New studies have shown that Japan’s “Tohoku earthquake” (March 11, 2011) was also felt and measured high above the Earth’s surface by ESA’s GOCE gravity satellite. This hyper-sensitive instrument is now known as the world’s first “seismometer in space” but it is more than that. What GOCE also showed is a way for seismologists to start looking into space to understand what’s going on under our feet.
Earthquakes produce seismic waves that travel through Earth’s interior and can be recorded by seizmometers all over the globe but stronger ones cause the surface of the planet to vibrate like a drum. This produces sound waves that travel upwards through the atmosphere. The size of those waves changes from centimeters on surface to kilometers in the thin atmosphere at altitudes of 200 – 300 km. Only low-frequency sound – infrasound – reaches these heights. It causes vertical movements that expand and contract the atmosphere by accelerating air particles.
As the animation shows, GOCE, circling Earth at almost 8 km/s crossed the infrasound travelling above the Pacific Ocean at the speed of seismic surface waves, around 4 km/s. Then, after having passed over the North Pole, GOCE caught the infrasound wavefront again above Europe. These observations are the first direct sounding of atmospheric waves created by earthquakes.
Exploiting GOCE data to the maximum, scientists from the Research Institute in Astrophysics and Planetology in France, the French space agency CNES, the Institute of Earth Physics of Paris and Delft University of Technology in the Netherlands, supported by ESA’s Earth Observation Support to Science Element, have been studying past measurements.
What is GOCE?
ESA’s dart-like Gravity field and Ocean Circulation Explorer (GOCE) Earth Explorer orbits as close to Earth as possible – just 260 km up – to maximise its sensitivity to variations in Earth’s gravity field.
Launched in 2009, GOCE’s state-of-the-art gradiometer is mapping Earth’s geoid to an unprecedented level of accuracy, opening a window into Earth’s interior structure as well as the currents circulating within the depths of it’s oceans. More on GOCE here.
When GOCE passed through sound waves created by massive earthquake in Japan, its accelerometers sensed the vertical displacements of the surrounding atmosphere in a way similar to seismometers on the surface of Earth. Wave-like variations in air density were also observed.
Raphael Garcia from the Research Institute in Astrophysics and Planetology said seismologists are particularly excited by this discovery because they were virtually the only Earth scientists without a space-based instrument directly comparable to those deployed on the ground.
“With this new tool they can start to look up into space to understand what is going on under their feet.”
Rune Floberghagen, ESA’s GOCE Mission Manager noted:
“The GOCE team has always known that our accelerometers are something truly special. They are about 100 times more sensitive than previous accelerometers. So good, in fact, it was obvious these data would find great uses outside the domain of the gravity field by, for example, probing the space environment in new ways. We are now of course excited to see such predictions turning into reality, with such an exciting application in the field of seismology.”
Source and featured image credit: ESA
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