Geologists shed light on formation of Alaska Range

Syracuse University Professor Paul Fitzgerald and a group of students have been studying the Alaska Range. -  Syracuse University
Syracuse University Professor Paul Fitzgerald and a group of students have been studying the Alaska Range. – Syracuse University

Geologists in Syracuse University’s College of Arts and Sciences have recently figured out what has caused the Alaska Range to form the way it has and why the range boasts such an enigmatic topographic signature. The narrow mountain range is home to some of the world’s most dramatic topography, including 20,320-foot Mount McKinley, North America’s highest mountain.

Professor Paul Fitzgerald and a team of students and fellow scientists have been studying the Alaska Range along the Denali fault. They think they know why the fault is located where it is and what accounts for the alternating asymmetrical, mountain-scale topography along the fault.

Their findings were the subject of a recent paper in the journal Tectonics (American Geophysical Union, 2014).

In 2002, the Denali fault, which cuts across south-central Alaska, was the site of a magnitude-7.9 earthquake and was felt as far away as Texas and Louisiana. It was the largest earthquake of its kind in more than 150 years.

“Following the earthquake, researchers flocked to the area to examine the effects,” says Fitzgerald, who serves as professor of Earth Sciences and an associate dean for the College. “They were fascinated by how the frozen ground behaved; the many landslides [the earthquake] caused; how bridges responded; and how the Trans-Alaska oil pipeline survived, as it was engineered to do so.”

Geologists were also surprised by how the earthquake began on a previously unknown thrust-fault; then propagated eastward, along the Denali fault, and finally jumped onto another fault, hundreds of kilometers away.

“From our perspective, the earthquake has motivated analyses of why the highest mountains in the central Alaska Range occur south of the Denali fault and the highest mountains in the eastern Alaska Range occur north of the fault–something that has puzzled us for years,” Fitzgerald adds. “It’s been an enigma staring us in the face.”

He attributes the Alaska Range’s alternating topographic signatures to a myriad of factors: contrasting lithospheric strength between large terranes (i.e., distinctly different rock units); the location of the curved Denali fault; the transfer of strain inland from southern Alaska’s active plate margin; and the shape of the controlling former continental margin against weaker suture-zone rocks.

It’s no secret that Alaska is one of the most geologically active areas on the planet. For instance, scientists know that the North American Plate is currently overriding the Pacific Plate at the latter’s southern coast, while the Yakutat microplate is colliding with North America.

As a result of plate tectonics, Alaska is an amalgamation of terranes that have collided with the North American craton and have accreted to become part of North America.

Cratons are pieces of continents that have been largely stable for hundreds of millions of years.

Terranes often originate as volcanic islands (like those of Hawaii) and, after colliding with one another or a continent, are separated by large discrete faults. When terranes collide and accrete, they form a suture, also known as a collision zone, which is made up of weak, crushed rock. During deformation, suture-zone rocks usually deform first, especially if they are adjacent to a strong rock body.

“Technically, the Denali fault is what we’d call an ‘intercontinental right-lateral strike-slip fault system,'” says Fitzgerald, adding that a strike-slip fault occurs when rocks move horizontally past one another, usually on a vertical fault. “This motion includes a component of slip along the fault and a component of normal motion against the fault that creates mountains. Hence, the shape of the fault determines which of the two components is predominant and where mountains form.”

In Alaska, the shape of the accreted terranes generally controls the location of the Denali fault and the mountains that form along it, especially at the bends in the trace of the fault.

Fitzgerald: “Mount McKinley and the central Alaska Range lie within the concave curve of the Denali fault. There, higher topography and greater exhumation [uplift of rock] occur south of the Denali fault, exactly where you’d expect a mountain range to form, given the regional tectonics. In the eastern Alaska Range, higher topography and greater exhumation are found north of the fault, on its convex side–not an expected pattern at all and very puzzling.”

Using mapped surface geology, geophysical data, and thermochronology (i.e., time-temperature history of the rocks), Fitzgerald and colleagues have determined that much of Alaska’s uplift and deformation began some 25 million years ago, when the Yakutat microplate first started colliding with North America. The bold, glacier-clad peaks comprising the Alaska Range actually derive from within the aforementioned “weak suture-zone rocks” between the terranes.

While mountains are high and give the impression of strength, they are built largely from previously fractured rock units. Rock movement along the Denali fault drives the uplift of the mountains, which form at bends in the fault, where previously fractured suture-zone rocks are pinned against the stronger former North American continental margin.

