The Lambesc Individual Seismogenic Source is responsible of the 11 June 1909 Lambesc earthquake, the largest event of the twentieth century in France. The source is included in the Database as a 10 km-long, N290Â°-striking, 60Â°N north-dipping fault. The Lambesc Source is located beneath the Trevaresse anticline (Baroux, et al., 2002, 2003; Chardon and Bellier, 2003; Lacassin et al. 2001). Based on its proposed fault length (10 km), the Lambesc Source could correspond to the western half of the Trevaresse anticline (Baroux, et al., 2003).
SURFACE EXPRESSION AND GEOMETRY:
The crustal profile proposed by Champion et al. (2000) and adapted by Chardon and Bellier (2003) (see pictures) and the orientation of the western part of the Trevaresse ridge are in agreement with the fault geometry proposed by Baroux et al. (2003). According to the waveform analyses performed by the latter investigators, who proposed an average Mw 5.8- 6.0, the fault area is about 60 km2.
The focal mechanism test proposed by Baroux et al. (2003) using the Goettingen and De Bilt seismograms indicates that the fault plane associated with the Lambesc Source has a steep dip (about 60Â°) and an orientation of about N110Â°.
The Lambesc Source is thus probably a steeply dipping fault (60Â°), about 10 km long and 6 km wide, probably rooted into the Triassic decollement level that lies at about 6 km depth beneath the Trevaresse ridge. According to Lacassin et al.
(2001), scarplet possibly due to the Lambesc earthquakecan be observed at the front of the Trevaresse anticline (see pictures).
The source kinematics is based on the best fitting focal mechanism proposed by Baroux et al. (2003). This mechanism envisioned a reverse fault with a dextral component estimated at about 45 degrees. We remark that Chardon and Bellier (2003) and Champion et al. (2000) propose for the Lambesc Source a small sinistral component (direction of compression of about N005Â°-N010Â°), on the basis of the inversion of microtectonic data in Miocene sediments on the Trevaresse fold. According to magnitude estimations of about M=6.0 and a rupture area of 60 km2, the average displacement must be in the order of 0.5 m (see pictures). No event prior to that of 1909 is known in relation with the Lambesc Source.
Lacassin et al. (2001) hypothesized that the scarp located at the base of the southern slope of the Trevaresse anticline could be in part due to the 1909 Lambesc event. Nonetheless, all paleoseismological trenches dug along this scarp did not emphasize any deformation in the recent sediments.
LATEST EARTHQUAKE MAGNITUDE:
The most recent earthquake caused by the Lambesc Source occurred on 11 June 1909. The recent reappraisal by Baroux et al. (2003) yielded Mw 5.7-6.1 (6.0 preferred) and Ms 6.0, consistent with the magnitude from intensity data (5.8) and with constraints derived from modelling of coseismic elevation changes. Borax et al. (2003) based their studies on 30 seismograms collected from 14 European observatories.
Elevation changes were measured just after the event by Lallemand (1911) and reassessed by Romieu et al. (1998). The source kinematics seems to be in good agreement with the trend of these leveling data.
1) Surface expression of the fault: does the Lambesc Source create a fault scarp?
2) Levelling data show vertical motions that can not be explained only by the 1909 Lambesc earthquake. Are they processing artefacts, or are they due to creeping, or do they represent some unknown tectonic signal?
3) Is the lateral component of motion on the Lambesc fault dextral or a sinistral?
Angot (1909a, 1909b, 1909c and 1910)
Theses articles correspond to the reports made for the French Academy of Sciences after the 11 June 1909 Lambesc earthquake. They show the queries that made possible the the analyses of felt reports and describe the event recordings principally at the Paris - Saint Maur Observatory.
This article completes the series of articles from Angot (1909a, 1909b, 1909c and 1910). It corresponds to report of the macroseismic survey made just after the 1909 earthquake for the French Academy of Sciences.
This investigator discussed a levelling survey made just after the 1909 earthquake around the epicentral area. The author shows a maximum uplift and subsidence of about 3 cm. Significant vertical motions occurred also outside the are of largest damage.
This is an unpublished master thesis of the French National School of Geographical Sciences (Ecole Nationale des Sciences Geographiques, Saint-Mande, France). The author reconsiders each individual line of the levelling survey made by Lallemand (1911) after the 1909 earthquake. The amplitude of vertical movement he proposes is significantly higher than previous estimations, in the order of +/- 6 cm, but the elevation changes do not taper to zero away from the epicentral area but remain quite significant. No significant vertical motions were observed around the Lambesc Source during the period 1910 to 1969, when the most recent levelling survey was performed.
Champion et al. (2000)
The investigators propose a geological cross-section running N-S from the Panneau Provencal and across the Trevaresse anticline, the southernmost structure that has been positively active after Miocene. Active compression along the N016Â°direction was measured in Miocene sediments along the Trevaresse anticline .
Lacassin et al. (2001)
These investigators propose the Trevaresse as responsible of the 1909 Lambesc earthquake, based on the VIII and VII isoseismal contours. They identified a cumulative scarp based on observations of the south flank of the fold, with a south- facing free-face scarplet that may mark the surface expression of successive surface ruptures. They propose a location of the Lambesc Source from empirical-statistical relationships according the different estimates of the magnitude: Me 5.5 from intensity data (Levret et al., 1996) and Ms 6.3 from an individual seismogram (Cara, 1987).
Baroux et al. (2003)
These investigators present a reappraisal of the Lambesc Source based on seismological, macroseismic and geodetic observations of the 1909 Lambesc earthquake. The reappraisal of the seismological data yield Mw 5.7-6.1 (6.0 preferred) and Ms 6.0, consistent with the magnitude from intensity data (5.8) and with constraints derived from modelling of coseismic elevation changes. The data they collected allowed them to obtain the focal mechanism of the 1909 Lambesc earthquake thanks to 2 and 3 component signals. As a consequence they propose the geometry and kinematics of the Lambesc Source, suggesting that the earthquake was generated by reverse-right lateral slip on a WNW-striking, steeply north-dipping fault lying beneath the western part of the Trevaresse anticline. The base of the fault plane could coincide with a Triassic decrement level that is known to exist throughout the study area. According to the estimated fault rupture area and seismic moment, the average slip on the fault would be about 0.5 m, in agreement with the order of magnitude of vertical motions detected by levelling data. Moreover, the sense of slip they suggest agrees with the regional stress field obtained from earthquake focal mechanisms and recent GPS data. Finally, the analysis of source directivity seems to suggest west-southwestward rupture propagation.
Chardon and Bellier (2003)
These investigators present the geological boundary conditions of the 1909 Lambesc earthquake through a reappraisal of the structure and evolution of the Trevaresse ridge Anticline. For the Trevaresse fault they calculate an average rate of reverse slip of 0.03 +/- 0.02 mm/y over the past 11 Ma, consistent with an homogeneous N005Â°- N010Â° shortening direction. The Trevaresse ridge anticline is composed by two major segments separated by a relay fault zone associated with an en-echelon fault pattern on the easternmost segment.