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One of the 73 quartz crystals used in the study. They averaged about one millimeter in diameter. Credit: Guilherme Gualda, Vanderbilt University
One of the 73 quartz crystals used in the study. They averaged about one millimeter in diameter. Credit: Guilherme Gualda, Vanderbilt University
Super-eruptions - volcanic events large enough to devastate the entire planet - give only about a year's warning before they blow.

That is the conclusion of a new microscopic analysis of quartz crystals in pumice taken from the Bishop Tuff in eastern California, which is the site of the super-eruption that formed the Long Valley Caldera 760,000 years ago.

The study is described in the paper "The year leading to a supereruption" by Guilherme Gualda, associate professor of earth and environment sciences at Vanderbilt University, and Stephen Sutton at the University of Chicago published July 20 in the journal PLOS One.

"The evolution of a giant, super-eruption-feeding magma body is characterized by events taking place at a variety of time scales," said Gualda. Tens of thousands of years are needed to prime the crust to generate sufficient eruptible magma. Once established, these melt-rich, giant magma bodies are unstable features that last for only centuries to few millennia. "Now we have shown that the onset of the process of decompression, which releases the gas bubbles that power the eruption, starts less than a year before eruption."

Gualda and Sutton analyzed dozens of small quartz crystals from the Bishop Tuff. Previous investigations of quartz crystals from several super-eruptions, including Long Valley, have noted that they have distinctive surface rims. These studies concluded that the rims formed in less than a century before eruption.

The new study uses a more accurate method for measuring rim growth times pinned on variations in the concentration of titanium in the crystal. Titanium is one of the few impurities that is incorporated into quartz in appreciable amounts and it diffuses fast enough to permit probing of time scales as short as minutes. However, it is extremely difficult to measure the small levels of titanium involved at sufficient spatial resolution. So the researchers established that the concentration of titanium in quartz directly correlates with the amount of light produced when a material is bombarded by electrons, an effect called cathodoluminescence. This allowed them to use cathodoluminescence images to make high-resolution measurements of variations in titanium concentration and, based on this, to determine rim growth times and growth rates.

"Maximum rim growth times span from approximately 1 minute to 35 years, with a median of approximately 4 days. More than 70 percent of rim growth times are less than 1 year, showing that quartz rims have mostly grown in the days to months prior to eruption... . Growth took place under conditions of high supersaturation suggesting that rim growth marks the onset of decompression and the transition from pre-eruptive to syn-eruptive conditions," the paper summarized.

According to Gualda, the decompression period would likely be accompanied by the expansion of the magma body which should have detectable effects on the Earth's surface. While more work is needed to understand what exactly the signs at the surface would be, the study suggests that signs of an impending super-eruption would start to be felt within a year of eruption, and they would intensify as the eruption neared.

Very large eruptions -- including super-eruptions -- have taken place in a number of places worldwide in the recent geological past. The Taupo Volcanic Zone in New Zealand was the site of the most recent super-eruption -- the Oruanui eruption at 26,500 years -- and it includes deposits from more than a dozen very large eruptions that took place in the last couple of million years. Campi Flegrei in Italy produced a very large eruption 40,000 years ago. Indonesia was the site of the Toba super-eruption in Sumatra 75,000 years ago and the Tambora eruption in 1815. In the United States, Yellowstone has experienced three super-eruptions over the last two million years. In light of this evidence, it seems inevitable that another super-eruption will strike the Earth in the future.

"As far as we can determine, none of these places currently house the type of melt-rich, giant magma body needed to produce a super-eruption," said Gualda. "However, they are places where super-eruptions have happened in the past so are more likely to happen in the future."

Gualda and Sutton's study provides new insights into the timescales over which the initiation of such a potentially civilization-ending event would take place.
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Source:
The above post is reprinted from materials provided by Vanderbilt University. The original item was written by David F Salisbury.

Reference:
Guilherme A. R. Gualda, Stephen R. Sutton. The Year Leading to a Supereruption. PLOS ONE, 2016; 11 (7): e0159200 DOI: 10.1371/journal.pone.0159200
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This is a historic photo of the San Francisco earthquake of 1906. Credit: USGS Earth Observatory.
This is a historic photo of the San Francisco earthquake of 1906. Credit: USGS Earth Observatory.
The triggering of small, deep earthquakes along California's San Andreas Fault reveals depth-dependent frictional behavior that may provide insight into patterns signaling when a major quake could be on the horizon, according to a paper released this week by the Proceedings of the National Academy of Sciences (PNAS).

The study, which was led by the U.S. Geological Survey and Los Alamos National Laboratory, reports that the deepest part of California's 800-mile-long San Andreas Fault is weaker than expected and produces small earthquakes in response to tidal forces.

"These findings provide previously inaccessible information about the San Andreas Fault activity and strength," said Los Alamos National Laboratory's Paul Johnson, a coauthor on the paper and geophysicist in the Lab's Earth and Environmental Sciences Division. "The study's discovery of low-frequency-earthquake (LFE) and tidal triggering of the San Andreas Fault gives seismologists new warning signals and information about slightly more predictable triggers of quakes to come."

Los Alamos maintains technical expertise in seismology and the behavior of Earth's crust as a part of its role monitoring underground nuclear testing globally and applies that expertise to other national challenges, including earthquakes.