“The patterns of deformation help us understand regional tectonics and the formation of the Alaska Range, which is fascinating to geologists and non-geologists alike,” says Fitzgerald. “Being able to determine patterns or how to reveal them, while others see chaos, is often the key to finding the answer to complex problems. … To us scientists, the real significance of this work is that it helps us understand the evolution of our planet, how faults and mountain belts form, and why earthquakes happen. It also provides a number of hypotheses about Alaskan tectonics and rock deformation that we can test, using the Alaska Range as our laboratory.”

In addition to Fitzgerald, the paper was co-authored by Sarah Roeske, a research scientist at the University of California, Davis; Jeff Benowitz, a research scientist at the Geophysical Institute at the University of Alaska Fairbanks; Steven Riccio and Stephanie Perry, graduate students in Earth Sciences at Syracuse; and Phillip Armstrong, professor and chair of geological sciences at California State University, Fullerton.

Housed in Syracuse’s College of Arts and Sciences, the Department of Earth Sciences offers graduate and undergraduate degree opportunities in crustal evolution and tectonics, environmental sciences and climate change, hydrogeology, sedimentology and paleolimnology, geochemistry, and paleobiology.

New tools reveal mysteries of an ancient Arctic terrane

This is a satellite image of northern and western Alaska, Bering Strait, and the Chukotka Peninsula of Russia. -  NASA Worldview satellite image,
This is a satellite image of northern and western Alaska, Bering Strait, and the Chukotka Peninsula of Russia. – NASA Worldview satellite image,

The evolution and origin of Earth’s Arctic realm and the nature, location, and age of its major tectonic boundaries remain subjects of considerable uncertainty. This new compilation of studies from The Geological Society of America demonstrates the power of modern research tools to penetrate the effects of orogenesis and reconstruct the area’s pre-deformational tectonic and paleogeographic history.

The largest piece of continental crust that plays a role in Arctic tectonics is the Arctic Alaska-Chukotka terrane or microplate. This microplate includes northern Alaska and northeastern-most Russia, along with the adjacent continental shelves. Because of its size, understanding its origin and movements during the Paleozoic and Mesozoic are critical components of tectonic and paleogeographic models.

This new GSA Special Paper, edited by Julie A. Dumoulin and Allison B. Till of the U.S. Geological Survey, examines the Arctic Alaska-Chukotka microplate from the Late Proterozoic (about 240 million years ago) to the Devonian (from 360 to 410 million years ago), and includes the first compelling evidence for a rift event that may have detached the Arctic Alaska-Chukotka microplate from the Timanide margin of Baltica.

Yellowstone geyser eruptions influenced more by internal processes

<IMG SRC="/Images/994664490.jpg" WIDTH="350" HEIGHT="396" BORDER="0" ALT="This is a map showing the location of Daisy and Old Faithful geysers in Yellowstone's Upper Geyser Basin. Inset map of Yellowstone National Park shows the weather station at Yellowstone Lake, seismic stations LKWY and H17A, and strainmeter B944. – Images taken from: Shaul Hurwitz, Robert A. Sohn, Karen Luttrell, Michael Manga, "Triggering and modulation of geyser eruptions in Yellowstone National Park by earthquakes, earth tides, and weather", Journal of Geophysical Research: Solid Earth, DOI:10.1002/2013JB010803″>
This is a map showing the location of Daisy and Old Faithful geysers in Yellowstone’s Upper Geyser Basin. Inset map of Yellowstone National Park shows the weather station at Yellowstone Lake, seismic stations LKWY and H17A, and strainmeter B944. – Images taken from: Shaul Hurwitz, Robert A. Sohn, Karen Luttrell, Michael Manga, “Triggering and modulation of geyser eruptions in Yellowstone National Park by earthquakes, earth tides, and weather”, Journal of Geophysical Research: Solid Earth, DOI:10.1002/2013JB010803

The intervals between geyser eruptions depend on a delicate balance of underground factors, such as heat and water supply, and interactions with surrounding geysers. Some geysers are highly predictable, with intervals between eruptions (IBEs) varying only slightly. The predictability of these geysers offer earth scientists a unique opportunity to investigate what may influence their eruptive activity, and to apply that information to rare and unpredictable types of eruptions, such as those from volcanoes.

Dr. Shaul Hurwitz took advantage of a decade of eruption data-spanning from 2001 to 2011-for two of Yellowstone’s most predictable geysers, the cone geyser Old Faithful and the pool geyser, Daisy.

