Seismic Microzonation Studies Research Articles

The importance of seismic microzonation studies on land governance: results from a fact-finding survey

The importance of seismic microzonation studies on land governance: results from a fact-finding survey

Possible measure of soil factors in the Italian seismic code

AbstractThe Italian seismic code provides a simplified approach to account for the effect of local seismostratigraphical configuration on the expected ground motion. This approach, common with other seismic codes, provides specific ‘soil factors’ as a function of a set of reference subsoil conditions (soil classes): these factors are considered in 1D subsoil configurations to modify the uniform probability hazard spectrum deduced from probabilistic seismic hazard at reference soil conditions. It is inferred that, to provide a coherent management of uncertainty affecting the response spectrum to be used for the design, the contribution of uncertainty affecting soil factors must be carefully considered to avoid biases in the hazard evaluation. In the present study, variability of soil factors representative of each soil class has been explored by numerical simulation relative to many seismostratigraphical configurations inferred from seismic microzonation studies available in Italy relative to 1689 municipalities. This analysis shows that variability of soil factors is of the same order of magnitude of variability affecting reference response spectra, which implies that the former cannot be neglected as presently happens in the common practice. It is also shown that neglecting this contribution can lead to underestimate the impact of subsoil configuration on the regularized response spectrum provided by the norm, in particular, in the short period range.

Mapping 1D seismic amplification effects in the range of periods of engineering interest based on geological data

Regional scale seismic hazard assessment including the effect of local seismo-stratigraphical conditions is a basic tool for seismic risk estimates. A novel physically based procedure is proposed for using geological maps to extensively estimate expected seismic amplification effects relative to spectral ordinates of main engineering interest (<0.8 s). Automatic GIS based analysis of geological maps, statistical data relative to the seismic/geotechnical properties of geological units and numerical modelling are combined to determine the probability distribution of expected amplification effects by accounting for uncertainty affecting the relevant parameters. To evaluate the feasibility of the proposed procedure, it has been applied to the Tuscany Region in Central Italy. Unbiasedness of outcomes has been tested by considering detailed microzonation studies available for the considered area. Results of the proposed approach could be easily implemented in extensive seismic risk analyses where detailed seismic microzonation studies are lacking.

Development of Seismic microzonation for Rancaekek Sub-District using microtremor method

Abstract Rancaekek sub-district located in the Bandung Regency is geologically characterized by soft and loose sediment layers such as clays, silts, and sands associated with the old Bandung Lake. Due to the existence of the Sesar Lembang Fault, the Rancaekek area is also vulnerable to seismic disasters. To provide knowledge on seismic vulnerability, therefore, a seismic microzonation study was conducted at 25 locations in the Rancaekek area using the microtremor method. Based on the available microtremor measurement data, the seismic vulnerability of the Rancaekek sub-district varies from one area to another. Amongst other areas Northern part of Bojongloa, Northern part of Rancaekek Wetan and Northern part of Jelegong areas are the most vulnerable to seismic damage because these areas have low dominant frequencies with medium amplification zones, and low Vs30 values ranging from 90 to 127 m/s. Thus, any building and infrastructure development in these areas should follow the national building code to reduce damage due to earthquake shaking.

A geological, petrophysical and ground motion model of the Pavoncelli bis hydraulic tunnel (Southern Apennines, Italy): Implications for reservoir analyses

The strong 1980 M6.81 Irpinia earthquake in Southern Italy critically damaged the Pavoncelli hydraulic tunnel. Based on geological, petrophysical and geomechanical investigations and seismological data, a multi-scale geological model was inferred. The detrital mode and key petrophysical properties (porosity, permeability, and nanopore volumes) data have been used for defining the water-reservoir potential of the arenitic successions, with a focus on defining the hydrogeological model. The model was then used to perform a novel back-analysis of the spatial distribution of the peak ground acceleration (PGA) caused by the 1980 earthquake accounting for site-effects in a robust manner. This analysis, which accounts for site-effects in a robust manner, shows high PGA values along the length of the Pavoncelli tunnel, which is located between the causative fault of the 1980 earthquake in the vicinity of the epicenter. The main outcomes of the study are that: (1) the multi-scale and interdisciplinary approach developed in this study can be used in future studies in this and other areas, (2) the spatial 2D-3D geological model must be accounted for when analyzing distributed systems, (3) the hydrogeological model coupled with porosity data allows for reservoir analyses and (4) the complex tectonic environment in the area hosting the Pavoncelli tunnel plays a strong role in the analysis of the spatial distribution of PGA, creating the need for site-specific seismic microzonation studies and the evaluation of near-fault effects.

