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20 January 2007 -- Gravity Meters & Seismometer Simultaneously Capture Japanese Earthquake in Luxembourg


A Micro-g LaCoste gPhone Gravimeter (formerly known as the Portable Earth Tide or "PET" meter), a Superconducting Gravity Meter, and a broadband seismometer, all measuring simultaneously in Walferedange, Luxembourg recorded the earthquake centered on the Kuril Islands north of Japan on 13 January 2007.

Discussed below are:

gPhone vs. Superconducting Gravity Meter

The plots below show the response of the gPhone and the Superconducting meter. Note that though the Superconducting meter's output is limited to +/-7500 nm/s2 (750 microGals), the instruments stay in phase and demonstrate remarkable agreement throughout the entire earthquake. The gPhone data have also been integrated to provide velocity and position information.

Gravity Velocity Position
The following time series plots show the above response in five minute segments, starting immediately before the earthquake. Scroll right to see the response during the earthquake, and scroll left to see the response during quiet seismic activity before the quake). Note that in general the vertical scales changes from frame to frame.


Gravity Velocity Position

gPhone vs. Broadband Seismometer

The gPhone response (velocity integrated from the acceleration signal) is compared with that of the Streckeisen STS-2 seismometer in the plots below. Note that the high sensitivity of the gPhone allows it to clearly detect the decaying Rayleigh waves which are not visible in the STS-2 data when plotted on the same scale.



The following time series plots again show the above response, but in five minute segments. Again, the plots start immediately before the earthquake. Scroll right to see the response during the earthquake, and scroll left to see the response during quiet seismic activity before the quake). Note that in general the vertical scales changes from frame to frame.


For a more detailed look at the results and correlations between the instruments, please click here. Also, for a related discussion of five gPhone Gravimeters measuring a November 2006 earthquake, please click here. These data are courtesy of Olivier Francis, Faculte des Sciences, de la Technologie et de la Communication, University of Luxembourg.

Detailed Analysis of the Rayleigh Wave Response

  • Distance from Japan to Luxembourg, forward arc: 8944km
  • Distance from Japan to Luxembourg, reverse arc: 31087km
  • Modeled Rayleigh wave velocities: 3.97km/s forward arc, 3.54km/s reverse arc (higher velocity across Eurasian craton)
  • Model travel times (forward, in hours): 0.70, 3.83, 6.96, 10.10
  • Model travel times (reverse, in hours): 2.43, 5.56, 8.70, 11.83



Plotted above is the gPhone response to the earthquake integrated twice to get the vertical displacement. Note the clear onset of the Rayleigh waves at approximately 0.7 hours.



In this plot, the green marks indicate the Rayleigh wave arrival times as observed by the gPhone, and the red marks indicate the model-predicted times.



Here the waves are plotted on a vertical log scale. Note the exponential fall off of the power.


Please click here for a pdf file of these Rayleigh response figures.