During the Christmas break, the hydrostatic level sensors (HLSs) in the ATLAS cavern revealed a new facet of their capabilities. Installed by the CERN survey group to monitor any deformation or movement of the structure on which the detector feet rest, these sensors with submicrometre resolution coupled to the heavy ATLAS mechanical infrastructure took on the function of a seismograph.
The signals recorded by the sensors are shown in the figure, which reveals two perturbations, one on 23 December starting at 15.45 GMT and the other on 26 December at 01.23 GMT. Seeing these unusual readings raised the question of whether they were connected with the earthquake off the Indonesian coast that gave rise to the devastating tsunami.
The Geneva Centre for the Study of Geological Risks was duly contacted and it confirmed that the earthquake off the coast of Sumatra, which measured 9.0 on the Richter scale, was indeed responsible for the large peak recorded at CERN. When a seismic event occurs, the resulting vibrations spread out in all directions and two types of wave can be distinguished: primary waves, which propagate through the earth at speeds of 6-8 km s-1, and the slower waves that are confined to the surface of the Earth (such as the horizontal Love wave, which can cause structural damage to buildings).
The epicentre of the Sumatra earthquake was some 9000 km from CERN and happened at 00:59 GMT (07:59 local time). The primary waves need about 20 minutes to reach the ATLAS cavern, which is consistent with the first perturbations recorded by the sensors at 01.23 GMT on 26 December.
The earlier, smaller perturbation is linked to another earthquake measuring 8.1 on the Richter scale, which is thought to have been correlated with the earthquake of 26 December. It happened at 14.59 GMT on 23 December north of Macquarie Island (between Australia and Antarctica), much further away from CERN.
