|
143.5 km |
Mountains |
Lourdes earthquake: The Hautacam seismic crisis. |
On November 17th, 2006, the departure city of today’s stage, Lourdes, was rattled by a ML 5.0 earthquake that was widely felt, but did not cause significant damage. The earthquake occurred about 10 km south of Lourdes (Figure 1) at a depth of 9.7 km and was felt from Bordeaux to Barcelona. It was followed by over 250 aftershocks, smaller earthquakes that highlight a source region with an area equivalent to the area of about 1250 football pitches.
The Hautacam seismic crisis explained
The earthquake occurred along a so-called ‘normal fault’, which is a fault that accommodates stretching, with an upper block that slides down from a lower block. In this case, the normal fault was dipping to the north, and shows that the Pyrenees are being stretched in a north-south direction. This seems a bit strange, since the Pyrenean mountain belt formed because of north-south shortening. However, this shortening stopped about 20 million years ago, and the mountain belt is slowly collapsing, like a pudding. This is probably the cause of the Hautacam seismic crisis.
Cause of earthquakes
Earthquakes are the results of rapid motion between large masses of rocks which is accompanied by a release of energy, some of which is radiated as seismic (elastic) waves which cause the ground to shake. A simple representation of this is shown in the video: the elastic strain stored in the spring is released suddenly when the block slips. That slip generates seismic waves that we then feel as an earthquake.
Earthquake size
The amount of shaking that is experienced at the surface depends on several factors, such as the depth at which motion occurred and the size of the ruptured area. The total energy of an earthquake can be determined from the seismic waves and is presented on a logarithmic scale. Since this represents the increase in the amplitude of the seismic waves and the energy scales as amplitude to the power 1.5, this means that a magnitude 6 earthquake has 32 times more energy than a magnitude 5.0 earthquake.
Historical seismicity in the Lourdes region - the Hautacam seismic crisis
Earthquakes are not uncommon in the Pyrenees and particularly in the Lourdes region. Historical records show an earthquake occurred close to Lourdes on June 21st 1660 which caused significant damage. The magnitude of this earthquake is estimated to be 6.1, so with more than 32 times more energy than the one in 2006. Unfortunately, we currently don’t have sufficient knowledge and tools to predict whether or when such an event is likely to occur. Let’s hope the ground shaking action is restricted to the race of today!
I am a geologist with expertise in earthquake and fault mechanics. I attempt to decipher how earthquakes start and stop by torturing rocks in the laboratory at high pressures and temperature. Occasionally, I spend time investigating exhumed faults in remote (e.g. New Zealand) and not so remote locations (e.g. U.K.). Check the Geo-TdF-team-2022.
André Niemeijer