Corso di Dottorato Regionale in Scienze della Terra
PhD Course in Earth Sciences
XXX Ciclo – 30th Cycle 2014 – 2017
Giacomo Peruzzi
Extensive Seismic Response Analysis by passive seismic methods
and numerical modelling
Tutore - Supervisor: Prof. Dario Albarello
Coordinatore del Corso di Dottorato Prof. Carlo Baroni
Extensive Seismic Response Analysis by passive seismic
methods and numerical modelling
Index
0. Extended Summary ... 4
1. Introduction... 10
2. Tools for Level II seismic microzoning studies... 14
2.1. The extensive assessment of amplification factors in Level II seismic microzonation studies... 14
2.1.1. Simplified Tools for computing 1D seismic response... 14
2.2. Abacuses for Tuscany... 16
2.2.1. Parameterization ... 17
2.2.2. Numerical Simulations... 19
2.2.3. Identification of characteristic parameters to identify representative AF values... 20
2.2.4. Statistical analysis... 21
2.2.5. Final considerations... 24
2.3. Limitations in the use of 1D local seismic response and abacuses: 2D effects... 25
2.3.1. The modelling ... 25
2.3.2. Comparison of different calculation codes... 26
2.3.3. Description of the performed simulations ... 31
2.3.4. Final considerations... 40
3. The role of vertical components in seismic response analysis... 41
3.1. Importance of vertical components of seismic hazard... 41
3.1.1. Input data ... 42
3.1.2. Numerical tools... 43
3.1.3. Numerical results... 44
3.1.4. Consideration on results... 48
3.2. Approximate approaches for assessing seismic response vertical components... 49
3.2.1. Observations and some final considerations... 54
4. Seismic Microzonation and deconvolution analysis: case studies ... 55
4.1. Level I Seismic Microzonation of San Gimignano (SI) ... 55
4.1.1. Geological and geomorphological constraints ... 55
4.1.4. Final considerations... 63
4.2. Level II Seismic Microzonation of Montecatini (PT)... 65
4.2.1. The geophysical survey... 65
4.2.2. The new geological model... 79
4.2.3. The determination of the Amplification Factors (AF)... 80
4.3. Level III Seismic Microzonation of Montegallo (AP) ... 83
4.3.1. Introduction... 83
4.3.2. Local seismic response analysis for level II and level III Seismic Microzonation ... 85
4.3.3. Some preliminary results... 86
4.4. Seismic characterization of RAN accelerometric sites and deconvolution analysis ... 91
4.4.1. Geological surveys and seismic prospecting at the accelerometric sites... 92
4.5. Deconvolution analysis ... 97
4.5.1. An example: the Peglio accelerometric site ... 97
4.5.2. Final considerations... 102
5. Passive seismic measurements supporting seismic risk assessment ... 103
5.1.1. SSR Technique... 103
5.2. Case study 1: San Gimignano Towers ... 104
5.3. Case study 2: San Marino Towers ... 106
5.3.1. The towers ... 106
5.3.2. Results... 110
6. Conclusions... 116
7. References... 117
Appendix
1 - Basin models used for 2D local seismic response simulations
2 - Seismic layering at the sites considered for the numerical analysis in Figure 26 3 - Earthquakes selected for seismic response analysis at the sites in Figure 27 4, 5, 6, 7, 8, 9, 10 - Level II Seismic Microzonation of Montecatini T.me
0. Extended Summary
Seismic hazard assessment (i.e., a forecast of seismic ground motion expected in the next tens of years in the Italian territory) represents a main goal of seismological studies since it represents the basic element for planning risk reduction policies by addressing available resources where they are more urgent and where their effectiveness can be maximized. Well-consolidated approaches exists for seismic hazard assessment at regional scale, whose outcomes constitute a general reference for anti-seismic design of structures. On the other hand, recent earthquakes occurred in Italy show that seismic effects show a high degree of lateral variability at the scale of hundreds to thousands of meters (local seismic response) that cannot be captured by regional scale hazard maps. This makes mandatory and urgent the definition of local hazard maps (at the scale of a single municipality) providing a more realistic picture of expected seismic ground motion. This poses a number of methodological problems that are generally ignored when regional scale hazard maps are developed. In fact, in these last maps, large scale propagation effects and geometry of seismic sources play a major role, while the effects of small-scale seismo-stratigraphical and morphological features is ignored by referring to a ‘standard’ configuration (rigid deposits) rarely found in actual situations. On the other hand, the former features are responsible for large modifications of ground motion at local scale and require very detailed survey procedures to be detected and characterized. To be effective, these last studies (Seismic Microzonation) must be performed extensively over wide areas in all municipalities exposed to potentially damaging earthquake. This prevents the application of costly procedures for subsoil exploration and requires the development of cheap and effective approaches allowing a general application of local seismic hazard assessment also where resources available on purpose are scarce. In the last years, on behalf of National Civil Protection and Regional Authorities, the Italian scientific community has been deeply involved in developing and applying such approaches, which have been initially formalized in the Italian Guidelines for Seismic Microzonation or IGSM (WGSM, 2008) providing guidelines for Seismic Microzonation. An important field application of these guidelines after the L’Aquila earthquake (2009) showed effectiveness and limitations of these guidelines and provided a new impulse to these studies. This evolution in presently going on and the studies summarized in this Thesis represent a contribution to improve and validate procedures and geophysical exploration tools on purpose developed for supporting and improving seismic microzoning studies.
The Thesis includes 6 chapters and respective contents reflect number of papers I published during the PhD activity.
