Jacques Descloitres, MODIS Rapid Response Team, NASA/GSFC
Recently, researchers detected a rare type of deep-Earth tremor. It was triggered by a hurricane. In this case, the source was a "weather bomb" strengthening rapidly over the North Atlantic Ocean.
“Weather bombs” are a type of extreme storm. Scientists can detect the seafloor quivers that these storms trigger. It starts as whipping winds stir up towering ocean swells. When two opposing ocean swells collide, the meet-up can send a pressure pulse down to the ocean floor. That pulse thumps the seafloor. The resulting Earth tremors — or seismic waves — penetrate deep into the planet.
Those seismic waves speed up, slow down or change direction as they move through the ground. Such motions depend on the type of material they pass through. In the past, scientists have carefully measured some of these movements caused by earthquake waves. That let them gather clues about the structure and composition of Earth’s deepest layers.
But some regions of Earth don’t see many earthquakes. One example is the middle of tectonic plates under the ocean. Luckily, weather bombs can generate their own seismic waves here.
Scientists previously had detected only one type of these storm-generated deep seismic waves. They are known as P waves. Such waves cause a material to compress and stretch in the same direction that the wave travels. Think of an accordion being stretched and compressed when someone plays it.
A second type of waves have proved more elusive. When storms form these S waves, they typically are weaker than P waves. They cause material to ripple perpendicular to the wave’s path. The effect is similar to when one end of a garden hose is jerked up and down. That action produces waves that travel along the hose’s length.
The new research tracked such S waves from a weather bomb. Details on the unusual waves appear in the August 26 in Science.
How they found them
Kiwamu Nishida is a seismologist at the University of Tokyo in Japan. Ryota Takagi is also a seismologist. He works at Tohoku University in Sendai, Japan. The pair hunted for the elusive S waves. To do this, they used a network of 202 seismic stations in Japan.
Typically, the S waves can be lost within Earth’s natural seismic background noise. But Nishida and Takagi combined and analyzed data from extra-sensitive seismometers. That let the researchers tease out the S-wave signals.
Those waves had been triggered by a cyclone — hurricane — in the North Atlantic, they found. That storm produced two types of S waves. SV waves shift material vertically (up and down) relative to Earth’s surface. They can form from P waves. SH waves shift material horizontally (left and right). Their origins are more of a mystery. Those SH waves may form from complex interactions between the ocean and the seafloor, Nishida says.
“We’re potentially getting a suite of new seismic source locations that can be used to investigate the interior of the Earth,” notes Peter Bromirski. He is an oceanographer at the Scripps Institution of Oceanography in La Jolla, Calif. He wrote a commentary on the new research in the same issue of Science. Further study, he says, should help science “refine our understanding of how useful these particular waves will be.”
Keith Koper is a seismologist at the University of Utah in Salt Lake City. Combining measurements of P, SV and SH waves will “ultimately provide better maps of Earth’s mantle and maybe even the core,” he says. (The mantle is the layer between Earth’s outer crust and its core.) Koper and his colleagues made similar observations of S waves generated in the Pacific Ocean. Those waves were detected by a Chinese seismic network.
Koper’s group reported these related findings in the September 1 Earth and Planetary Sciences Letters. “It’s nice to see someone else get similar results,” Koper says. “It makes me feel more confident about what we observed.”
(for more about Power Words, click here)
Atlantic One of the world’s five oceans, it is second in size only to the Pacific. It separates Europe and Africa to the east from North and South America to the west.
colleague Someone who works with another; a co-worker or team member.
commentary (in science) An opinion piece, often written to accompany — and add perspective to — a paper by others, which describes new research findings.
core In geology, Earth’s innermost layer. Or, a long, tube-like sample drilled down into ice, soil or rock. Cores allow scientists to examine layers of sediment, dissolved chemicals, rock and fossils to see how the environment at one location changed through hundreds to thousands of years or more.
crust (in geology) Earth’s outermost surface, usually made from dense, solid rock.
cyclone A strong, rotating vortex, usually made of wind. Notable examples include a tornado or hurricane.
data Facts and/or statistics collected together for analysis but not necessarily organized in a way that give them meaning. For digital information (the type stored by computers), those data typically are numbers stored in a binary code, portrayed as strings of zeros and ones.
earthquake magnitude A measurement of the intensity of the ground-shaking associated with an earthquake. The scale is logarithmic. So for every 1 point increase in magnitude (such as from 3 to 4), there is a 10 fold increase in ground motion (how far the land shakes back and forth) and a roughly 33-fold increase in the amount of energy released.
mantle (in geology) The thick layer of the Earth beneath its outer crust. The mantle is semi-solid and generally divided into an upper and lower mantle.
network A group of interconnected people or things.
oceanography The branch of science that deals with the physical and biological properties and phenomena of the oceans. People who work in this field are known as oceanographers .
Pacific The largest of the world’s five oceans. It separates Asia and Australia to the west from North and South America to the east.
perpendicular An adjective that describes two things that are situated approximately 90 degrees to each other. In the letter “T,” the top line of the letter is perpendicular to the bottom line.
pressure Force applied uniformly over a surface, measured as force per unit of area.
seismic wave A wave traveling through the ground produced by an earthquake or some other means.
seismometer (also known as a seismograph ) An instrument that detects and measures tremors (known as seismic waves) as they pass through Earth.
tectonic Surface activity on a large rocky body (such as a planet or moon) as liquid rock flows up to the surface where it solidifies, then slowly drifts atop molten rock, carrying surface features with it.
tectonic plates The gigantic slabs — some spanning thousands of kilometers (or miles) across — that make up Earth’s outer layer.
wave A disturbance or variation that travels through space and matter in a regular, oscillating fashion.
weather Conditions in the atmosphere at a localized place and a particular time. It is usually described in terms of particular features, such as air pressure, humidity, moisture, any precipitation (rain, snow or ice), temperature and wind speed. Weather constitutes the actual conditions that occur at any time and place. It’s different from climate, which is a description of the conditions that tend to occur in some general region during a particular month or season.
weather bomb A type of extreme storm that can pound the ocean floor, sending tremors that can travel huge distances.
Journal: K. Nishida and R. Takagi. Teleseismic S wave microseisms. Science. Vol. 353, August 26, 2016, p. 919. doi: 10.1126/science.aaf7573.
Journal: P. Gerstof and P.D. Bromirski. ‘Weather bomb’ induced seismic signals. Science. Vol. 353, August 26, 2016, p. 869. doi: 10.1126/science.aag1616.
Journal: Q. Liu et al. Source locations of teleseismic P, SV, and SH waves observed in microseisms recorded by a large aperture seismic array in China. Earth and Planetary Science Letters. Vol. 449, September 1, 2016, p. 39. doi: 10.1016/j.epsl.2016.05.035.