Gravity Satellite Could 'Hear" Japanese Earthquake in Space

Earthquakes not only create the seismic waves that travel through rock and show up on vibration sensitive seismometers, but large quakes can even produce sound waves that travel upwards through the atmosphere by causing the surface of the planet to vibrate like a drum.

These kind of sound waves have now been sensed in space for the first time by ESA’s hyper-sensitive GOCE gravity satellite.

The size of these waves changes from centimetres at the surface to kilometres in the thin atmosphere at altitudes of 200–300 km.

On Monday, Japan remembers the 20 000 people who died in the earthquake and tsunami that devastated its northeastern coast two years ago. New studies have revealed that this massive quake, about the strongest ever recorded, was sensed in Earth's orbit by ESA’s GOCE satellite.

Since it was launched in 2009, GOCE has been mapping Earth’s gravity with unrivalled precision, orbiting at the lowest altitude of any observation satellite. But at less than 270 km up, it has to cope with air drag as it cuts through the remnants of the atmosphere.

The cleverly designed satellite carries an innovative ion engine that instantly compensates for any drag by generating carefully calculated thrusts. These measurements are provided by very precise accelerometers.

While the measurements ensure that GOCE remains ultra-stable in its low orbit to collect ultra-precise measurements of Earth’s gravity, atmospheric density and vertical winds along its path can be inferred from the thruster and accelerometer data.

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, have been studying past measurements.

When GOCE passed through the sound waves caused by the quake 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.”