The team used a data set of 81,000 LFEs since 2008 to match LFEs to tides. They determined in addition to being modulated with the semidiurnal (twice daily) tides, LFEs are also modulated by fortnightly tides. The contrasting relationship between the LFE responses observed at two different tidal timescales should serve as a powerful constraint on understanding frictional behavior and stress transfer on the deep San Andreas.

"The findings provide new information regarding the fault zone structure with depth," Johnson said. The authors found that deep, small, low-frequency earthquakes (LFEs) on the San Andreas Fault are most likely to occur during the waxing period approaching a full or new moon within the fortnightly tide period of 14.7 days. The fortnightly tide modulates the semidiurnal (twice a day) tide. LFEs preferentially occur not when the tidal amplitude is highest, as might be expected, but when the tidal amplitude most exceeds its previous value, the authors found. LFEs correlate more strongly with larger-amplitude shear stress.

Previous studies have found stronger tidal semidiurnal variation for deeper, continuously active LFE families. The team used two models to explain variations: One, based on friction studies, posited LFEs occur when stress accelerates slip. The other model suggests LFEs occur by simple threshold failure but are driven indirectly by tidally modulated creep. Regardless of which tidal triggering model is correct, the inverse relationship between the strength of the semidiurnal and fortnightly modulations provides a key insight into the mechanics of LFEs and the structure of the deep fault, according to the paper.

"The pattern of LFEs tells us something about loading rates and stress conditions in the deep part of the fault," said Andrew Delorey, a seismologist with Los Alamos. "We don't know to what extent the deep part of the fault where LFEs occur is coupled to the shallow part of the fault where regular earthquakes occur. We may find that as stress increases and approaches failure in the shallow fault, where large earthquakes occur, it will affect the pattern of LFEs in a way that allows us to use LFE behavior to infer conditions in the shallow fault."

While tidal triggering of earthquakes is found only for select environments, triggering of tremor has been found almost everywhere that tectonic tremor is observed, generating insights into the mechanics of the brittle transition zones. The response to the tidal stress carries otherwise inaccessible information about fault strength and rheology.

Source:
The above post is reprinted from materials provided by DOE/Los Alamos National Laboratory.

Reference:
Nicholas J. van der Elst, Andrew A. Delorey, David R. Shelly, Paul A. Johnson. Fortnightly modulation of San Andreas tremor and low-frequency earthquakes. Proceedings of the National Academy of Sciences, 2016; 201524316 DOI: 10.1073/pnas.1524316113

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Schematic summary of research findings showing the sequence of slip behavior. Credit: UC Riverside
Schematic summary of research findings showing the sequence of slip behavior. Credit: UC Riverside
Preparation and good timing enabled Gareth Funning and a team of researchers to collect a unique data set following the 2014 South Napa earthquake that showed different parts of the fault, sometimes only a few kilometers apart, moved at different speeds and at different times.

Aided by GPS measurements made just weeks before the earthquake and data from a new radar satellite, the team found post-earthquake fault movement, known as afterslip, was concentrated in areas of loosely packed sediment. Areas where the fault passed through bedrock tended to slip more during the actual earthquake.

Sections of Highway 12, which runs through the earthquake zone, were broken during the initial 6.0 magnitude earthquake and were further damaged in the coming days due to afterslip. In some areas the afterslip damage exceeded the initial damage from the earthquake.

"No one has seen variability in afterslip like we saw," said Funning, an associate professor of earth sciences at the University of California, Riverside. "This helps us address a big question: Can we use geology as a proxy for fault behavior? Our findings suggest there is a relationship between those two things."

The findings could have significant implications for earthquake hazard models, and also for planning earthquake response. If geological information can give a guide to the likely extent of future earthquakes, better forecasts of earthquake damage will be possible. And if areas likely to experience afterslip can be identified in advance, it can be taken into account when building or repairing infrastructure that crosses those faults

California, in particular the Hayward and Calaveras Faults, which run along the east side of the San Francisco Bay, seems more susceptible to afterslip than other earthquake-prone regions throughout the world, Funning said.

The findings on the South Napa earthquake were recently published in paper, "Spatial variations in fault friction related to lithology from rupture and afterslip of the 2014 South Napa, California, earthquake," in the journal Geophysical Research Letters.

Funning's work in the region just north of San Francisco dates back to 2006, when he was a post-doctoral researcher at UC Berkeley and noticed the area wasn't that well studied, at least compared to the central Bay Area.

He continued the research after he was hired at UC Riverside and received funding from the United States Geological Survey to conduct surveys using GPS sensors in earthquake prone areas throughout Marin, Napa, Sonoma, Mendocino and Lake counties.

He began the most recent survey in July 2014. When the South Napa earthquake struck on Aug. 24, 2014, he and three other researchers were in Upper Lake, CA in Lake County, about 70 miles north of the earthquake's epicenter, making additional measurements.

The earthquake occurred at 3:20 a.m. By noon, Funning and the other researchers, Michael Floyd (a former post-doctoral researcher with Funning who is now a research scientist at the Massachusetts Institute of Technology), Jerlyn Swiatlowski (a graduate student working with Funning) and Kathryn Materna (a graduate student at UC Berkeley), had deployed additional GPS sensors in the earthquake zone in locations that they had, fortuitously, measured just seven weeks earlier.

