World’s largest subwoofer: Earthquakes ‘pump’ ground to produce infrasound

Earthquakes sway buildings, buckle terrain, and rumble – both audibly and in infrasound, frequencies below the threshold of human hearing. New computer modeling by a team of researchers indicates that most of the low-frequency infrasound comes from an unexpected source: the actual “pumping” of the Earth’s surface. The researchers confirmed their models by studying data from an actual earthquake.

“It’s basically like a loudspeaker,” said Stephen Arrowsmith, a researcher with the Geophysics Group at Los Alamos National Laboratory in Santa Fe, N.M., who presents his team’s findings at the 164th meeting of the Acoustical Society of America (ASA), held Oct. 22 – 26 in Kansas City, Missouri. “In much the same way that a subwoofer vibrates air to create deep and thunderous base notes, earthquakes pump and vibrate the atmosphere producing sounds below the threshold of human hearing.”

Infrasound can reveal important details about an earthquake. In particular, it may be used to measure the amount of ground shaking in the immediate region above the source, which would normally require an array of many seismometers to measure. There is therefore potential to use infrasound to assess damage in the immediate aftermath of an earthquake.

To better understand the relationship between earthquakes and infrasound, the researchers used the basic idea that the Earth’s surface above the earthquake pumps the atmosphere like a piston. They were then able to apply the same modeling approach used on loudspeaker dynamics.

The researchers tested their model by comparing its predictions to actual data collected from a magnitude 4.6-earthquake that occurred on January 3, 2011, in Circleville, Utah. The University of Utah maintains seismograph stations across the state supplemented with infrasound sensors, which recorded the infrasound produced during that event. Their predictions were in good agreement with the actual data, suggesting that earthquakes generate most of their sound by pumping the atmosphere like a loudspeaker.

“This was very exciting because it is the first such clear agreement in infrasound predictions from an earthquake,” said Arrowsmith. “Predicting infrasound is complex because winds can distort the signal and our results also suggest we are getting better at correcting for wind effects.”

Until now, seismologists have not understood the relative importance of the simple pumping of the ground versus other mechanisms for generating infrasound.

Additional members of the research team include Relu Burlacu and Kristine Pankow, University of Utah; Brian Stump and Chris Haward, Southern Methodist University; and Richard Stead and Rod Whitaker, Los Alamos National Laboratory.

World’s largest subwoofer: Earthquakes ‘pump’ ground to produce infrasound

Earthquakes sway buildings, buckle terrain, and rumble – both audibly and in infrasound, frequencies below the threshold of human hearing. New computer modeling by a team of researchers indicates that most of the low-frequency infrasound comes from an unexpected source: the actual “pumping” of the Earth’s surface. The researchers confirmed their models by studying data from an actual earthquake.

“It’s basically like a loudspeaker,” said Stephen Arrowsmith, a researcher with the Geophysics Group at Los Alamos National Laboratory in Santa Fe, N.M., who presents his team’s findings at the 164th meeting of the Acoustical Society of America (ASA), held Oct. 22 – 26 in Kansas City, Missouri. “In much the same way that a subwoofer vibrates air to create deep and thunderous base notes, earthquakes pump and vibrate the atmosphere producing sounds below the threshold of human hearing.”

Infrasound can reveal important details about an earthquake. In particular, it may be used to measure the amount of ground shaking in the immediate region above the source, which would normally require an array of many seismometers to measure. There is therefore potential to use infrasound to assess damage in the immediate aftermath of an earthquake.

To better understand the relationship between earthquakes and infrasound, the researchers used the basic idea that the Earth’s surface above the earthquake pumps the atmosphere like a piston. They were then able to apply the same modeling approach used on loudspeaker dynamics.

