Earthquake scars Earth's gravity

Dec 04, 2013
Gravity scar over Japan. Credit: DGFI/TU Delft

(Phys.org) —ESA's GOCE satellite has revealed that the devastating Japanese earthquake of 2011 left its mark in Earth's gravity – yet another example of this extraordinary mission surpassing its original scope.

GOCE mapped Earth's gravity with unrivalled precision for over four years, but nobody really expected the data to show changes over time.

Now, careful analysis shows the effects of the 9.0 earthquake that struck east of Japan's Honshu Island on 11 March 2011 are clearly visible in GOCE's .

Large earthquakes not only deform Earth's crust, but can also cause tiny changes in local gravity.

The strength of gravity varies from place to place on our planet's surface and it was GOCE's task to map these variations very precisely.

There are a number of reasons why values of gravity differ, but one is a consequence of material inside Earth being inhomogeneous and unevenly distributed. Since earthquakes shift around rock and other material tens of km below the surface, they also cause small changes in the local gravity.

Earthquakes under oceans, as in the 2011 Japanese quake, can also change the shape of the sea bed. This displaces water and changes the sea level, which in turn also affects gravity.

GOCE. Credit: ESA /AOES Medialab

After more than doubling its planned life in orbit, the recently ran out of fuel and reentered the atmosphere, largely disintegrating in the process. Although it is no longer in orbit, the real mission is only just starting because scientists will be analysing the data for years to come to help understand many aspects of our world.

Information from GOCE is being used to understand how oceans transport huge quantities of heat around the planet and to develop a global height reference system, for example.

The mission has already shed new light on different aspects of Earth – from atmospheric density and winds, to mapping the boundary between the crust and upper mantle, and to understand geodynamic processes occurring in these layers far below our feet.

In a surprising discovery earlier this year unrelated to gravity changes, the satellite's accelerometer and ion thruster also revealed that GOCE had 'felt' sound waves in space from the Japanese quake.

Recently, scientists from the German Geodetic Research Institute, DGFI, and from Delft University of Technology in the Netherlands analysed the high-resolution vertical gravity gradients measured over Japan. They discovered that the quake had clearly ruptured the .

This is the first time that GOCE has been shown to have found changes over time. This work was carried out through ESA's Earth Observation Support to Science Element.

GOCE gravity results (left) compared to model. Credit: DGFI/TU Delft

Moreover, the gravity change measured by GOCE differs in size and location compared to those predicted by standard models.

GOCE's results are consistent with coarser observations from the NASA–German Grace satellite, which is designed to measure changes over time. This suggests that GOCE data will be important in improving models and will therefore contribute to our understanding of earthquakes.

Martin Fuchs from DGFI said, "Thus, we see that GOCE gravity gradients complement other types of data such as seismic, GPS and GRACE satellite gravimetry.

"We are now working in an interdisciplinary team to combine GOCE data with other information to obtain a better picture of the actual rupture in the field than is currently available."

Explore further: Satellite's gravity-mapping mission is over, ESA says

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kevin_hingwanyu
1 / 5 (2) Dec 04, 2013
the satellite's accelerometer and ion thruster also revealed that GOCE had 'felt' sound waves in space from the Japanese quake

Layperson's view. Is it possible that the sound waves 'felt' by the satellite's instruments and ion thruster (in space) were some sort of varying electric fields, where the earthquake compressed and stretched the crystals or quartz in rocks to generate the changing electric fields, e.g. piezoelectric effect.
careful analysis shows the effects of the 9.0 earthquake that struck east of Japan's Honshu Island on 11 March 2011 are clearly visible in GOCE's gravity data

I realise that today's instrumental sensitivity could be sufficient to measure gravitational waves with two or few (or one) GOCE spacecrafts.
RobertKarlStonjek
1 / 5 (1) Dec 04, 2013
I am wondering if, as is reported above, Earth's gravity profile changed as result of the Earthquake, if this in turn created a gravity wave?

Though the wave would be extremely small by cosmological standards, it would be bigger as measured by an orbiting gravity wave detector than those from the further reaches of the universe ie just as a dim light close by can be brighter than a far off bright search light.
kevin_hingwanyu
5 / 5 (1) Dec 05, 2013
If some instruments (theoretically) can detect very faint gravitational waves from distant galaxies where the sources with enormous amplitude were produced by some bizarre objects e.g. the distribution of mass collapsed/expanded suddenly for black holes or supernovae, then, on planetary scale the sudden redistribution of mass (earthquake) is sufficiently to generate although "weaker" gravitational waves but ultimately can be detected by nearby instruments or satellites. The logic is, 'gravitational waves' have no specialness, are just common natural phenomena, not discovered yet. (I think) It doesn't make sense why only blackholes supernovae can producing it but to exclude other objects or events. Or magnitude too weak not detectable.

A question is, does the wavelength, frequency or spectrum of gravitational waves matter? i.e. to detect radio, infrared or visible waves the instruments are very different, using an inappropriate instrument (high sensitivity though) will detect nothing!
kevin_hingwanyu
5 / 5 (1) Dec 25, 2013
A conjecture, no answer yet(?). Assume two GOCE spacecrafts, their distances to the source of an earthquake were different, d1 and d2. Assume their sensitivity could distinguish the difference before / and after the earthquake. The questions are: at the instant when the earthquake occurs, are the two GOCE detect the change (e.g. after earthquake) of mass distribution at the **same time/instant** ? If not the same time, how long would it take for the information (e.g. the change of distribution of mass at the source of earthquake) to travel from d1 to d2 ? The time it would take to travel from d1 to d2, is this the speed of gravity or gravitational wave ?
kevin_hingwanyu
not rated yet Dec 26, 2013
@ Zephir_fan. Thx for your support. Science advances very fast. I update knowledge from Physorg, this forum, other sites and forums. Perhaps other peoples/users of this forum too. How one can know the others' mind/thinking (e.g. pretending . . . etc)?

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