Dr. Hurwitz’s team focused their statistical analysis on possible correlations between the geysers’ IBEs and external forces such as weather, earth tides and earthquakes. The authors found no link between weather and Old Faithful’s IBEs, but they did find that Daisy’s IBEs correlated with cold temperatures and high winds. In addition, Daisy’s IBEs were significantly shortened following the 7.9 magnitude earthquake that hit Alaska in 2002.

The authors note that atmospheric processes exert a relatively small but statistically significant influence on pool geysers’ IBEs by modulating heat transfer rates from the pool to the atmosphere. Overall, internal processes and interactions with surrounding geysers dominate IBEs’ variability, especially in cone geysers.

Network for tracking earthquakes exposes glacier activity

Alaska’s seismic network records thousands of quakes produced by glaciers, capturing valuable data that scientists could use to better understand their behavior, but instead their seismic signals are set aside as oddities. The current earthquake monitoring system could be “tweaked” to target the dynamic movement of the state’s glaciers, suggests State Seismologist Michael West, who will present his research today at the annual meeting of the Seismological Society of America (SSA).

“In Alaska, these glacial events have been largely treated as a curiosity, a by-product of earthquake monitoring,” said West, director of the Alaska Earthquake Center, which is responsible for detecting and reporting seismic activity across Alaska.

The Alaska seismic network was upgraded in 2007-08, improving its ability to record and track glacial events. “As we look across Alaska’s glacial landscape and comb through the seismic record, there are thousands of these glacial events. We see patterns in the recorded data that raise some interesting questions about the glaciers,” said West.

As a glacier loses large pieces of ice on its leading edge, a process called calving, the Alaska Earthquake Center’s monitoring system automatically records the event as an earthquake. Analysts filter out these signals in order to have a clear record of earthquake activity for the region. In the discarded data, West sees opportunity.

“We have amassed a large record of glacial events by accident,” said West. “The seismic network can act as an objective tool for monitoring glaciers, operating 24/7 and creating a data flow that can alert us to dynamic changes in the glaciers as they are happening.” It’s when a glacier is perturbed or changing in some way, says West, that the scientific community can learn the most.

Since 2007, the Alaska Earthquake Center has recorded more than 2800 glacial events along 600 km of Alaska’s coastal mountains. The equivalent earthquake sizes for these events range from about 1 to 3 on the local magnitude scale. While calving accounts for a significant number of the recorded quakes, each glacier’s terminus – the end of any glacier where the ice meets the ocean – behaves differently. Seasonal variations in weather cause glaciers to move faster or slower, creating an expected seasonal cycle in seismic activity. But West and his colleagues have found surprises, too.

In mid-August 2010, the Columbia Glacier’s seismic activity changed radically from being relatively quiet to noisy, producing some 400 quakes to date. These types of signals from the Columbia Glacier have been documented every single month since August 2010, about the time when the Columbia terminus became grounded on sill, stalling its multi-year retreat.

That experience highlighted for West the value of the accidental data trove collected by the Alaska Earthquake Center. “The seismic network is blind to the cause of the seismic events, cataloguing observations that can then be validated,” said West, who suggests the data may add value to ongoing field studies in Alaska.

Many studies of Alaska’s glaciers have focused on single glacier analyses with dedicated field campaigns over short periods of time and have not tracked the entire glacier complex over the course of years. West suggests leveraging the data stream may help the scientific community observe the entire glacier complex in action or highlight in real time where scientists could look to catch changes in a glacier.

“This is low-hanging fruit,” said West of the scientific advances waiting to be gleaned from the data.

Magnitude of quake scales with maturity of fault, suggests new study by German scientist

The oldest sections of transform faults, such as the North Anatolian Fault Zone (NAFZ) and the San Andreas Fault, produce the largest earthquakes, putting important limits on the potential seismic hazard for less mature parts of fault zones, according to a new study to be presented today at the Seismological Society of America (SSA) 2014 Annual Meeting in Anchorage, Alaska. The finding suggests that maximum earthquake magnitude scales with the maturity of the fault.

Identifying the likely maximum magnitude for the NAFZ is critical for seismic hazard assessments, particularly given its proximity to Istanbul.

“It has been argued for decades that fault systems evolving over geological time may unify smaller fault segments, forming mature rupture zones with a potential for larger earthquake,” said Marco Bohnhoff, professor of geophysics at the German Research Center for Geosciences in Potsdam, Germany, who sought to clarify the seismic hazard potential from the NAFZ. “With the outcome of this study it would in principal be possible to improve the seismic hazard estimates for any transform fault near a population center, once its maturity can be quantified,” said Bohnhoff.

