Holocene debris flows recognized in a lacustrine sedimentary succession: sedimentology, chronostratigraphy and cause of triggering

This study focuses on the sedimentary characteristics and the chronostratigraphy of Holocene massflow deposits recognized in a lake-fill sedimentary succession. These deposits in lake Ulvadalsvatnet, western Norway, are discrete, sharp-bounded units of sand-sized sediment, running from gravelly and graded to silt-rich, and characterized by low total carbon and water contents. They are rich in terrestrial macroflora detritus, dark brown in colour, and interpreted as high-density turbidity current deposits attributed to subaerial debris flows that plunged into the lake. Thirty-three C-14 AMS dates were derived from three cores, and though the ages are somewhat inconsistent (macroflora invariably younger than bulk sediment samples), they indicate a marked increase in debris-flow processes after c. 2200 cal. yr BP, considered to reflect increased occurrence of heavy rainstorms.</p

Holocene debris flows recognized in a lacustrine sedimentary succession: sedimentology, chronostratigraphy and cause of triggering

This study focuses on the sedimentary characteristics and the chronostratigraphy of Holocene massflow deposits recognized in a lake-fill sedimentary succession. These deposits in lake Ulvådalsvatnet, western Norway, are discrete, sharp-bounded units of sand-sized sediment, running from gravelly and graded to silt-rich, and characterized by low total carbon and water contents. They are rich in terrestrial macroflora detritus, dark brown in colour, and interpreted as high-density turbidity current deposits attributed to subaerial debris flows that plunged into the lake. Thirty-three 14C AMS dates were derived from three cores, and though the ages are somewhat inconsistent (macroflora invariably younger than bulk sediment samples), they indicate a marked increase in debris-flow processes after c. 2200 cal. yr BP, considered to reflect increased occurrence of heavy rainstorms

Analysis of Microseismic Activity Within Unstable Rock Slopes

This chapter illustrates the concept of passive seismics as a method for monitoring the propagation of cracks within a rock mass as a result of load stress or water freezing in view of the use of this technique for rockfall early warning. The methodology is still far from being a standard and consolidated technique. The research is making progress, but just a few real case studies are documented. They are shortly overviewed in the introduction. Then, an interesting field test where crack propagation was artificially triggered up to full rock detachment, while a small sensor network was active, is discussed to show the existence and the characteristics of precursory signals. It follows the illustration of the microseismic monitoring methodology through the description of the Mt. San Martino (Lecco, Italy) sensor network and the discussion of the preliminary results obtained during the initial months of activity. Apparently, the preliminary results show some correlation with rainfalls, but not with temperature. Microseismic spectra are mainly concentrated in the first 100 Hz. This probably means that the hypocentre distances from the sensors are quite longer than 10 m. Electromagnetic interferences are also observed as mentioned by other authors who have analyzed data sets from other microseismic networks installed in mountain regions. They are automatically discriminated from significant signals by a classification software which works on the time/ frequency properties of these events. Hypocenter localization and clustering analysis of the significant events are the planned near- future activities

Analysis of microseismic activity within unstable rock slopes

This chapter illustrates the concept of passive seismics as a method for monitoring the propagation of cracks within a rock mass as a result of load stress or water freezing in view of the use of this technique for rockfall early warning. The methodology is still far from being a standard and consolidated technique. The research is making progress, but just a few real case studies are documented. They are shortly overviewed in the introduction. Then, an interesting field test where crack propagation was artificially triggered up to full rock detachment, while a small sensor network was active, is discussed to show the existence and the characteristics of precursory signals. It follows the illustration of the microseismic monitoring methodology through the description of the Mt. San Martino (Lecco, Italy) sensor network and the discussion of the preliminary results obtained during the initial months of activity. Apparently, the preliminary results show some correlation with rainfalls, but not with temperature. Microseismic spectra are mainly concentrated in the first 100 Hz. This probably means that the hypocentre distances from the sensors are quite longer than 10 m. Electromagnetic interferences are also observed as mentioned by other authors who have analyzed data sets from other microseismic networks installed in mountain regions. They are automatically discriminated from significant signals by a classification software which works on the time/ frequency properties of these events. Hypocenter localization and clustering analysis of the significant events are the planned near- future activities