Coronal Heating Mystery!

SDO/AIA in extreme ultra-violet light (false color)

Imagine standing around a roaring campfire, roasting s’mores. You feel the warmth of the flames as the marshmallows crackle. Now back away. You get cooler, right?

That's not how it works on the sun. The visible surface of the sun has a temperature of 10,000° F.  Backing away from the inferno should cool things down, but it doesn’t.  Instead, the sun's upper atmosphere, or corona, sizzles at millions of degrees - a temperature 200 to 500 times higher than that of the roaring furnace below.

For more than a half-century, astronomers have tried to figure out what causes the corona to be so hot.  It is one of the most vexing problems in astrophysics.

Solar physicist Bart De Pontieu of the Lockheed Martin Solar & Astrophysics Laboratory says, “The problem of coronal heating was first discovered in the 1940s. The problem involves a variety of complex physical processes that are difficult to directly measure or capture in theoretical models.”

On June 27, 2013, with campfires blazing around the USA, NASA launched the Interface Region Imaging Spectrograph (IRIS) - a space-based solar observatory designed to get to the bottom of how the solar atmosphere is heated.

“IRIS studies the transition region between the sun’s surface and the corona,” explains De Pontieu, who is the science lead of the observatory. “It can track the temperature and motions of hot gas at unprecedented spatial (0.33 arcsec), temporal (2 s) and spectral (2 mi/s) resolution.”

Most researchers agree that the corona is probably heated in several different ways. For instance, plasma waves from the sun can rise into the corona and crash, depositing their energy there. At the same time, “heat bombs” could be going off. These explosions happen when magnetic fields in the corona criss-cross and realign, exploding like a miniature solar flare.

One of the big questions of coronal heating has been: Is the corona heated everywhere at once, or is heat delivered in discrete, bomb-like events?

De Pontieu says, “These two possibilities are very different, but the distinction can be difficult to observe.”

The problem is the corona is a great thermal conductor. If a heat bomb goes off, the resulting heat rapidly spreads out over a large region. Blink, and it looks much the same as uniform heating.

Fortunately, IRIS never blinks. A recent observation by the observatory’s spectrographs has found evidence for these discrete, explosive events.

Paola Testa of the Harvard-Smithonian Center for Astrophysics, lead author of the paper reporting the results says, “Because IRIS can resolve the transition region ten times better than previous instruments, we were able to see hot material rushing up and down magnetic fields in the low corona. This is compatible with models from the University of Oslo, in which magnetic reconnection sets off heat bombs in the corona.”

Testa emphasizes that other heating mechanisms may be at work, too. Even so, these new observations could help tease out how much of the heating comes from discrete heating events, helping researchers sort out a decades-old puzzle of great complexity. 

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A giant cloud of solar particles, called a coronal mass ejection, explodes off the sun on Jan. 7, 2014, as seen in the light halo to the lower right in this image captured by ESA/NASA's Solar and Heliospheric Observatory. The sun is obscured to better see the tenuous atmosphere around it.

The Jan. 7, 2014, X-class flare was also associated with a coronal mass ejection, or CME, another solar phenomenon that can send billions of tons of particles into space that can reach Earth one to three days later. These particles cannot travel through the atmosphere to harm humans on Earth, but they can affect electronic systems in satellites and on the ground.
The European Space Agency and NASA's Solar and Heliospheric Observatory, or SOHO, captured an image of the giant particle cloud as it burst away from the sun.
To see how this event may impact Earth, please visit NOAA's Space Weather Prediction Center at http://spaceweather.gov, the U.S. government's official source for space weather forecasts, alerts, watches and warnings.
Updates will be provided as needed.
The sun emitted a significant solar flare peaking at 1:32 p.m. EST on Jan.7, 2014. This is the first significant flare of 2014, and follows on the heels of mid-level flare earlier in the day. Each flare was centered over a different area of a large sunspot group currently situated at the center of the sun, about half way through its 14-day journey across the front of the disk along with the rotation of the sun.
Solar flares are powerful bursts of radiation. Harmful radiation from a flare cannot pass through Earth's atmosphere to physically affect humans on the ground, however -- when intense enough -- they can disturb the atmosphere in the layer where GPS and communications signals travel. This disrupts the radio signals for as long as the flare is ongoing, anywhere from minutes to hours.
To see how this event may impact Earth, please visit NOAA's Space Weather Prediction Center at http://spaceweather.gov, the U.S. government's official source for space weather forecasts, alerts, watches and warnings.
This flare is classified as an X1.2-class flare. X-class denotes the most intense flares, while the number provides more information about its strength. An X2 is twice as intense as an X1, an X3 is three times as intense, etc.
Updates will be provided as needed.