Shallow geothermal potential and numerical modelling of the geo-exchange for a sustainable post-earthquake building reconstruction (Potenza River valley, Marche Region, Central Italy)

The southern part of the Marche Region (Italy) was hit in 2016 by dramatic seismic events that caused damages and fatalities. After these events, the national and local administrations started a development plan to improve the energy efficiency of the restored or reconstructed buildings. Shallow geothermal energy, consisting of closed-loop Borehole Heat Exchangers (BHEs) coupled with heat pumps, could represent a first-order renewable choice for indoor air conditioning (heating and cooling) systems. The feasibility and potential of the BHEs also match this technology's null visual outdoor impact which is paramount in preserving the landscape of the earthquake-affected areas. However, the exploitation, sustainability, and correct design of these systems require detailed knowledge of the ground's geological, hydrogeological, and thermophysical properties. In this framework, the present study was carried out to map and test, through the G.POT algorithm, the shallow geothermal potential and sustainability of the Potenza River valley through the publicly available data from the Italian Seismic Microzonation studies — an extensive database of geognostic drillings and other geological and geophysical surveys. This step allowed us to assign the main thermal parameters (thermal conductivity, volumetric heat capacity, specific heat extracted) to each geological layer and averaged them over the first 100 m (i.e., a typical depth for BHEs). All the data were then interpolated to produce geothermal thematic maps to visualize and compare the potential and adequacy of the territory for the installation of closed-loop BHEs. Finally, finite elements numerical models (FEFLOW® software) were developed in a balanced mode through the annual heating and cooling cycles to verify the sustainability of this renewable concerning the thermal impact induced to the ground by the BHEs during a long-term (20 years) operation.

Seismic microzonation for Lembang fault hazard area in West Bandung Regency based on microtremor measurement

Lembang district in Bandung Regency is prone to seismic risk because it is in the vicinity of a very well-known active fault called the Lembang Fault. The effort to reduce the seismic risk requires knowledge of the dynamic properties of the soils, mainly derived from young volcanic rocks. This paper presents the results of a seismic microzonation study based on microtremor measurements. In total 195 microtremor data from single-station measurements were processed using the HVSR method to estimate the dominant frequency and amplification factors. Furthermore, the Rayleigh wave ellipticity inversion has also been used to estimate the corresponding Vs profiles. The HVSR results show that the dominant frequency for the Lembang District is 0.2 Hz or higher, and the amplification factors are dominantly higher than 3. Some areas also have a lower dominant frequency than 2 Hz, especially the area with thick soft sediments. Results of the inversion show that the shear wave velocities of subsoil layers for the upper 30 m vary from 250 m/s to 300 m/s, indicating site class D. Meanwhile, the areas with Vs30 less than 250 m/s are especially located north of the Lembang Fault. Thus, the area is likely to have a stronger ground response to earthquake shaking than the other areas in Lembang District.

NC92Soil: A computer code for deterministic and stochastic 1D equivalent linear seismic site response analyses

The extensive evaluation of the impact of local seismo-stratigraphic configurations on seismic ground motion presents significant challenges due to the necessity of considering the combined effects of uncertainty and small-scale lateral variability of the relevant parameters. To effectively explore these sources of uncertainty, a new Python-based computer program is proposed for one-dimensional seismic site response simulations, adopting the equivalent linear viscoelastic approach in the frequency domain. With respect to existing software, the code introduces new pre- and post-processing features, which also meet the specific requirements of seismic microzonation studies. Within the code, the complete spectrum of uncertainties related to local seismo-stratigraphic configurations, including lithotype successions, layer thicknesses, and seismic and geotechnical properties for the considered lithotypes, is managed by considering user-defined constraints and statistical properties of the relevant parameters. Additionally, a batch approach is offered, enabling the application of the procedure to an unlimited number of different scenarios. To demonstrate the potentiality of the proposed code, a comprehensive set of 90,000 local seismic site response analyses was conducted, showing a clear correlation between the amplification factors, the mean shear wave velocity in the upper 30 m, the fundamental frequency of the deposit and the depth to the seismic bedrock.