After a short introduction (Chapter 1) and a general summary of main problems involved in the extensive application of seismic microzoning tools, in Chapter 2, a methodology is presented for the definition of an effective tool to estimate local amplification factors based on low-cost geophysical prospecting tools. In fact, an extensive application of seismic microzoning studies requires the definition of specific proxies to allow local practitioners to quantify litho-stratigraphical amplification phenomena on the basis of procedures simple enough to allow a widespread application. These tools should be specialized to the litho-stratigraphical configurations representative of the study area. A procedure is here described to provide such a tool based on extensive numerical simulations taking advantage of geological/geotechnical information made available by regional/national Authorities. This procedure is quite general and could be applied in several contexts and, in particular, in developing countries characterized by low seismicity rates or where extensive accelerometric databases are lacking. As an example, an application of the above procedure developed on behalf of the Tuscany Regional Administration (in Central Italy) is presented. In Chapter 3 an important methodological aspect is addressed, concerning the role of vertical ground motion components in seismic response analysis. This element has been generally discarded in common approaches but revealed to be of great importance during the seismic sequence that stroke the Po Plain in 2012. In fact, in the seismic codes currently applied, vertical components accounted for in anti-seismic design are considered to be a fraction of the horizontal ones and are supposed to play a minor role in the definition of damages expected in existing structures. However, observations carried out during last important seismic events in Italy (L’Aquila in 2009 and Emilia-Romagna in 2012) demonstrate that vertical components were larger than those expected on the basis of current hazard estimates and played an
important role in damaging specific structures (masonry buildings, churches, industrial warehouses, etc.) located in the epicentral area. In order to provide a more correct estimate of seismic hazard in the near field, numerical simulations have been carried out to explore features of the vertical ground motion at the top of a sedimentary cover overlying a buried seismogenic sources. Numerical modelling is considered to evaluate the possible effect of vertical ground motion components of input motion on the horizontal seismic response at the surface of a stack of homogenous sedimentary layers. This analysis has been performed at four Italian sites where the local Vs profile was available down to seismic bedrock. Computations show that the effect of vertical components on horizontal ground seismic response is frequency dependent and changes as a function of the local Vs profile and of the accelerometric time series. These outcomes suggest that the common practice of considering only horizontal components of input motion may result ineffective and provides in some cases of possible practical interest underconservative outcomes.
After that in Chapter 4 some case studies are presented, which respectively represent seismic microzoning studies performed on behalf of local and national Authorities at different levels by following the IGSM. The first example concerns the development of a preliminary Microzonation study (Level I, by following IGSM) developed in the frame of the project ‘Rischio Sismico negli Edifici Monumentali-RiSEM’ (Seismic Risk in Monumental Buildings). In this case, a seismo-geological reference model was developed for the historical centre of the town of San Gimignano (Central Italy) in order to evaluate possible small-scale lateral variation of seismic hazard. To this aim, an on-purpose geological and geomorphological survey was performed along with ambient vibration measurements (both in a single station and array configurations) to characterize seismic response in the study area. Expected seismic amplification effects were quantified by considering a simplified approach developed on purpose and described in Chapter 2. This study provides preliminary information supporting site-specific analyses of the local seismic response and makes it possible to identify most critical situations where eventual seismic retrofitting interventions are more urgent to reduce seismic risk.
A second example concerns a more advanced Microzonation study (Level II by following IGSM) relative to the municipality of Montecatini Terme (Central Italy). Here, a previous Microzonation study was improved by implementing a more detailed geophysical survey as required for the second level of microzonation. The third case study concerns a more advanced microzoning study (Level III by following IGSM) developed in support of seismic reconstruction in the area struck by the recent disastrous seismic sequence (2016-2107) in Central Italy. In that context, the Department of Physics, Earth and Environmental Sciences of the University of Siena was involved in the activities for the reconstruction project such as geophysical survey, but also for supporting the realization of level I, level II and level III Microzonation. In particular in the municipality of Montegallo (near Mount Vettore), a geological model was realized and the relevant geological sections were used to find reference seismostratigraphical log for each residential area. Each log was then used to estimate a one-dimensional seismic response and then to calculate some amplification factors. It was also used a two-dimensional seismic response analysis to evaluate possible topographic and stratigraphic amplification effects.
The fourth case study concerns an extensive study aiming at evaluating seismic response at accelerometric sites to retrieve ground motion at reference soil conditions by deconvolution analysis. To allow a generalized application to large areas where borehole data are generally lacking or appear inadequate for the seismic characterization for soils down to the reference seismic bedrock, cost-effectiveness of the considered procedures is a main issue. Thus, major efforts have been devoted to optimize available information and exploit fast and cheap surface geophysical prospecting. In particular, geological/geomorphological survey and passive seismic prospecting (both in single and multi-station configurations) were jointly considered to reconstruct seismo-stratigraphical site conditions. This information was then used to feed numerical modelling aiming at computing the local seismic response and performing a deconvolution analysis to reconstruct ground motion at reference soil conditions. Major attention was devoted to evaluate and manage uncertainty involved in the procedure and to quantify its effect on final outcomes. An application of this procedure to a set of sites included in the Italian Accelerometric Network is presented.