This labeled image taken by SDO's Helioseismic and Magnetic Imager shows the location of two active regions on the sun, labeled AR1944 and AR1943, which straddle a giant sunspot complex. A Jan. 17, 2014, X1.2-class flare emanated from an area closer to AR1943.

This pictures combines two images from NASA's Solar Dynamics Observatory captured on Jan. 7, 2013. Together, the images show the location of a giant sunspot group on the sun, and the position of an X-class flare that erupted at 1:32 p.m. EST.

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NASA scientists have revealed the inner workings of the ozone hole that forms annually over Antarctica and found that declining chlorine in the stratosphere has not yet caused a recovery of the ozone hole.
More than 20 years after the Montreal Protocol agreement limited human emissions of ozone-depleting substances, satellites have monitored the area of the annual ozone hole and watched it essentially stabilize, ceasing to grow substantially larger. However, two new studies show that signs of recovery are not yet present, and that temperature and winds are still driving any annual changes in ozone hole size.
"Ozone holes with smaller areas and a larger total amount of ozone are not necessarily evidence of recovery attributable to the expected chlorine decline," said Susan Strahan of NASA's Goddard Space Flight Center in Greenbelt, Md. "That assumption is like trying to understand what's wrong with your car's engine without lifting the hood."
To find out what's been happening under the ozone hole's hood, Strahan and Natalya Kramarova, also of NASA Goddard, used satellite data to peer inside the hole. The research was presented Wednesday at the annual meeting of the American Geophysical Union in San Francisco.
Kramarova tackled the 2012 ozone hole, the second-smallest hole since the mid 1980s. To find out what caused the hole's diminutive area, she turned to data from the NASA-NOAA Suomi National Polar-orbiting Partnership satellite, and gained the first look inside the hole with the satellite's Ozone Mapper and Profiler Suite's Limb Profiler. Next, data were converted into a map that shows how the amount of ozone differed with altitude throughout the stratosphere in the center of the hole during the 2012 season, from September through November.
The map revealed that the 2012 ozone hole was more complex than previously thought. Increases of ozone at upper altitudes in early October, carried there by winds, occurred above the ozone destruction in the lower stratosphere.
"Our work shows that the classic metrics based on the total ozone values have limitations – they don't tell us the whole story," Kramarova said.

 (A look inside the 2012 ozone hole with the Ozone Mapper and Profiler Suite shows how the build-up of ozone (parts per million by volume) in the middle stratosphere masks the ozone loss in the lower stratosphere.)

The classic metrics create the impression that the ozone hole has improved as a result of the Montreal protocol. In reality, meteorology was responsible for the increased ozone and resulting smaller hole, as ozone-depleting substances that year were still elevated. The study has been submitted to the journal of Atmospheric Chemistry and Physics.
Separate research led by Strahan tackled the holes of 2006 and 2011 – two of the largest and deepest holes in the past decade. Despite their similar area, however, Strahan shows that they became that way for very different reasons.
Strahan used data from the NASA Aura satellite's Microwave Limb Sounder to track the amount of nitrous oxide, a tracer gas inversely related to the amount of ozone depleting chlorine. The researchers were surprised to find that the holes of 2006 and 2011 contained different amounts of ozone-depleting chlorine. Given that fact, how could the two holes be equally severe?
The researchers next used a model to simulate the chemistry and winds of the atmosphere. Then they re-ran the simulation with the ozone-destroying reactions turned off to understand the role that the winds played in bringing ozone to the Antarctic. Results showed that in 2011, there was less ozone destruction than in 2006 because the winds transported less ozone to the Antarctic – so there was less ozone to lose. This was a meteorological, not chemical effect. In contrast, wind blew more ozone to the Antarctic in 2006 and thus there was more ozone destruction. The research has been submitted to the journal Geophysical Research Letters.
This work shows that the severity of the ozone hole as measured by the classic total column measurements does not reveal the significant year-to-year variations in the two factors that control ozone: the winds that bring ozone to the Antarctic and the chemical loss due to chlorine.
Until chlorine levels in the lower stratosphere decline below the early 1990s level – expected sometime after 2015 but likely by 2030 – temperature and winds will continue to dictate the variable area of the hole in any given year. Not until after the mid 2030s will the decline stratospheric chlorine be the primary factor in the decline of ozone hole area.