Shear modulus reduction and damping ratios curves joined with engineering geological units in Italy

Numerical simulations of seismic site response require the characterization of the nonlinear behaviour of shallow subsoil. When extensive evaluations are of concern, as in the case of seismic microzonation studies, funding problems prevent from the systematic use of laboratory tests to provide detailed evaluations. For this purpose, 485 shear modulus reduction, G\\G0(γ) and damping ratio, D(γ) curves were collected from multiple literature sources available in Italy. Each curve was associated with the related engineering geological units considered in seismic microzonation studies. A statistical analysis of the data was carried out with the aim of shedding light on the significant difference between the laboratory classification of samples and the macroscopic/engineering geological one, provided during seismic microzonation studies. Since the engineering geological classification plays a prominent role in extensive site response evaluations, the outcomes of the present work may be of help at least when preliminary seismic response estimates are of concern. The dataset provides reference information that can serve as key data for large-scale hazard assessments worldwide.

Seismic amplification effects induced by ancient shallow cavities underneath the urban area of the historical city center of Chieti, Italy

Anthropogenic cavities excavated just beneath the walkable floor (about 1–6 m depth) are present in many ancient urban centres, particularly in Europe and along the Mediterranean basin. The oldest caves are Roman constructions and have been used for different purposes over time (i.e., water storage, secret pathways, shelters against sieges, and wars). For this reason, Seismic Microzonation (SM) studies performed in urban areas must consider the presence of these holes in the subsoil, with their effects predicted through numerical simulations of local seismic response on the surface. In this study, we investigate the seismic effects of ancient anthropogenic cavities in Chieti city centre (Italy) through the cavity amplification factor (CAF). A parametric study was carried out on several different dynamic subsoil models. We define CAF as the ratio between the Acceleration Spectral Intensities on the surface and at the bedrock calculated in the period range of 0.1–0.5 s. The 2-dimensional parametric simulations were performed adopting the soil equivalent linear constitutive law by varying: (1) shear wave velocity values of subsoil, (2) cavity dimensions, (3) cavity depths, (4) cavity lining shear wave velocity values. The CAF plots were calculated on the surface for cavity sizes (width x height) varying from 4 × 5 m to 7 × 9 m. Our study demonstrates that CAF values are relevant only in the period range of 0.1–0.5 s. In particular, results show that: (1) CAF values vary from 1% to 15%; (2) the maximum CAF is felt away from the cavity axis at a distance equal to about 1.5 times the cavity width (L); (3) the maximum difference in CAF values at the surface is equal to 40% within a horizontal distance from the cavity axis ranging from 6 to 14 m. Finally, the case study of the city centre of Chieti (Abruzzo Region) is considered to show how to draw CAF maps for microzonation purposes.

Seismic microzonation study for two mining cities in the SW of Kyrgyzstan

Seismic microzonation study for two mining cities in the SW of Kyrgyzstan

Geotechnical characterization of Viña del Mar for areas where buildings were affected by mega-earthquakes in Central Chile

Over the course of little more than a century, several large-magnitude earthquakes have struck Central Chile. In the area within the Marga-Marga basin in the city of Viña del Mar, notable damage to buildings suggests the occurrence of local seismic site effects. While seismic microzonation studies have attempted to explain such effects, the geotechnical characteristics adopted are generally oversimplified. This article addresses this with a thorough geotechnical characterization of the areas in Viña del Mar that sustained outsized damage to structures. The article provides a robust and updated geotechnical and geophysical database of the area, with additional analysis from records of the city’s urban development, geomorphology, and geology features. The study builds on geotechnical data from previous seismic zonation studies following the 3 March 1985 earthquake; building design projects undertaken between 1977 and 2018, including 89 standard penetration test (SPT) boreholes, 58 dynamic cone penetration tests (DCPT), 18 refraction microtremor surveys (ReMI), and 16 Down-Hole tests; and rock depth estimates from 63 horizontal to vertical spectral ratio (HVSR) tests. These data were used in data spatialization techniques to model geotechnical units along one (1-D) and two-dimensional (2-D) profiles, for the foundations of buildings damaged after the 1906, 1985, and 2010 earthquakes. The results show the existence of deep the existence of deep soft soils, high impedance contrast between geotechnical units shape of rock, and the dynamic soil properties can explain the damage observed in the studied area damage in the studied area. The study provides significant background and experimental information for the area, where there is enough seismic potential available to produce a large earthquake in the coming decades of similar characteristics as the 1730 Valparaiso earthquake (9.1–9.3 Mw).

