Understanding Infrasound: Nature's Secret Signals
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Chapter 1: The Role of Infrasound in Disaster Awareness
Infrasound, which consists of low-frequency sounds, serves as a vital tool for scientists to uncover the mysteries of various natural disasters, including volcanoes, tornadoes, tsunamis, glaciers, and avalanches.
The Alps, despite experiencing climate changes at elevations above 3500 meters, maintain very low air temperatures. However, perennial ice is receding from ridges and slopes situated at altitudes of 2500–3500 meters. This area is particularly vulnerable to landslides and rockslides. As many climatologists anticipate, if the average annual temperatures in the Alps rise by several degrees over the next fifty years, these processes could intensify dramatically. A significant concern is the Swiss peak, Eiger (3970 m), which features a renowned 1800 m high north face that draws not only adventurous climbers but also spectators eager to ride Europe’s highest cog railway to the Jungfraujoch pass, standing at 3454 m above sea level. This pass boasts a substantial visitor center, while the cable car path is carved through the Eiger and the neighboring Mönch peak (4107 m). Glaciers descending from these mountains, along with the nearby Jungfrau (4158 m), are diminishing, revealing eroded rock. Notably, in the summer of 2006, a massive rock block weighing around 5 million tons detached from Eiger’s north face.
With even greater apprehension, researchers are monitoring the glacier perched on the western slope of Eiger, situated at 3400 meters above sea level, where climate change is most impactful. Each year, it shrinks, and rather than disappearing quietly, it does so dramatically. Enormous ice blocks regularly break off from its face, which stands as tall as a ten-story building and is approximately 200 meters wide. These colossal chunks first plummet before sliding down steep inclines, transforming into avalanches composed not of snow, but of ice fragments, resembling landslides. The icy cascade heads toward the Jungfraujoch cable car station named Eigergletscher, located at 2320 m above sea level. While ice avalanches have yet to reach this station, that may soon change if the climate in the Alps continues to warm.
Infrasound Detection and Location: Insights from David Fee
In 2016, Emanuele Marchetti from the Università degli Studi di Firenze, an expert in low-frequency acoustic wave analysis, visited the Eiger after being invited by two Swiss researchers from ETHZ Polytechnic University in Zurich. After their site assessment, the scientists set up four sensors near the Eigergletscher station to record infrasound. The sensors were positioned 1.5 km horizontally and nearly 1 km vertically from the hanging glacier. A year later, on May 29, 2017, a significant chunk of ice more than 100 meters long and several meters high broke away from the glacier, crashing half a kilometer down and shattering into billions of ice crystals, some of which reached the cable car station, blanketing Marchetti's sensors with crystalline debris. Fortunately, the sensors remained intact, and they accurately detected the glacier's front cracking, accompanied by the release of infrasound.
After three years of thorough analysis of this and smaller ice avalanches from below Eiger, scientists concluded they had discovered a universal, cost-effective, and straightforward method for monitoring the disintegration rates of mountain glaciers. Instead of relying on expensive seismometers or radar systems, they found that relatively simple acoustic pressure sensors could suffice. From a few kilometers away, these sensors can determine the size of a breaking ice block, its fall location, and the fate of the ice particles. Ultimately, Marchetti and his team aspire to establish such infrasound monitoring stations near numerous alpine glaciers experiencing intense heat.
Infrasound offers an array of applications beyond glacier monitoring. Inaudible to human ears due to its frequency below 20 Hz, infrasound is prevalent in nature and can also be artificially generated. It can arise from earthquakes, storms, volcanic activity, large avalanches, or breaking glaciers. In recent decades, a dedicated scientific field has emerged to study infrasound, focusing on its potential to collect environmental data. Applications are rapidly expanding—from examining the Earth's crust and upper atmosphere to tracking severe weather patterns and installing avalanche warning systems in mountainous regions. Additionally, there are ongoing attempts to harness infrasound for military and medical applications.
Section 1.1: The Historical Context of Infrasound
The first documentation of natural infrasound occurred following the 1883 eruption of the Indonesian volcano Krakatau, with sensitive barometers capturing its presence. A similar phenomenon was recorded during the infamous Tunguska meteorite explosion in June 1908, which devastated a 40 km radius in central Siberia. The explosion was visible from 500 km away and audible from twice that distance, with the infrasound waves circling the globe. However, serious interest in these acoustic waves surged post-World War II, amid the Cold War arms race. Hundreds of nuclear tests, including those conducted in the atmosphere, generated powerful infrasound sources, with frequencies below 1 Hz traveling globally and detected by specialized microphones.