In Chapter 5, a step is made in the direction of a more comprehensive estimate of seismic effects by considering the possible impact of ground motion on structures. In fact, the expected soil-structure interaction is very important in evaluating seismic risk, because damages enhance when dynamic properties of buildings induce resonance phenomena at frequencies corresponding to those locally amplified by subsoil configuration. Thus, beyond information about seismic response of the local subsoil, retrieving information about dynamic characteristics of building exposed to future events is of paramount importance to identify most critical situations. To this purpose, passive seismic methods may be of great importance due to the relative easiness of their application in several contexts. In this Thesis, two case studies are presented concerning two historical hamlets, which are famous for their historical buildings (in particular for their towers) and they are both included in the UNESCO World Heritage List. The sites are located in regions characterized by medium level of seismic hazard. Therefore, mitigating earthquake damages is an important goal for conservation of historical buildings. From an economic point of view, this issue is fundamental in order to sustain tourism, which is a relevant source of local economy. A first step in this direction is the evaluation of dynamic response to seismic loads at least in the domain of small strain levels corresponding to the beginning of damage. Several recent studies demonstrate that this task can be achieved efficiently by using suitable single station asynchronous ambient vibration measurements.
The first case study is performed in San Gimignano (Tuscany, Central Italy). In order to improve the knowledge about the seismic risk of this cultural heritage, the Regione Toscana (Tuscany Regional Administration) promoted the RISEM project (RIschio Sismico negli Edifici Monumentali - Seismic Risk in Monumental Buildings), involving the universities of Florence and Siena. A part of this project was devoted to the estimate of possible damages expected at each tower as a consequence of possible future earthquakes. In this frame, ambient-vibration monitoring was used along with measurements inside the buildings to contribute to evaluate dynamical response of the towers. The second case study was the San Marino Historical centre. The same technique described in the previous case has been applied to three medieval towers located in San Marino and allowed identifying fundamental (elastic) resonance frequency, a key parameter for assessing seismic behaviour of historical buildings.
Un’analisi estensiva della risposta sismica locale attraverso
tecniche di sismica passiva e modellazione numerica
La valutazione della pericolosità sismica (ovvero la previsione del moto sismico del terreno atteso nelle prossime decine di anni nel territorio italiano) rappresenta l’obiettivo principale degli studi sismologici, poiché è l'elemento base per pianificare le politiche di riduzione del rischio sismico, impiegando le risorse disponibili dove c’è maggiore necessità e dove la loro efficacia può essere massimizzata. Esistono approcci ben consolidati per la valutazione della pericolosità sismica su scala regionale, che costituiscono un riferimento generale per la progettazione antisismica delle strutture. D'altra parte, i recenti terremoti verificatisi in Italia hanno mostrato che gli effetti sismici hanno un alto grado di variabilità laterale alla scala di centinaia e di migliaia di metri (si parla di ”Risposta Sismica Locale”), che non può essere ben descritta da mappe di pericolosità a scala regionale. Ciò rende obbligatoria ed urgente la definizione di mappe di pericolosità locale (a scala comunale) che forniscano un quadro più realistico del moto sismico previsto. Ciò pone una serie di problemi metodologici, che sono generalmente ignorati quando vengono sviluppate mappe di pericolosità su scala regionale. Infatti, in queste ultime giocano un ruolo importante gli effetti di propagazione su larga scala e la geometria delle sorgenti sismiche, mentre gli effetti delle caratteristiche sismo-stratigrafiche e morfologiche a piccola scala vengono ignorati, facendo riferimento ad una configurazione "standard" (suolo rigido e pianeggiante) raramente assimilabile a situazioni reali. D'altra parte, le caratteristiche sismo-stratigrafiche e morfologiche posso essere responsabili di grandi variazioni del movimento del terreno a scala locale e richiedono procedure di rilevamento molto dettagliate per essere valutate e caratterizzate. Per essere efficaci, questi ultimi studi (detti di ‘Microzonazione Sismica’) devono essere eseguiti estensivamente su vaste aree, comprendenti tutti i comuni esposti a terremoti potenzialmente dannosi. Ciò rende difficile l'applicazione di procedure costose per l'esplorazione del sottosuolo e richiede quindi lo sviluppo di approcci a basso costo per unità di volume di sottosuolo esplorato che consentano un'applicazione anche laddove le risorse disponibili sono scarse. Negli ultimi anni, grazie all’impegno del Dipartimento della Protezione Civile della Presidenza del Consiglio dei Ministri ed alcune Autorità regionali, la comunità scientifica italiana è stata profondamente coinvolta nello sviluppo e nell'applicazione di tali approcci, che sono stati inizialmente formalizzati negli Indirizzi e Criteri per la Microzonazione Sismica o ICMS (WGSM, 2008) ovvero delle linee guida per la realizzazione di studi di Microzonazione Sismica. Queste linee guida hanno trovato un'importante applicazione sul campo dopo il terremoto de L'Aquila (2009), che ne ha mostrato l’ efficacia ma anche i limiti, fornendo allo stesso tempo un nuovo impulso a questo tipo di studi. Questa evoluzione è tutt’ora in corso e gli studi riassunti in questa Tesi rappresentano un contributo per migliorare e convalidare le procedure e gli strumenti di esplorazione geofisica appositamente sviluppati per sostenere e migliorare gli studi di Microzonazione Sismica.
La Tesi comprende 6 capitoli, i contenuti dei quali riflettono alcuni articoli che ho pubblicato durante l'attività di dottorato.
