Marc Cushing

Local erosion rates versus active tectonics: cosmic ray exposure modelling in Provence (south-east France)

Over the past decade, in situ-produced cosmogenic nuclides have revolutionised the study of landscape evolution. In particular, numerous studies have demonstrated that, in active tectonic settings, cosmic ray exposure dating of deformed or displaced geomorphic features makes it possible to quantify long-term deformation rates. In western European countries, erosion due to climatically driven processes and human activities is probably the factor that most limits the accuracy of exposure ages and landscape modification rates. In this study, we present the results of a depth-profiling technique applied to alluvial terraces located along the Rhone and the Moyenne Durance rivers. The expected decrease with depth of the measured 10Be concentrations has been modelled using a χ2 inversion method in order to constrain the exposure history of the alluvial sediments. The results suggest that: (1) over the Quaternary, the local surface erosion rates including both regional uplift and climatically driven processes acting on landforms are on the order of 30 m/Myr in southeastern France, and (2) providing a fairly good bracketing of the exposure age, the modelled abandonment age of alluvial terraces affected by the Moyenne Durance Fault allows estimating incision rates, comparing the alluvial terrace elevations with topographic river profiles, and a minimum vertical slip rate value of roughly 0.02 mm/yr for the southern segment of the Moyenne Durance Fault.

Co-seismic ruptures and deformations recorded by speleothems in the epicentral zone of the Basel earthquake

The study of growth anomalies of speleothems in a karstic environment can provide potential evidence for palaeoearthquakes. These data are used to study the recurrence times of major earthquakes in areas where evidence for historic seismicity is lacking. A study has been carried out in the epicentral area of the 1356 Basel earthquake (epicentral intensity = VII–VIII, macroseismic magnitude = 6.2). The Battlerloch and Dieboldslochli caves, situated in the area of greatest damage, show growth anomalies of speleothems possibly related to a seismic event (several breaks of speleothems and offsets of the axis of the regrowths). The first U/Th disequilibrum measurements by alpha spectrometry show recent ages (less than several tens of thousands of years and probably historic). 14C dating by AMS of carbonate laminations taken on both sides of the anomalies confirm the evidence of a seismic event around 1300 AD. More accurate darings by U/Th TIMS are carried out in order to compare the information provided by the two different dating methods.

Determination of geomechanical site effects in France from macroseismic intensities and reliability of macroseismic magnitude of historical events

Results from a detailed analysis of the French macroseismic database SIRENE96 show that geomechanical site effects (i.e. site effects due to superficial geology) can be statistically detected and quantified from macroseismic data. Our results are used to derive a predictive map of potential site effects in France and to check if site effects induce a significant overestimation of macroseismic magnitude of historical events. The key hypothesis of our work is that geological formations of similar age produce similar site effects because one expects the younger sediments to be less compacted, and therefore more prone to cause site effects. A geological formation is associated with each intensity using a Geographic Information System (GIS) and the digital geological map of France. Two approaches are developed in order to characterise the intensity increase associated with geological formations having the same age. The first approach is based on the intensities that differ significantly from the average pattern, intensities which are labelled as anomalies. In the second approach, a theoretical intensity attenuation law is computed for each selected event only from observations located on formations older than 2 My. According to the results of the first approach, these formations are unlikely to cause site effects, and for the sake of simplicity they are labelled as rocks (younger formations are defined as sediments). The derived attenuation laws are then used to compute the residuals for each observation. Residuals are classified as a function of the age of the formations on which they are located. The results of the two approaches are in good agreement. No intensity amplification is detected for formations older than 1 My. For younger formations, the younger the formations, the larger the average intensity increase. The average intensity increase reaches 0.6±0.2 degree on the MSK scale for 0 My sediments. The results are better expressed in terms of probability: for 0 My sediments, there is an estimated 83% probability of observing an intensity increase; for 0.01 My and 0.75 My sediments, this probability is about 52%. The magnitude of the intensity increase is variable, but it is generally small: it has a 26% probability to reach 1 degree or above on the MSK scale for 0 My sediments and this probability falls to about 5% for 0.01 and 0.75 My formations. A predictive map of geomechanical site effects is produced by extrapolating the characteristics of intensity increases to all geologic formations of similar age in France. Glacial and fluvial valleys, already known as being subject to site effects, clearly appear on this map, as well as less expected areas such as the South Atlantic coast. Lastly, new estimations of macroseismic magnitude that exclude intensities potentially affected by geomechanical site effects are realised. A magnitude overestimation due to site effects can possibly exist, but it remains limited (<0.2). This result tends to demonstrate that geomechanical site effects in France do not significantly distort our knowledge of historical seismicity.

Traces d'activité pléistocène de failles dans le Nord du fossé du Rhin supérieur (plaine d'Alsace, France)

Abstract In the Upper Rhine Graben, trenches excavated across a 100-km-long geomorphic lineament show that normal faults vertically offset Late Cromerian formations (400–450 ka) by 16 m and Saalian formations (150–300 ka) by 6 m. The re-interpretation of seismic profiles corroborates the existence of normal faults cutting through the sedimentary cover. However, the connection between superficial observations and deep structures remains unclear. This analysis suggests that displacements were coseismic. An average recurrence interval of 25 000 years is obtained for a M =6.0–6.5 magnitude earthquake (single metric displacement) with an inferred vertical deformation rate of 0.04 mm-yr −1 .