In 1963, a global treaty banning atmospheric nuclear weapons testing reduced interest in infrasound within geoacoustic defense. However, scientists began leveraging infrasound to study environmental phenomena, although the process was complex. Recordings obtained from sensitive equipment contained numerous enigmatic "noises," each with a distinct origin. Decades passed before these sounds were decoded into meaningful signals, as exemplified by research on the Eiger glacier breakup. Scientists are gradually unraveling the complexities of infrasound, learning that different natural events produce unique sound signatures. For instance, avalanches, auroras, and foehn winds each emit distinct infrasound patterns. Some experts suggest that infrasound fluctuations during foehn winds may correlate with increased rates of heart attacks and suicides, as our bodies can sense infrasound even when our ears cannot.
The FEAR Frequency: Infrasound's Impact on Human Health
Section 1.2: Infrasound's Unique Propagation Properties
One notable aspect of infrasound is its ability to travel vast distances with minimal interference. Lower frequencies are often heard over greater distances than higher frequencies. For instance, when lightning strikes nearby, the accompanying thunder consists of high-frequency crackling sounds, but those farther away perceive only the deep rumble of distant thunder. Generally, sounds we hear seldom travel beyond a few tens of kilometers due to atmospheric absorption, especially at higher frequencies. A 1,000 Hz tone loses about 90% of its energy within 7 km. In contrast, a 1 Hz tone can travel approximately 3,000 km, while a 0.01 Hz tone can circle the globe without distortion.
This understanding proved invaluable when the Comprehensive Nuclear Test Ban Treaty (CTBT) was established in September 1996. Ratified by most countries, the treaty remains unimplemented until all 44 negotiating nations approve it. As of now, eight countries, including the United States, China, India, Pakistan, Iran, and North Korea, have not ratified it. Nevertheless, over two decades ago, a global monitoring network was initiated to swiftly detect any nuclear weapons use worldwide. The International Monitoring System (IMS) comprises stations that measure radioactive substances, seismographs, hydroacoustic equipment on the ocean floor, and infrasound sensors.
The infrasound sensors are located in 37 countries and Antarctica, utilizing microbarometers in long pipe systems to suppress wind-induced vibrations. These sensors are ideally situated far from urban areas, factories, ports, and airports, preferably in forests that mitigate wind noise. However, this is not always feasible, as seen with the American Windless Bight station, one of the first in the network, located on the Ross Ice Shelf. Despite challenges, this station proved effective in capturing signals from the nearby active volcano Erebus and ice streams transitioning into floating shelf glaciers. This discovery has provided volcanologists and glaciologists with a new tool for gathering data on natural phenomena of interest.
Chapter 2: Infrasound in Volcanic Monitoring
Scientists are also deploying sensitive microbarometers to monitor infrasound from volcanoes. For instance, Cotopaxi, one of the highest active volcanoes globally, is under close scrutiny as it frequently shows signs of activity. Researchers have installed equipment atop Cotopaxi to capture infrasound generated when volcanic gases escape from a magma chamber, creating a "natural organ" that produces low-frequency melodies.
Johnson and his colleagues have found that the infrasound emitted by Cotopaxi varies with its activity levels, serving as a potential warning system for impending eruptions. Similarly, they are examining the infrasound from Chile's Villarrica volcano, where the bubbling lava lake produces distinct sound patterns based on lava levels.
Meanwhile, NASA has employed balloons equipped with microbarometers to detect infrasound in the upper atmosphere. A notable mission in 2016 observed infrasound signals from diverse sources, including lightning, ocean waves, and meteoroids entering the atmosphere.
Infrasound also plays a role in detecting meteorite explosions. On June 21, 2018, a bolide exploded near several Russian cities, with infrasound recorded across Russia and Europe, allowing researchers to estimate its explosive power. Notably, the Chelyabinsk meteorite explosion in 2013, documented by the IMS network, highlighted the capacity of infrasound to track smaller celestial bodies entering Earth's atmosphere.
Section 2.1: Infrasound in Animal Communication
In addition to its geological applications, infrasound is utilized by various animal species. Whales, for example, communicate using low-frequency acoustic signals that can travel extensive distances underwater. Scientists have begun monitoring the migration patterns of Antarctic blue whales using hydroacoustic devices within the IMS network.
Similarly, elephants communicate over distances exceeding 10 km using infrasound, which plays a crucial role in their social interactions. Other species, including pigeons and crocodiles, also utilize infrasound for communication. Some studies suggest that certain dinosaurs may have possessed the ability to generate low-frequency sounds, potentially aiding in navigation, mating, and prey location.
In conclusion, infrasound is a fascinating phenomenon with myriad applications in monitoring natural disasters, tracking wildlife, and understanding environmental changes. The ongoing research into infrasound continues to reveal its potential to provide critical insights into our world.