Luca Lenti

Strong Ground Motion in the 2011 Tohoku Earthquake: A One‐Directional Three‐Component Modeling

Local wave amplification due to strong seismic motions in surficial multilayered soil is influenced by several parameters such as the wavefield polarization and the dynamic properties and impedance contrast between soil layers. The present research aims at investigating seismic motion amplification in the 2011 Tohoku earthquake through a one-directional three-component (1D-3C) wave propagation model. A 3D nonlinear constitutive relation for dry soils under cyclic loading is implemented in a quadratic line finite element model. The soil rheology is modeled by mean of a multi-surface cyclic plasticity model of the Masing-Prandtl-Ishlinskii-Iwan (MPII) type. Its major advantage is that the rheology is characterized by few commonly measured parameters. Ground motions are computed at the surface of soil profiles in the Tohoku area (Japan) by propagating 3C signals recorded at rock outcrops, during the 2011 Tohoku earthquake. Computed surface ground motions are compared to the Tohoku earthquake records at alluvial sites and the reliability of the 1D-3C model is corroborated. The 1D-3C approach is compared with the combination of three separate one-directional analyses of one motion component propagated independently (1D-1C approach). The 3D loading path due to the 3C-polarization leads to multiaxial stress interaction that reduces soil strength and increases nonlinear effects. Time histories and spectral amplitudes, for the Tohoku earthquake, are numerically reproduced. The 1D-3C approach allows the evaluation of various parameters of the 3C motion and 3D stress and strain evolution all over the soil profile.

Soil Liquefaction During the Emilia, 2012 Seismic Sequence: Investigation and Analysis

In the framework of a Project issued by the Italian National Institute of Geophysics and Volcanology (INGV) a Research Unit (RU) has been granted with the commitment to provide a link between the seismic shaking and the triggering of ground failures such as liquefaction. The main goals have regarded both the enlargement of the base of observables for a better constrain of the seismic hazard assessments and the analysis of the triggering and causative factors of permanent ground deformations. Nevertheless, when analyzing the non-linear soil response under which liquefaction occur, some insights into site-effects have been also provided, thus contributing to the general task of the site-specific hazard. The paper illustrates the analyses and investigations carried out within the aim of the project, some of them are still provisional due to the huge amount of data produced and the strong effort required to analyze all the matters related to the observed phenomena.

Strong Ground Motion in the 2011 Tohoku Earthquake: a 1Directional - 3Component Modeling

Local wave amplification due to strong seismic motions in surficial multilayered soil is influenced by several parameters such as the wavefield polarization and the dynamic properties and impedance contrast between soil layers. The present research aims at investigating seismic motion amplification in the 2011 Tohoku earthquake through a one-directional three-component (1D-3C) wave propagation model. A 3D nonlinear constitutive relation for dry soils under cyclic loading is implemented in a quadratic line finite element model. The soil rheology is modeled by mean of a multi-surface cyclic plasticity model of the Masing-Prandtl-Ishlinskii-Iwan (MPII) type. Its major advantage is that the rheology is characterized by few commonly measured parameters. Ground motions are computed at the surface of soil profiles in the Tohoku area (Japan) by propagating 3C signals recorded at rock outcrops, during the 2011 Tohoku earthquake. Computed surface ground motions are compared to the Tohoku earthquake records at alluvial sites and the reliability of the 1D-3C model is corroborated. The 1D-3C approach is compared with the combination of three separate one-directional analyses of one motion component propagated independently (1D-1C approach). The 3D loading path due to the 3C-polarization leads to multiaxial stress interaction that reduces soil strength and increases nonlinear effects. Time histories and spectral amplitudes, for the Tohoku earthquake, are numerically reproduced. The 1D-3C approach allows the evaluation of various parameters of the 3C motion and 3D stress and strain evolution all over the soil profile.