In total, there were more than 20 GPS sensors set up by Funning's team and scientists from the United States Geological Survey. They left the equipment out for four weeks following the earthquake.

They then combined the GPS sensor data with remote sensing data. The South Napa earthquake was the first major earthquake to be imaged by Sentinel-1A, a European radar imaging satellite launched in 2014 that provides higher resolution information than was previously available.
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Source:
The above post is reprinted from materials provided by University of California - Riverside. The original item was written by Sean Nealon.

Reference:
Michael A. Floyd, Richard J. Walters, John R. Elliott, Gareth J. Funning, Jerry L. Svarc, Jessica R. Murray, Andy J. Hooper, Yngvar Larsen, Petar Marinkovic, Roland Bürgmann, Ingrid A. Johanson, Tim J. Wright. Spatial variations in fault friction related to lithology from rupture and afterslip of the 2014 South Napa, California, earthquake. Geophysical Research Letters, 2016; DOI: 10.1002/2016GL069428
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Layout of the network of acoustic transponders (French in red, German in yellow) in the Sea of Marmara, on either side of the submarine segment of the North Anatolian fault (NAF), whose assumed trace is shown by the dashed line. Credit: © J-Y Royer / CNRS-UBO LDO
Layout of the network of acoustic transponders (French in red, German in yellow) in the Sea of Marmara, on either side of the submarine segment of the North Anatolian fault (NAF), whose assumed trace is shown by the dashed line. Credit: © J-Y Royer / CNRS-UBO LDO
To monitor a segment of the North Anatolian seismic fault near Istanbul, an international team of researchers, in particular from CNRS and Université de Bretagne Occidentale, has installed a network of transponders on the floor of the Sea of Marmara. The aim is to measure motion of the sea floor on either side of this segment. The data collected during the first six months reveals that the fault is probably locked in the region of this segment, suggesting that there is a progressive build-up of energy that could be released suddenly. This could cause a major earthquake in the Istanbul area.

The study, carried out by a collaboration of researchers from France, Germany and Turkey, is published in Geophysical Research Letters.

The North Anatolian fault, which caused destructive earthquakes in Turkey in 1999, is comparable to the San Andreas fault in California. It marks the boundary between the Eurasian and Anatolian tectonic plates, which move relative to each other at a speed of around 2 cm per year. The behavior of one underwater segment of the fault, located a few tens of kilometers from Istanbul in the Sea of Marmara, particularly intrigues researchers, since there has apparently been no seismic activity there since the eighteenth century. How does this segment behave? Does it continuously creep? Does it regularly give way, occasionally causing small, low-magnitude quakes? Or is it locked, making it likely that it will one day rupture and cause a major earthquake?

Observing the motion of a submarine fault in situ over a period of several years is no easy matter. To meet this challenge, the researchers are testing an innovative underwater remote sensing method, using active, autonomous acoustic transponders remotely accessible from the sea surface. Placed on the sea floor on either side of the fault at a depth of 800 meters, the transponders take it in turns to interrogate each other in pairs, and measure the round-trip time of an acoustic signal between them. These time lapses are then converted into distances between the transponders. The variation in these distances over time is used to detect motion of the sea floor and any deformation of the network of transponders, and thus infer the displacement of the fault. Specifically, a network of ten French and German transponders was set up during an initial sea cruise1 in October 2014. The first six months of data (travel time, temperature, pressure and stability)2 have confirmed that the system is performing well. Following calculations, the data show no significant motion of the monitored fault, within the network's resolution limits. The distances between the transponders, which are between 350 and 1700 meters apart, are measured with a resolution of 1.5 to 2.5 mm. The segment is therefore probably locked or nearly locked, and is accumulating stress that could trigger an earthquake. However, it will be necessary to acquire data for several years in order to confirm this observation or show that this part of the fault has a more complex behavior.

Going beyond this specific demonstration, if this approach, known as acoustic seafloor geodesy, proves to be robust in the long term (in this case, three to five years are planned, within the limits of the autonomy of the batteries), it could be included within a permanent underwater observatory as an addition to other observations (seismology, gas bubble emission, etc) for in situ real-time monitoring of the activity of this particular fault, or of other active submarine faults elsewhere in the world.

The work was carried out by the Laboratoire Domaines Océaniques3 (LDO, CNRS/Université de Bretagne Occidentale), in collaboration with the Laboratoire Littoral Environnement et Sociétés (CNRS/Université de La Rochelle), GEOMAR (Kiel, Germany), Centre Européen de Recherche et d'Enseignement de Géosciences de l'Environnement (CNRS/Collège de France/AMU/IRD), the IFREMER's Laboratoire Géosciences Marines, the Eurasian Institute of Earth Sciences at the Istanbul Technical University (Turkey), and the Kandilli Observatory and Earthquake Research Institute at Bogazici University, Istanbul. This paper is dedicated to the memory of the Principal Investigator of the project, Anne Deschamps, CNRS researcher at LDO, who passed away shortly after leading the successful deployment of the acoustic transponders.

