Seismic Bedrock Research Articles

Seismic soil classification of Italy based on surface geology and shear-wave velocity measurements

During an earthquake the seismic wave amplification related to local site conditions can have a significant impact on the ground motion. In order to account for these local effects some proxies for the soil characteristics exist; e.g., the average shear-wave velocity of the upper 30 m (VS,30), or the equivalent shear-wave velocity from the ground to the depth of the seismic bedrock when this is less than 30 m VS,eq.The aim of this paper is to provide maps of seismic shallow soil classification for Italy accounting for two sources of information: site-specific measurements and large-scale geological maps. The soil maps are obtained via a four-step procedure: (1) a database of available site-specific investigations is built, covering (unevenly) the whole national territory; (2) twenty geo-lithological complexes are identified from the available geological maps; (3) the investigations are grouped as a function of the geo-lithological complex and the distribution of measured VS,30 and VS,eq are estimated; (4) medians and standard deviations of such distributions are assumed to be representative of the corresponding complexes. The statistics of investigations are used to derive the large-scale soil maps. To make the results of the study available, a stand-alone software has been developed. Despite not being adequate substitutes of site-specific studies such as microzonation and local site response analyses, the provided results can be useful for large-scale seismic risk studies.

Seismic response across the Tronto Valley (at Acquasanta Terme, AP, Marche) based on the geophysical monitoring of the 2016 Central Italy seismic sequence

Since August 2016, Central Italy has been struck by three strong earthquakes (Mw 6.0 Amatrice, Mw 5.9 Visso and Mw 6.5 Norcia). This seismic sequence resulted in ground failure, infrastructural damages and destruction of several villages. This work aims to analyze the site effects on the ground shaking along the valley of the Tronto River across the Acquasanta Terme municipality, where some historical sites were located. These suffered heavy and differentiated damage during the seismic sequence. Following the Amatrice earthquake of August 24, authors managed the deployment and acquisition of seismic stations located in an array along the valley of the Tronto River approximately 25 km from the epicenter. By recording the two stronger earthquakes (i.e., Mw 5.9 Visso and Mw 6.5 Norcia) and 30 aftershocks with varying magnitudes and also ambient vibration, the site effects were preliminarily studied using the horizontal-to-vertical spectral ratio (HVSR). Furthermore, the amplification effects on the ground motion were also evaluated with reference to the earthquakes in terms of the standard spectral ratio (SSR) and the normalized energy content, in both cases using the bottom of the valley where the seismic bedrock outcrops as a reference. The comparison between the spectral accelerations due to the two strong earthquakes and those provided by ground motion prediction equations displayed values that were usually inadequately precautionary for periods lower than 0.35 s. High spectral accelerations were detected in period ranges corresponding to those predominant for masonry structures that are available in the literature. According to the current knowledge about surface geology, the local site effects seem to have significantly influenced ground shaking along the slopes of the valley, thus producing a larger seismic effect on ancient structures with more than two floors. The results can provide useful information for undertaking a possible future microzonation study that would be able to support urban planning and seismic designs in the rebuilding phase.

Effects of choice of soil constitutive model on seismic performance of moment-resisting frames experiencing foundation rocking subjected to near-field earthquakes

