Considering large earthquake clustering in seismic risk analysis

Earthquakes cluster in space and time resulting in nonlinear damage effects. We compute earthquake interactions using the Coulomb stress transfer theory and dynamic vulnerability from the concept of ductility capacity reduction. We combine both processes in the generic multi-risk framework where risk scenarios are simulated using a variant of the Markov chain Monte Carlo method. We apply the proposed approach to the thrust fault system of northern Italy, considering earthquakes with characteristic magnitudes in the range ~[6, 6.5], different levels of tectonic loading \(\dot{\tau }\) = {10⁻⁴, 10⁻³, 10⁻²} bar/year and a generic stock of fictitious low-rise buildings with different ductility capacities μ _Δ = {2, 4, 6}. We describe the process’ stochasticity by non-stationary Poisson earthquake probabilities and by binomial damage state probabilities. We find that earthquake clustering yields a tail fattening of the seismic risk curve, the effect of which is amplified by damage-dependent fragility due to clustering. The impact of clustering alone is in average more important than dynamic vulnerability, the spatial extent of the former phenomenon being greater than of the latter one.

     

A water retention model accounting for void ratio changes in double porosity clays

This paper presents a constitutive model that predicts the water retention behaviour of compacted clays with evolving bimodal pore size distributions. In line with previous research, the model differentiates between the water present inside the saturated pores of the clay aggregates (the microstructure) and the water present inside the pores between clay aggregates (the macrostructure). A new formulation is then introduced to account for the effect of the macrostructural porosity changes on the retention behaviour of the soil, which results in a consistent evolution of the air-entry value of suction with volumetric deformations. Data from wetting tests on three different active clays (i.e. MX-80 bentonite, FEBEX bentonite, and Boom clay), subjected to distinct mechanical restraints, were used to formulate, calibrate, and validate the proposed model. Results from free swelling tests were also modelled by using both the proposed double porosity model and a published single porosity model, which confirmed the improvement in the predictions of degree of saturation by the present approach. The proposed retention model might be applied, for example, to the simulation of the hydromechanical behaviour of engineered bentonite barriers in underground nuclear waste repositories, where compacted active clays are subjected to changes of both suction and porosity structure under restrained volume conditions.

     

Magma chambers and localization of deformation during thrusting

Terra Nova, 22, 390–395, 2010 Abstract We present the results of coupled analogue and numerical models that provide new insights into the relationships between volcanoes and thrusts. The effects of both upper‐crustal magma chambers and the load of volcanoes on the geometry of thrust systems were investigated. Analogue modelling points to a strong influence exerted by a magma chamber on thrust geometry, which, as suggested by the numerical models used to rationalize these results, is related to the stress redistribution around the weak heterogeneity. The low‐viscosity body below a volcanic edifice localizes compressional deformation and causes a curvature of the thrusts towards the magma chamber, opposite to the direction of tectonic transport. In these conditions, the volcanic load has a negligible effect on the structural geometry. These results are in contrast with those of previous studies, where intrusions or the load of major volcanoes generated a curvature of the thrusts away from volcanic edifices in the direction of tectonic transport.     

A dispersion scale effect in model calibrations and field tracer experiments

Field tracer experiments and model calibrations indicate that the magnitude of dispersivity increases as a function of the scale at which observations are made. Calculations presented in this study suggest that some part of this scaling may be explained as an artifact of the models used. Specifically, a scaling-up of dispersivity will occur whenever an (n − 1)-dimensional model is calibrated or otherwise employed to describe an n-dimensional system. The calibrated coefficients for such models will depend not only on size of the contaminant plume or tracer experiment at the time of calibration, but will exhibit a size dependency beyond the calibration period. The magnitude of scaling appears to be sufficient to encompass the range of differences between laboratory measurements of dispersivity and model calibrations.     

Rate of soil erosion processes on vineyards in central Piedmont (NW Italy)

The ‘Tertiary Basin of Piedmont’ is a hilly region, mainly composed of marine sediments, such as marls, silts and sands. The slopes, largely devoted to grape production, are usually kept bare of vegetation and are thus prone to soil erosion processes. For two years we have measured soil loss in relation to rainfall on experimental plots located in vineyards. In all the plots considered erosion started with low rainfall intensities (0·07 mm/min), and above 0·4 mm/min the amount of soil loss dramatically increased (to over 1800 g/m in one event). Most of the erosion occurs during summer rainstorms, but the behaviour of the soil under erosive rainfalls is very different from one site to another, depending only in part upon the various rainfall rates, soil and geometrical characteristics of the plots. It can be seen that soil loss varies from nearly negligible values (20 g/m/yr) to unacceptably high levels (to over 4·7 kg/m/yr), according to the different types of land cultivation. Deep ploughing and heavy herbicide treatment give rise to accelerated erosion processes leading to soil losses much higher than in other vineyard plots in Europe.     

Earthquakes in Switzerland and surrounding regions during 2006

This report of the Swiss Seismological Service summarizes the seismic activity in Switzerland and surrounding regions during 2006. During this period, 572 earthquakes and 91 quarry blasts were detected and located in the region under consideration. Of these earthquakes, two occurred in conjunction with the construction of the new Gotthard railway tunnel and 165 were induced artificially by the stimulation of a proposed geothermal reservoir beneath the city of Basel. With 20 events with M _L ≥ 2.5, five of which were artificially induced, the seismic activity in the year 2006 was far below the average over the previous 31 years. Nevertheless, six events were felt by the public, most prominently the strongest of the induced Basel events (M _L 3.4), which caused some non-structural building damage. Noteworthy are also the two earthquakes near Cortaillod (M _L 3.2), on the shore of Lake Neuchatel, and in Val Mora (M _L 3.5), between the Engadin and Val Müstair, as well as the 42 aftershocks of the M _L 4.9 Vallorcine earthquake, between Martigny and Chamonix, of September 2005. Editorial handling: Stefan Bucher     

Analysis of sustained long-period activity at Etna Volcano, Italy

Following the installation of a broadband network on Mt. Etna, sustained Long-Period (LP) activity was recorded accompanying a period of total quiescence and the subsequent onset of the 2004–2005 effusive episode. From about 56000 events detected by an automatic classification procedure, we analyse a subset of about 3000 signals spanning the December 17th, 2003–September 25th, 2004, time interval. LP spectra are characterised by several, unevenly-spaced narrow peaks spanning the 0.5–10 Hz frequency band. These peaks are common to all the recording sites of the network, and different from those associated with tremor signals. Throughout the analysed time interval, LP spectra and waveforms maintain significant similarity, thus indicating the involvement of a non-destructive source process that we interpret in terms of the resonance of a fluid-filled buried cavity. Polarisation analysis indicates radiation from a non-isotropic source involving large amounts of shear. Concurrently with LP signals, recordings from the summit station also depict Very-Long-Period (VLP) pulses whose rectilinear motion points to a region located beneath the summit craters at depths ranging between 800 and 1100 m beneath the surface. Based on a refined repicking of similar waveforms, we obtain robust locations for a selected subset of the most energetic LP events from probabilistic inversion of travel-times calculated for a 3D heterogenous structure. LP sources cluster in a narrow volume located beneath the summit craters, and extending to a maximum depth of ≈ 800 m beneath the surface. No causal relationships are observed between LP, VLP and tremor activities and the onset of the 2004–2005 lava effusions, thus indicating that magmatic overpressure played a limited role in triggering this eruption. These data represent the very first observation of LP and VLP activity at Etna during non-eruptive periods, and open the way to the quantitative modelling of the geometry and dynamics of the shallow plumbing system.