Dopo una breve introduzione (Capitolo 1) ed una sintesi generale dei principali problemi coinvolti nell'applicazione estensiva degli strumenti di Microzonazione Sismica, nel Capitolo 2 è trattata una metodologia per la definizione di uno strumento efficace per la stima dei fattori di amplificazione locali basato su tecniche di prospezione geofisica a basso costo. Infatti, un'estesa applicazione degli studi di Microzonazione Sismica richiede la definizione di specifici approcci speditivi (proxy) per consentire ai professionisti locali di quantificare i fenomeni di amplificazione litostratigrafica sulla base di procedure abbastanza semplici da consentirne un'applicazione diffusa. Viene descritta una procedura volta alla realizzazione di uno di questi strumenti e basata su estese simulazioni numeriche e sfruttando le informazioni geologiche e geotecniche rese disponibili dalle autorità regionali e nazionali. Tale procedura è abbastanza generale e potrebbe trovare applicazione anche in diversi contesti e, in particolare, nelle aree a bassa sismicità dei paesi in via di sviluppo, o dove manchino estesi database accelerometrici. Come esempio, viene presentata un'applicazione della procedura sopra descritta, che è stata sviluppata per conto della Regione Toscana.
trascurato nelle comuni metodologie di analisi. In particolare, nella normativa sismica vigente, la componente verticale considerata nella progettazione è stimata come una frazione di quella orizzontale, presumendo così che svolga un ruolo minore nella definizione dei danni attesi. Tuttavia, le osservazioni effettuate durante gli ultimi importanti eventi sismici in Italia (L'Aquila, 2009; Emilia-Romagna, 2012) dimostrano come le componenti verticali del moto siano state maggiori di quelle attese sulla base delle attuali stime di pericolosità ed abbiano svolto un ruolo importante nel danneggiamento di certi tipi di strutture (edifici in muratura, chiese, magazzini industriali, ecc.) situate nell'area epicentrale. Al fine di fornire una stima più corretta della pericolosità sismica (in particolare nelle aree epicentrali dove l’entità della componente verticale del moto sismico è presumibilmente maggiore), sono state effettuate simulazioni numeriche per esplorare il possibile effetto del moto verticale del terreno alla base delle coperture sedimentarie sulle componenti orizzontali del moto alla superficie Questa analisi è stata eseguita in quattro siti italiani, per i quali si disponeva del profilo locale di Vs fino alla profondità del substrato sismico. I calcoli svolti mostrano che l'effetto delle componenti verticali sulla risposta sismica orizzontale al suolo dipende dalla frequenza e cambia in funzione del profilo locale di Vs e degli accelerogrammi usati. Questi risultati suggeriscono che la pratica comune di considerare solo le componenti orizzontali del moto di sismico alla base di una coltre sedimentaria può produrre una sottostima degli effetti attesi in superficie. Nel Capitolo 4 sono successivamente presentati alcuni casi studio, che rappresentano studi pilota di Microzonazione Sismica svolti a diversi livelli di approfondimento per conto delle autorità locali e nazionali, seguendo gli ICMS.
Il primo esempio riguarda lo sviluppo di uno studio preliminare di Microzonazione (Livello I, secondo gli ICMS) sviluppato nell'ambito del progetto Rischio Sismico negli Edifici Monumentali (RiSEM). In questo studio è stato sviluppato un modello geologico e sismico di riferimento per il centro storico della città di San Gimignano (in Toscana), al fine di valutare possibili variazioni laterali di pericolosità sismica su piccola scala. A tale scopo, è stato eseguito un rilevamento geologico e geomorfologico di dettaglio, insieme a numerose misure di vibrazioni ambientali (sia nella configurazione a stazione singola, sia con antenne) per la determinazione del profilo sismostratigrafico locale fino al substrato sismico. Gli effetti di amplificazione sismica attesi sono stati quantificati considerando un approccio semplificato sviluppato in Italia per la Microzonazione Sismica a scala comunale (di cui è stato accennato relativamente al capitolo 2). Questo studio ha fornito informazioni preliminari a supporto di analisi sito - specifiche della risposta sismica locale ed ha consentito di identificare le situazioni più critiche in cui eventuali interventi di adeguamento sismico sono più urgenti per ridurre il rischio sismico.
Un secondo esempio riguarda uno studio di microzonazione più avanzato (Livello II seguendo gli ICMS), che riguarda il comune di Montecatini Terme (sempre in Toscana). Qui era già stato sviluppato un precedente studio di microzonazione, ma per il secondo livello si è resa necessaria una campagna di misure geofisiche più dettagliata che permettesse la definizione di un profilo sismostratigrafico su ciascuna delle configurazioni presenti e, su questa base, fornire una stima del livello di amplificazione sismica attesa mediante l’approccio semplificato descritto in precedenza.
Il terzo caso studio riguarda una Microzonazione Sismica ancora più avanzata (Livello III, secondo gli ICMS) sviluppata a supporto della ricostruzione nell'area colpita dalla recente sequenza sismica (2016-2107) dell'Italia Centrale. Al termine della sequenza, il Dipartimento di Scienze Fisiche, della Terra e dell'Ambiente dell'Università di Siena è stato coinvolto nelle attività per il progetto di ricostruzione, tra le quali il rilevamento geofisico, ma anche a supporto della realizzazione dei livelli I, II e III di Microzonazione Sismica. In particolare, nel comune di Montegallo (nelle Marche, vicino al Monte Vettore) è stato realizzato un modello geologico di dettaglio, in cui le sezioni geologiche sono state utilizzate per identificare la colonna sismostratigrafica di riferimento per ogni zona residenziale. Ogni colonna stratigrafica è stata anche utilizzata per stimare una risposta sismica locale monodimensionale e per calcolare i fattori di amplificazione da attribuire alla zona indagata. È stata anche eseguita un'analisi di risposta sismica bidimensionale per identificare eventuali effetti indotti dalle variazioni morfologiche della superficie del terreno e delle eterogeneità litostratigrafiche presenti nel sottosuolo.