The current study investigated the extent to which the choice of the soil constitutive models can impact the predicted seismic performance of a 20-story reinforced concrete moment-resisting building with a mat foundation considering the Seismic Soil-Structure Interaction (SSSI). Since the soil, in general, is the weakest material, involved in the commonplace geotechnical engineering projects, a soil constitutive model would be able to rule the dynamic response of the system. In this research, the hardening plasticity-based soil constitutive model, named “hyperbolic hardening with hysteretic damping”, in conjunction with the two simple, conventional soil models, namely, the isotropic elastic with hysteretic damping model, and elastic-perfectly plastic Mohr-Coulomb with hysteretic damping model, were invoked in the three-dimensional coupled soil-structure numerical simulations using FLAC3D software. The direct method of analysis was used for analyzing the soil-foundation-structure system in one single step without a need to separately analyze each part of the domain. The cherry-picked earthquake excitations, viz, the 1999 Chi-Chi (Taiwan), and 2011 Kohriyama (Japan), were scaled by means of the widely-used response spectrum matching method as per the design response spectrum of a strong rock. The plastic moment concept was employed so as to assign the elastic-perfectly plastic model to the superstructure and its foundation. Additionally, the strain-compatible shear modulus and damping dependency on the cyclic shear strain were considered via the programmed hysteretic damping algorithm. The numerical predictions included the response spectra at the seismic bedrock and ground surface, base shear forces, shear force distributions along the building height, maximum and permanent foundation displacements, and foundation rocking, plus the flooring lateral deflections and inter-story drifts. The life safety limits for the transient and residual total inter-story drift ratios were not met whilst considering the soil pre-failure plasticity. The concluded remarks herein on the significant effects of the soil hardening plasticity on the seismic performance of the adopted 20-story moment-resisting frame would be applicable to the other structures on a mat foundation, potentially experiencing the foundation rocking amidst the earthquakes.

Mapping the Seismic Bedrock of the Po Plain (Italy) through Ambient‐Vibration Monitoring

Mapping the Seismic Bedrock of the Po Plain (Italy) through Ambient‐Vibration Monitoring

VS models in the historical centre of Naples (Southern Italy) from noise cross-correlation

Ambient noise measurements have been performed in the historical centre of Naples (Southern Italy) along 40 paths (inter-receiver distances) with a length ranging from ~300 m to ~1550 m, for recording time variable from few hours to several months.Synchronous noise recordings at two receivers, 1-bit normalized, have been cross-correlated in order to estimate the empirical Green's function (surface wave train) of the medium between them. The fundamental-mode Raleigh waves have been extracted at periods ranging from 0.15 s to 1 s by using the frequency-time analysis technique. Average group velocity dispersion curves with error bar have been evaluated along 28 paths and inverted with a non-linear approach to get VS profiles extending to depths of ~50–300 m.Comparison of VS with some deep drillings and two down-hole measurements lets to argue important findings. First of all there is a good agreement with shallow (about 35 m) point down-hole measurements. The historical centre of Naples results to be characterized by a cover of man-made ground and pyroclastics of recent activity with thickness of 15–30 m and VS lower than 400 m/s. The products of the Neapolitan Yellow Tuff eruption have a thickness of about 80 m and are characterized by VS of 500–700 m/s in the shallower, altered and fractured part and of 700–900 m/s in the deeper, compact part. Below them, separated by a thin layer of the Campanian Ignimbrite that cannot be resolved by the data, ancient tuffs or marine sands are present which are locally responsible of velocity decrease. VS of 1100–1300 m/s, ascribable to intercalations of volcanic and sedimentary (marls, conglomerates and marly sands) rocks, are detected at ~200 m depth in the ancient part of the historical centre, rising to ~100 m depth, in the north-east of it.This study also gives an important contribution to microzoning studies. The top of the lithoid Neapolitan Yellow Tuff cannot be considered as the top of the seismic bedrock, being characterized by 500 m/s < VS < 800 m/s. The seismic bedrock with VS ≥ 800 m/s is deeper, sometimes at depths >100 m.

Modeling of the Subsurface Structure from the Seismic Bedrock to the Ground Surface for a Broadband Strong Motion Evaluation in Kumamoto Plain

During the 2016 Kumamoto earthquakes, two earthquakes of seismic intensity 7 were observed in Mashiki Town, the foreshock (MJMA6.5) of April 14 and the main shock (MJMA7.3) of April 16, resulting in significant damage to structures near the fault. The distribution of damage of houses and other buildings [1] showed a tendency in which damage was concentrated in areas near the surface earthquake fault where the main shock presumably occurred. However, there were locations with slight damage even though they were immediately above the fault and locations with a relatively significant damage even though they were far from the fault. These phenomena are highly likely to be a result of soil structure. First, we built an initial geologic model by collecting boring data in areas of the Kumamoto plain near the fault where damage was severe. Next, we observed microtremors, collected earthquake observational records, and adjusted the layer thickness and S-wave velocity of the initial geologic model. Finally, we built a shallow and deep integrated ground model, compared it to the building damage distribution, and discussed the implications.