The researchers tested their model by comparing its predictions to actual data collected from a magnitude 4.6-earthquake that occurred on January 3, 2011, in Circleville, Utah. The University of Utah maintains seismograph stations across the state supplemented with infrasound sensors, which recorded the infrasound produced during that event. Their predictions were in good agreement with the actual data, suggesting that earthquakes generate most of their sound by pumping the atmosphere like a loudspeaker.

“This was very exciting because it is the first such clear agreement in infrasound predictions from an earthquake,” said Arrowsmith. “Predicting infrasound is complex because winds can distort the signal and our results also suggest we are getting better at correcting for wind effects.”

Until now, seismologists have not understood the relative importance of the simple pumping of the ground versus other mechanisms for generating infrasound.

Additional members of the research team include Relu Burlacu and Kristine Pankow, University of Utah; Brian Stump and Chris Haward, Southern Methodist University; and Richard Stead and Rod Whitaker, Los Alamos National Laboratory.

Cleaner fracking

The technology that opened a wealth of new natural gas resources in the U.S. is producing millions of gallons of dirty water – enough from one typical gas well to cover a football field to a depth of 9-15 feet. Cleaning up that byproduct of “fracking” is the topic of the cover story of the current issue of Chemical & Engineering News. C&EN is the weekly newsmagazine of the American Chemical Society, the world’s largest scientific society.

Melody M. Bomgardner, C&EN senior business editor, explains that hydraulic fracturing, or fracking, uses a mixture of water and chemicals injected into the ground to break open rock and release natural gas. Some of that water comes back out of the ground, laden with various salts, minerals, heavy metals and other substances that pose health and environmental risks. The article describes how water treatment firms are responding to that challenge, developing new ways to treat fracking wastewater and competing for business.

Some companies have developed chemical treatments to remove contaminants and microbes from the wastewater, which can then be reused, while others use evaporators to separate fresh water from the brine. Bomgardner notes that treating the wastewater is a special challenge in the Marcellus Shale area of the Appalachian Basin, where wastewater – millions of gallons per well – must be trucked away for disposal. The cost of disposal is spurring oil and gas companies to adopt these and other technologies that could limit the amount of contaminated water that reaches people, plants and animals, the article notes.

Cleaner fracking

The technology that opened a wealth of new natural gas resources in the U.S. is producing millions of gallons of dirty water – enough from one typical gas well to cover a football field to a depth of 9-15 feet. Cleaning up that byproduct of “fracking” is the topic of the cover story of the current issue of Chemical & Engineering News. C&EN is the weekly newsmagazine of the American Chemical Society, the world’s largest scientific society.

Melody M. Bomgardner, C&EN senior business editor, explains that hydraulic fracturing, or fracking, uses a mixture of water and chemicals injected into the ground to break open rock and release natural gas. Some of that water comes back out of the ground, laden with various salts, minerals, heavy metals and other substances that pose health and environmental risks. The article describes how water treatment firms are responding to that challenge, developing new ways to treat fracking wastewater and competing for business.

Some companies have developed chemical treatments to remove contaminants and microbes from the wastewater, which can then be reused, while others use evaporators to separate fresh water from the brine. Bomgardner notes that treating the wastewater is a special challenge in the Marcellus Shale area of the Appalachian Basin, where wastewater – millions of gallons per well – must be trucked away for disposal. The cost of disposal is spurring oil and gas companies to adopt these and other technologies that could limit the amount of contaminated water that reaches people, plants and animals, the article notes.

Study advances understanding of volcanic eruptions

Volcanic eruptions vary from common, small eruptions that have little impact on humans and the environment to rare, large-to-gigantic eruptions so massive they can threaten civilizations.

While scientists don’t yet fully understand the mechanisms that control whether an eruption is large or small, they do know that eruptions are driven by the rapid expansion of bubbles formed from water and other volatile substances trapped in molten rock as it rises beneath a volcano. The mechanism is much the same as that involved in shaking a bottle of a carbonated drink and then opening the lid. Whether the volcano or the drink erupts dramatically or slowly loses its gas depends on the interplay of bubble growth and gas loss. Investigating the formation and growth of bubbles and their effects on magma properties thus provides a key to understanding volcanic eruptions, and could lead to better predictions of their scale.