Il quarto esempio riguarda invece un ampio studio volto a valutare la risposta sismica in alcuni siti dove sono presenti le stazioni accelerometriche della rete italiana (RAN). Lo studio ha avuto lo scopo di ricavare il moto del terreno al substrato sismico di riferimento a partire dalle registrazioni in superficie mediante analisi numeriche di deconvoluzione. Data la necessità di operare su aree vaste laddove dati geologici sono generalmente carenti o inadeguati per la caratterizzazione sismica dei terreni fino al substrato sismico, un elemento chiave è stato la messa a punto di procedure caratterizzate da un buon rapporto costo-efficacia. Sono stati quindi compiuti notevoli sforzi per ottimizzare le informazioni disponibili e sfruttare prospezioni geofisiche di superficie veloci ed economiche. In particolare, il rilevamento geologico e geomorfologico e le indagini di sismica passiva (sia nelle configurazioni a stazione singola, sia con antenne sismiche) sono state considerate congiuntamente per ricostruire le condizioni sismostratigrafiche del sito. Questa informazione è stata quindi utilizzata per fornire dati per la modellazione numerica, allo scopo di calcolare la risposta sismica locale e di eseguire un'analisi di deconvoluzione, ricostruendo il moto del terreno al substrato sismico affiorante. Una grande attenzione è stata dedicata alla valutazione ed alla gestione delle incertezze coinvolte nella procedura ed alla quantificazione del loro effetto sui risultati finali.
Nel Capitolo 5, viene compiuto un ulteriore passo nella direzione di una stima più completa degli effetti sismici, considerando le possibili conseguenze del movimento del terreno sulle strutture. Infatti, nella valutazione del rischio sismico, ciò che è importante è l'interazione terreno-struttura ed è possibile prevedere che i danni aumentino quando le proprietà dinamiche degli edifici inducono fenomeni di risonanza a frequenze corrispondenti a quelle localmente amplificate dalla stratigrafia del sottosuolo. Pertanto, al di là delle informazioni sulla risposta sismica del sottosuolo, il recupero delle informazioni sulle caratteristiche dinamiche dell'edificio esposto a probabili eventi sismici è di fondamentale importanza per identificare le situazioni più critiche. A tale scopo, i metodi sismici passivi possono essere molto importanti per la facilità della loro applicazione in vari contesti. In questa tesi vengono presentati due casi di studio riguardanti due centri storici, famosi per i loro edifici di alto valore culturale ed in particolare per le loro torri: San Gimignano e San Marino, entrambi inclusi nella lista del patrimonio UNESCO dell’umanità ed entrambi in aree caratterizzate da una pericolosità sismica media. La mitigazione dei danni dei terremoti è pertanto un obiettivo importante per la conservazione degli edifici storici. Inoltre da un punto di vista economico, questa questione è fondamentale per sostenere il turismo, di notevole importanza per l’economia locale. Anche se l'adeguamento sismico di edifici antichi è un compito particolarmente complesso, un primo passo in questa direzione è la valutazione della risposta dinamica ai carichi sismici, almeno nel dominio dei piccoli livelli di deformazione corrispondente all'inizio del danno. Diversi studi recenti hanno dimostrato che questo compito può essere raggiunto in modo efficiente utilizzando misure di vibrazioni ambientali asincrona a stazione singola.
Il primo caso studio riguarda San Gimignano (in Toscana). Per migliorare la conoscenza del rischio sismico di del suo patrimonio culturale, la Regione Toscana ha promosso il progetto RiSEM (RIschio Sismico negli Edifici Monumentali), coinvolgendo le Università di Firenze e Siena. Una parte del progetto è stata dedicata alla stima dei possibili danni attesi a ciascuna torre in conseguenza di possibili futuri terremoti. In questo contesto sono state eseguite misurazioni di vibrazioni ambientali al’interno degli edifici per contribuire a valutare la risposta dinamica delle torri. Il secondo caso studio è stato il centro storico di San Marino, anch’esso incluso nella lista del patrimonio mondiale dell'UNESCO. In questo caso le tecniche già citate sono state applicate alle tre torri medievali ed hanno permesso di identificare la frequenza di risonanza fondamentale (elastica), un parametro chiave per la valutazione del comportamento sismico degli edifici storici.
1. Introduction
Providing information about future earthquakes is one of the major goals of seismological research and is the basis for effectively promoting preparedness of local communities (small and large) to cope with their possible effects. Beyond the adoption of effective rules for anti-seismic design of new buildings, coping with the effects of future earthquake requires:
o planning and supporting building retrofitting focusing on most critical situations,
o developing anti-seismic city plans aiming at reducing the level of exposure in the most
hazardous areas,
o developing emergency plans calibrated on the specific situations met in the area of concern. These actions are costly (both in terms of direct costs and of lack of incomes due to limitations in the use of land) and require a long times to be completed. Thus, to be affordable and sustainable, such activities require the consensus of involved populations, political Authorities and stakeholders. This implies that hazard assessment cannot be considered a merely scientific problem and its strong political and social implications should not be ignored by scientists and technicians involved in the assessment.
