feet within seconds.

Although such accumulated strain may be released in a series of smaller, less hazardous rumbles, Floyd says that given the historical pattern of major quakes along the North Anatolian Fault, it would be reasonable to expect a large earthquake off the coast of Istanbul within the next few decades.

“Earthquakes are not regular or predictable,” Floyd says. “They’re far more random over the long run, and you can go many lifetimes without experiencing one. But it only takes one to affect many lives. In a location like Istanbul that is known to be subject to large earthquakes, it comes back to the message: Always be prepared.”

Foreshock series controls earthquake rupture

A long lasting foreshock series controlled the rupture process of this year’s great earthquake near Iquique in northern Chile. The earthquake was heralded by a three quarter year long foreshock series of ever increasing magnitudes culminating in a Mw 6.7 event two weeks before the mainshock. The mainshock (magnitude 8.1) finally broke on April 1st a central piece out of the most important seismic gap along the South American subduction zone. An international research team under leadership of the GFZ German Research Centre for Geosciences now revealed that the Iquique earthquake occurred in a region where the two colliding tectonic plates where only partly locked.

The Pacific Nazca plate and the South American plate are colliding along South America’s western coast. While the Pacific sea floor submerges in an oceanic trench under the South American coast the plates get stressed until occasionally relieved by earthquakes. In about 150 years time the entire plate margin from Patagonia in the south to Panama in the north breaks once completely through in great earthquakes. This cycle is almost complete with the exception of a last segment – the seismic gap near Iquique in northern Chile. The last great earthquake in this gap occurred back in 1877. On initiative of the GFZ this gap was monitored in an international cooperation (GFZ, Institut de Physique du Globe Paris, Centro Sismologico National – Universidad de Chile, Universidad de Catolica del Norte, Antofagasta, Chile) by the Integrated Plate Boundary Observatory Chile (IPOC), with among other instruments seismographs and cont. GPS. This long and continuous monitoring effort makes the Iquique earthquake the best recorded subduction megathrust earthquake globally. The fact that data of IPOC is distributed to the scientific community in near real time, allowed this timely analysis.

Ruptures in Detail

The mainshock of magnitude 8.1 broke the 150 km long central piece of the seismic gap, leaving, however, two large segments north and south intact. GFZ scientist Bernd Schurr headed the newly published study that appeared in the lastest issue of Nature Advance Online Publication: “The foreshocks skirted around the central rupture patch of the mainshock, forming several clusters that propagated from south to north.” The long-term earthquake catalogue derived from IPOC data revealed that stresses were increasing along the plate boundary in the years before the earthquake. Hence, the plate boundary started to gradually unlock through the foreshock series under increasing stresses, until it finally broke in the Iquique earthquake. Schurr further states: “If we use the from GPS data derived locking map to calculate the convergence deficit assuming the ~6.7 cm/yr convergence rate and subtract the earthquakes known since 1877, this still adds up to a possible M 8.9 earthquake.” This applies if the entire seismic gap would break at once. However, the region of the Iquique earthquake might now form a barrier that makes it more likely that the unbroken regions north and south break in separate, smaller earthquakes.

International Field Campaign

Despite the fact that the IPOC instruments delivered continuous data before, during and after the earthquake, the GFZ HART (Hazard And Risk Team) group went into the field to meet with international colleagues to conduct additional investigations. More than a dozen researchers continue to measure on site deformation and record aftershocks in the aftermath of this great rupture. Because the seismic gap is still not closed, IPOC gets further developed. So far 20 multi-parameter stations have been deployed. These consist of seismic broadband and strong-motion sensors, continuous GPS receivers, magneto-telluric and climate sensors, as well as creepmeters, which transmit data in near real-time to Potsdam. The European Southern astronomical Observatory has also been integrated into the observation network.

Scientists use ‘virtual earthquakes’ to forecast Los Angeles quake risk

Stanford scientists are using weak vibrations generated by the Earth’s oceans to produce “virtual earthquakes” that can be used to predict the ground movement and shaking hazard to buildings from real quakes.

The new technique, detailed in the Jan. 24 issue of the journal Science, was used to confirm a prediction that Los Angeles will experience stronger-than-expected ground movement if a major quake occurs south of the city.