Notes:
  1. By the oceanographic vessel Pourquoi pas?, with the help of IFREMER's Laboratoire Géosciences Marines
  2. The data was collected from the surface during the campaign of the German oceanographic vessel Poseidon in April 2015.
  3. This laboratory is attached to the Institut Universitaire Européen de la Mer -- IUEM (CNRS/UBO/IRD)
Source:
The above post is reprinted from materials provided by Le Centre national de la recherche scientifique (CNRS).

Reference:
P. Sakic, H. Piété, V. Ballu, J.-Y. Royer, H. Kopp, D. Lange, F. Petersen, M. S. Özeren, S. Ergintav, L. Geli, P. Henry, A. Deschamps. No significant steady state surface creep along the North Anatolian Fault offshore Istanbul: Results of 6 months of seafloor acoustic ranging. Geophysical Research Letters, 2016; DOI: 10.1002/2016GL069600
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City of Naples with Mount Vesuvius at sunset.
Credit: Antonsusi
New work by Italian geochemists seems to indicate that the current ground movement around one of the world's most dangerous volcano systems may be due to gas pressure, and not because of a surge of volcanic magma. This work was recently presented at the Goldschmidt conference in Yokohama, Japan (30 June 2016).

The Campi Flegrei (Phlegraean Fields), just across the Bay of Naples from the famous Vesuvius volcano, is amongst the most dangerous volcanos on Earth. In the past it has been capable of a "VEI 7" eruption (Volcanic Explosivity Index of 7, meaning that it has produced an explosive eruption even bigger than the famous Krakatoa eruption of 1883). However, this was around 40,000 years ago. The last eruption, "VEI 2" occurred in 1538 AD.

Because of the geological instability in the area, the land in this area can rise and fall by several metres over just a few years, a phenomenon known as Bradyseism. The last few years have seen the ground in the area begin to rise again, with a 38 cm rise recorded since late 2005. There have been worries that this may presage an eruption.

The last serious geological unrest in the area was in 1982-84, which saw ground levels rise by up to 1.8m. Most scientists think that the movement in this period was caused by mixed magmatic- hydrothermal activity (although some recent papers in the geochemical literature have suggested a major role for hydrothermal processes supported by deep magmatic gases, with pressurised water causing the land to rise). On the other hand, consensus exists that the current activity is caused by molten magma movement and accumulation under the Campi -- which carries a greater risk of an eruption. Now however, a group of Geochemists from Second University of Naples and the Vesuvius Observatory think that the consensus has got it exactly the wrong way round.

Lead researcher, Professor Roberto Moretti (Seconda Università degli Studi di Napoli) commented: "Everyone accepts the geochemical evidence that current activity has different causes to that of 1982-84. Most geochemists are now showing that the 1982-84 movement was caused by hydrothermal activity and the current activity is caused by magma, but we think that it's exactly the other way round. We have checked geochemical records going back over more than 30 years, and our ongoing interpretation -- looking at released gases and physical signals -- seem to be consistent with current activity being hydrothermal, with the support of deep magmatic gases, rather than due to magma migration or growth of a shallow (3-4 km deep) magma chamber. We believe that this magma dynamics characterized the 1982-84 episode.

This is apparently better news, at least for now; activity in which magma moves upward and accumulates tends to be associated with an increased chance of an eruption. However the change from hydrothermal to magmatic activity can take place at any time, so we're not in a position to say that everything is well under the Campi Flegrei. The Campi Flegrei is still a very volatile place. What it does show is the difficulty in interpreting the data, even from one of the most-studied volcanic areas in the world. Reconciling all of the data is a major issue, despite our efforts.

Achieving such a unique and consistent interpretation would probably require direct access to underground geochemical, geophysical and geochemical information in the areas of interest. However, there is still a debate over the safety of drilling in such a volatile area."

Commenting, Professor Jon Blundy (University of Bristol) said: "Interpreting the causes of ground movement at restless volcanoes is an enduring problem for volcanologists. Both hot gases (steam) and magma are candidate causes, but with quite different implications for future eruptive activity. Moretti and others make a compelling case for gas, rather than magma, as the cause of the latest bradyseisms at Campi Flegrei. Their methods could be used at other restless volcanoes where there is evidence of ground uplift".
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Source:
The above post is reprinted from materials provided by Goldschmidt Conference.
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Potential quake zone. Solid red line indicates an area of about 24,000 square miles that could move during a subduction-zone earthquake, affecting 140 million people or more. The dashed line represents a scenario in which the slip might take place along a separate fault. Credit: Chris Small/Lamont-Doherty Earth Observatory
Potential quake zone. Solid red line indicates an area of about 24,000 square miles that could move during a subduction-zone earthquake, affecting 140 million people or more. The dashed line represents a scenario in which the slip might take place along a separate fault. Credit: Chris Small/Lamont-Doherty Earth Observatory
A huge earthquake may be building beneath Bangladesh, the most densely populated nation on earth. Scientists say they have new evidence of increasing strain there, where two tectonic plates underlie the world's largest river delta. They estimate that at least 140 million people in the region could be affected if the boundary ruptures; the destruction could come not only from the direct results of shaking, but changes in the courses of great rivers, and in the level of land already perilously close to sea level.

