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Hervé
28th August 2016, 16:17
Teleseismic S wave microseisms (http://science.sciencemag.org/content/353/6302/919)


Kiwamu Nishida1 (http://science.sciencemag.org/content/353/6302/919#aff-1),* (http://science.sciencemag.org/content/353/6302/919#corresp-1),
Ryota Takagi2 (http://science.sciencemag.org/content/353/6302/919#aff-2)

Science 26 Aug 2016:
Vol. 353, Issue 6302, pp. 919-921
DOI: 10.1126/science.aaf7573

A seismic “weather bomb” detector
Seismic tomography is like an x-ray of Earth's interior, except that it uses earthquakes for the illumination. Earthquakes are imperfect illuminators because they are clustered on plate boundaries, leaving much of the interior in the shadows. Using a seismic array in Japan, Nishida and Takagi detected seismic waves that they attribute to a severe and distant North Atlantic storm called a “weather bomb” (see the Perspective by Gerstoft and Bromirski). The seismic energy traveling from weather bombs through the Earth appears to be capable of illuminating the many dark patches of Earth's interior.
Science, this issue p. 919 (http://science.sciencemag.org/lookup/doi/10.1126/science.aaf7573); see also p. 869 (http://science.sciencemag.org/lookup/doi/10.1126/science.aag1616)

Abstract
Although observations of microseisms excited by ocean swells were firmly established in the 1940s, the source locations remain difficult to track. Delineation of the source locations and energy partition of the seismic wave components are key to understanding the excitation mechanisms. Using a seismic array in Japan, we observed both P and S wave microseisms excited by a severe distant storm in the Atlantic Ocean. Although nonlinear forcing of an ocean swell with a one-dimensional Earth model can explain P waves and vertically polarized S waves (SV waves), it cannot explain horizontally polarized S waves (SH waves). The precise source locations may provide a new catalog for exploring Earth’s interior.


'Weather Bombs' Could 'X-Ray' Earth to Help Detect Quakes (http://www.livescience.com/55889-weather-bombs-could-x-ray-earth.html)

By Kacey Deamer, Staff Writer | August 25, 2016 02:17pm ET

A "weather bomb" is a storm over the ocean with intense central pressure, producing strong winds and ocean swell that result in some wave-generated seismic activity.
Credit: Carl Milner/Flickr Small, intense storms known as "weather bombs" may trigger rare tremors deep within the Earth, offering scientists a new way to study the mysterious structure and inner workings of the planet, according to a new study.

A "weather bomb" is an extratropical (outside of the tropical zone) storm in which the central pressure intensifies (http://www.livescience.com/39004-weather-fronts-definition-facts.html) rapidly. These storms produce very strong winds that cause the ocean to swell, generating powerful waves. Some of the wave energy from these storms interacts with the seafloor, causing wave-generated seismic activity.


http://static.latercera.com/20160826/2327087.jpg
Deep-Earth tremor. Earthquakes and storms both trigger two types of disturbance in the Earth;


Known as microseisms, these seismic waves (http://www.livescience.com/21486-earthquakes-causes.html) are detectable anywhere in the world, because they penetrate the Earth deeply and can be observed at faraway land seismic stations, the researchers said.

However, observations and analysis of microseismic activity have focused mostly on P waves — the first set of waves in an earthquake that deliver a sharp jolt — because of their larger amplitudes. This gives scientists only a narrow view of the Earth's structure, because P waves typically travel in straight lines.


http://epeak.in/uploads/attach/867/ae/ae0e3f8cb9953a2.jpg
The faint and deep tremors "weather bombs" can cause in the oceanic crust run through the earth and can be detected in places as far away as Japan. Credit: Kiwamu Nishida and Ryota Takagi


In their new study, the scientists detected so-called S wave microseisms (http://www.livescience.com/50457-gps-smartphones-earthquake-early-warning.html), which travel much more slowly and curve through the ground but are generally more difficult to observe. The previously unobserved S waves were generated under a weather bomb between Greenland and Iceland in December 2014.

The researchers detected both P-wave and S-wave microseisms triggered by the severe North Atlantic storm at their station in Japan. In their paper, the authors described the direction and distance to the waves' origins.

Mapping the microseisms provides insight into the planet's deep structure. As seismic energy from the weather-bomb storm travels through the deep Earth, the planet's interior structure is revealed (http://www.livescience.com/31279-seismic-waves-earth-mantle.html), the researchers said. This is especially beneficial in areas where such monitoring is limited — such as the oceans.

"We would like to explore the Earth's interior beneath the storm in oceanic areas, where no earthquakes and no stations exist," study co-author Kiwamu Nishida, a professor at the Earthquake Research Institute at the University of Tokyo, told Live Science.

With more acute observations of these storms and the microseisms they cause, scientists can better understand the Earth's internal structure. And understanding the precise locations of P waves and S waves and how they move can also help scientists learn how the seismicity occurs, the researchers explained in their paper.

"Delineation of the source locations and energy partition of the seismic wave components are key to understanding the excitation mechanisms," the authors wrote. As such, the findings could contribute to more accurate detection of earthquakes (http://www.livescience.com/31412-earthquake-warnings-explained.html) and oceanic storms.

The new study was published online today (Aug. 25) in the journal Science (http://science.sciencemag.org/cgi/doi/10.1126/science.aaf7573).

Original article on Live Science (http://www.livescience.com/55889-weather-bombs-could-x-ray-earth.html).

Related:Weather bombs could help us see deep inside Earth (https://www.newscientist.com/article/2102519-weather-bombs-could-help-us-see-deep-inside-earth/)