The 2006 November, M L = 5.0 earthquake near Lourdes (France): new evidence for NS extension across the Pyrenees

SUMMARY A widely felt, M L = 5.0 earthquake occurred in the central French Pyrenees on 2006 November 17, close to the pilgrimage city of Lourdes, in a region where strong historical earthquakes produced severe damage and casualties in the 17th and 18th centuries. Seismic recordings performed by dense permanent networks and temporary stations allowed an exhaustive study of this event and its aftershock sequence, revealing a great coherency of all the parameters which characterize the rupture. More than 250 aftershock hypocentres, located in a 3‐D tomographic model, are remarkably distributed on a 10 km 2 quasi‐planar surface which extends at depth between 6 and 10 km. This surface coincides with one of the main shock nodal planes, as inferred from P ‐wave polarities and body waveform modelling. The tectonic structure responsible for the earthquake is identified as an E–W oriented normal fault, dipping 56° north, a few kilometres south of the North Pyrenean Fault, recognized as the former boundary between the Iberian and Eurasian Plates. The mechanisms of the strongest aftershocks are also clearly extensional ( Tables 1 and 2). 1 Focal solution parameters for the main shock, the seven strongest aftershocks, the 2006 December 16 and the 2007 November 15 events.Date Origin time Lat (°N) Lon (°E) Depth ML Plane 1 Plane 2 P ‐axis T ‐axis (km) St Dip Rk St Dip Rk St Pln St Pln2006 November 17 18:19 43.0282 0.0032 9.7 5.0 284 56 −84 93 34 −99 215 78 10 11 2006 November 18 20:34 43.0123 0.0027 6.9 3.3 122 30 −74 284 61 −99 172 72 20 16 2006 November 18 22:17 43.0120 −0.0002 6.6 3.0 137 29 −60 284 65 −105 166 66 25 19 2006 November 19 05:10 43.0227 −0.0063 8.0 2.8 299 40 −90 119 50 −90 29 85 209 05 2006 November 19 09:14 43.0285 −0.0035 8.9 2.5 322 64 −69 101 33 −126 268 65 37 16 2006 November 19 13:16 43.0223 −0.0095 8.4 3.3 260 65 −86 71 25 −99 178 70 347 20 2006 November 20 04:01 43.0190 −0.0082 7.7 2.9 104 70 −72 240 27 −130 41 61 180 23 2006 November 22 13:55 43.0255 −0.0057 8.7 2.8 249 55 −90 69 35 −90 159 80 339 10 2006 December 02 06:23 43.0213 −0.0005 8.0 2.9 285 28 −90 105 62 −90 15 73 195 17 2006 December 16 08:17 43.0208 −0.1100 9.7 4.0 312 58 −89 130 32 −92 225 77 41 13 2007 November 15 13:47 43.0207 −0.0022 7.8 4.1 296 30 −50 72 67 −110 310 62 177 20Notes: Projection on the lower hemisphere. The strike (St), dip and rake (Rk) angles for each event are reported (in degrees) for each plane, as well as the strike (St) and plunge (Pln) angles for the P ‐ and T ‐axes.2 Source parameters obtained from body waveform modelling, with their uncertainties, for the main shock, the three strongest aftershocks, and the December 16 event.Date‐time NM0 Plane 1 Plane 2 (Nm) Strike Dip Rake Strike Dip Rake2006 November 17–18:19 25 5.32 × 10 15 ± 5.00 × 10 13 267 ± 1 52 ± 1 −107 ± 1 114 ± 1 40 ± 1 −69 ± 1 2006 November 18–20:34 6 2.29 × 10 13 ± 5.60 × 10 12 231 ± 9 72 ± 6 −132 ± 2 122 ± 4 45 ± 3 −25 ± 7 2006 November 18–22:17 7 1.14 × 10 13 ± 1.95 × 10 12 148 ± 5 51 ± 1 −25 ± 9 254 ± 4 71 ± 6 −139 ± 3 2006 November 19–13:16 10 3.64 × 10 13 ± 3.82 × 10 12 332 ± 95 31 ± 1 −128 ± 23 195 ± 7 66 ± 2 −69 ± 13 2006 December 16–08:17 16 7.80 × 10 13 ± 3.57 × 10 12 350 ± 4 67 ± 2 −59 ± 4 112 ± 3 38 ± 2 −141 ± 6Note: N : number of stations used.Moment tensors and other source parameters (corner frequency, stress drop and source radius) are determined from body wave modelling for the strongest events of the sequence ( Table 3). For the main shock, a seismic moment of 5.32 × 10 15 N m is found, and the source size (9.4 km 2 ) is consistent with the aftershock distribution. Stress drops are about four times smaller for the aftershocks than for the main shock, except for a late event (2006 December 16) located 10 km to the west. We show that it could have been triggered by the main shock through viscoelastic stress transfer in the lower crust. Hydrological manifestations such as water level changes in hydrothermal wells are also discussed. 3 Source parameters obtained for the main events of the seismic sequence of 2006 November–December, assuming a Brune‐type source.mo. dy‐hr:mn MLM0 (Nm) Mw1Mw2fc (Hz) Δσ (10 5 Pa) S (km 2 ) D (cm)11.17–18:19 5.0 5.32 × 10 15 a 4.4 a 4.4 0.75 4.36 9.4 1.67 11.18–20:34 3.3 1.8 × 10 13 2.8 2.9 3.16 1.10 0.5 0.10 11.18–22:17 3.0 5.0 × 10 12 2.4 2.6 3.65 0.47 0.4 0.04 11.19–13:16 3.3 2.0 × 10 13 2.8 3.0 3.25 1.33 0.5 0.12 12.16–08:17 4.0 9.8 × 10 13 3.3 3.2 2.92 4.74 0.6 0.46aValue taken from body wave modelling.Notes: ML : local magnitude (OMP determination); M w1 : moment magnitude from displacement spectra; M w2 : moment magnitude from body wave modelling; f c : corner frequency; Δσ: static stress drop; S : surface of rupture ( S =π r s 2 ) and D : displacement on the fault plane.The rupture occurred on a segment of an E–W trending normal fault. It is the clearest active structure identified in the whole Pyrenean range, and might be the cause of large historical earthquakes. Although paradoxical in an active mountain belt generally depicted as enduring a compressional regime, its extensional mechanism is consistent with recent geomorphological evidences of quaternary normal faults, and with the present‐day strain and stress fields inferred from permanent GPS measurements and CMT moment tensors. This new seismic sequence provides an additional support to the rising idea of extensional stress and strain regimes across the Pyrenees.