Contribution of HVSR measures for seismic microzonation studies

The HVSR method applied to seismic noise can be a very useful technique to map the site effects of the territory, to identify the thickness of the soft covering and so the depth of the seismic bedrock. The case of the urban area of Oliveri is presented. Because of its high seismic hazard this area has been subject of first level seismic microzonation. The town lies on a large coastal plain made of mixed fluvial/marine sediments, overlapping a deformed substrate. In order to identify points on the area probably suffering of relevant site effects and to define a preliminary Vs subsurface model, 23 HVSR measurements were performed. A clustering technique of continuous signals has been used to optimize the calculation of the HVSR curves and 42 reliable peaks, in the frequency range 0.1-20 Hz, have been identified. A second clustering technique has been applied to the set of 42 vectors, containing coordinates, frequency and amplitude of each peak, to identify subsets attributed to the same seismic discontinuities. Three main clusters have been identified. The two characterized by lower frequencies have been considered in the HVSR data inversion, as stratigraphic peaks probably caused by the seismic bedrock. Finally, the morphology of the top of the seismic bedrock has been mapped. The deepening of the seismic bedrock below the mouth of the Elicona Torrent suggests the possible presence of a buried paleo-valley.

Towards the revision of EC8: Proposal for an alternative site classification scheme and associated intensity dependent spectral amplification factors

The paper presents an alternative site classification scheme and associated intensity dependent spectral amplification factors, aiming to contribute to the ongoing revision of Eurocode 8. The new classification scheme, introducing among the main classification parameters the fundamental period T0 of the site, is an evolution of the recent work by Pitilakis et al. (2013) [1], while the main features of the amplification factors and the new seismic design actions are summarized in the use of two anchoring spectral values, for short (0.2–0.3 s) and intermediate (1.0 s) periods, instead of only one of the present version of Eurocode 8 (i.e. effective ground acceleration), and the scalar intensity variation of site amplification factors to account for soil nonlinearity. The work is based on a comprehensive analysis of a worldwide database of strong ground motion records from sites which dispose a very well-documented soil profile down to the seismic bedrock (Vs>800 m/s).

Seismo-stratigraphic model of "La Bandita" area in the Palermo Plain (Sicily, Italy) through HVSR inversion constrained by stratigraphic data

Ambient noise Horizontal-to-Vertical Spectral Ratio (HVSR) technique is commonly used approach to obtain 1D models of the shear-wave velocity in the shallow surface of an investigated area. However, obtained models can have a wide margin of uncertainty if inversions have not been appropriately constrained by detailed stratigraphic information. An application of HVSR inversion constrained by lithostratigraphic data is presented in order to verify the effectiveness of this technique for purposes of geological and geophysical reconstruction of a sedimentary basin in a densely urbanized area. This is often the case of seismic microzonation studies, in which almost all the information derives from near surface stratigraphic drillings, since other geophysical methods are logistically difficult to carry out. In our work, we used stratigraphic constraints derived from 93 superficial bore-holes whose depth rarely exceeds 30 m. In an area called La Bandita, located in Palermo Plain (Sicily, Italy), a geophysical survey was performed by means of 55 microtremor recordings. Part of these was distributed randomly, while others very close to the available stratigraphic perforations. The reconstruction of the stratigraphy in the studied area has been obtained by a review of the main stratigraphic sequences and by a consequent stratigraphic three-dimensional modelling. HVSR curves have been interpreted taking care the thicknesses of the near surface successions derived by the stratigraphic 3-D model. The results, in terms of vertical profiles of the shear-wave velocity, have been interpolated to obtain a 3D seismic model. This has been used to extract basic information to identify and reconstruct the seismic bedrock and the main geological boundaries that were not directly identifiable by means of only stratigraphic logs. It results that the bedrock is affected by a fault system that generated adjacent depressions where Quaternary successions deposited.

