Long-term monitoring of a large deep-seated landslide (La Clapiere,South-East French Alps): initial study

The large-scale deformation of high mountain slopes finds its origin in many phenomena (inherent parameters, external stresses) with very different time constants (instantaneous to geological scale). Gravitational effect, tectonic forces and water infiltration are generally the principal causes of slope instability. However, it can be very difficult to distinguish which cause is dominant and which are their respective effects. To gain a better understanding of the complex processes taking place during the evolution of an unstable slope and separate the causes responsible of the landslide dynamic, an observational study based on geodetic, meteorological, seismological and electrical data has been performed on the La Clapière rockslide (Southern French Alps). This deep-seated landslide (DSL) is known for many years as one of the largest and fastest rock slide in Europe (60 million m³ of highly weathered metamorphic material, moving at 1 to 3 m year^?1). The set-up of the “Observatoire Multidisciplinaire des Instabilités de Versants” (OMIV, http://omiv.osug.fr) in 2011 has allowed the production and availability of an important and original data set over several years of accurate monitoring. Thus, for the first time, the long-term study of geodetic data permitted us to highlight acceleration phases in the general movement of the landslide that affect its dynamic. These modifications are associated with variations of the velocity by a factor 3 to 6. The characterization of the origin of these variations was possible due to the comparison with meteorological, electrical and seismological data. Based on these various signals, we were able to establish correlations and contributions of meteorological water infiltration in the dynamic evolution of the La Clapière slope. We determine several response times to the meteorological stress for seismic endogenous events (mainly rockfalls), the resistivity of the ground (quasi-instantaneous) and the kinematics of the slope (from 2 weeks to 2.5 months). Moreover, our results strongly suggest the existence of rainfall threshold of 3.5?±?1 mm day^?1 from which the number of seismic endogenous events is highly increased.     

The use of low frequencies in a full‐waveform inversion and impedance inversion land seismic case study

Velocity model building and impedance inversion generally suffer from a lack of intermediate wavenumber content in seismic data. Intermediate wavenumbers may be retrieved directly from seismic data sets if enough low frequencies are recorded. Over the past years, improvements in acquisition have allowed us to obtain seismic data with a broader frequency spectrum. To illustrate the benefits of broadband acquisition, notably the recording of low frequencies, we discuss the inversion of land seismic data acquired in Inner Mongolia, China. This data set contains frequencies from 1.5–80 Hz. We show that the velocity estimate based on an acoustic full‐waveform inversion approach is superior to one obtained from reflection traveltime inversion because after full‐waveform inversion the background velocity conforms to geology. We also illustrate the added value of low frequencies in an impedance estimate.     

Geochemical evidence for a large methane release during the last deglaciation from Marmara Sea sediments

Recent methane inventories have revealed the potential impact of gas hydrates on the global carbon cycle, and hence in climate change (Milkov, 2004). However, only a few studies have traced methane release in the geologic record. Here, we show geochemical evidence for a large scale methane release at mid-latitudes during the last deglaciation. The Sea of Marmara, an enclosed sea between the Mediterranean and Black Seas, is located in a tectonically active basin with gas hydrate expulsion and the formation of shallow gas hydrates. Since depths in the basin are shallower than 1100 m, future global temperatures are expected to have a great influence in destabilizing methane clathrates. Among the suite of biomarkers, we have focused on diplopterol and diploptene profiles in core MD012430, retrieved from the central basin in the Marmara Sea. Our results indicate that during the last 15,000 years, hopanoids showed important concentration variations with a pronounced peak during the deglaciation.The lack of a relationship between diplopterol/diploptene and phytoplanktonic biomarker concentrations, as well as a depleted isotopic composition, have linked the hopanoid maxima to methanotrophic activity, suggesting that an intense methane release occurred at the onset of deglaciation in the Marmara Sea. The vulnerability of the hydrate stability zone to changes in temperature and pressure under this range of shallow water depths, as well as the relative timing of the hopanoid maxima and sea surface temperature rise, points to thermal destabilization of hydrates as a trigger for methane release in the water column.