 "We are still in the period where small changes in chlorine do not affect the area of the ozone hole, which is why it's too soon to say the ozone hole is recovering," Strahan said. "We're going into a period of large variability and there will be bumps in the road before we can identify a clear recovery."

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                                Two Solar Flares Say Goodbye 2013 and Welcome 2014

Several wavelengths of light are combined in this New Year's Day solar flare image, categorized as an M9.9 and peaking at 1:52 p.m. EST on Jan. 1, 2014. Each wavelength represents material at a different temperatures, helping scientists understand how it is moved and heated through these events

The sun ushered out 2013 and welcomed 2014 with two mid-level flares on Dec. 31, 2013 and Jan. 1, 2014. Solar flares are powerful bursts of radiation. Harmful radiation from a flare cannot pass through Earth's atmosphere to physically affect humans on the ground, however -- when intense enough -- they can disturb the atmosphere in the layer where GPS and communications signals travel. This disrupts the radio signals for as long as the flare is ongoing, anywhere from minutes to hours.
To see how this event may impact Earth, please visit NOAA's Space Weather Prediction Center at http://spaceweather.gov, the U.S. government's official source for space weather forecasts, alerts, watches and warnings.
The first flare (below) was categorized as an M6.4 and it peaked at 4:58 p.m EST on Dec. 31. The second (above) was categorized as an M9.9 and peaked at 1:52 p.m. EST on Jan. 1. Both flares emerged from the same active region on the sun, AR1936.
Imagery of the flares was captured by NASA's Solar Dynamics Observatory, which keeps a constant watch on the sun, collecting new data every 12 seconds.

An M6.4 class solar flare erupts from the sun in this image from NASA's Solar Dynamics Observatory, which was captured on Dec. 31, at 4:59 p.m. EST. The image shows light in the 304 Angstrom wavelength, which is typically colorized in red.

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                         Communications Tests Go the Distance for MAVEN

Engineers work on the MAVEN spacecraft, which is dominated by the high-gain antenna that is crucial to communications with NASA's Deep Space Network.

By Steven Siceloff,
Kennedy Space Center
It's not easy to simulate millions of miles electronically, but that's what engineers did recently as they tested the all-important communications system the MAVEN spacecraft will use to relay its study results from Mars orbit to Earth-bound researchers.

One of the antennas at the Canberra, Australia, station of NASA's Deep Space Network used to communicate with spacecraft operating far from Earth.

Working from their consoles at NASA's Kennedy Space Center, a team of test engineers from the Jet Propulsion Laboratory in California, better known as JPL, conducted more than a week of evaluations on the antennas and circuitry aboard the spacecraft.
They beamed signals to the low-gain and high-gain antennas on MAVEN and basically treated the machine as though it really were flying on a 10-month journey from Earth to Mars and then studying the upper atmosphere of the Red Planet.
Such work is critical, mission managers said, because there is no way to fix a spacecraft's communications system once it leaves Earth.
"It doesn't matter what we do out there if we can't get the data back to Earth," said Jeff Coyne, Lockheed Martin's Assembly Test and Launch Operations manager for the project.
MAVEN is short for Mars Atmosphere and Volatile Evolution. It is scheduled to launch in November aboard a United Launch Alliance Atlas V.
"I say this is one of the most important things, because if we can't talk to it . . . ," said Sheryl Bergstrom, manager of JPL's Cape Operations Office at Kennedy.
The testing was standard stuff for the engineers, but nonetheless mind-bending considering that the spacecraft will operate millions of miles from Earth and rely on commands from operators at Goddard Space Flight Center in Maryland.
To mimic the distances between the spacecraft and Earth, the electronic signals sent between the two during testing are run through a cabling system that quickly ramps down the power by going through various wiring networks.
"We'll try to squeeze the signal down to its lowest possible point," said Chris Green, an engineer with Exelis who supervised the testing. "It's a machine and we test its actual flight performance -- every scenario of flight configuration it would be in is what we go through in testing."
"We allow the project to get online and do what they would do on a normal day so they can perform all the tasks through us just as if MAVEN was actually in space," said Lorenzo Morgan, one of the engineers operating and evaluating the procedures.
Although every spacecraft goes through intense communications testing, the work is not considered routine because every spacecraft has unique requirements.
"Every mission is different, every mission has its own peculiarities," said Albert Ibarra, also a test engineer for the communications system. "You have to know the details on every spacecraft design and so you become familiar with it as soon as they start putting the spacecraft together."
NASA has an intricate system of antennas in California, Spain and Australia to pick up and transmit signals to its fleet of spacecraft that now reaches out beyond the solar system in the form of Voyager 1.
Called the Deep Space Network and referred to by its acronym, DSN, the system uses antennas almost as big as a football field to communicate with the spacecraft that are using their own much smaller antennas and more limited power sources.
It's the system NASA uses to communicate with all of its interplanetary probes and some of the spacecraft studying Earth, as well. In addition to Voyager 1, whose signal is incredibly weak because of the vast distance it is from Earth, the network is picking up signals from newer spacecraft such as New Horizons, which is speeding toward Pluto. Cassini in orbit around Saturn, Juno on its way to Jupiter and the Curiosity rover operating on Mars all relay their data to Earth on the DSN and get their commands from ground operators through the same network.
Kennedy's portion of the DSN structure is a testing facility called MIL-71, a reference to the time when the space center was known as the Merritt Island Launch Annex. Every time a spacecraft comes to Kennedy for launch preparations, a team of engineers sets up racks of equipment and computer servers before beginning several days of 12-hour shifts to make sure the mission's communications system and interface with the Deep Space Network will work.
With the spacecraft checked out, the team takes its gear back to California and gets ready for launch day, knowing very well that it won't hear anything from the spacecraft until well after liftoff. In the case of MAVEN, the engineers and scientists won't find out if the testing really was successful until 54 minutes after launch from Cape Canaveral when MAVEN makes its initial contact with the DSN.
"When the signal is acquired," said Bergstrom, a veteran of long wait-times for good missions and bad, "that's when we get to breathe."
 