Influence of Lateral Heterogeneities on Strong‐Motion Shear Strains: Simulations in the Historical Center of Rome (Italy)

Abstract The influence of lateral heterogeneities on alluvial deposits is a topic of particular interest in the field of urban planning and engineering design of structures and infrastructures. This work focuses on the effects of such heterogeneities on the shear strains produced within the recent alluvial deposits of the Tiber River in the historical center of Rome in case of the worst expected earthquake scenario. To this aim, a 3D engineering‐geology model of the subsoil is used to derive four geological sections across the Tiber River valley as well as 48 soil columns to perform numerical simulations. Various models are considered: a viscoelastic equivalent linear rheology in a 1D finite‐difference model for one‐component horizontal input, a nonlinear elastoplastic model in a 1D finite‐element scheme for three‐component input, and a nonlinear viscoelasto‐plastic rheology in a 2D finite‐difference model under one‐component horizontal input. After comparing these different simulations, results have shown that lateral heterogeneities play a key role with respect to the expected shear strains within multilayered soils. To this aim, some specific indexes are introduced to estimate the maximum shear strain (MSS) concentration within the soil layers as well as to highlight their effect due to the stratigraphic position of the layers, within the soil column, independently from its depth. A final differential index leads to the evaluation of the lateral heterogeneity effect on the estimated MSS, demonstrating their prevalent role with respect to the bedrock shape (i.e., the angle of inclination of the buried valley slopes). From these results, an MSS zoning map is obtained for the historical center of Rome, showing that the local seismic response should be modeled by assuming 1D or 2D conditions depending on the location considered.

Seismic Analysis of the Gran Sasso Catastrophic Rockfall (Central Italy)

The paper focuses on the description of a landslide event occurred along the Gran Sasso d’Italia Massif (Central Apennines, Italy) and on the interpretation of the possible causes that determined some unusual secondary effects occurred at the base of the slope. The study was carried out by coupling the results of field surveys with the processing of an available seismic record generated by the rockfall. The recorded velocimetric data were processed in order to assess the evolution of the rockfall in terms of number of main impacts and energy dissipation.

A Characteristic-Period Based Approach for Evaluating Earthquake-Induced Displacements of the Large Büyükçekmece Landslide (Turkey)

The Buyukcekmece landslide is located in Turkey, W of Istanbul, about 15 km northward from the North Anatolian Fault Zone (NAFZ) and involves upper Oligocene to lower Miocene deposits, consisting of silty clays, tuffs and sands. No earthquake-induced re-activations are testified so far but due to the high seismicity of the area (struck by the 17th August 1999 Mw 7.4 Kocaeli and by the 12th November Mw 7.2 Duzce earthquakes) it was selected as the case-study in the framework of the European project “MARSite—Marmara Supersite: new directions in seismic hazard assessment through focused Earth observation in the Marmara Supersite”. Due to the existence of several secondary scarps, the original geological setting of the sedimentary deposits is significantly modified as many counter-slope-tilted landslide sub-masses can be identified in the landslide mass. Earthquake-induced displacements of the landslide were evaluated through a characteristic-period based (CPB) approach. To this aim, a stress-strain dynamic numerical modelling was carried out using several seismic inputs with Arias intensity (AI) values ranging from 1 and 0.01 m/s and characteristic periods, Tm, ranging from 0.3 to 16.5 s. Depending on the landslide dimensions (thickness and length), characteristic periods for thicknesses (Ts) and lengths (Tl), were computed to define the corresponding ratios over Tm. The obtained results indicate that the effective characteristic period of the landslide (Tl *) corresponds to the length of a single sub-mass, and not to the total length of the landslide. Moreover, for the lowest AI values the maximum earthquake-induced displacements correspond to Tm values close to resonance period of the landslide while for higher AI values, such a 2D interaction between landslide mass and seismic waves is much more evident.