Bohnhoff and colleagues investigated the maximum magnitude of historic earthquakes along the NAFZ, which poses significant seismic hazard to northwest Turkey and, specifically, Istanbul.

Relying on the region’s extensive literary sources that date back more than 2000 years, Bohnhoff and colleagues used catalogues of historical earthquakes in the region, analyzing the earthquake magnitude in relation to the fault-zone age and cumulative offset across the fault, including recent findings on fault-zone segmentation along the NAFZ.

“What we know of the fault zone is that it originated approximately 12 million years ago in the east and migrated to the west,” said Bohnhoff. “In the eastern portion of the fault zone, individual fault segments are longer and the offsets are larger.”

The largest earthquakes of approximately M 8.0 are exclusively observed along the older eastern section of the fault zone, says Bohnhoff. The younger western sections, in contrast, have historically produced earthquakes of magnitude no larger than 7.4.

“While a 7.4 earthquake is significant, this study puts a limit on the current seismic hazard to northwest Turkey and its largest regional population and economical center Istanbul,” said Bohnhoff.

Bohnhoff compared the study of the NAFZ to the San Andreas and the Dead Sea Transform Fault systems. While the earlier is well studied instrumentally with few historic records, the latter has an extensive record of historical earthquakes but few available modern fault-zone investigations. Both of these major transform fault systems support the findings for the NAFZ that were derived based on a unique combination of long historical earthquake records and in-depth fault-zone studies.

Bohnhoff will present his study, “Fault-Zone Maturity Defines Maximum Earthquake Magnitude,” today at the SSA Annual Meeting. SSA is an international scientific society devoted to the advancement of seismology and the understanding of earthquakes for the benefit of society. Its 2014 Annual Meeting will be held Anchorage, Alaska on April 30 – May 2, 2014.

Warm US West, cold East: A 4,000-year pattern

<IMG SRC="/Images/485889256.jpg" WIDTH="350" HEIGHT="262" BORDER="0" ALT="University of Utah geochemist Gabe Bowen led a new study, published in Nature Communications, showing that the curvy jet stream pattern that brought mild weather to western North America and intense cold to the eastern states this past winter has become more dominant during the past 4,000 years than it was from 8,000 to 4,000 years ago. The study suggests global warming may aggravate the pattern, meaning such severe winter weather extremes may be worse in the future. – Lee J. Siegel, University of Utah.”>
University of Utah geochemist Gabe Bowen led a new study, published in Nature Communications, showing that the curvy jet stream pattern that brought mild weather to western North America and intense cold to the eastern states this past winter has become more dominant during the past 4,000 years than it was from 8,000 to 4,000 years ago. The study suggests global warming may aggravate the pattern, meaning such severe winter weather extremes may be worse in the future. – Lee J. Siegel, University of Utah.

Last winter’s curvy jet stream pattern brought mild temperatures to western North America and harsh cold to the East. A University of Utah-led study shows that pattern became more pronounced 4,000 years ago, and suggests it may worsen as Earth’s climate warms.

“If this trend continues, it could contribute to more extreme winter weather events in North America, as experienced this year with warm conditions in California and Alaska and intrusion of cold Arctic air across the eastern USA,” says geochemist Gabe Bowen, senior author of the study.

The study was published online April 16 by the journal Nature Communications.

“A sinuous or curvy winter jet stream means unusual warmth in the West, drought conditions in part of the West, and abnormally cold winters in the East and Southeast,” adds Bowen, an associate professor of geology and geophysics at the University of Utah. “We saw a good example of extreme wintertime climate that largely fit that pattern this past winter,” although in the typical pattern California often is wetter.

It is not new for scientists to forecast that the current warming of Earth’s climate due to carbon dioxide, methane and other “greenhouse” gases already has led to increased weather extremes and will continue to do so.

The new study shows the jet stream pattern that brings North American wintertime weather extremes is millennia old – “a longstanding and persistent pattern of climate variability,” Bowen says. Yet it also suggests global warming may enhance the pattern so there will be more frequent or more severe winter weather extremes or both.

“This is one more reason why we may have more winter extremes in North America, as well as something of a model for what those extremes may look like,” Bowen says. Human-caused climate change is reducing equator-to-pole temperature differences; the atmosphere is warming more at the poles than at the equator. Based on what happened in past millennia, that could make a curvy jet stream even more frequent and-or intense than it is now, he says.