Basin Edge Effect on Seismic Ground Response: A Parametric Study for Duzce Basin Case, Turkey

In this study, edge effect on the spatial variation of surface ground motion was evaluated by conducting one (1D) and two-dimensional (2D) dynamic analyses on the numerical model of Duzce basin. Duzce is a province in northwestern Turkey and was hit by a devastating earthquake in November 1999. In order to estimate the seismic behavior of Duzce basin, 1D and 2D dynamic response analyses were carried out on its numerical model by using the seismic bedrock motion data derived from the deconvolution analysis of the N–S component of 1999 Duzce earthquake accelerogram. In addition, a series of different accelerograms and slope values were utilized to investigate the possible effects of change in strong ground motion intensity level and edge geometry on the dynamic response of soil layers over inclined bedrock. The finite element model for the edge section of the flat sedimentary basin was set up by using the 2D shear wave velocity profile, geotechnical and geological site conditions in Duzce. The variation of 2D/1D spectral acceleration ratios and aggravation factors with distance from basin edge was obtained for different period values. The results indicate that the aggravation factors reached their maximum values at a specific zone near basin edge and they generally converged to 1 regardless of the bedrock inclination or period values with increasing distance from the edge section. The highest aggravation factor values were calculated as 2.0–2.5 for the period interval of $$T=$$ 0.2–0.6 s. A relationship was proposed between the aggravation factor and basin edge geometry.

3D Bedrock Structure of Bornova Plain and Its surroundings (İzmir/Western Turkey)

An earthquake record is needed on engineering bedrock to perform soil deformation analysis. This record could be obtained in different ways (seismographs on engineering bedrock; by the help of the soil transfer function; scenario earthquakes). S-wave velocity (V s) profile must be known at least till engineering bedrock for calculating soil transfer functions true and completely. In addition, 2D or 3D soil, engineering–seismic bedrock models are needed for soil response analyses to be carried out. These models are used to determine changes in the amplitude and frequency content of earthquake waves depending on the seismic impedance from seismic bedrock to the ground surface and the basin effects. In this context, it is important to use multiple in situ geophysical techniques to create the soil–bedrock models. In this study, 2D and 3D soil–bedrock models of Bornova plain and its surroundings (Western Turkey), which are very risky in terms of seismicity, were obtained by combined survey of surface wave and microgravity methods. Results of the study show that the engineering bedrock depths in the middle part of Bornova plain range from 200 to 400 m and the southern and northern parts which are covered limestone and andesite show the engineering bedrock (V s > 760 m/s) feature. In addition, seismic bedrock (V s < 3000 m/s) depth changes from 550 to 1350 m. The predominant period values obtained from single station microtremor method change from 0.45 to 1.6 s while they are higher than 1 s in the middle part of Bornova plain where the basin is deeper. Bornova Plain has a very thick sediment units which have very low V s values above engineering bedrock. In addition, it is observed sudden changes at the interfaces of the layer in horizontal and vertical directions.

An empirical relationship between seismic bedrock depth and fundamental frequency for Değirmendere (Kocaeli-Turkey)

Sediments’ thickness is one of the important parameters in seismic hazard assessment. This study aims to establish a new empirical relationship to determine the overburden layer thickness on seismic bedrock by using a relationship between the fundamental frequencies obtained from microtremor measurements and seismic bedrock depth obtained from microgravity measurements and subsequently produce an approximate seismic bedrock depth distribution map for the study area. In this context, ambient noise analyses were performed to estimate the fundamental frequency of the ground. Within this scope, noise measurements at 43 points were established in the study area. Determined fundamental frequencies and seismic bedrock depths obtained by microgravity measurements were iteratively processed to calculate the best-fitting exponential function for the empirical depth–frequency relation. This relation was established as $$m = 338.2 \cdot f_{0}^{ - 0.6558}$$ (m is seismic bedrock depth, f 0 is fundamental frequency) for the study area.