An international research team led by Prof. Don R. Baker of McGill University’s Department of Earth and Planetary Sciences has published a new study in Nature Communications that suggests the difference between a small or large eruption depends on the first 10 seconds of bubble growth in molten rocks. The findings point to a need to develop volcanic monitoring systems that can measure rapid changes in gas flux and composition during those brief, crucial moments.

The researchers examined the growth of volcanic bubbles in real time by heating water-bearing molten rock with a recently developed laser heating system at the Swiss Light Source facility in Villigen, Switzerland, where they could perform three-dimensional X-ray microtomography (CAT scans) of the samples during the first 18 seconds of bubble growth and foaming. With these images, the researchers were able to measure the number and size of bubbles, investigate the geometry of the connections between bubbles, and calculate how quickly gas flowed out of the sample and how the foam strength dropped.

The researchers found that initially thousands of small bubbles per cubic centimetre formed, trapping gas inside them, but that they swiftly coalesced into a foam of larger bubbles whose strength rapidly decreased while the rate of gas loss increased. All of these changes occurred within the first 15 seconds of bubble growth. They then determined which conditions of bubble formation and growth lead to failure in the rock.

From these results, Baker and his team hypothesized that even molten rocks with small amounts of water have the potential to create devastating, large eruptions. In most cases gas escapes rapidly enough to outpace bubble growth, resulting in smaller eruptions; but under exceptional rates of bubble expansion, or conditions where the bubbles cannot coalesce, large eruptions may result.

The findings represent a small but important step toward the goal of being able to predict the type of eruption that will occur in various volcanic regions of the world. “Future work will need to concentrate on the first few seconds of bubble growth and the effect of crystals on the bubble growth,” Baker said.

Study advances understanding of volcanic eruptions

Volcanic eruptions vary from common, small eruptions that have little impact on humans and the environment to rare, large-to-gigantic eruptions so massive they can threaten civilizations.

While scientists don’t yet fully understand the mechanisms that control whether an eruption is large or small, they do know that eruptions are driven by the rapid expansion of bubbles formed from water and other volatile substances trapped in molten rock as it rises beneath a volcano. The mechanism is much the same as that involved in shaking a bottle of a carbonated drink and then opening the lid. Whether the volcano or the drink erupts dramatically or slowly loses its gas depends on the interplay of bubble growth and gas loss. Investigating the formation and growth of bubbles and their effects on magma properties thus provides a key to understanding volcanic eruptions, and could lead to better predictions of their scale.

An international research team led by Prof. Don R. Baker of McGill University’s Department of Earth and Planetary Sciences has published a new study in Nature Communications that suggests the difference between a small or large eruption depends on the first 10 seconds of bubble growth in molten rocks. The findings point to a need to develop volcanic monitoring systems that can measure rapid changes in gas flux and composition during those brief, crucial moments.

The researchers examined the growth of volcanic bubbles in real time by heating water-bearing molten rock with a recently developed laser heating system at the Swiss Light Source facility in Villigen, Switzerland, where they could perform three-dimensional X-ray microtomography (CAT scans) of the samples during the first 18 seconds of bubble growth and foaming. With these images, the researchers were able to measure the number and size of bubbles, investigate the geometry of the connections between bubbles, and calculate how quickly gas flowed out of the sample and how the foam strength dropped.

The researchers found that initially thousands of small bubbles per cubic centimetre formed, trapping gas inside them, but that they swiftly coalesced into a foam of larger bubbles whose strength rapidly decreased while the rate of gas loss increased. All of these changes occurred within the first 15 seconds of bubble growth. They then determined which conditions of bubble formation and growth lead to failure in the rock.