In general, hazard assessment for design is performed at “national” scale (Figure 1). This kind of estimate accounts for the distribution and level of activity of seismogenic sources and of long-range seismic wave propagation pattern. On the other hand, seismic hazard is inherently «local» since events are essentially experienced at the scale of small communities. This is mainly true in countries, such as Italy, where a huge amount of distributed settlements exist, each characterized by small dimensions and strong historical identity. This last view of the earthquake should not be ignored by scientists, which, instead, tend to have a global view of earthquakes that, along with other communication troubles (e.g., Albarello et al., 2015), may hamper the correct communication of hazard to the population exposed to the future earthquakes.
Recent experiences in Italy (see, e.g., Cultrera et al., 2011 and references therein) put in evidence that hazard is «local» also from the seismological point of view. It is well known in fact, that seismic waves interact with «local» geomorphological and seismo-stratigraphical features by proving strong heterogeneities in observed seismic effects (e.g., Kramer, 1998). In particular, two groups of effects can be expected respectively relative to stable and unstable soil conditions. The first group concerns transient phenomena (e.g., seismic resonance) able to enhance the local seismic ground motion. The second group includes induced soil instability having permanent effects on the ground configuration (landslides, liquefaction, etc.).
Amplification under “stable conditions” is the effect of the interference of seismic waves (mainly Vs phases) trapped within geological bodies bounded by large seismic impedance contrasts (soft soil/bedrock, soil/free surface, etc.) irrespective to the absolute impedance values. The dimension of geological bodies and discontinuities to be analyzed for characterizing the relevant phenomena are of the order of the seismic wavelengths responsible for resonance of man-made structures. By considering that most buildings are characterized by natural oscillation frequency in the range 0.8-5 Hz, and that shear waves are responsible for most damages in the epicentral area, a rough estimate of the actual dimensions “local scale” R can be provided in the form
R≈Vs/ν
where Vs is the average shear wave velocity in the shallow subsoil (the uppermost 30 m to say) and ν is the natural vibration frequency of the structures of possible interest. By assuming a typical Vs value around 200 m/s, one can estimate the “local scale” R in the range 50 - 250 m: eventual seismostratigraphical heterogeneities in this dimensional range may be responsible for effects to be accounted for when prevention actions or anti-seismic design are planned.
In this situation, regional scale hazard assessment cannot be considered as a satisfactory basis for risk reduction activities. This, of course, implies that a very detailed knowledge of subsoil and local geomorphological situation is mandatory to provide effective seismic hazard assessment. This is the target of seismic studies focusing on configurations (at the scale of few hundreds of meters) potentially responsible for the local enhancement of expected seismic ground motion.
On the other hand, to be useful in reducing the impact of future earthquakes, hazard assessment should focus on the huge amount of small-medium dimension villages (thousands inhabitants) and small towns (less than hundred thousand inhabitants) that characterize the Italian (and European) territory. The basic political and administrative “unit” of these settlements is the Municipality (more than 8100 in Italy): at this scale, prevention actions can be actually managed by also directly involving resident people. It is worth to note that such a “local” hazard assessment (Seismic Microzonation) is inherently different from the Seismic Response Analysis typical of seismic codes (e.g., Eurocode 8, NTC08). In fact, Seismic Microzonation is extensive in character since it focuses on entire settlements (while Seismic Response Analysis is intensive in that it mainly focuses on single buildings). On the other side, Seismic Microzonation represents a basic tool for planning prevention activities and land management and does not aim (at least not primarily) to support the design of single structures. This suggests that Seismic Microzonation requires specific methodologies and approaches to warrant its feasibility and effectiveness. In particular, it must:
o be cost effective (to be applied over wide areas),
o be operated by practitioners and public technicians (one cannot expect that Academic and
Research institutions may be charged of this task when thousands of settlements are of concern),
o be technically effective (main expected phenomena must be captured in the analysis),
o provide outcomes useful for seismic risk reduction and effectively applied in city management
and emergency planning.
In the last years, many efforts have been devoted in Italy to develop a coherent approach to Seismic Microzonation. Although seismic microzonation studies were performed in many countries (USA, Japan,
Microzonation or IGSM (WGSM, 2008) have strong peculiarities. First of all, they have been developed having in mind their extensive application also in the lack of strong financial supports. Economical sustainability of the microzoning studies is a basic element to warrant its actual large-scale application. Furthermore, since most part of the work is expected to be performed by practitioners with a minor role universities or research centres, most diffuse field procedures and analytical approaches have been preferred to more advanced techniques. These last ones are only reserved to most critical or complex situations. As a third, maximum exploitation of information provided by local technical Authorities are strongly encouraged. These basic elements represent the backbone of the whole procedure.
To fit these requirements, a modular structure has been adopted for IGSM, mimicking the one previously proposed in the TC4 manual (1999). In particular, three levels have been indicated that are characterized by growing in-depth analysis (and costs). In the first level, a reference geological model is built based on available information (borehole data when available), new geological/geomorphological surveys and low-cost geophysical measurements (ambient vibrations, resistivity, gravity, etc.). This model will represent the basis for all the subsequent analyses and is semi-qualitative in essence. The main outcome of the level I Microzonation is the identification of “homogenous micro-zones in the seismic perspective” (MOPS, using the Italian acronym) that are areas characterized by the same seismic effects. In particular, three kinds of situations are identified:
1. Stable areas: where no permanent modifications of the soil configuration (lands slope instability, liquefaction, surface faulting) is expected due to expected earthquakes; in these areas, the reference ground motion is the one provided by national reference hazard map accounting for source and large scale propagation phenomena; stable area corresponds to “reference” soil conditions.
