The Asian earthquakes detected in the ATLAS cavern

At the end of December, mysterious vibrations were picked up by the deformation sensors under the feet that are to support the ATLAS detector. It transpired that they had detected waves produced by the earthquakes responsible for the terrible tsunami in Asia.


Six hydrostatic level sensors installed under the feet of ATLAS to monitor any deformation of the structure on which the experiment will rest detected seismic waves linked to the Asian earthquakes last December.


During the Christmas break, the hydrostatic level sensors in the ATLAS cavern revealed a new facet of their capabilities. Installed by the CERN surveyors to monitor over time any deformation of the structure on which the feet that will support the detector rest, these sensors with sub-micron resolution took on the function of a seismograph. The signals recorded by the sensors at the end of the year are shown in the graph below, which reveals two perturbations, one on 23 December starting at 15.45 and the other on 26 December at 01.23. Seeing these unusual readings on 26 December, the CERN surveyors immediately wondered whether they were connected with the earthquake off the Indonesian coast.




The recordings taken by the six sensors revealed abnormal perturbations on 23 and 26 December 2004.


Dr Corine Frischknecht of the Geneva Centre for the Study of Geological Risks confirmed that the seaquake off the coast of Sumatra, which measured 9.0 on the Richter scale, was indeed responsible for the perturbations recorded at CERN. When a seismic event occurs, the resulting vibrations spread out in all directions and two types of wave can be distinguished, namely body waves, which propagate through the earth (such as P-waves or compressional waves, which are the fastest and propagate at a speed of 6 to 8 km/s, and S-waves or shear waves) and the generally slower surface waves, which are confined to the surface of the Earth (such as the horizontal Love wave, which causes much structural damage to foundations, and the Rayleigh wave).

The epicentre of the Sumatra quake was 9700 km from CERN. The primary (P) waves took about 20 minutes to reach us, which is consistent with the first perturbations recorded by the sensors at 1.23 a.m. (second graph). Thereafter it is hard to distinguish one wave type from another as they travel rather like rays of light that can be deflected each time they encounter a different medium, thus making their trajectories, which dictate the travelling time, very complex.

Once the cause of the perturbations on 26 December had been clarified, the CERN surveyors began to wonder what was behind those that had occurred at 15.45 UTC on 23 December. Various articles on the Sumatra tsunami mention a first quake at 14.59 UTC on 23 December north of the Macquarie islands (between Australia and Antarctica), which measured 8.1 on the Richter scale. This first quake is thought to have caused the second quake on 26 December, which was associated with the rupture of a friction zone between two plates, in all likelihood the one that occurred on 23 December.