Japan Earthquake Changed Earth's Gravitational Field

The orbits of satellites were altered after the 9.1-magnitude earthquake shook Japan.

Last week, we learned that the massive Tohuku earthquake that rocked Japan earlier this year shifted the sea floor of the Pacific Ocean by about 50 metres. Today, New Scientist informs us that the same earthquake was so powerful that it altered the planet's gravitational field and, in turn, the orbits of satellites circling Earth. The findings were made by two satellites that make up NASA's Gravity Recovery and Climate Experiment, or GRACE, which has the express purpose of monitoring fluctuations in the Earth's gravitational field, which are caused by adjustments in the planet's mass, such as those caused by the melting of polar ice caps. How did GRACE come to the conclusion that satellites were knocked slightly off course?

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.

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.

Only two earthquakes since 2002 had been recorded as altering the gravitational field – the 2004 Indian Ocean quake and last year's megaquake in Chile – and now the Tohuku quake has joined that rarified company. The GRACE team also found that monitoring shifts in satellite orbits could give a more accurate reading of how powerful an earthquake was, as their findings suggest the quake was a 9.1 on the Richter scale, compared to the 9.0 that Earth-based sensors arrived at.