The newly identified threat is a subduction zone, where one section of earth's crust, or a tectonic plate, is slowly thrusting under another. All of earth's biggest known earthquakes occur along such zones; these include the Indian Ocean quake and tsunami that killed some 230,000 people in 2004, and the 2011 Tohoku quake and tsunami off Japan, which swept away more than 20,000 and caused the Fukushima nuclear disaster. Up to now, all known such zones were only under the ocean; this one appears to be entirely under the land, which greatly multiplies the threat. The findings appear in this week's issue of Nature Geoscience.

Subduction-zone quakes generally occur where plates of heavy ocean crust slowly dive offshore beneath the lighter rocks of adjoining continents, or under other parts of the seafloor. Sometimes sections get stuck against each other over years or centuries, and then finally slip, moving the earth. Scientists knew of the plate boundary in and around Bangladesh, but many assumed it to be sliding only horizontally near the surface, where it sometimes causes fairly large, but less damaging earthquakes in areas that are not as densely populated. However, the authors of the new research say movements on the surface over the past decade show that subduction is taking place below, and that part of the plate juncture is locked and loading up with stress. They are not forecasting an imminent great earthquake, but say it is an "underappreciated hazard."

"Some of us have long suspected this hazard, but we didn't have the data and a model," said lead author Michael Steckler, a geophysicist at Columbia University's Lamont-Doherty Earth Observatory. "Now we have the data and a model, and we can estimate the size." He said strain between the plates has been building for at least 400 years -- the span of reliable historical records, which lack reports of any mega-quake. When an inevitable release comes, the shaking is likely to be larger than 8.2, and could reach a magnitude of 9, similar to the largest known modern quakes, said Steckler. "We don't know how long it will take to build up steam, because we don't know how long it was since the last one," he said. We can't say it's imminent or another 500 years. But we can definitely see it building."

The newly identified zone is an extension of the same tectonic boundary that caused the 2004 Indian Ocean undersea quake, some 1,300 miles south. As the boundary reaches southeast Asia, the complexity of the motions along it multiply, and scientists do not completely understand all of them. But basically, they say, a giant plate comprising India and much of the Indian Ocean has been thrusting northeasterly into Asia for tens of millions of years. This collision has caused the Himalayas to rise to the north, bringing events like the 2015 Nepal quake that killed 8,000 people. Bangladesh, India's neighbor, lies on the far eastern edge of this plate, but pressure from the collision seems to be warping Asia clockwise around the top of Bangladesh, ending up largely in the next country over, Myanmar. This wraparound arrangement has resulted in a crazy quilt of faults and quakes in and around Bangladesh. Among the largest, a 1762 subduction-zone quake near the southern coast killed at least 700 people. This January, a magnitude 6.7 event in adjoining eastern India killed more than 20. There have been dozens of large quakes in between, but the assumption was that no actual subduction was taking place under Bangladesh itself, seeming to insulate the region from a truly gigantic one. The new study undercuts this idea.

Starting in 2003, U.S. and Bangladeshi researchers set up about two dozen ground-positioning (GPS) instruments linked to satellites, capable of tracking tiny ground motions. Ten years of data now show that eastern Bangladesh and a bit of eastern India are pushing diagonally into western Myanmar at a rapid clip -- 46 millimeters per year, or about 1.8 inches. Combined with existing GPS data from India and Myanmar, the measurements show that much of the resulting strain has been taken up by several known, slowly moving surface faults in Myanmar and India. But the rest of the movement -- about 17 millimeters, or two-thirds of an inch per year -- is shortening the distance from Myanmar to Bangladesh. This has been going on for a long time, and the results are clearly visible: neatly parallel north-south ranges of mountains draping the landscape, like a carpet being shoved against a wall. The researchers interpret the shortening pattern to mean that subduction is taking place below, and that a huge zone -- about 250 kilometers by 250 kilometers, more than 24,000 square miles -- is locked and building pressure, just a few miles below the surface. The zone includes Bangladesh's densely packed capital of Dhaka, a megalopolis of more than 15 million.

Steckler says that, assuming fairly steady motion over the last 400 years, enough strain has built for the zone to jump horizontally by about 5.5 meters, or 18 feet, if the stress is released all at once. If strain has been building longer, it could be up to 30 meters, or almost 100 feet. The land would also move vertically, to a lesser extent. This is the worst-case scenario; in the best case, only part would slip, and the quake would be smaller and farther from Dhaka, said Steckler.

In any case, Bangladesh and eastern India sit atop a landscape vulnerable even to moderate earthquakes: the vast delta of the Ganges and Brahmaputra rivers. This is basically a pile of mud as deep as 12 miles, washed from the Himalayas to the coast, covering the subduction zone. In a quake, this low-lying substrate would magnify the shaking like gelatin, and liquefy in many places, sucking in buildings, roads and people, said study coauthor Syed Humayun Akhter, a geologist at Dhaka University. The great rivers -- 10 miles across in places -- could jump their banks and switch course, drowning everything in the way; there is in fact evidence that such switches have happened in previous centuries.

Akhter says that fast-growing, poor Bangladesh is unprepared; no building codes existed before 1993, and even now, shoddy new construction flouts regulations. Past quake damages and deaths are no indicator of what could happen now, he said; population and infrastructure have grown so fast that even fairly moderate events like those of past centuries could be mega-disasters. "Bangladesh is overpopulated everywhere," he said. "All the natural gas fields, heavy industries and electric power plants are located close to potential earthquakes, and they are likely to be destroyed. In Dhaka, the catastrophic picture will be beyond our imagination, and could even lead to abandonment of the city."