Non-linear Seismic Ground Response Analysis Considering Two-dimensional Topographic Irregularities

In the event of an earthquake, local site conditions such as soil characteristics, dimension of topographic irregularities, seismic bedrock depth, etc. and also characteristics of incident wave have important effects on seismic ground response. In this study, Finite Element Method (FEM) coupled to Viscous Boundaries is used to evaluate the effect of empty two-dimensional valleys on amplification or attenuation of seismic waves. Parametric studies carry out and the effects of dimension of the topography, frequency of the incident wave and bedrock depth on the seismic ground response are considered using non-linear method in a time domain analysis. Results are shown by means of horizontal and vertical amplification ratio in valley span and its surrounding area. It is concluded that displacement variation on ground surface due to topographical effects is a considerable factor to select a site location or design structures in the valley mount and its surrounding area.

Geophysical and geologic surveys of the areas struck by the August 26th 2016 Central Italy earthquake: The study case of Pretare and Piedilama

A MW6.0 earthquake struck the Central Italian Apennines on August 24th 2016, followed by an important seismic sequence characterized by hundreds of aftershocks. The earthquake caused fatalities and partially destroyed several towns surrounding the epicentral area. In this context the Italian Centre for Seismic Microzonation and its applications was involved for the seismic characterization of the struck area. Using ambient vibrations spectral ratios at seventy-five sites and shear wave velocity profiles obtained through surface waves dispersion properties we assessed seismic site response properties in the Pretare and Piedilama villages located in the territory of Arquata del Tronto. The survey performed inside the valley in which the two villages are located set into evidence that the spectral ratios often show a predominant frequency ranging between 3.0 and 5.0Hz or several peaks that can be related to some alternating layers with different velocities. Conversely, along the valley flanks, where the geological substratum outcrop, spectral ratios tend towards low amplitude values. The soft sediments inside the valley are characterized by shear wave velocity lower than 350m/s, whereas the geological substratum has shear wave velocity ranging between 600 and 1200m/s. The integrated interpretation of both geophysical and geological data, made possible to infer subsoil models of the investigated areas identifying the depth of the seismic bedrock and characterizing the buried morphology.

Geometry and seismic characterization of the subsoil below the Amphitheatrum Flavium, Rome

The seismic-risk assessment of archeological and monumental sites is extraordinarily challenging from the earthquake engineer’s point of view: the monumental structures are generally unconventional and of great value, the geology and ground conditions are often poorly known and the presence of tourists in the area can be massive. In addition, the effects of anthropic and artificial deposits on seismic ground motion are particularly relevant within urban historicalsites and should be addressed thoroughly. Under these conditions, non-destructive geophysical methods can be successfully employed to identify key parameters for seismic verification. The main focus of this paper is the application of non-invasive geophysical methods to investigatea famous monumental structure: the Colosseum or Amphiteatrum Flavium (Rome, Italy). Because of the complexity of the subsoil under the Colosseum, a comprehensive 3D approach should be encompassed to fulfil the lack of information still pending on the complex subsoil geometry and characterization, as well on the shape of the monument’s foundations. Our investigations are mostly focused on the southern sector of the monument, basically for budget reasons. Despite these limitations our findings do however contribute to link the reference geologic model to the specific conditions of the anthropic layer and the seismic bedrock at this site, as well as the foundations and the buried parts of the monument below the former arena, providingkey input data for the assessment of the response of the Colosseum under dynamic loading.