Geotechnical characterization of the estuarine deltaic deposits in the Guayaquil city through in situ and laboratory tests

According to previously available research and seismic microzonation studies a large area of the Guayaquil (Ecuador, South America) sits on estuarine deltaic deposits which consist of weak and highly compressible clays with diatoms. The nature of these fine-grained deposits may determine difficulties in a proper estimation of the soil properties. In this respect, the paper provides a detailed geotechnical and geophysical characterization of these soft clays, carried out in the estuarine complex of the Ecuadorean city. Borehole logs, standard penetration tests (SPT), piezocone tests (CPTu), a seismic dilatometer test (SDMT), a non-invasive geophysical survey, and laboratory tests were performed and then compared to analyze the static and dynamic geotechnical parameters of these deposits. The interpretation of the results highlighted the higher reliability of CPTu and SDMT rather than SPT and characterization lab testing to estimate soil shear strength, compressibility and stress history due to the soft nature of these clays, underlining also a certain sensitivity to the presence of the diatoms.

Constraints for the Vs profiles from engineering-geological qualitative characterization of shallow subsoil in seismic microzonation studies

A statistical analysis has been performed aiming at determining to what extent a qualitative engineering-geological characterization of subsoil may provide constraints to the shear wave velocity (Vs) profile. For this purpose, a large database of experimental values provided by seismic microzonation studies carried out in the last 10 years in Italy has been considered. This analysis suggests that statistical constraints can be provided to Vs values as a function of depth by considering main lithological characteristics of cover terrains and geological bedrock. The effectiveness and unbiasedness of resulting relationships are tested against some experimental situations. This information can be used to feed stochastic numerical simulations aiming at preliminary assessment of probability distribution of the expected seismic response in wide areas where an experimental geophysical/geotechnical characterization of the local subsoil is scarce or unavailable.

Proposal procedure to evaluate the seismic stability of underground cavities in seismic microzonation studies

Under seismic actions, underground cavities play a not negligible role for seismic site effect and, at the same time, they represent objects exposed to seismic risk. Thus, their possible instability up to the collapse could have effects on the above-ground structures and infrastructures, as revealed by recent and past earthquakes. In consideration of the fact that the presence of underground cavities is very common in many urban and historical areas in Italy, it represents issue of concern for our territory and the related seismic risk should be considered both for planning and management issues.Being a rather complex topic, the cavity instability under seismic actions is little covered in the technical literature, thus the present work represents a first attempt to frame the problem at different scales. An extensive analysis at national scale was firstly carried out to evaluate the relevance of the problem by using the data stored in the Italian database and catalogue of co-seismic effects and those reported in the database of Seismic Microzonation. This analysis allowed to create a database of Italian co-seismic cavities instabilities, to identify the most susceptible zones in Italy and to preliminarily evaluate some elements of screening. Afterwards, the problem was addressed on an urban scale by proposing a possible procedure to be adopted within the Italian Guidelines for Seismic Microzonation to identify areas potentially prone to seismic cavities collapse.Currently, the Italian Guidelines for Seismic Microzonation do not address this kind of co-seismic effect by means of specific guidelines as for other instabilities (i.e., liquefactions, landslides, active and capable faults, seismic densification), although underground cavities are regularly reported in the studies. Therefore, the proposed methodology represents a first attempt to define the requirements of Seismic Microzonation studies in areas potentially prone to this kind of co-seismic instability.According to the SM-Working Group (2008), the proposing procedure defines three levels of zonation, i) attention zones, ii) susceptible zones and iii) respect zones, corresponding to three different cavity seismic instability-prone zones for which, an increasing knowledge levels of the cavity and the geological/geotechnical conditions of the surrounding rock\soil from one hand, and increasing accuracy of the methods adopted to assess the seismic susceptibility of cavity from another, are expected.The proposed procedure includes for the level 1 of SM a preliminary screening of some predisposing and triggering factors to define the attention zones, while at level 3 of SM the introduction of a simplified approach based on the use of abacuses to preliminary quantify the cavity seismic instability, and an advanced approach to define the respect zones by means of specific numerical analysis.

Multiscale geothematic maps for using the database from the Italian Seismic Microzonation Project: an example of application in the Calabria Region (Southern Italy)