From these results, Baker and his team hypothesized that even molten rocks with small amounts of water have the potential to create devastating, large eruptions. In most cases gas escapes rapidly enough to outpace bubble growth, resulting in smaller eruptions; but under exceptional rates of bubble expansion, or conditions where the bubbles cannot coalesce, large eruptions may result.

The findings represent a small but important step toward the goal of being able to predict the type of eruption that will occur in various volcanic regions of the world. “Future work will need to concentrate on the first few seconds of bubble growth and the effect of crystals on the bubble growth,” Baker said.

An extremely brief reversal of the geomagnetic field, climate variability and a super volcano

41,000 years ago, a complete and rapid reversal of the geomagnetic field occured. Magnetic studies of the GFZ German Research Centre for Geosciences on sediment cores from the Black Sea show that during this period, during the last ice age, a compass at the Black Sea would have pointed to the south instead of north. Moreover, data obtained by the research team formed around GFZ researchers Dr. Norbert Nowaczyk and Prof. Helge Arz, together with additional data from other studies in the North Atlantic, the South Pacific and Hawaii, prove that this polarity reversal was a global event. Their results are published in the latest issue of the scientific journal “Earth and Planetary Science Letters“.

What is remarkable is the speed of the reversal: “The field geometry of reversed polarity, with field lines pointing into the opposite direction when compared to today’s configuration, lasted for only about 440 years, and it was associated with a field strength that was only one quarter of today’s field,” explains Norbert Nowaczyk. “The actual polarity changes lasted only 250 years. In terms of geological time scales, that is very fast.” During this period, the field was even weaker, with only 5% of today’s field strength. As a consequence, the Earth nearly completely lost its protection shield against hard cosmic rays, leading to a significantly increased radiation exposure.

This is documented by peaks of radioactive beryllium (10Be) in ice cores from this time, recovered from the Greenland ice sheet. 10Be as well as radioactive carbon (14C) is caused by the collision of high-energy protons from space with atoms of the atmosphere.

The Laschamp event

The polarity reversal now found with the magnetisation of Black Sea sediments has already been known for 45 years. It was first discovered after the analysis of the magnetisation of several lava flows near the village Laschamp near Clermont-Ferrand in the Massif Central, which differed significantly from today’s direction of the geomagnetic field. Since then, this geomagnetic feature is known as the ‘Laschamp event’. However, the data of the Massif Central represent only some point readings of the geomagnetic field during the last ice age, whereas the new data from the Black Sea give a complete image of geomagnetic field variability at a high temporal resolution.

Abrupt climate changes and a super volcano

Besides giving evidence for a geomagnetic field reversal 41,000 years ago, the geoscientists from Potsdam discovered numerous abrupt climate changes during the last ice age in the analysed cores from the Black Sea, as it was already known from the Greenland ice cores. This ultimately allowed a high precision synchronisation of the two data records from the Black Sea and Greenland. The largest volcanic eruption on the Northern hemisphere in the past 100 000 years, namely the eruption of the super volcano 39400 years ago in the area of today’s Phlegraean Fields near Naples, Italy, is also documented within the studied sediments from the Black Sea. The ashes of this eruption, during which about 350 cubic kilometers of rock and lava were ejected, were distributed over the entire eastern Mediterranean and up to central Russia. These three extreme scenarios, a short and fast reversal of the Earth’s magnetic field, short-term climate variability of the last ice age and the volcanic eruption in Italy, have been investigated for the first time in a single geological archive and placed in precise chronological order.

An extremely brief reversal of the geomagnetic field, climate variability and a super volcano

41,000 years ago, a complete and rapid reversal of the geomagnetic field occured. Magnetic studies of the GFZ German Research Centre for Geosciences on sediment cores from the Black Sea show that during this period, during the last ice age, a compass at the Black Sea would have pointed to the south instead of north. Moreover, data obtained by the research team formed around GFZ researchers Dr. Norbert Nowaczyk and Prof. Helge Arz, together with additional data from other studies in the North Atlantic, the South Pacific and Hawaii, prove that this polarity reversal was a global event. Their results are published in the latest issue of the scientific journal “Earth and Planetary Science Letters“.