Bowen and his co-authors analyzed previously published data on oxygen isotope ratios in lake sediment cores and cave deposits from sites in the eastern and western United States and Canada. Those isotopes were deposited in ancient rainfall and incorporated into calcium carbonate. They reveal jet stream directions during the past 8,000 years, a geological time known as middle and late stages of the Holocene Epoch.

Next, the researchers did computer modeling or simulations of jet stream patterns – both curvy and more direct west to east – to show how changes in those patterns can explain changes in the isotope ratios left by rainfall in the old lake and cave deposits.

They found that the jet stream pattern – known technically as the Pacific North American teleconnection – shifted to a generally more “positive phase” – meaning a curvy jet stream – over a 500-year period starting about 4,000 years ago. In addition to this millennial-scale change in jet stream patterns, they also noted a cycle in which increases in the sun’s intensity every 200 years make the jet stream flatter.

Bowen conducted the study with Zhongfang Liu of Tianjin Normal University in China, Kei Yoshimura of the University of Tokyo, Nikolaus Buenning of the University of Southern California, Camille Risi of the French National Center for Scientific Research, Jeffrey Welker of the University of Alaska at Anchorage, and Fasong Yuan of Cleveland State University.

The study was funded by the National Science Foundation, National Natural Science Foundation of China, Japan Society for the Promotion of Science and a joint program by the society and Japan’s Ministry of Education, Culture, Sports, Science and Technology: the Program for Risk Information on Climate Change.

Sinuous Jet Stream Brings Winter Weather Extremes

The Pacific North American teleconnection, or PNA, “is a pattern of climate variability” with positive and negative phases, Bowen says.

“In periods of positive PNA, the jet stream is very sinuous. As it comes in from Hawaii and the Pacific, it tends to rocket up past British Columbia to the Yukon and Alaska, and then it plunges down over the Canadian plains and into the eastern United States. The main effect in terms of weather is that we tend to have cold winter weather throughout most of the eastern U.S. You have a freight car of arctic air that pushes down there.”

Bowen says that when the jet stream is curvy, “the West tends to have mild, relatively warm winters, and Pacific storms tend to occur farther north. So in Northern California, the Pacific Northwest and parts of western interior, it tends to be relatively dry, but tends to be quite wet and unusually warm in northwest Canada and Alaska.”

This past winter, there were times of a strongly curving jet stream, and times when the Pacific North American teleconnection was in its negative phase, which means “the jet stream is flat, mostly west-to-east oriented,” and sometimes split, Bowen says. In years when the jet stream pattern is more flat than curvy, “we tend to have strong storms in Northern California and Oregon. That moisture makes it into the western interior. The eastern U.S. is not affected by arctic air, so it tends to have milder winter temperatures.”

The jet stream pattern – whether curvy or flat – has its greatest effects in winter and less impact on summer weather, Bowen says. The curvy pattern is enhanced by another climate phenomenon, the El Nino-Southern Oscillation, which sends a pool of warm water eastward to the eastern Pacific and affects climate worldwide.

Traces of Ancient Rains Reveal Which Way the Wind Blew

Over the millennia, oxygen in ancient rain water was incorporated into calcium carbonate deposited in cave and lake sediments. The ratio of rare, heavy oxygen-18 to the common isotope oxygen-16 in the calcium carbonate tells geochemists whether clouds that carried the rain were moving generally north or south during a given time.

Previous research determined the dates and oxygen isotope ratios for sediments in the new study, allowing Bowen and colleagues to use the ratios to tell if the jet stream was curvy or flat at various times during the past 8,000 years.

Bowen says air flowing over the Pacific picks up water from the ocean. As a curvy jet stream carries clouds north toward Alaska, the air cools and some of the water falls out as rain, with greater proportions of heavier oxygen-18 falling, thus raising the oxygen-18-to-16 ratio in rain and certain sediments in western North America. Then the jet stream curves south over the middle of the continent, and the water vapor, already depleted in oxygen-18, falls in the East as rain with lower oxygen-18-to-16 ratios.

When the jet stream is flat and moving east-to-west, oxygen-18 in rain is still elevated in the West and depleted in the East, but the difference is much less than when the jet stream is curvy.

By examining oxygen isotope ratios in lake and cave sediments in the West and East, Bowen and colleagues showed that a flatter jet stream pattern prevailed from about 8,000 to 4,000 years ago in North America, but then, over only 500 years, the pattern shifted so that curvy jet streams became more frequent or severe or both. The method can’t distinguish frequency from severity.

The new study is based mainly on isotope ratios at Buckeye Creek Cave, W. Va.; Lake Grinell, N.J.; Oregon Caves National Monument; and Lake Jellybean, Yukon.