2. Stable areas characterized by amplification of the ground motion due to the local litho-stratigraphic or morphological configuration.
3. Unstable areas: where earthquakes may produce permanent modification of the soil.
The second level aims at providing the first level map with quantitative elements. In particular, as concerns stable areas susceptible to seismic amplification, any quantification of effects induced by local stratigraphy or morphology must be supplied. Since it is expected that practitioners operating in the study area will provide this quantification, simplified tools are provided to compute expected amplification values. Briefly, these tools for a simplified local seismic response analysis are tables of correspondences (seismic “abacuses” i.e., literally, “tables for computation”) between “soil classes” (representative of seismo-stratigraphical/morphological configurations present in the area where the abacus will be applied) and any parameterization of the expected amplification effects. The user, based on a small number of informative elements concerning geological features or seismological parameters, is required to determine the soil class of concern. However, this tool only allows estimate of amplification factors in specific situations, i.e., when seismic heterogeneities only exist in the vertical direction (one dimensional configuration) and cannot be used in the presence of sharp lateral variations in the subsoil seismic configurations or when rough surface morphologies are present.
While the first and the second levels are defined for a widespread application, the third level only concerns specific situations (presence of sharp lateral heterogeneities, 2D-3D subsoil configurations, etc.) and strongly non-linear seismic phenomena (liquefaction, seismically induced landslide, surface faulting, etc.). The application of third level studies needs higher professional and economical resources that can be afforded only in specific situation where seismic risk is particularly high.
In this case, more advanced (and expensive) surveys are required for a more detailed characterization of subsoil properties (including borehole sampling, laboratory experiments, the application of advanced numerical tools form managing 2D-3D heterogeneities). However, this approach becomes mandatory only when actually necessary and is not required to cover the entire area of concern.
work-in-progress and that IGSM only represent single steps towards a more comprehensive approach. In fact, field activities are continuously suggesting possible improvements and adjustments required to face with specific situations not expected at the beginning or reducing the effects of possible biases induced applications of simplified approaches in complex contexts. Some of these improvements have been published after the main document (e.g., WGSMLA, 2010; Various Authors, 2011) but they only cover a small part of more troublesome situations. In this Thesis, some important problematic aspects of Seismic Microzonation have been explored.
In the second chapter, a procedure is illustrated to define robust abacuses to be used for Level II Microzoning studies. The propose approach is more extensively applicable than the one originally proposed in IGSM and has been actually included in the seismic rules emanated by several regional authorities in Italy (Tuscany, Marche, Apulia). In the same chapter, the limits of these abacuses have been also explored by considering situations where the existence of lateral variations of subsoil properties may give wrong estimates of amplification effects provided by the abacuses.
In the third chapter, an important aspect generally ignored in the study of local amplification phenomena (both in microzonation and seismic response analysis) is addressed, that is the role of vertical input ground motion components (i.e., the reference ground motion at the bottom of the sedimentary cover) in the definition of the horizontal ground motion at the surface of the sedimentary layer. It is demonstrated that ignoring this effect (as occurs in the common practice) may provide severe underestimates of local seismic response. In the same chapter a preliminary attempt is introduced to estimate vertical components, by applying a procedure developed by Zaho and Horike, 2003.
In the fourth chapter, the feasibility of IGSM has been explored by considering its extensive application in three different contexts by considering the three levels of microzonation. In the same chapter, microzoning approach and local seismic response studies are jointly applied to face the problem of reconstructing input motion from earthquake recorded at the surface of a sedimentary cover (deconvolution) at a number of accelerometric sites.
In the fifth chapter an attempt is performed to push onward the application of a typical microzoning tool (ambient vibration measurement) by preliminarily evaluating resonance properties of buildings. This could allow a very fast and cheap way to evaluate extensively seismic risk (at least in the risk component relative to building damages) by rapidly identifying most critical situations where double-resonance phenomena (soil and building resonance in the same frequency band) may occur.
Many of the arguments discussed in the following have been the subject of a number of scientific publications (Peruzzi et al., 2013; Lunedei et al., 2014, 2015; Paolucci et al., 2015; Peruzzi et al., 2016a,b; Peruzzi and Albarello, 2017; Peruzzi et al., 2017a,b; Guerra et al., 2017). What is reported here is in most cases just a summary of what has been actually published.
2. Tools for Level II seismic microzoning studies
2.1. The extensive assessment of amplification factors in Level II seismic microzonation studies
As stated previously the second level of microzoning studies aims at providing the first level map with quantitative elements. In fact these tools are useful for a simplified local seismic response analysis are tables of correspondences between “soil classes” (representative of seismo-stratigraphical/morphological configurations present in the area where the abacus will be applied) and any parameterization of the expected amplification effects.
Of course, such kind of tool cannot be expected to provide reliable results where complex situations (2D-3D resonance phenomena, etc.) exist. Thus, areas where these kind of phenomena are expected are excluded from this analysis and demanded to a further level of analysis (third level). When applicable, second level analysis estimates seismic amplification in terms of a synthetic integral parameter (the Amplification Factor, thereafter AF) which defines the amount of seismic amplification expected in the relevant site area for any range of vibration periods of interest for anti-seismic design. This simple parameterisation may result acceptable because it only aims at providing a relative estimate of expected effects over wide areas. To be effective, these abacuses should be representative of the specific litho/stratigraphical configuration of the study area. On the other hand, they must be applied over wide areas. Thus, abacuses must be the result of a compromise between generalization and specialization.