Ensuring very shallow-water sediment properties: case study from Capo Granitola harbour, Sicily (Italy)

We present high-resolution Vp models of the Capo Granitola harbor, Sicily (Italy) obtained by first arrival traveltime tomography. Seismic data were collected along four hydrophone arrays on the sea-bottom and via a Watergun as seismic source, in order to plan dredging operations in the harbor. Using a hydrophone spacing of 2.5 m and shot spacing of 5 m, very high resolution quality data were recorded. Seismic tomography expands existing knowledge of the harbour subsoil with a penetration of about 20 m, illuminating the Lower Pleistocene bedrock (Marsala calcarenites) that corresponds to high-Vp regions (Vp > 4.5 km/s). Low Vp (1.8–4.5 km/s) deposits belonging to terraced calcarenites (Upper Pleistocene in age) are also well imaged; they are about 8 m thick and lie below loose sand deposits (Vp = 1.5 km/s). The substratum has an articulated morphology; Vp images unravel small steps in the basement probably related to structural discontinuities (e.g., faults). Processing data with 3D techniques enables images of the structure and the thickness of the lithotypes to be reconstructed, thus leading to large-scale, realistic estimates of the total quantity of material to be excavated or dredged. Tomographic profiles permit clear discrimination of the soft sediment above the basement and thus allow the determination of the total volume of sediment above the seismic bedrock, estimated at about 265,000 m3.

Long‐Period Amplification in Deep Alluvial Basins and Consequences for Site‐Specific Probabilistic Seismic‐Hazard Analysis: An Example from the Po Plain (Northern Italy)

This study deals with the impact of deep‐soil discontinuities on ground‐motion amplification in the Po alluvial basin (northern Italy), and consequently, on long‐period probabilistic seismic‐hazard analysis estimates. The focus is on Castelleone, where a seismic station of the Italian National Seismic Network has been deployed since 2009. To define a detailed shear‐wave velocity ( V S ) profile, extensive active and passive geophysical surveys were carried out. The active measurements included seismic profiles examined through standard refraction techniques and multichannel analysis of surface waves. The passive methods included ambient vibration measurements in both single and array configurations. In particular, three microtremor arrays with increasing apertures were performed to capture the experimental dispersion curve down to 0.2 Hz. The horizontal‐to‐vertical spectral ratio (HVSR) showed two main peaks, at 0.17 and 0.70 Hz. A joint inversion of the experimental phase‐velocity dispersion and HVSR curves was performed to obtain the V S profile. The data show two main discontinuities at ∼160 and 1350 m in depth. According to the Italian and European seismic codes, the shallow V S discontinuity can be ascribed to the seismic bedrock (i.e., V S >800 m/s), whereas the deeper one is observed at the transition between Pliocene–Quaternary deposits and Miocene marls. Preliminary site‐specific seismic‐hazard analysis in terms of displacement response spectra for periods up to 10 s shows that neglecting the effects of the deeper discontinuity implies underestimation in hazard estimates of up to about 49% for mean return period (MRP) of 475 years and about 57% for MRP of 2475 years, with possible consequences regarding the design of very tall buildings and large bridges.

2D soil and engineering-seismic bedrock modeling of eastern part of Izmir inner bay/Turkey

Soil-bedrock models are used as a base when the earthquake-soil common behaviour is defined. Moreover, the medium which is defined as bedrock is classified as engineering and seismic bedrock in itself. In these descriptions, S-wave velocity is (Vs) used as a base. The mediums are called soil where the Vs is <760m/s, the bigger ones are called bedrock as well. Additionally, the parts are called engineering bedrock where the Vs is between 3000m/s and 760m/s, the parts where are bigger than 3000m/s called seismic bedrock. The interfacial's horizontal topography where is between engineering and seismic bedrock is effective on earthquake's effect changing on the soil surface. That's why, 2D soil-bedrock models must be used to estimate the earthquake effect that could occur on the soil surface. In this research, surface wave methods and microgravity method were used for occuring the 2D soil-bedrock models in the east of İzmir bay. In the first stage, velocity values were obtained by the studies using surface wave methods. Then, density values were calculated from these velocity values by the help of the empiric relations. 2D soil-bedrock models were occurred based upon both Vs and changing of density by using these density values in microgravity model. When evaluating the models, it was determined that the soil is 300–400m thickness and composed of more than one layers in parts where are especially closer to the bay. Moreover, it was observed that the soil thickness changes in the direction of N-S. In the study area, geologically, it should be thought the engineering bedrock is composed of Bornova melange and seismic bedrock unit is composed of Menderes massif. Also, according to the geophysical results, Neogene limestone and andesite units at between 200 and 400m depth show that engineering bedrock characteristic.