What is remarkable is the speed of the reversal: “The field geometry of reversed polarity, with field lines pointing into the opposite direction when compared to today’s configuration, lasted for only about 440 years, and it was associated with a field strength that was only one quarter of today’s field,” explains Norbert Nowaczyk. “The actual polarity changes lasted only 250 years. In terms of geological time scales, that is very fast.” During this period, the field was even weaker, with only 5% of today’s field strength. As a consequence, the Earth nearly completely lost its protection shield against hard cosmic rays, leading to a significantly increased radiation exposure.

This is documented by peaks of radioactive beryllium (10Be) in ice cores from this time, recovered from the Greenland ice sheet. 10Be as well as radioactive carbon (14C) is caused by the collision of high-energy protons from space with atoms of the atmosphere.

The Laschamp event

The polarity reversal now found with the magnetisation of Black Sea sediments has already been known for 45 years. It was first discovered after the analysis of the magnetisation of several lava flows near the village Laschamp near Clermont-Ferrand in the Massif Central, which differed significantly from today’s direction of the geomagnetic field. Since then, this geomagnetic feature is known as the ‘Laschamp event’. However, the data of the Massif Central represent only some point readings of the geomagnetic field during the last ice age, whereas the new data from the Black Sea give a complete image of geomagnetic field variability at a high temporal resolution.

Abrupt climate changes and a super volcano

Besides giving evidence for a geomagnetic field reversal 41,000 years ago, the geoscientists from Potsdam discovered numerous abrupt climate changes during the last ice age in the analysed cores from the Black Sea, as it was already known from the Greenland ice cores. This ultimately allowed a high precision synchronisation of the two data records from the Black Sea and Greenland. The largest volcanic eruption on the Northern hemisphere in the past 100 000 years, namely the eruption of the super volcano 39400 years ago in the area of today’s Phlegraean Fields near Naples, Italy, is also documented within the studied sediments from the Black Sea. The ashes of this eruption, during which about 350 cubic kilometers of rock and lava were ejected, were distributed over the entire eastern Mediterranean and up to central Russia. These three extreme scenarios, a short and fast reversal of the Earth’s magnetic field, short-term climate variability of the last ice age and the volcanic eruption in Italy, have been investigated for the first time in a single geological archive and placed in precise chronological order.

Scientists identify trigger for explosive volcanic eruptions

This is the Las Cañadas volcano. -  Barry Marsh
This is the Las Cañadas volcano. – Barry Marsh

Scientists from the University of Southampton have identified a repeating trigger for the largest explosive volcanic eruptions on Earth.

The Las Cañadas volcanic caldera on Tenerife, in the Canary Islands, has generated at least eight major eruptions during the last 700,000 years. These catastrophic events have resulted in eruption columns of over 25km high and expelled widespread pyroclastic material over 130km. By comparison, even the smallest of these eruptions expelled over 25 times more material than the 2010 eruption of Eyjafjallajökull, Iceland.

By analysing crystal cumulate nodules (igneous rocks formed by the accumulation of crystals in magma) discovered in pyroclastic deposits of major eruptions, the scientists found that pre-eruptive mixing within the magma chamber – where older cooler magma mixed with younger hotter magma – appears to be the repeating trigger in large-scale eruptions.

These nodules trapped and preserved the final magma beneath the volcano immediately before eruption. Dr Rex Taylor, Senior Lecturer in Ocean and Earth Science at the University of Southampton, investigated nodules and their trapped magma to see what caused the eruptions. He found that the nodules provide a record of the changes occurring in the magma plumbing right through to the moment the volcano erupted.

