Le fond d'une parcelle d'habitat de l'agglomération de Koenigshoffen et un fossé médiéval, Route des Romains - rocade ouest (Strasbourg, Bas-Rhin): Rapport d'opération

La fouille, au niveau du 2 route des Romains (superficie : 2520 m²), a été réalisée en 2018 en préalable à l’aménagement de la rocade ouest conduit par la CTS (Compagnie des Transports strasbourgeois).À l’époque romaine, cet espace est localisé dans la partie orientale de l’agglomération antique de Koenigshoffen, à 25 m au nord de la route des Romains qui constituait dès le 1er siècle apr. J.-C. un axe antique important de Strasbourg-Argentorate. Quelques aménagements situés à l’arrière d’au moins une parcelle d’habitation ont été fouillés, notamment une cave mise en place dans la première moitié du 2e siècle et des latrines utilisées à partir de la seconde moitié du 2e siècle. Le remplissage de ces dernières a fait l’objet de plusieurs analyses combinées, carpologiques, archéozoologiques, parasitologiques et moléculaires, nous informant sur les consommations végétales et l’état sanitaire des occupants. Les fosses romaines ont été abandonnées et comblées au même moment, au milieu du 3e siècle environ. Dans la cave ont été jetés un ensemble de sculptures funéraires (deux lions et une colonne au serpent) et des éclats d’éléments d’architecture qui signalent probablement la redécouverte, au 3e siècle, de restes de monuments de l’allée des tombeaux du 1er siècle encore en usage au début du 2e siècle. Dans le comblement de plusieurs fosses ont été retrouvés des fragments de parois de fours et des ratés de cuisson, provenant certainement de l’aire de fours de potier fouillée dans le cadre d’une autre fouille réalisée la même année, à quelques mètres au sud.La fouille a également permis d’étudier plus précisément le fossé post-antique large de 4 m et profond d’1,8 m, observé à plusieurs reprises lors d’interventions archéologiques précédentes, sur une longueur totale de 250 m. Les datations radiocarbone réalisées sur les premiers niveaux du remplissage sont concordants et indiquent que le fossé a été mis en place entre le 11e et le 12e siècle. Il est resté ouvert un moment, peut-être un siècle ou deux avant d’être obturé. Cette limite perdure encore au 19e siècle sous la forme du chemin Halbenhoffenweg

Dynamics of microseismicity and its relationship with the active structures in the western Corinth Rift (Greece)

International audienceWe analyse the complete earthquake archive of the western Corinth Rift using both cross-correlations between pairs of event waveforms and accurate differential traveltimes observed at common stations, in order to identify small-scale fault structures at depth. The waveform database was generated by the dense Corinth Rift Laboratory network and includes about 205 000 events between 2000 and 2015. Half of them are accurately relocated using double-difference techniques. The novelty of this relocated catalogue is the integration of the recent westernmost earthquakes due to the extension of the network in 2010 to the western extremity of the Corinth Rift and the consideration of the whole database over more than 15 yr. The total relocated seismicity exhibits well-defined clusters at the root of the main normal faults mainly between 5 and 10 km depth in the middle of the gulf and illuminates thin active structure planes dipping north about 20° under the northern coast. Some seismicity is observed in the footwall of the main active faults, along the West and East Helike faults. We also built a multiplet database based on waveform similarity taking into account cross-correlation coefficients weighted by signal-to-noise ratios. Short-term multiplets are concentrated in the middle of the gulf along the Kamarai fault system, in a 1–2 km thick layer at 6–8 km depth, interpreted as a highly fractured geological layer. They are often associated to slow seismic migration velocities occurring in this zone during strong swarm episodes and are thus likely to be triggered by pore pressure variations. On the other hand, most long-term and regular multiplets are located deeper (7–10 km), under the northern coast, within a layer less than 0.3 km thick. They occur at the border of nearly planar structures with low seismicity rate, which we identify as fault planes, and they may be explained by aseismic slip on the fault surface around them. This supports the existence of an immature structure growing downdip towards the north at the base of the active geological layer, which possibly connects to the ductile middle crust around 15 km depth, as suggested by the occurrence of deeper events in the continuity of the 1995–fault plane. The different migration velocities (from 0.05 km d−1 to several km d−1) highlighted during the western 2014–swarms indicate that both pore pressure and creep diffusion are operating in the fault zone. The fast migrations observed in the Psathopyrgos fault zone, where a slow slip event was detected by dilatometers in 2002, compare with that for creeping faults. To the west, from spatial distribution of events, we show that the Rion–Patras fault connecting the western extremity of the Corinth Rift fault system to the Patras Rift, is dipping around 60° north–west with a rake angle of −115°. Finally, we identified two new areas within the central active zone which may correspond to large scale, locked asperities on active fault surfaces, similar in size to the main asperity broken during the 1995, MW 6.3, Aigion earthquake

Reassessment of the rifting process in the Western Corinth Rift from relocated seismicity

International audienceThe seismic activity in the western part of the Corinth Rift (Greece) over the period 2000– 2007, monitored by a dense network of three-component stations, is analysed in terms of multiplets and high precision relocation using double difference techniques. This detailed analysis provides new insights into the geometry of faults at depth, the nature and the structure of the active zone at 6–8 km depth previously interpreted as a possible detachment, and more generally into the rifting process. The seismicity exhibits a complex structure, strongly varying along the rift axis. The detailed picture of the seismic zone below the rift indicates that its shallower part (at depths of 6–8 km) is 1–1.5 km thick with a complex microstructure, and that its deeper part (at depths of 9–12 km) gently dipping to the north (10–20°) is 0.1–0.3 km thick with a microstructure consistent with the general slope of the structure. Although the nature of this seismic zone remains an open question, the presence of seismicity beneath the main active area, the strong variability of the structure along the rift over short distances and the complex microstructure of the shallower part revealed by the multiplet analysis are arguments against the hypothesis of a mature detachment under the rift: this active zone more likely represents a layer of diffuse deformation. The geometry of the mapped active faults is not well defined at depth, as no seismicity is observed between 0 and 4 km, except for the Aigion Fault rooting in the seismic layer at 6 km depth with a dip of 60°. A distinct cloud of seismicity may be associated with the antithetic Kalithea Fault, on which the 1909 Fokis earthquake (M s = 6.3) may have occurred. The link between the 1995 rupture (M s = 6.2) and the faults known at the surface has been better constrained, as the relocated seismicity favours a rupture on an offshore, blind fault dipping at 30° , rather than on the deeper part of the East Helike Fault. Consequently, the 1995 event is expected to have decreased the Coulomb stress on the East Helike Fault. To explain these seismic observations along with the geodetic observations, a new mechanical model for the rifting process in this region is proposed, involving non-elastic, mostly aseismic uniform NS opening below the rift axis, coupled with the downward and northward growth of a yet immature detachment: the reported GPS rates would mainly result from this deep, silent source, and the seismicity would reveal the detachment position, not yet connected to the ductile lower crust. In such a model, the strong fluctuations of microseismicity would result from small strain instabilities, undetected by continuous GPS and possibly related to pore pressure transients