This work is part of a broader project that aims to define a methodology for assessing the local seismic response on a large scale through a series of cognitive phases. Here we present the first phase that illustrates a procedure for the creation of standardised geothematic maps, at a national, regional and sub-regional scale, showing the geological and engineering-geological features that potentially induce stratigraphic amplification of the ground seismic motion. The main goal is to create a national reference legend to adopt, at different scales and detail, for complete coverage of the national territory. The method consists of consecutive steps that can be followed to obtain more detailed documents, starting from national scale geological maps, taking into account all the possible geological contexts characterising the Italian Peninsula. The first step is the reclassification of the lithostratigraphic units from the most detailed, reliable and updated available geological maps, in order to compose homogeneous geological and engineering geological maps at national scale. The construction of the legend and the elements contained in it plays a fundamental role in the process and constitutes the reference for more detailed maps at larger scales. We consider the age and the genetic domain to identify 10 main geological categories and from these, considering the prevailing lithological characters, to distinguish 27 geological groups. The engineering-geological map at national scale derives from the combination of the geological groups with the engineering-geological groups, representing the main characters of the cover terrains on the base of the main grain size and the geological substratum categories, adopted in the seismic microzonation studies (Technical Commission for Seismic Microzonation, 2020). At regional and sub-regional scale, we reclassified a more detailed geological map to convert it in an engineering-geological map of territorial context, characterised by the engineering-geological units adopted in the seismic microzonation studies of Italy. In order to reconstruct all the possible stratigraphic columns in term of the engineering-geological units in the territorial context representative of the shallower subsoil (< 50 m in depth), we proposed a representation in matrix form that combines the outcropping engineering-geological units with the units of the subsoil. Where seismic microzonation studies are not available, the multiscale engineering-geological maps could represent a useful tool for the seismic hazard assessment for the territorial planning at regional and sub-regional scale. Furthermore our results will be able to use the data of dynamic parameters (i.e., the shear wave velocity Vs) stored in DB-SM (National Seismic Microzonation Database, www.webms.it), in order to characterise the engineering-geological units of the reconstructed successions. Despite the different observation scales of the national SM project, the proposed methodology can provide rules for applying the SM standards with the aim of carrying out a “review” of the studies already completed and directing those in the implementation phases towards homogeneity. At international level this methodology could be taken as a reference for planning SM studies.

Developing effective subsoil reference model for seismic microzonation studies: Central Italy case studies

Developing effective subsoil reference model for seismic microzonation studies: Central Italy case studies

Correction to: The possible use of equivalent homogeneous subsoil models for 1D seismic response analyses in seismic microzonation studies

Correction to: The possible use of equivalent homogeneous subsoil models for 1D seismic response analyses in seismic microzonation studies

The possible use of equivalent homogeneous subsoil models for 1D seismic response analyses in seismic microzonation studies

The possibility is here explored to use an ‘equivalent’ homogeneous configuration to simulate 1D seismic response of heterogeneous engineering-geological bodies when relatively weak seismic impedance contrasts internal to the bodies (it was assumed a shear wave velocity variation between the alternating layers equal to 150 m/s) only exist above the seismic bedrock. This equivalent configuration is obtained by considering an equivalent Vs value the harmonic average of the actual Vs values and a linear combination of G/G0 and D curves relative to the lithotechnical components present in the actual configuration. To evaluate feasibility of this approach, a wide set of numerical simulations was carried out by randomly generating subsoil layering including sequences of alternating thin layers of geotechnical units (e.g. sands and clays) each characterized by a characteristic nonlinear curve. Outcomes of these simulations are compared with those provided by considering a single homogeneous layer characterized by equivalent nonlinear curves obtained as a weighted average of the original curves. By comparing the heterogeneous and the homogeneous columns seismic response in terms of amplification factors and fundamental period, the results confirm the possibility to model a 1D column characterized by a generic lithostratigraphic succession with an equivalent one without introducing significative errors that, at least for the studied cases, do not exceed the 6%. This conclusion is substantially confirmed by extending the comparison to a real case, i.e., the 113 m-thick heterogeneous soil profile at Mirandola site (Northern Italy), presented in the last part.

Seismic microzonation of a region with complex surficial geology based on different site classification approaches

A seismic microzonation study was conducted to refine the seismic hazard model for the city of Saguenay, Canada. The Quaternary geology underlying Saguenay shows complex glacial and post-glacial stratigraphy with a number of buried valleys filled with fluvioglacial and glaciomarine sediments. High impedance contrast between rock formations and surficial sediments is prone to seismic amplification. To evaluate their applicability, advantages and limitations in capturing the geological specificity of the study area, four site classification methods were applied: the current National Building Code of Canada (NBCC) and Eurocode 8, both mainly based on the average shear-wave velocity for the surficial sediments (VS,avg) and for the top 30 m (VS,30); a method based on the fundamental site period (T0); and a hybrid method based on the combination of VS,30, T0 and VS,avg. The study specifically aimed to evaluate the importance of the site classification parameters on the resulting microzonation maps. VS,30 is capable to present the geological and geotechnical site conditions, however, the results may be further improved by considering Vs,avg in shallow and T0 in thick layers of soil sediments as secondary parameters. The T0 method gives also satisfactory results with T0 showing a better correlation to Vs,30 than to Vs,avg. The versatile hybrid method may be challenging to apply in certain cases with its nine different site categories and parameters.