Roger Bilham, a geophysicist at the University of Colorado who has studied the region but was not involved in the new paper, said its "data are unassailable, the interpretation is sound." Bilham said the research "ties an enormous amount of structural interaction together. We have seen in recent history only modest seismicity responding to those interactions. The Indian subcontinent is effectively being pushed into a tight corner."

Susan Hough, a U.S. Geological Survey seismologist who also studies the region and was not involved in the study, said that in recent years, "we've been surprised by big earthquakes that have not been witnessed during historical times, or witnessed so long ago, they were forgotten. Studies like this are critical for identifying those zones."

Scientists in Bangladesh and neighboring countries continue to assess the hazards. James Ni, a seismologist at New Mexico State University, said he and colleagues hope to deploy 70 seismometers across Myanmar in 2017, to get a better image of the apparently subducting slab. "We don't have a good idea of its geometry, we don't know how far it goes down," said Ni. He said that if the study authors are right, and the slab is building strain, a quake would probably turn urban areas in eastern India "into ruins," and effects likely would extend into Myanmar and beyond. "We need more data," he said.

The other authors of the study are Dhiman Ranjan Mondal of the City University of New York; Leonardo Seeber, Jonathan Gale and Michael Howe of Lamont-Doherty Earth Observatory; and Lujia Feng and Emma Hill of Singapore's Nanyang Technological University. The research was supported by the U.S. National Science Foundation.
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Source:
The above post is reprinted from materials provided by Lamont-Doherty Earth Observatory, Columbia University.

Reference:
Michael S. Steckler, Dhiman Ranjan Mondal, Syed Humayun Akhter, Leonardo Seeber, Lujia Feng, Jonathan Gale, Emma M. Hill & Michael Howe. Locked and loading megathrust linked to active subduction beneath the Indo-Burman Ranges. Nature Geoscience, 2016; DOI: 10.1038/ngeo2760
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Kyoto University researchers show that details about fault dip direction can be extracted from tsunami-borne electromagnetic fields. Such details may contribute to tsunami early warning systems that are more informative for residents of coastal areas. Credit: Eiri Ono/Kyoto University (K-CONNEX)
Kyoto University researchers show that details about fault dip direction can be extracted from tsunami-borne electromagnetic fields. Such details may contribute to tsunami early warning systems that are more informative for residents of coastal areas. Credit: Eiri Ono/Kyoto University (K-CONNEX)
Could electromagnetic fields be used in tsunami early warning? New research shows that important focal parameters of tsunamigenic earthquakes -- particularly fault dip direction -- can be extracted from tsunami-borne EM fields.

"It's been five years since we discovered that tsunamis generate EM fields," says Hiroaki Toh, who led the Kyoto University study. "We've now demonstrated that tsunami-generated EM fields are a reliable and useful source of information for seismology,"

Tsunamis consist of large volumes of electrically conductive seawater, generating EM fields through the coupling of synchronous seawater motion with the Earth's geomagnetic field. In a previous study, Toh's team found that those tsunami-generated fields revealed information such as the height of the tsunami, its direction of travel, and its type (a rise wave or a backwash).

"This time we aimed to extract information about hypocenters of tsunamigenic earthquakes," explains Toh.

Knowing the direction in which the fault dips could be helpful for tsunami early warning, as the direction sometimes determines whether a rise wave or a backwash hits a particular costal area.

"With backwash, residents of coastal areas get more time to evacuate. The real disaster is when rise waves come in your direction; you can't afford to lose a single moment."

"But fault dips are one of the most difficult characteristics to investigate. Even with modern techniques in seismology, seismic waves don't always tell us the direction in which the fault is dipping. In these instances, we have to wait for aftershocks to occur and make inferences from them."

Toh and former graduate student Issei Kawashima analyzed waves from a 2007 tsunami earthquake at the Kuril Trench, off the northeast coast of Hokkaido. With improvements to preexisting methods in calculating tsunamis' phase velocity, they found that the fault dip lay to the southeast direction.

"EM fields have been measured on the ocean floor of the northwest Pacific since 2001," says Toh. "This research further proves that EM fields from tsunamis are rich in information that can eventually be applied to global tsunami early warning."
***

Source:
The above post is reprinted from materials provided by Kyoto University.

Reference:
Issei Kawashima, Hiroaki Toh. Tsunami-generated magnetic fields may constrain focal mechanisms of earthquakes. Scientific Reports, 2016; 6: 28603 DOI: 10.1038/srep28603

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The latest Research, Reviews, News and information about Geology / Earth Science from around the web. GEOLOGY INFO
Photo of Tilca volcano in Nicaragua erupting. Credit: Diana Roman
Photo of Tilca volcano in Nicaragua erupting. Credit: Diana Roman
When dormant volcanoes are about to erupt, they show some predictive characteristics--seismic activity beneath the volcano starts to increase, gas escapes through the vent, or the surrounding ground starts to deform. However, until now, there has not been a way to forecast eruptions of more restless volcanoes because of the constant seismic activity and gas and steam emissions. Carnegie volcanologist Diana Roman, working with a team of scientists from Penn State, Oxford University, the University of Iceland, and INETER has shown that periods of seismic quiet occur immediately before eruptions and can thus be used to forecast an impending eruption for restless volcanoes. The duration of the silence can indicate the level of energy that will be released when eruption occurs. Longer quiet periods mean a bigger bang.