“We used our virtual earthquake approach to reconstruct large earthquakes on the southern San Andreas Fault and studied the responses of the urban environment of Los Angeles to such earthquakes,” said lead author Marine Denolle, who recently received her PhD in geophysics from Stanford and is now at the Scripps Institution of Oceanography in San Diego.

The new technique capitalizes on the fact that earthquakes aren’t the only sources of seismic waves. “If you put a seismometer in the ground and there’s no earthquake, what do you record? It turns out that you record something,” said study leader Greg Beroza, a geophysics professor at Stanford.

What the instruments will pick up is a weak, continuous signal known as the ambient seismic field. This omnipresent field is generated by ocean waves interacting with the solid Earth. When the waves collide with each other, they generate a pressure pulse that travels through the ocean to the sea floor and into the Earth’s crust. “These waves are billions of times weaker than the seismic waves generated by earthquakes,” Beroza said.

Scientists have known about the ambient seismic field for about 100 years, but it was largely considered a nuisance because it interferes with their ability to study earthquakes. The tenuous seismic waves that make up this field propagate every which way through the crust. But in the past decade, seismologists developed signal-processing techniques that allow them to isolate certain waves; in particular, those traveling through one seismometer and then another one downstream.

Denolle built upon these techniques and devised a way to make these ambient seismic waves function as proxies for seismic waves generated by real earthquakes. By studying how the ambient waves moved underground, the researchers were able to predict the actions of much stronger waves from powerful earthquakes.

She began by installing several seismometers along the San Andreas Fault to specifically measure ambient seismic waves.

Employing data from the seismometers, the group then used mathematical techniques they developed to make the waves appear as if they originated deep within the Earth. This was done to correct for the fact that the seismometers Denolle installed were located at the Earth’s surface, whereas real earthquakes occur at depth.

In the study, the team used their virtual earthquake approach to confirm the accuracy of a prediction, made in 2006 by supercomputer simulations, that if the southern San Andreas Fault section of California were to rupture and spawn an earthquake, some of the seismic waves traveling northward would be funneled toward Los Angeles along a 60-mile-long (100-kilometer-long) natural conduit that connects the city with the San Bernardino Valley. This passageway is composed mostly of sediments, and acts to amplify and direct waves toward the Los Angeles region.

Until now, there was no way to test whether this funneling action, known as the waveguide-to-basin effect, actually takes place because a major quake has not occurred along that particular section of the San Andreas Fault in more than 150 years.

The virtual earthquake approach also predicts that seismic waves will become further amplified when they reach Los Angeles because the city sits atop a large sedimentary basin. To understand why this occurs, study coauthor Eric Dunham, an assistant professor of geophysics at Stanford, said to imagine taking a block of plastic foam, cutting out a bowl-shaped hole in the middle, and filling the cavity with gelatin. In this analogy, the plastic foam is a stand-in for rocks, while the gelatin is like sediments, or dirt. “The gelatin is floppier and a lot more compliant. If you shake the whole thing, you’re going to get some motion in the Styrofoam, but most of what you’re going to see is the basin oscillating,” Dunham said.

As a result, the scientists say, Los Angeles could be at risk for stronger, and more variable, ground motion if a large earthquake – magnitude 7.0 or greater – were to occur along the southern San Andreas Fault, near the Salton Sea.

“The seismic waves are essentially guided into the sedimentary basin that underlies Los Angeles,” Beroza said. “Once there, the waves reverberate and are amplified, causing stronger shaking than would otherwise occur.”

Beroza’s group is planning to test the virtual earthquake approach in other cities around the world that are built atop sedimentary basins, such as Tokyo, Mexico City, Seattle and parts of the San Francisco Bay area. “All of these cities are earthquake threatened, and all of them have an extra threat because of the basin amplification effect,” Beroza said.

Because the technique is relatively inexpensive, it could also be useful for forecasting ground motion in developing countries. “You don’t need large supercomputers to run the simulations,” Denolle said.

In addition to studying earthquakes that have yet to occur, the technique could also be used as a kind of “seismological time machine” to recreate the seismic signatures of temblors that shook the Earth long ago, according to Beroza.

“For an earthquake that occurred 200 years ago, if you know where the fault was, you could deploy instruments, go through this procedure, and generate seismograms for earthquakes that occurred before seismographs were invented,” he said.