How Do Tsunamis Work?

Hiroshi Kawahara, AFP/Getty Images
Hiroshi Kawahara, AFP/Getty Images

Tsunamis have been wreaking havoc on the world's coastlines for centuries. Since 1850 alone, tsunamis have been responsible for taking 420,000 lives and causing billions of dollars in damage. How do these monster waves work?

DON'T CALL IT A TIDAL WAVE

Tsunamis have nothing to do with the wind-generated waves we're used to seeing, or the tides—they’re a set of ocean waves caused by the rapid displacement of water. Most commonly, this happens when large underwater earthquakes push up the seabed; the larger and shallower the earthquake, the bigger the potential tsunami. Once generated, the waves split: A distant tsunami travels out into the open ocean, while a local tsunami travels toward the nearby coast. The speed of the waves depends on the depth of the water, but typically, waves roll across the ocean at speeds between 400 and 500 mph.

It’s not only the method of generation that differentiates tsunamis from wind-generated waves. On average, wind waves have a crest-to-crest wavelength—the distance over which the wave’s shape repeats—of approximately 330 feet and a height of 6.6 feet. A deep ocean tsunami will have a wavelength of 120 miles and amplitude (the distance from the peak of the wave to its trough) of only about 3.3 feet. This is why tsunamis are difficult to detect in the open ocean.

As a tsunami approaches the shore, the wave compresses: Its speed and wavelength decrease while its amplitude grows enormously. Most waves arrive on-shore not as a huge wave but as a fast-moving tidal bore that floods the shoreline. However, if the trough of the wave arrives before the ridge, or peak, the sea will recede from the shore, exposing normally submerged areas, as the trough builds into a ridge. This can serve as a brief warning that a tsunami is about to occur.

Other causes of tsunamis include underwater landslides and explosions. Another type of wave, called a mega-tsunami, is caused by above-water landslides or glacier calving. The largest recorded mega-tsunami struck in Alaska’s Lituya Bay in 1958; an earthquake triggered a landslide that displaced so much water that the waves created were 470 feet taller than the Empire State Building.

MONITORING WAVES

Like earthquakes, tsunamis can’t be predicted—but that doesn’t mean scientists aren’t trying to figure out ways to warn people before the flooding starts. Using a system of buoys called DART—Deep-Ocean Assessment and Reporting of Tsunamis—researchers can monitor ocean wave height in real time. When an earthquake occurs that scientists believe is likely to trigger a tsunami, these strategically placed buoys send reports on sea level change back to tsunami warning centers. There, scientists use that data to create a model of the potential tsunami’s effects and decide whether to issue a warning or make populations evacuate.

In the 2012 action film Battleship, the DART system took a star turn. Director Peter Berg used it as a method of creating the game’s iconic grid. (The Hollywood version of DART is much more robust than the real-world version, which has just 39 buoys.)

LOCATION, LOCATION, LOCATION

Tsunamis are mostly generated by quakes that occur in subduction zones: areas where denser oceanic plates slide underneath lighter continental plates, causing vertical displacement of the seafloor and water column above it. The majority of the world's subduction zones are in the Pacific Ocean bordering Oceania, Asia, North America, and South America. This highly unsettled loop is nicknamed the "ring of fire" for its concentration of geologic upheavals.

Because the Atlantic Ocean has far fewer subduction zones than the Pacific, Atlantic tsunamis are rare, but possible. The most likely cause would be an earthquake creating a submarine landslide that would displace a huge volume of water and trigger the wave.

In 2001, geophysicists Steven N. Ward and Simon Day suggested that an Atlantic mega-tsunami could be generated by a massive landslide off La Palma, the most active volcano in the Canary Islands archipelago. The theory was based on modeling a number of worst-case scenarios, the authors said. Others have argued that the danger is overblown.

9 Fascinating Facts About the Vagus Nerve

The vagus nerve is so named because it “wanders” like a vagabond, sending out sensory fibers from your brainstem to your visceral organs. The vagus nerve, the longest of the cranial nerves, controls your inner nerve center—the parasympathetic nervous system. And it oversees a vast range of crucial functions, communicating motor and sensory impulses to every organ in your body. New research has revealed that it may also be the missing link to treating chronic inflammation, and the beginning of an exciting new field of treatment for serious, incurable diseases. Here are nine facts about this powerful nerve bundle.

1. THE VAGUS NERVE PREVENTS INFLAMMATION.

A certain amount of inflammation after injury or illness is normal. But an overabundance is linked to many diseases and conditions, from sepsis to the autoimmune condition rheumatoid arthritis. The vagus nerve operates a vast network of fibers stationed like spies around all your organs. When it gets a signal for incipient inflammation—the presence of cytokines or a substance called tumor necrosis factor (TNF)—it alerts the brain and draws out anti-inflammatory neurotransmitters that regulate the body’s immune response.