The research is published in Earth and Planetary Science Letters.

The team monitored a sequence of eruptions at the Telica Volcano in Nicaragua in 2011. It is a so-called stratovolcano, with a classic-looking cone built up by many layers of lava and ash. They started monitoring Telica in 2009 with various instruments and by 2011 they had a comprehensive network within 2.5 miles (4 kilometers) of the volcano's summit.

The 2011 eruptive event was a month-long series of small to moderate ash explosions. Prior to the eruption, there was a lack of deep seismicity or deformation, and small changes in sulfur dioxide gas emissions, indicating that the eruption was not driven by fresh magma. Instead, the eruption likely resulted from the vents being sealed off so that gas could not escape. This resulted in an increase in the pressure that eventually caused the explosions.

Of the 50 explosions that occurred, 35 had preceding quiet periods lasting 30 minutes or longer. Thirteen explosions were preceded by quiet intervals of at least five minutes. Only two of the 50 did not have any quiet period preceding the explosion.

"It is the proverbial calm before the storm," remarked Roman. "The icing on the cake is that we could also use these quiet periods to forecast the amount of energy released."

The researchers did a "hindsight" analysis of the energy released. They found that the longer the quiet phase preceding an explosion, the more energy was released in the ensuing explosion. The quiet periods ranged from 6 minutes before an explosion to over 10 hours (619 minutes) for the largest explosion.

The researchers were also able to forecast a minimum energy for impending explosions based on the data from the previous quiet/explosion pairs and the duration of the particular quiet period being analyzed. The correlation between duration of quiet periods and amount of energy released is tied to the duration of the gas pathways being blocked. The longer the blockage, the more pressure builds up resulting in more energy released. Sealing might be occurring due to mineral precipitation in cracks that previously acted as gas pathways, or due to the settling of the rock near the volcano's surface.

"What is clear is that this method of careful monitoring of Telica or other similar volcanoes in real time could be used for short-term forecasts of eruptions," Roman said. "Similar observations of this phenomenon have been noted anecdotally elsewhere. Our work has now quantified that quiet periods can be used for eruption forecasts and that longer quiet periods at recently active volcanoes could indicate a higher risk of energetic eruptions."

The paper's other authors are Mel Rodgers of Oxford University, Peter LaFemina of Penn State University, Halldor Geirsson of the University of Iceland, and Virginia Tenorio of the Instituto Nicaraguense de Estudios Territoriales.

This work was supported by the National Science Foundation and the Nicaraguan Institute of Earth Sciences (INETER).
***

Source:
The above post is reprinted from materials provided by Carnegie Institution for Science.

Reference:
Diana C. Roman, Mel Rodgers, Halldor Geirsson, Peter C. LaFemina, Virginia Tenorio. Assessing the likelihood and magnitude of volcanic explosions based on seismic quiescence. Earth and Planetary Science Letters, 2016; DOI: 10.1016/j.epsl.2016.06.020
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The latest Research, Reviews, News and information about Geology / Earth Science from around the web. GEOLOGY INFO
Measurements from satellite radar images of two giant West Texas sinkholes (dark black areas) shows the ground around them is sinking, including indications a much larger potential new sinkhole is developing. The rates of east-west deformation of the ground (cm/year) are indicated in blue (eastward) and red (westward). Credit: Jin-woo Kim, SMU
Measurements from satellite radar images of two giant West Texas sinkholes (dark black areas) shows the ground around them is sinking, including indications a much larger potential new sinkhole is developing. The rates of east-west deformation of the ground (cm/year) are indicated in blue (eastward) and red (westward). Credit: Jin-woo Kim, SMU
Residents of Wink and neighboring Kermit have grown accustomed to the two giant sinkholes that sit between their small West Texas towns.

But now radar images taken of the sinkholes by an orbiting space satellite reveal big changes may be on the horizon.

A new study by geophysicists at Southern Methodist University, Dallas, finds the massive sinkholes are unstable, with the ground around them subsiding, suggesting the holes could pose a bigger hazard sometime in the future.

The two sinkholes -- about a mile apart -- appear to be expanding. Additionally, areas around the existing sinkholes are unstable, with large areas of subsidence detected via satellite radar remote sensing. That leaves the possibility that new sinkholes, or one giant sinkhole, may form, said geophysicists and study co-authors Zhong Lu, professor, Shuler-Foscue Chair, and Jin-Woo Kim, a research scientist, in the Roy M. Huffington Department of Earth Sciences at SMU.

"This area is heavily populated with oil and gas production equipment and installations, hazardous liquid pipelines, as well as two communities. The intrusion of freshwater to underground can dissolve the interbedded salt layers and accelerate the sinkhole collapse," said Kim, who leads the SMU geophysical team reporting the findings. "A collapse could be catastrophic. Following our study, we are collecting more high-resolution satellite data over the sinkholes and neighboring regions to monitor further development and collapse."

Lu and Kim reported the findings in the scientific journal Remote Sensing, in the article "Ongoing deformation of sinkholes in Wink, Texas, observed by time-series Sentinel-1A SAR Interferometry."

