Abstract. HIRESSS (HIgh REsolution Slope StabilitySimulator) is a physically based distributed slope stability simulator for analyzing shallow landslide triggering conditions in real time and on large areas using parallel computational techniques. The physical model proposed is composed of two parts: hydrological and geotechnical. The hydrological model receives the rainfall data as dynamical input and provides the pressure head as perturbation to the geotechnical stability model that computes the factor of safety (FS) in probabilistic terms. The hydrological model is based on an analytical solution of an approximated form of the Richards equation under the wet condition hypothesis and it is introduced as a modeled form of hydraulic diffusivity to improve the hydrological response. The geotechnical stability model is based on an infinite slope model that takes into account the unsaturated soil condition. During the slope stability analysis the proposed model takes into account the increase in strength and cohesion due to matric suction in unsaturated soil, where the pressure head is negative. Moreover, the soil mass variation on partially saturated soil caused by water infiltration is modeled.The model is then inserted into a Monte Carlo simulation, to manage the typical uncertainty in the values of the input geotechnical and hydrological parameters, which is a common weak point of deterministic models. The Monte Carlo simulation manages a probability distribution of input parameters providing results in terms of slope failure probability. The developed software uses the computational power offered by multicore and multiprocessor hardware, from modern workstations to supercomputing facilities (HPC), to achieve the simulation in reasonable runtimes, compatible with civil protection real time monitoring.A first test of HIRESSS in three different areas is presented to evaluate the reliability of the results and the runtime performance on large areas.
Abstract. PREVIEW is an European Commission FP6 Integrated Project with the aim of developing, at an European level, innovative geo-information services for atmospheric, geophysical and man-made risks. Within this framework, the Landslides Platform Service 2 (forecasting of shallow rapid slope movements) has developed an integrated procedure for the forecasting and warning of distributed shallow landsliding to be used for civil protection purposes.The Service consists of an automated end-to-end forecasting chain which uses data from a probabilistic downscaled short-term rainfall forecast, soil saturation estimates and meteorological radar outputs. The above data are entered into a hydro-geological model that makes use of an infinite slope approach to calculate the distributed Factor of Safety over the entire basin. All outputs, and much of the input data, are shown on a WebGIS system so that end-users can interactively access and download data. A distinctive feature of the service is the use of an innovative soil depth model for predicting the distributed thickness of the regolith cover within the basin, which is one of the most important parameters controlling shallow landslide triggering.The service was developed in a pilot test site in NE Italy, the Armea basin. Validation makes use of two rainfall events: one that occurred in 2000 and a smaller, more recent event (2006) that caused fewer landslides. Rainfall data have been used to compute a distributed factor-of-safety map that has been overlaid onto the landslide inventory. Instead of a traditional validation approach based on the number count of correctly identified landslides, we carried out an alternative procedure based on the landslides area that gave outcomes which, for this preliminary stage of the research, can be considered promising.
Ocean Drilling Program Legs 170 and 205 offshore Costa Rica provide structural observations which support a new model for the geometry and deformation response to the seismic cycle of the frontal sedimentary prism and décollement. The model is based on drillcore, thin section, and electron microscope observations. The décollement damage zone is a few tens of meters in width, it develops mainly within the frontal prism. A clear cm-thick fault core is observed 1.6 km from the trench. The lower boundary of the fault core is coincident with the lithological boundary between the frontal prism and the hemipelagic and pelagic sediment of the Cocos plate. Breccia clast distributions in the upper portion of the décollement damage zone were studied through fractal analysis. This analysis shows that the fractal dimension changes with brecciated fragment size, implying that deformation was not accommodated by self-similar fracturing. A higher fractal dimensionality correlates with smaller particle size, which indicates that different or additional grain-size reduction processes operated during shearing. The co-existence of two distinct fracturing processes is also confirmed by microscopic analysis in which extension fracturing in the upper part of the damage zone farthest from the fault core is frequent, while both extension and shear fracturing occur approaching the fault core.The coexistence of extensional and shear fracturing seems to be best explained by fluid pressure variations in response to variations of the compressional regime during the seismic cycle. During the co-seismic event, sub-horizontal compression and fluid pressure increase, triggering shear fracturing and fluid expulsion. Fractures migrate upward with fluids, contributing to the asymmetric shape of the décollement, while slip propagates. In the inter-seismic interval the frontal prism relaxes and fluid pressure drops. The frontal prism goes into diffuse extension during the interval when plate convergence is accommodated by creep along the ductile fault core. The fault core is typically a barrier to deformation, which is explained by its weak, but impermeable, nature. The localized development of a damage zone beneath the fault core is characterized by shear fracturing that appears as the result of local strengthening of the detachment.
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