Dr Taylor says: “These nodules are special because they were ripped from the magma chamber before becoming completely solid – they were mushy, like balls of coarse wet sand. Rims of crystals in the nodules grew from a very different magma, indicating a major mixing event occurred immediately before eruption. Stirring young hot magma into older, cooler magma appears to be a common event before these explosive eruptions.”

Co-author of the study, Dr Tom Gernon, Lecturer in Ocean and Earth Science at the University of Southampton, says: “The analysis of crystal nodules from the volcano documents the final processes and changes immediately prior to eruption – those triggering the catastrophic eruptions. The very presence of mushy nodules in the pyroclastic deposits suggests that the magma chamber empties itself during the eruption, and the chamber then collapses in on itself forming the caldera.”

The Las Cañadas volcano is an IAVCEI (International Association of Volcanology and Chemistry of the Earth’s Interior) Decade Volcano – identified by the international community as being worthy of particular study in light of their history of large, destructive eruptions and proximity to populated area.

Dr Gernon, who is based at the National Oceanography Centre at Southampton’s waterfront campus with Dr Taylor, adds: “Our findings will prove invaluable in future hazard and risk assessment on Tenerife and elsewhere. The scale of the eruptions we describe has the potential to cause devastation on the heavily populated island of Tenerife and major economic repercussions for the wider European community.”

Scientists identify trigger for explosive volcanic eruptions

This is the Las Cañadas volcano. -  Barry Marsh
This is the Las Cañadas volcano. – Barry Marsh

Scientists from the University of Southampton have identified a repeating trigger for the largest explosive volcanic eruptions on Earth.

The Las Cañadas volcanic caldera on Tenerife, in the Canary Islands, has generated at least eight major eruptions during the last 700,000 years. These catastrophic events have resulted in eruption columns of over 25km high and expelled widespread pyroclastic material over 130km. By comparison, even the smallest of these eruptions expelled over 25 times more material than the 2010 eruption of Eyjafjallajökull, Iceland.

By analysing crystal cumulate nodules (igneous rocks formed by the accumulation of crystals in magma) discovered in pyroclastic deposits of major eruptions, the scientists found that pre-eruptive mixing within the magma chamber – where older cooler magma mixed with younger hotter magma – appears to be the repeating trigger in large-scale eruptions.

These nodules trapped and preserved the final magma beneath the volcano immediately before eruption. Dr Rex Taylor, Senior Lecturer in Ocean and Earth Science at the University of Southampton, investigated nodules and their trapped magma to see what caused the eruptions. He found that the nodules provide a record of the changes occurring in the magma plumbing right through to the moment the volcano erupted.

Dr Taylor says: “These nodules are special because they were ripped from the magma chamber before becoming completely solid – they were mushy, like balls of coarse wet sand. Rims of crystals in the nodules grew from a very different magma, indicating a major mixing event occurred immediately before eruption. Stirring young hot magma into older, cooler magma appears to be a common event before these explosive eruptions.”

Co-author of the study, Dr Tom Gernon, Lecturer in Ocean and Earth Science at the University of Southampton, says: “The analysis of crystal nodules from the volcano documents the final processes and changes immediately prior to eruption – those triggering the catastrophic eruptions. The very presence of mushy nodules in the pyroclastic deposits suggests that the magma chamber empties itself during the eruption, and the chamber then collapses in on itself forming the caldera.”

The Las Cañadas volcano is an IAVCEI (International Association of Volcanology and Chemistry of the Earth’s Interior) Decade Volcano – identified by the international community as being worthy of particular study in light of their history of large, destructive eruptions and proximity to populated area.

Dr Gernon, who is based at the National Oceanography Centre at Southampton’s waterfront campus with Dr Taylor, adds: “Our findings will prove invaluable in future hazard and risk assessment on Tenerife and elsewhere. The scale of the eruptions we describe has the potential to cause devastation on the heavily populated island of Tenerife and major economic repercussions for the wider European community.”