Additional data supporting increasing curviness of the jet stream over recent millennia came from seven other sites: Crawford Lake, Ontario; Castor Lake, Wash.; Little Salt Spring, Fla.; Estancia Lake, N.M.; Crevice Lake, Mont.; and Dog and Felker lakes, British Columbia. Some sites provided oxygen isotope data; others showed changes in weather patterns based on tree ring growth or spring deposits.

Simulating the Jet Stream

As a test of what the cave and lake sediments revealed, Bowen’s team did computer simulations of climate using software that takes isotopes into account.

Simulations of climate and oxygen isotope changes in the Middle Holocene and today resemble, respectively, today’s flat and curvy jet stream patterns, supporting the switch toward increasing jet stream sinuosity 4,000 years ago.

Why did the trend start then?

“It was a when seasonality becomes weaker,” Bowen says. The Northern Hemisphere was closer to the sun during the summer 8,000 years ago than it was 4,000 years ago or is now due to a 20,000-year cycle in Earth’s orbit. He envisions a tipping point 4,000 years ago when weakening summer sunlight reduced the equator-to-pole temperature difference and, along with an intensifying El Nino climate pattern, pushed the jet stream toward greater curviness.

Computer models solve geologic riddle millions of years in the making

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

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

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

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

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

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

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

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

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

Earthquakes caused by clogged magma a warning sign of eruption, study shows

These images are public domain; just please credit the Alaska Volcano Observatory. The photographer was Cyrus Read. -  Alaska Volcano Observatory / Cyrus Read
These images are public domain; just please credit the Alaska Volcano Observatory. The photographer was Cyrus Read. – Alaska Volcano Observatory / Cyrus Read

New research in Geophysical Research Letters examines earthquake swarms caused by mounting volcanic pressure which may signal an imminent eruption. The research team studied Augustine Volcano in Alaska which erupted in 2006 and found that precursory earthquakes were caused by a block in the lava flow.

36 hours before the first magmatic explosions, a swarm of 54 earthquakes was detected across the 13-station seismic network on Augustine Island. By analyzing the resulting seismic waves, the authors found that the earthquakes were being triggered from sources within the volcano’s magma conduit.

“Our article talks about a special type of volcanic earthquake that we think is caused by lava breaking, something that usually can’t happen because lava is supposed to flow more like a liquid, rather than crack like a piece of rock,” said Dr. Helena Buurman from the University of Alaska Fairbanks. “Much like breaking a piece of chewing gum by stretching it really fast, lab tests show that hot lava can break when stretched quickly enough under certain pressures like those that you might find in the conduit of a volcano. “

The authors found that over the course of the two hour swarm, the earthquakes’ focus moved 35 meters deeper down into the magma conduit, an indication that the conduit was becoming clogged. The resulting buildup of pressure may have contributed to the explosive eruption the next day.

“We think that these earthquakes happened within the lava that was just beginning to erupt at the top of Augustine. The earthquakes show that the lava flow was grinding to a halt and plugging up the system. This caused pressure to build up from below, and resulted in a series of large explosions 36 hours later,” concluded Dr. Buurman. “We believe that these types of earthquakes can be used to signal that a volcano is becoming pressurized and getting ready to explode, giving scientists time to alert the public of an imminent eruption. ”

Nearby Georgia basin may amplify ground shaking from next quake

Tall buildings, bridges and other long-period structures in Greater Vancouver may experience greater shaking from large (M 6.8 +) earthquakes than previously thought due to the amplification of surface waves passing through the Georgia basin, according to two studies published by the Bulletin of the Seismological Society of America (BSSA). The basin will have the greatest impact on ground motion passing over it from earthquakes generated south and southwest of Vancouver.

“For very stiff soils, current building codes don’t include amplification of ground motion,” said lead author Sheri Molnar, a researcher at the University of British Columbia. “While the building codes say there should not be any increase or decrease in ground motion, our results show that there could be an average amplification of up to a factor of three or four in Greater Vancouver.”

The research provides the first detailed studies of 3D earthquake ground motion for a sedimentary basin in Canada. Since no large crustal earthquakes have occurred in the area since the installation of a local seismic network, these studies offer refined predictions of ground motion from large crustal earthquakes likely to occur.

Southwestern British Columbia is situated above the seismically active Cascadia subduction zone. A complex tectonic region, earthquakes occur in three zones: the thrust fault interface between the Juan de Fuca plate, which is sliding beneath the North America plate; within the over-riding North America plate; and within the subducting Juan de Fuca plate.

