According to a new ESA study, the Sumatran earthquake of 26 December 2004 not only devastated a huge area, but it slightly altered the gravity of that area.
When the floor of the Indian Ocean shifted along a 1000 km fault line during the earthquake, it created a ledge 6 metres high. This enormous movement of rock instantly changed the local gravity.
This is not unusual. Maps made from satellite data show that the pull of gravity varies from place to place. One of the reasons for this is the Earth’s rotation. As the planet spins, it bulges outward at the equator. Since the Earth is 42 km wider at the equator, the force of gravity is weaker there than at the poles.
High mountains, deep ocean trenches and different types of rock inside the Earth also have an impact. Even tides, ocean currents and melting ice sheets cause the value of gravity to vary.
At present, such small gravity shifts are difficult to measure, but this will change with next year’s launch of ESA’s Gravity Field and Ocean Circulation Explorer (GOCE).
The sensitive spacecraft will study gravity in great detail. As it passes over regions of stronger and weaker gravitational pull, the spacecraft will bob up and down and its instruments will report the changing gravity field along its path.
This will enable scientists to measure how much material is being moved around, either suddenly in an earthquake or gradually, as in a melting ice sheet. The data will even help to reveal shifts in ocean circulation that are related to climate change.