The research was supported by the U.S. Geological Survey Land Remote Sensing Program, the NASA Earth Surface & Interior Program, and the Shuler-Foscue Endowment at Southern Methodist University.

Unstable ground linked to rising, falling groundwater

The sinkholes were originally caused by the area's prolific oil and gas extraction, which peaked from 1926 to 1964. Wink Sink No. 1, near the Hendricks oil well 10-A, opened in 1980. Wink Sink No. 2, near Gulf WS-8 supply well, opened 22 years later in 2002.

It appears the area's unstable ground now is linked to changing groundwater levels and dissolving minerals, say the scientists. A deep-seated salt bed underlies the area, part of the massive oil-rich Permian Basin of West Texas and southeastern New Mexico.

With the new data, the SMU geophysicists found a high correlation between groundwater level in the underlying Ogallala Aquifer and further sinking of the surface area during the summer months, influenced by successive roof failures in underlying cavities.

Satellite images and groundwater records indicate that when groundwater levels rise, the ground lifts. But the presence of that same groundwater then speeds the dissolving of the underground salt, which then causes the ground surface to subside.

Everything's bigger in Texas, and the Wink sinkholes are no exception

Officials have fenced off the two sinkholes near Wink, a town of about 940 people, and Kermit, a town of about 6,000 people. The giant holes are notable features on the area's vast plains, which are dotted mostly with oil pump jacks, storage facilities, occasional brush and mesquite trees.

Based on modeling of satellite image datasets, SMU's researchers report that Wink Sink No. 1, which is closer to the town of Kermit, appears to be the most unstable. The smaller hole of the two, it has grown to 361 feet (110 meters) across -- the length of a football field.

"Even though Wink No. 1 collapsed in 1980, its neighboring areas are still subsiding," say the authors, "and the sinkhole continues to expand." An oval-shaped deformation circling the sinkhole measures three-tenths of a mile (500 meters) wide and is subsiding up to 1.6 inches (4 centimeters) a year.

Wink Sink No. 2, which is nine-tenths of a mile south of No. 1 and which sits closer to the town of Wink, is the larger of the sinkholes. It varies from 670 feet to 900 feet across.

Wink No. 2 is not experiencing as much subsidence as Wink No. 1. However, its eastern side is collapsing and eroding westward at a rate of up to 1.2 inches (3 centimeters) a year.

"Wink No. 2 exhibits depression associated with the ongoing expansion of the underground cavity," the authors report.

Some ground that doesn't even border the edges of the two sinkholes is also subsiding, the scientists observed. An area more than half a mile (1 kilometer) northeast of No. 2 sank at a rate of 1.6 inches (4 centimeters) in just four months.

Ground northeast of sinkholes is subsiding, suggesting new ones forming

The largest rate of ground subsidence is not at either sinkhole, but at an area about seven-tenths of a mile (1.2 kilometers) northeast of No. 2. Ground there is subsiding at a rate of more than 5 inches (13 centimeters) a year.

It's aerial extent, the researchers report, has also enlarged over the past eight years when a previous survey was done.

"The enlarged deformation could be an alarming precursor to the potential future development of hazards in the vicinity," said the authors.

Additionally, ground along a road traveled by oil field vehicles, about a quarter mile (400 meters) directly north of No. 2, is subsiding about 1.2 inches (3 centimeters) a year.

Ground's movement detected with radar technique

The satellite radar datasets were collected over five months between April 2015 and August 2015. With them, the geophysicists observed both two-dimension east-west deformation of the sinkholes, as well as vertical deformation.

The SMU scientists used a technique called interferometric synthetic aperture radar, or InSAR for short, to detect changes that aren't visible to the naked eye.

"From 435 miles above the Earth's surface, this InSAR technique allows us to measure inch-level subsidence on the ground. This is a monumental human achievement, and scientists will not stop endeavoring to improve this technique for more precise measurements," said Lu, who is known for leading scientists in InSAR applications. Lu is a member of the Science Definition Team for the dedicated U.S. and Indian NASA-ISRO InSAR mission, set for launch in 2020 to study hazards and global environmental change.

InSAR accesses a series of images captured by a read-out radar instrument mounted on the orbiting satellite Sentinel-1A. Sentinel-1A was launched in April 2014 as part of the European Union's Copernicus program.

Simply put, Sentinel-1A bounces a radar signal off the earth, then records the signal as it bounces back, delivering measurements. The measurements allow geophysicists to determine the distance from the satellite to the ground, revealing how features on the Earth's surface change over time.

"Sinkhole formation has previously been unpredictable, but satellite remote sensing provides a great means to detect the expansion of the current sinkholes and possible development of new sinkholes," said Kim. "Monitoring the sinkholes and modeling the rate of change can help predict potential sinkhole development."

Sentinel-1A data were obtained from Sentinels Scientific Data Hub -- Copernicus. Groundwater well data came from the Texas Water Development Board.
***

Source:
The above post is reprinted from materials provided by Southern Methodist University.

Reference:
Jin-Woo Kim, Zhong Lu, Kimberly Degrandpre. Ongoing Deformation of Sinkholes in Wink, Texas, Observed by Time-Series Sentinel-1A SAR Interferometry (Preliminary Results). Remote Sensing, 2016; 8 (4): 313 DOI: 10.3390/rs8040313
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