Molnar and her colleagues investigate the effect the three dimensional (3D) deep basin beneath Greater Vancouver has on the earthquake-generated waves that pass through it. The Georgia basin is one in a series of basins spanning form California to southern Alaska along the Pacific margin of the North America and is relatively wide and shallow. The basin is filled with sedimentary layers of silts, sands and glacial deposits.

While previous research suggested how approximately 100 meters of material near the surface would affect ground shaking, no studies had looked at the effect of the 3D basin structure on long period seismic waves.

To fill in that gap in knowledge, Molnar and colleagues performed numerical modeling of wave propagation, using various scenarios for both shallow quakes (5 km in depth) within the North America plate and deep quakes (40 – 55 km in depth) within the Juan de Fuca subducting plate, the latter being the most common type of earthquake. The authors did not focus on earthquakes generated by a megathrust rupture of the Cascadia subduction zone, a scenario studied previously by co-author Kim Olsen of San Diego State University.

For these two studies, the authors modeled 10 scenario earthquakes for the subducting plate and 8 shallow crustal earthquakes within the North America plate, assuming rupture sites based on known seismicity. The computational analyses suggest the basin distorts the seismic radiation pattern – how the energy moves through the basin – and produces a larger area of higher ground motions. Steep basin edges excite the seismic waves, amplifying the ground motion.

The largest surface waves generated across Greater Vancouver are associated with earthquakes located approximately 80 km or more, south-southwest of the city, suggest the authors.

“The results were an eye opener,” said Molnar. “Because of the 3D basin structure, there’s greater hazard since it will amplify ground shaking. Now we have a grasp of how much the basin increases ground shaking for the most likely future large earthquakes.”

In Greater Vancouver, there are more than 700 12-story and taller commercial and residential buildings, and large structures – high-rise buildings, bridges and pipelines – that are more affected by long period seismic waves, or long wavelength shaking. “That’s where these results have impact,” said Molnar.

Longmanshen fault zone still hazardous, suggest new reports

The 60-kilometer segment of the fault northeast of the 2013 Lushan rupture is the place in the region to watch for the next major earthquake, according to research published in Seismological Research Letters (SRL). Research papers published in this special section of SRL suggest the 2008 Wenchuan earthquake triggered the magnitude 6.6 Lushan quake.

Guest edited by Huajian Yao, professor of geophysics at the University of Science and Technology of China, the special section includes eight articles that present current data, description and preliminary analysis of the Lushan event and discuss the potential of future earthquakes in the region.

More than 87,000 people were killed or went missing as a result of the 2008 magnitude 7.9 Wenchuan earthquake in China’s Sichuan province, the largest quake to hit China since 1950. In 2013, the Lushan quake occurred ~90 km to the south and caused 203 deaths, injured 11,492 and affected more than 1.5 million people.

“After the 2008 magnitude 7.9 Wenchuan earthquake along the Longmenshan fault zone in western Sichuan of China, researchers in China and elsewhere have paid particular attention to this region, seeking to understand how the seismic hazard potential changed in the southern segment of the fault and nearby faults,” said Yao. “Yet the occurrence of this magnitude 6.6 Lushan event surprised many. The challenge of understanding where and when the next big quake will occur after a devastating seismic event continues after this Lushan event, although we now have gained much more information about this area.”

Preliminary rupture details

The southern part of the Longmenshan fault zone is complex and still only moderately understood. Similar to the central segment where the 2008 Wenchuan event occurred, the southern segment, which generated the Lushan rupture, includes the Wenchuan-Maoxian fault, Beichuan-Yingxiu fault, the Pengxian-Guanxian fault and Dayi faults, a series of sub-parallel secondary faults.

Although the Lushan earthquake’s mainshock did not break to the surface, the strong shaking still caused significant damage and casualties in the epicentral region. Three papers detail the rupture process of the Lushan quake. Libo Han from the China Earthquake Administration and colleagues provide a preliminary analysis of the Lushan mainshock and two large aftershocks, which appear to have occurred in the upper crust and terminated at a depth of approximately 8 km. While the Lushan earthquake cannot be associated with any identified surface faults, Han and colleagues suggest the quake may have occurred on a blind thrust fault subparallel to the Dayi fault, which lies at and partly defines the edge of the Chengdu basin. Based on observations from extensive trenching and mapping of fault activity after both the Wenchuan and Lushan earthquakes, Chen Lichun and colleagues from the China Earthquake Administration suggest the Lushan quake spread in a “piggyback fashion” toward the Sichuan basin, but with weaker activity and lower seismogenic potential than the Wenchuan quake. And Junju Xie, from the China Earthquake Administration and Beijing University of Technology, and colleagues examined the vertical and horizontal near-source strong motion from the Mw 6.8 Lushan earthquake. The vertical ground motion is relatively weak for this event, likely due to the fact that seismic energy dissipated at the depth of 12-25 km and the rupture did not break through the ground surface.

