Feeling quakes (Image: NASA)

The Tōhoku earthquake that rattled Japan on 11 March changed Earth’s gravitational field enough to affect the orbits of satellites. The satellites’ altered courses suggest that the earthquake was stronger and deeper than instruments on Earth indicated.

These weren’t just any satellites: they are the twin spacecraft of the Gravity Recovery and Climate Experiment (GRACE), which fly 220 kilometres apart in a polar orbit about 500 kilometres above Earth. GRACE’s job is to map the Earth’s gravity field, and it does this by monitoring the effect of minute variations in the field on the trajectories of the satellites and the changing distance between them.

Earth’s gravity field changes whenever there is a redistribution of mass on its surface. This may be a result of snowfall, flood, melting of ice caps – or earthquakes. “That perturbed gravitational field affects the satellite orbits,” explains Shin-Chan Han of NASA’s Goddard Space Flight Center in Greenbelt, Maryland.


Megaquakes measured

Han and colleagues have already studied the effects of two previous megaquakes – the Sumatra-Andaman earthquake in 2004 and last year’s earthquake in Chile. After GRACE was launched in 2002, these were the only earthquakes that had had a measurable effect on the satellites’ orbits – until the Tōhoku earthquake.

Now Han’s team has used that most recent megaquake to show that the disturbances of satellite orbits can be used to independently estimate the magnitude and location of earthquakes, along with estimates based on surface seismographs and GPS measurements.

The researchers calculated how the relative velocity of the two satellites changed as they passed over the affected region. GRACE records variations in the gravity field due to other processes too, but these background signals change over larger timescales than that of the quake, and so could be identified and subtracted.

New mission

The leftover signal showed that the rate at which the distance between the two GRACE satellites changed – the so-called range rate – was twice as high in the month after the earthquake as in the month prior to the event.

The researchers then built models of the earthquake using data from seismographs and surface GPS instruments, and estimated what the satellites’ range rate would be in these models. They found that a model in which the earthquake was of magnitude 9.1 and occurred in Earth’s lower crust came closest to the true range rate. By contrast, conventional estimates have put the Tōhoku earthquake’s strength at 9.0 and located it in the upper crust.

NASA and DLR, the German space agency, which are joint partners in GRACE, are planning a new mission to measure with greater precision how Earth’s gravity field changes. This would allow the satellites to monitor earthquakes with magnitudes as low as 7.5, which occur nearly every month somewhere on Earth.

Han presented the results at the fall meeting of the American Geophysical Union in San Francisco this week.