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                          Voyager Reaches Interstellar Space

You Are Here, Voyager: This artist's concept puts huge solar system distances in perspective. The scale bar is measured in astronomical units (AU), with each set distance beyond 1 AU representing 10 times the previous distance. Each AU is equal to the distance from the sun to the Earth. It took from 1977 to 2013 for Voyager 1 to reach the edge of interstellar space

Whether and when NASA's Voyager 1 spacecraft, humankind's most distant object, broke through to interstellar space, the space between stars, has been a thorny issue. For the last year, claims have surfaced every few months that Voyager 1 has "left our solar system." Why has the Voyager team held off from saying the craft reached interstellar space until now?

"We have been cautious because we're dealing with one of the most important milestones in the history of exploration,” said Voyager Project Scientist Ed Stone of the California Institute of Technology in Pasadena.  “Only now do we have the data -- and the analysis -- we needed."
Basically, the team needed more data on plasma, which is ionized gas, the densest and slowest moving of charged particles in space. (The glow of neon in a storefront sign is an example of plasma.) Plasma is the most importantmarker that distinguishes whether Voyager 1 is inside the solar bubble, known as the heliosphere, which is inflated by plasma that streams outward from our sun, or in interstellar space and surrounded by material ejected by the explosion of nearby giant stars millions of years ago. Adding to the challenge: they didn't know how they'd be able to detect it. "We looked for the signs predicted by the models that use the best available data, but until now we had no measurements of the plasma from Voyager 1," said Stone.
Scientific debates can take years, even decades to settle, especially when more data are needed. It took decades, for instance, for scientists to understand the idea of plate tectonics, the theory that explains the shape of Earth's continents and the structure of its sea floors. First introduced in the 1910s, continental drift and related ideas were controversial for years. A mature theory of plate tectonics didn't emerge until the 1950s and 1960s. Only after scientists gathered data showing that sea floors slowly spread out from mid-ocean ridges did they finally start accepting the theory. Most active geophysicists accepted plate tectonics by the late 1960s, though some never did.
Voyager 1 is exploring an even more unfamiliar place than our Earth's sea floors -- a place more than 11 billion miles (17 billion kilometers) away from our sun. It has been sending back so much unexpected data that the science team has been grappling with the question of how to explain all the information. None of the handful of models the Voyager team uses as blueprints have accounted for the observations about the transition between our heliosphere and the interstellar medium in detail. The team has known it might take months, or longer, to understand the data fully and draw their conclusions.
"No one has been to interstellar space before, and it's like traveling with guidebooks that are incomplete," said Stone. "Still, uncertainty is part of exploration. We wouldn't go exploring if we knew exactly what we'd find."
The two Voyager spacecraft were launched in 1977 and, between them, had visited Jupiter, Saturn, Uranus and Neptune by 1989. Voyager 1's plasma instrument, which measures the density, temperature and speed of plasma, stopped working in 1980, right after its last planetary flyby. When Voyager 1 detected the pressure of interstellar space on our heliosphere in 2004, the science team didn't have the instrument that would provide the most direct measurements of plasma. Instead, they focused on the direction of the magnetic field as a proxy for source of the plasma. Since solar plasma carries the magnetic field lines emanating from the sun and interstellar plasma carries interstellar magnetic field lines, the directions of the solar and interstellar magnetic fields were expected to differ.
Most models told the Voyager science team to expect an abrupt change in the magnetic field direction as Voyager switched from the solar magnetic field lines inside our solar bubble to those in interstellar space. The models also said to expect the levels of charged particles originating from inside the heliosphere to drop and the levels of galactic cosmic rays, which originate outside the heliosphere, to jump.
In May 2012, the number of galactic cosmic rays made its first significant jump, while some of the inside particles made their first significant dip. The pace of change quickened dramatically on July 28, 2012. After five days, the intensities returned to what they had been.  This was the first taste of a new region, and at the time Voyager scientists thought the spacecraft might have briefly touched the edge of interstellar space.
By Aug. 25, when, as we now know, Voyager 1 entered this new region for good, all the lower-energy particles from inside zipped away. Some inside particles dropped by more than a factor of 1,000 compared to 2004. The levels of galactic cosmic rays jumped to the highest of the entire mission.  These would be the expected changes if Voyager 1 had crossed the heliopause, which is the boundary between the heliosphere and interstellar space. However, subsequent analysis of the magnetic field data revealed that even though the magnetic field strength jumped by 60 percent at the boundary, the direction changed less than 2 degrees. This suggested that Voyager 1 had not left the solar magnetic field and had only entered a new region, still inside our solar bubble, that had been depleted of inside particles.