Analysis of earthquake - Induced strain effects in a recently urbanized alluvial valley (Rome)

This study is focused on evaluating local response and the earthquake–induced strains in a highly heterogeneous alluvial valley through numerical modelling approach. The case study is located S of Rome, in correspondence to the Fosso di Vallerano alluvial valley. A high-resolution engineering-geological model of the alluvial deposits that fill the valley was derived by field investigations. Several tens of borehole log-stratigraphies were also taken into account. Outputs of 1D and 2D numerical models were analysed in terms of maximum shear strain (MSS) and Shear Strain Concentration Index (SSCI) to highlight where shear strain concentrates in the engineering-geological model as well as role of lateral heterogeneities in the distribution of shear strain within the alluvial deposits. The here reported preliminary results show that earthquake-induced strain effects: i) can be mainly referred to 2D conditions, ii) are mainly influenced by the heterogeneity of the alluvial deposits, iii) are poorly influenced by the shape of the bedrock.

Diezma Landslide (Southern Spain): Geological Model and Seismic Response

The Diezma landslide (Southern Spain) occurred on March 18 of 2001 after a heavy rainfall period, interrupting an important motorway. Landslide involves a heterogeneous soil laying on a metamorphic substratum (shale). A geological model has been made using 34 boreholes drilled between 1998 and 2012 in this area. As it results from this model, the landslide mechanism is mainly controlled by a complex system of tear faults which dislodge a main thrust line. Since the landslide is located in a high seismicity area, it is of interest to know its seismic response. Ambient noise was measured within and around the landslide and the Nakamura’s method was applied for studying local seismic response. The obtained results show that: (i) the landslide mass is characterized by a clear seismic response respect to the surrounding stable areas, and (ii) a relation seems to exist between the frequency of peak amplitudes retrieved by Nakamura’s method and thickness of landslide mass in the mid-upper part of landslide body.

The high damaging mw 5.1 lorca 2011 earthquake: Possible role of local seismic amplification

On May 11, 2011, a seismic series struck the city of Lorca (SE Spain) causing widespread damage and 9 fatalities. The series was characterized by a Mw 4.5 foreshock occurred at 15:05 (GMT), a main event occurred at 16:47, Mw 5.1, and many other aftershocks of smaller magnitude. All focuses were shallow and located about 3 km N of this city. The acceleration records of both Mw 4.5 and 5.1 events at the station located at Lorca (LOR) show that main direction of the motion was almost perpendicular to the strike of the north segment of the Lorca fault (N50oE). To understand the local seismic response of the city area in general, and of the LOR site in particular, ambient noise and Vs30 was measured through the city and the whole set of accelerograms recorded at LOR was analyzed (period 1993–2013; epicentral distances from 2 to 150 km). The HVSRs do not show a significant amplification while from the accelerograms recorded at LOR, a possible “fault effect” related to the near fault shaking, as well as to a “fault-zone” effect, i.e. due to the dynamic properties of the fault zone rocks with respect to the adjacent rock masses was recognized. These effects are only visible in the case of earthquake records whose epicenters are closer than 15 km (i.e. produced by the fault zone). Such effect combined with large accelerations that occurred may explain the damages produced by the main event.

Experimental and Numerical Investigations of Nonlinearity in Soils Using Advanced Laboratory-Scaled Models (ENINALS Project): From a Site-Test to a Centrifuge Model

The SERIES project “Experimental and Numerical Investigations of Nonlinearity in soils using Advanced Laboratory-Scaled models” (ENINALS) was focused on the centrifuge modeling of seismically-induced strains vs. stratigraphic features and it was applied to the heterogeneous alluvia of the Tiber River in the Rome historical centre. Four soil samples, representing two homogeneous soil columns (only clay and only sand respectively) and two heterogeneous soil columns (including a clay level between two sand ones) were tested in the IFSTTAR centrifuge. The applied dynamic input represents the maximum expected seismic action in Rome and it was reproduced at the centrifuge shaking device following three approaches: (i) a natural time history, (ii) an equivalent sinusoidal signal, and (iii) a multifrequential equivalent signal derived by the recently proposed LEMA_DES approach. The here reported preliminary results: (i) demonstrate the reliability of a “cut and install” procedure for realizing the saturated multilayer samples; (ii) give new insights on the reliability of multifrequencial dynamic equivalent signals for scale-reduced analogical modeling.