2. IT HELPS YOU MAKE MEMORIES.

A University of Virginia study in rats showed that stimulating their vagus nerves strengthened their memory. The action released the neurotransmitter norepinephrine into the amygdala, which consolidated memories. Related studies were done in humans, suggesting promising treatments for conditions like Alzheimer’s disease.

3. IT HELPS YOU BREATHE.

The neurotransmitter acetylcholine, elicited by the vagus nerve, tells your lungs to breathe. It’s one of the reasons that Botox—often used cosmetically—can be potentially dangerous, because it interrupts your acetylcholine production. You can, however, also stimulate your vagus nerve by doing abdominal breathing or holding your breath for four to eight counts.

4. IT'S INTIMATELY INVOLVED WITH YOUR HEART.

The vagus nerve is responsible for controlling the heart rate via electrical impulses to specialized muscle tissue—the heart’s natural pacemaker—in the right atrium, where acetylcholine release slows the pulse. By measuring the time between your individual heart beats, and then plotting this on a chart over time, doctors can determine your heart rate variability, or HRV. This data can offer clues about the resilience of your heart and vagus nerve.

5. IT INITIATES YOUR BODY'S RELAXATION RESPONSE.

When your ever-vigilant sympathetic nervous system revs up the fight or flight responses—pouring the stress hormone cortisol and adrenaline into your body—the vagus nerve tells your body to chill out by releasing acetylcholine. The vagus nerve’s tendrils extend to many organs, acting like fiber-optic cables that send instructions to release enzymes and proteins like prolactin, vasopressin, and oxytocin, which calm you down. People with a stronger vagus response may be more likely to recover more quickly after stress, injury, or illness.

6. IT TRANSLATES BETWEEN YOUR GUT AND YOUR BRAIN.

Your gut uses the vagus nerve like a walkie-talkie to tell your brain how you’re feeling via electric impulses called “action potentials". Your gut feelings are very real.

7. OVERSTIMULATION OF THE VAGUS NERVE IS THE MOST COMMON CAUSE OF FAINTING.

If you tremble or get queasy at the sight of blood or while getting a flu shot, you’re not weak. You’re experiencing “vagal syncope.” Your body, responding to stress, overstimulates the vagus nerve, causing your blood pressure and heart rate to drop. During extreme syncope, blood flow is restricted to your brain, and you lose consciousness. But most of the time you just have to sit or lie down for the symptoms to subside.

8. ELECTRICAL STIMULATION OF THE VAGUS NERVE REDUCES INFLAMMATION AND MAY INHIBIT IT ALTOGETHER.

Neurosurgeon Kevin Tracey was the first to show that stimulating the vagus nerve can significantly reduce inflammation. Results on rats were so successful, he reproduced the experiment in humans with stunning results. The creation of implants to stimulate the vagus nerve via electronic implants showed a drastic reduction, and even remission, in rheumatoid arthritis—which has no known cure and is often treated with the toxic drugs—hemorrhagic shock, and other equally serious inflammatory syndromes.

9. VAGUS NERVE STIMULATION HAS CREATED A NEW FIELD OF MEDICINE.

Spurred on by the success of vagal nerve stimulation to treat inflammation and epilepsy, a burgeoning field of medical study, known as bioelectronics, may be the future of medicine. Using implants that deliver electric impulses to various body parts, scientists and doctors hope to treat illness with fewer medications and fewer side effects.

How to Relieve a Tension Headache in 10 Seconds, According to a Physical Therapist

iStock.com/SIphotography
iStock.com/SIphotography

The source of a pounding headache isn't always straightforward. Sometimes over-the-counter painkillers have no effect, and in other cases all you need is a glass of water to ease the pain. When it comes to a specific type of a headache, Prevention recommends a treatment that takes about 10 seconds—no fancy medications or equipment required.

If you're experiencing pain throughout your head and neck, you may have a tension headache. This type of headache can happen when you tense the muscles in your jaw—something many people do when stressed. This tightening triggers a chain reaction where the surrounding muscles in the head and neck become tense, which results in a painful, stiff feeling.

Fortunately, there's a way to treat tension headaches that's even easier than popping an Advil. David Reavy, a physical therapist known for his work with NFL and NBA athletes, recently suggested a solution to Prevention writer Christine Mattheis called the masseter release. To practice it yourself, look for the masseter muscle—the thick tissue that connects your jawbone to your cheekbone on either side of your face—with your fingers. Once you've found them, press the spots gently, open your mouth as wide as you can, close it, and repeat until you feel the muscle relax. Doing this a few times a day helps combat whatever tension is caused by clenching your jaw.

If that doesn't work, it's possible that the masseter muscle isn't the source of your headache after all. In that case, read up on the differences among popular pain killers to determine which one is the best match for your pain.

[h/t Prevention]

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