Small Earthquakes May Cause Surprisingly Big Tsunamis

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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White Dwarfs Are Eating 'Earth-like' Planets for Dinner

There is one doomsday scenario that will, without a doubt, come true.
In 4-5 billion years time, when the sun runs out of fuel, it will become a bloated red giant star. During this violent phase, it will blowtorch the Earth before shedding huge quantities of mass and disintegrating into a planetary nebula. A tiny white dwarf star will remain -- the remnant of our sun's core -- with the dust cloud of pulverized inner solar system planets raining down onto it.
Now, using data from the Hubble Space Telescope, astronomers from the University of Warwick have discovered four white dwarf stars containing dust in their atmospheres, giving us a rare glimpse into the future death of our own solar system.
 Although dusty white dwarfs are a well-known astronomical phenomenon -- the extreme tidal shear and dynamical instability produced by a white dwarf will pulverize planetary bodies in orbit through a demolition derby of epic proportions -- these four new examples may be what our solar system will look like in a few billion years time.

In each case, the researchers have detected oxygen, magnesium, iron and silicon hanging in their stellar atmospheres. The presence of these elements are a telltale sign that rocky worlds used to exist in orbit. Interestingly, these four elements make up the composition of approximately 93 percent of the Earth.
In addition to these key elements is the detection of small quantities of carbon in proportions that closely match the proportion of carbon found inside the solar system's rocky planets. This is the first time such a proportion of carbon has been detected in the dusty debris surrounding white dwarfs.

Although the term "Earth-like" is often misrepresented in the field of exoplanetary studies, the Warwick astronomers are acutely aware of the implications of spotting these elements around distant stars. "What we are seeing today in these white dwarfs several hundred light-years away could well be a snapshot of the very distant future of the Earth," said lead researcher Boris Gänsicke.
Although we have little clue about the physical characteristics of the exoplanets before they were pulverized, all the components that make up the terrestrial planets -- Mercury, Venus, Earth, Mars and the asteroids -- are present in the white dwarfs' dust. The proportions of these elements are about as "Earth-like" as it gets.
There is one white dwarf, called PG0843+516, that stands out from the other three; it has an overabundance of iron, nickel and sulfur in its atmosphere. These particular elements are found in the cores of rocky planets. During planetary evolution, gravity pulls these elements into the core -- a process known as "differentiation." Differentiation will occur in large rocky worlds like Earth, forming a core, mantle, crust and, probably, tectonic activity.

Also, as the white dwarfs' gravity should consume these elements very quickly, the fact that they have been spotted in the star's atmosphere indicates a rocky planetary body is being ripped to shreds right now.
In all four white dwarfs, the researchers estimate 1 million kilograms of planetary material must be raining down into the stars every second. This is significant as they are witnessing the final stages of these star systems' death throes.
One can't help but wonder, if true "Earth-like" worlds are being pulverized and eaten by white dwarf stars, are the remnants of ancient extraterrestrial civilizations also being consumed?
This research has been accepted for publication in the Monthly Notices of the Royal Astronomical Society.
Source: University of Warwick

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