Down-hole geophysical characterization of middle-upper Quaternary sequences in the Apennine Foredeep, Mirabello, Italy

The 2012 earthquakes sequence stroke a wide area of the alluvial plain in the Emilia-Romagna Region and triggered a new research interest on the role of the subsurface stratigraphic architecture and petrophysical property distribution in the modulation of the local seismic effects. Few direct shear wave velocity V_S data were however available below the depth of 50m. The only available V_S measurements were obtained from an anticline area, characterized by a reduced stratigraphic thickness and peculiar sedimentary facies, hardly representative of the majority of the alluvial plain subsurface. The study provides the first V_S profile available from middle-upper Quaternary successions deposited into a fast subsiding syncline area of the Apennine Foredeep Basin. The P-wave velocity V_P and the S-wave velocity V_S logs fill in the previous data gap on the geophysical parameters needed for the estimation of the local seismic response. Both V_P and V_S logs were continuously acquired to the depth of 265 m. The log records a velocity increase with depth, punctuated by sharp increases at some stratigraphic discordance surfaces. The value of 800 m/s that characterizes the “seismic bedrock”, as defined by the Italian building code [NTC 2008] was never reached at any depth. The investigated succession records a depositional evolution from deltaic-marine to alluvial plain conditions, punctuated by six glacio-eustatic depositional cycles, developed in Middle-Upper Quaternary times. The stratigraphic units described in the syncline log were correlated at a regional scale, with the thinner anticline succession of Mirandola. Correlatable units deposited into syncline and anticline areas reveal similar shear wave velocity values, supporting the regional extrapolation of the measured values.&lt;br /&gt;&lt;br /&gt;&lt;br /&gt;&lt;em&gt;&lt;/em&gt;

Fieldwork on local-site seismic response in the Po Plain: examples from ambient vibration array and single station analyses

In the framework of the S2 DPC-INGV 2014–2015 project (https://sites.google.com/site/ingvdpc2014progettos2/), several field surveys were designed with the aim to investigate the local seismic site response of selected Italian Accelerometric Network strong motion stations. In this study the results obtained at Sorbolo, Lesignano de’ Bagni, and Castelfranco Veneto, three sites located in the Po alluvial basin (northern Italy), were presented. In particular, at each site, ambient vibration surveys were performed both in single stations and two dimensional arrays configurations. The ambient vibration arrays were analyzed to obtain at each site the shear-wave velocity (Vs) profile, until depths in correspondence of which the Vs show values higher than 800 m/s (i.e. lower bound for the soil category A, as now defined in the Italian seismic code for building). In this way, data were processed through a joint inversion computed using both the Rayleigh waves dispersion curve and the ellipticity curve deduced from the horizontal-to-vertical spectral ratios calculated from the microtremor single station measurements. In general, the obtained profiles show an increasing depth of the main Vs discontinuity (i.e. the seismic bedrock) from 60 to 320 m, moving from the edge to the central part of the Po alluvial basin. To support the outcomes of the field experiments, the available earthquakes recorded at each station in the last decades were processed in terms of the horizontal-to-vertical spectral ratios. Although in many cases the resonance frequencies are comparable with those obtained from microtremor, a high variability is observable depending on the selected portions of each single earthquake. Finally, residual analysis were carried out comparing the recorded ground motion to predicted data as provided by the actual Italian ground motion prediction equations, calibrated both at national and at regional scale.