Then, in April 2013, scientists got another piece of the puzzle by chance. For the first eight years of exploring the heliosheath, which is the outer layer of the heliosphere, Voyager's plasma wave instrument had heard nothing. But the plasma wave science team, led by Don Gurnett and Bill Kurth at the University of Iowa, Iowa City, had observed bursts of radio waves in 1983 to 1984 and again in 1992 to 1993. They deduced these bursts were produced by the interstellar plasma when a large outburst of solar material would plow into it and cause it to oscillate.  It took about 400 days for such solar outbursts to reach interstellar space, leading to an estimated distance of 117 to 177 AU (117 to 177 times the distance from the sun to the Earth) to the heliopause. They knew, though, that they would be able to observe plasma oscillations directly once Voyager 1 was surrounded by interstellar plasma.
Then on April 9, 2013, it happened: Voyager 1's plasma wave instrument picked up local plasma oscillations. Scientists think they probably stemmed from a burst of solar activity from a year before, a burst that has become known as the St. Patrick's Day Solar Storms. The oscillations increased in pitch through May 22 and indicated that Voyager was moving into an increasingly dense region of plasma. This plasma had the signatures of interstellar plasma, with a density more than 40 times that observed by Voyager 2 in the heliosheath.
Gurnett and Kurth began going through the recent data and found a fainter, lower-frequency set of oscillations from Oct. 23 to Nov. 27, 2012. When they extrapolated back, they deduced that Voyager had first encountered this dense interstellar plasma in August 2012, consistent with the sharp boundaries in the charged particle and magnetic field data on August 25.
Stone called three meetings of the Voyager team. They had to decide how to define the boundary between our solar bubble and interstellar space and how to interpret all the data Voyager 1 had been sending back. There was general agreement Voyager 1 was seeing interstellar plasma, based on the results from Gurnett and Kurth, but the sun still had influence. One persisting sign of solar influence, for example, was the detection of outside particles hitting Voyager from some directions more than others. In interstellar space, these particles would be expected to hit Voyager uniformly from all directions.
"Now that we had actual measurements of the plasma environment – by way of an unexpected outburst from the sun – we had to reconsider why there was still solar influence on the magnetic field and plasma in interstellar space," Stone said.
"The path to interstellar space has been a lot more complicated than we imagined."
Stone discussed with the Voyager science group whether they thought Voyager 1 had crossed the heliopause. What should they call the region were Voyager 1 is?
"In the end, there was general agreement that Voyager 1 was indeed outside in interstellar space," Stone said. "But that location comes with some disclaimers – we're in a mixed, transitional region of interstellar space. We don't know when we'll reach interstellar space free from the influence of our solar bubble."
So, would the team say Voyager 1 has left the solar system? Not exactly – and that's part of the confusion. Since the 1960s, most scientists have defined our solar system as going out to the Oort Cloud, where the comets that swing by our sun on long timescales originate. That area is where the gravity of other stars begins to dominate that of the sun. It will take about 300 years for Voyager 1 to reach the inner edge of the Oort Cloud and possibly about 30,000 years to fly beyond it. Informally, of course, "solar system" typically means the planetary neighborhood around our sun. Because of this ambiguity, the Voyager team has lately favored talking about interstellar space, which is specifically the space between each star's realm of plasma influence.
"What we can say is Voyager 1 is bathed in matter from other stars," Stone said. "What we can't say is what exact discoveries await Voyager's continued journey. No one was able to predict all of the details that Voyager 1 has seen. So we expect more surprises."
Voyager 1, which is working with a finite power supply, has enough electrical power to keep operating the fields and particles science instruments through at least 2020, which will mark 43 years of continual operation. At that point, mission managers will have to start turning off these instruments one by one to conserve power, with the last one turning off around 2025.
Voyager 1 will continue sending engineering data for a few more years after the last science instrument is turned off, but after that it will be sailing on as a silent ambassador. In about 40,000 years, it will be closer to the star AC +79 3888 than our own sun. (AC +79 3888 is traveling toward us faster than we are traveling towards it, so while Alpha Centauri is the next closest star now, it won't be in 40,000 years.) And for the rest of time, Voyager 1 will continue orbiting around the heart of the Milky Way galaxy, with our sun but a tiny point of light among many.
The Voyager spacecraft were built and continue to be operated by NASA's Jet Propulsion Laboratory, in Pasadena, Calif. Caltech manages JPL for NASA. The Voyager missions are a part of NASA's Heliophysics System Observatory, sponsored by the Heliophysics Division of the Science Mission Directorate at NASA Headquarters in Washington.
For more information about Voyager, visit: http://voyager.jpl.nasa.gov