Possible link between Lushan and Wenchuan earthquakes

Were the Lushan and Wenchuan earthquakes related? And if so, what is the relationship? Some researchers consider the Lushan quake to be a strong aftershock of the Wenchuan quake, while others see them as independent events. In this special section, researchers tackled the question from various perspectives.

To discover whether the Lushan earthquake was truly independent from the Wenchuan quake, researchers need to have an accurate picture of where the Lushan quake originated. Yong Zhang from the GFZ German Research Centre for Geosciences and the China Earthquake Administration and colleagues begin this process by confirming a new hypocenter for Lushan. To find this place where the fault first began to rupture, the researchers analyze near-fault strong-motion data (movements that took place at a distance of up to a few tens of kilometers away from the fault) as well as long distance (thousands of kilometers ) teleseismic data.

Using their newly calculated location for the hypocenter, Zhang and colleagues now agree with earlier studies that suggest the initial Lushan rupture was a circular rupture event with no predominant direction. But they note that their calculations place the major slip area in the Lushan quake about 40 to 50 kilometers apart from the southwest end of the Wenchuan quake fault. This “gap” between the two faults may hold increased seismic hazards, caution Zhang and colleagues.

Ke Jia of Beijing University and colleagues explore the relationship of the two quakes with a statistical analysis of aftershocks in the region as well as the evolution of shear stress in the lower crust and upper mantle in the broader quake region. Their analyses suggest that the Wenchuan quake did affect the Lushan quake in an immediate sense by changing the overall background seismicity in the region. If these changes in background seismicity are taken into account, the researchers calculate a 62 percent probability that Lushan is a strong aftershock of Wenchuan.

Similarly, Yanzhao Wang from the China Earthquake Administration and colleagues quantified the stress loading of area faults due to the Wenchuan quake and suggest the change in stress may have caused the Lushan quake to rupture approximately 28.4 to 59.3 years earlier than expected. They conclude that the Lushan earthquake is at least 85 percent of a delayed aftershock of the Wenchuan earthquake, rather than due solely to long-term tectonic loading.

After the Wenchuan quake, researchers immediately began calculating stress changes on the major faults surrounding the rupture zone, in part to identify where dangerous aftershocks might occur and to test how well these stress change calculations might work to predict new earthquakes. As part of these analyses, Tom Parsons of the U.S. Geological Survey and Margarita Segou of GeoAzur compared data collected from the Wenchuan and Lushan quakes with data on aftershocks and stress change in four other major earthquakes, including the M 7.4 Landers and Izmit quakes in California and Turkey, respectively, and the M 7.9 Denali quake in Alaska and the M 7.1 Canterbury quake in New Zealand.

Their comparisons reveal that strong aftershocks similar to Lushan are likely to occur where there is highest overall aftershock activity, where stress change is the greatest and on well-developed fault zones. But they also note that by these criteria, the Lushan quake would only have been predicted by stress changes, and not the clustering of aftershocks following the 2008 Wenchuan event.

Future earthquakes in this region

After Wenchuan and Lushan, where should seismologists and other look for the next big quake in the region? After the 2008 Wenchuan quake, seismologists were primed with data to help predict where and when the next rupture might be in the region. The data suggested that the Wenchuan event would increase seismic stress in the southern Longmenshan fault that was the site of the 2013 Lushan quake. But that information alone could not predict that the southern Longmenshan fault would be the next to rupture after Wenchuan, say Mian Liu of the University of Missouri and colleagues, because the Wenchuan earthquake also increased the stress on numerous others faults in the region

Additional insights can be gained from seismic moment studies, according to Liu and colleagues. Moment balancing compares how much seismic strain energy is accumulated along a fault over a certain period with the amount of strain energy released over the same period. In the case of the Longmenshan fault, there had been a slow accumulation of strain energy without release by a major seismic event for more than a millennium. After the Wenchuan quake, the southern part of the Longmenshan fault became the fault with the greatest potential for a quake. And now, after Lushan, Liu and colleagues say that the 60 kilometer-long segment of the fault northeast of the Lushan rupture is the place in the region to watch for the next major earthquake.