The European Space Agency’s (ESA) Gravity Field and Steady-State Ocean Circulation Explorer (GOCE) satellite was launched on March 17, 2009, as the first of a series of Earth Explorer satellites. Its mission is to capture high-resolution gravity measurements and produce an accurate gravity map – or geoid – of Earth. To increase the resolution of its measurements, GOCE was put into an unusually low orbit, which has also helped it to become the first satellite to sense sound waves from an earthquake from space.
March 11 marks the two-year anniversary of the 2011 Tohoku earthquake and tsunami that devastated the northeastern coast of Japan, but researchers studying past measurements taken by GOCE have only recently discovered that the satellite detected sound waves from the earthquake.
While earthquakes create seismic waves that travel through the Earth’s layers, large earthquakes can also cause the Earth’s surface to vibrate, producing sound waves that travel upwards through the atmosphere. While these waves measure only centimeters at the Earth’s surface, they expand to measure kilometers in the thin atmosphere at altitudes of 200-300 km.
The low-frequency sound, or infrasound – that reaches these heights causes vertical movements that expand and contract the atmosphere. It is these movements that GOCE was able to detect thanks to its three pairs of accelerometers, which are so precise they can detect accelerations to within one part in 10,000,000,000,000 of Earth’s gravity.
Because GOCE is flying at an altitude of less than 270 km – giving it the lowest altitude of any observation satellite – it passes through the remnants of the atmosphere. To compensate for the drag resulting from changes in the atmosphere and keep GOCE ultra-stable in its low orbit, its ion engine generates carefully calculated thrusts based on measurements provided by the satellite’s accelerometers.
Analysis of past thruster and accelerometer data by 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, revealed that GOCE detected sound waves from the March 11, 2011 earthquake, making it the first satellite to sense an earthquake from space.
Around 30 minutes after the earthquake, GOCE passed through sound waves with the vertical displacement of the surrounding atmosphere detected by its accelerometers, not unlike the way in which seismometers detect seismic waves. GOCE also detected variations in air density.
“With this new tool they (seismologists) can start to look up into space to understand what is going on under their feet,” said Raphael Garcia from the Research Institute in Astrophysics and Planetology.
The ripples in the atmosphere caused by the 2011 Tohoku earthquake and their effect on GOCE can be seen in the following animation.