 

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Mysterious small tremors in the most earthquake-prone areas on Earth may be the cause of surprisingly large tsunamis, researchers say.

These findings might also shed light on the huge tsunami generated by the disastrous magnitude 9.0 quake that hit Japan in 2011.

Nearly all of the 10 largest recorded earthquakes on Earth happened along subduction zones, where one of the tectonic plates making up the planet's surface is diving beneath another. The shallow regions of these zones are often not seismically active by themselves, but occasionally strange tremors are recorded from these locales that are rich in very-low-frequency seismic waves.
These shallow areas also seem to be home to so-called tsunami earthquakes, which generate tsunamis far stronger than one would expect for the amount of seismic energy they release. The Keicho quake of 1605 that caused disastrous tsunamis in Japan and killed thousands might have been one such earthquake.
To see if there were any links between the very-low-frequency events and tsunami earthquakes seen in the shallows of subduction zones, scientists in Japan used three ocean-bottom seismometers to analyze a swarm of very-low-frequency events in 2009. These occurred in the shallowest parts of the Nankai Trough, a part of a subduction zone near southwestern Japan that is rocked by giant earthquakes every century or so — most recently in 1946, when a magnitude 8.2 event killed an estimated 1,300 people.

The researchers discovered that the very-low-frequency quakes — ranging from magnitudes of 3.8 to 4.9 — can last 30 to 100 seconds. This is unusually long when compared with the 1-to-2 second duration of ordinary earthquakes with comparable magnitudes.
Although these very-low-frequency quakes get their name from seismic waves detected on land, the researchers discovered these events are actually rich in high-frequency waves as well. High-frequency waves tend to weaken with distance as they go through matter, which is why land seismometers did not detect these waves but ocean seismometers closer to the quakes did. The long duration of the quakes and the high-frequency waves now seen from them suggest these events may be caused by fluid seeping into fractures in the rock, making it easier for parts of the earth to slip past each other and generate tsunami earthquakes.
These findings suggest that authorities should keep a closer eye on the shallow areas of subduction zones. For instance, the huge tsunamis generated by the magnitude 9.0 quake that struck Japan in 2011 might be due in significant part to a slip in the shallow parts of the Japan Trench lying east of the country's main island.

"It is very important for us to monitor continuously seismic activities close to the trench," researcher Hiroko Sugioka, a seismologist at the Japan Agency for Marine-Earth Science and Technology at Yokosuka, told OurAmazingPlanet. "It is mitigation against unexpectedly large tsunami disasters."

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