This kind of simplified tool is widespread and implemented in many seismic codes throughout the World. The following section will be devoted to review most diffuse abacuses. Then, the procedure adopted for the formulation of abacuses to be used for microzoning studies in Tuscany (Central Italy) will be described in details. This approach is quite similar to the one formerly adopted in the Emilia-Romagna region (Pagani et al., 2006) and to the ones that are going to be applied in Liguria (Northern Italy), Marche (Central Italy) and Apulia (Southern Italy) regions. Due to its general character, the proposed approach could be of interest for similar applications in other countries.
2.1.1. Simplified Tools for computing 1D seismic response
Mainly, two kinds of abacuses exist that are relative to seismo-stratigraphical and morphological effects respectively. The first one (the only considered in the following) aims at quantifying amplification effects induced by the mutual interference of waves propagating vertically through a stack of layers overlying any reference seismic bedrock. The reference seismic ground motion is expected to be known at the bedrock by national hazard maps (e.g., Stucchi et al., 2011). The amplification is here considered as the ratio between the spectral amplitude of the ground motion at the outcropping bedrock and the one at the top of the sedimentary stack of concern. In the frame of microzoning studies, amplification is generally summarized through a single integral parameter (the AF cited above) representative of spectral amplification of ground motion over fixed ranges of frequencies of major concern for engineering purposes.
In general, compilation of abacuses for fast characterization seismo-stratigraphical amplification effects is performed by considering two approaches: empirical and computational.
Empirical approaches
In this case, a huge amount of seismic observations is collected, that is representative of a number of specific subsoil configurations (soil classes) defined in advance. Then, statistical properties of the seismic records (e.g., in terms of the respective response spectrum) belonging to each class are determined to retrieve representative AF values.
Medvedev (1962) and more recently Astroza and Monge (1991), basing on observations of the effects of earthquakes, developed an “abacus” having the outcropping geological unit as “input” parameter and as
outcrop. In addiction Medvedev (1962) tried to make correlations between the intensity increments and “seismic impedance” i.e. the product of shear wave velocities (Vs) and bulk density of outcropping rocks. Shima (1978) and Midorikawa (1987) built their tools for computing seismic response always using geological unit, but the seismic-intensity increment was replaced by the relative amplification factor, which defines the site amplification with respect to a reference outcrop motion or reference ground and it is more useful to evaluate the effect of site geology more quantitatively. These amplification factors are shown to correlate with the ratio between Vs of outcropping rocks and those of the reference rigid outcrop (Shima, 1978), while similar correlations were also found between the amplification factor for Peak Ground Velocity (PGV) or the average horizontal spectral amplification in the period range of 0.4 to 2.0 s (ASHA) and the average Vs up to a reference depth (Joyner and Fumal, 1984; Midorikawa, 1987; Borcherdt et al., 1991).
A simpler and quantitative diagnostic proxy to estimate 1D amplification effects was proposed by Borcherdt (1994) in the form of the average Vs velocity of the sedimentary stack up to a depth of 30 meters (Vs30). This proxy was adopted by the National Earthquake Hazard Reduction Program, NEHRP (2000), the International Building Code, IBC (2000), the Eurocode-8, EC-8 (2003) and the Italian building code (NTC 08, derived from EC-8). Up to now, Vs30 represents the most common single input parameter to estimate seismic amplification. This parameter has been used to classify accelerometric sites in terms of a set of “soil classes” (varying in the different implementations). Then accelerometric registrations are gathered and classified for each soil class to determine representative AF values (e.g., Rey, Faccioli and Bommer, 2002). The wide acceptance of this parameter hid its empirical character and a number of “second order proxies” were proposed to indirectly estimate this proxy (e.g., Boore, 2004; Wald and Allen, 2007) or by considering empirical relationships with other geotechnical parameters (e.g., Standard Penetration Tests, etc.).
Beyond its widespread application in the current regulatory codes, however, there is no general agreement about the effectiveness of Vs30 as a proxy to estimate (by alone) seismic amplification. For example in a paper by Castellaro et al. (2008) a reassessment of the original data derived from Borcherdt (1994) showed the weakness empirical basis supporting the correlation of Vs30 with AF.
Many Authors suggest that accompanying Vs30 with other observable may significantly improve the effectiveness of this proxy by considering soil frequency besides the velocity profile (e.g., Gallipoli and Mucciarelli, 2009). Rodriguez-Marek et al. (2001) and Pitilakis et al. (2003) proposed abacuses including as input parameter the site predominant period. More recently, Lang and Schwartz (2006) proposed an implementation of the NEHRP code that requires also ambient vibration measurements to estimate soil resonance period. Lang and Schwarz (2006) developed an hybrid classification scheme that availed oneself of the site class of the German Earthquake code DIN 4149:2005 and the subdivided NEHRP site classes according to ranges of sedimentary layer thickness as proposed by Bray and Rodriguez-Marek (1997). The input parameters on the base of which the spectral shape is modified are: soil description, Vs30 and the fundamental frequency.
Computational approaches
In this kind of approach, numerical simulations are considered to evaluate the seismic response of each subsoil configuration to the earthquakes expected in the study area. These simulations should account variability of relevant parameters relative to the considered soil class and result in a set of possible seismic responses. Statistical properties of this set are then analyzed to retrieve any representative AF value. In general, empirical approaches are preferred when a large amount of seismic records is available. In the other cases, the use of numerical simulations is mandatory to account for expected amplification phenomena. In this respect, they are more flexible than empirical approaches but entirely rely on numerical and modelling issues that can be biased by unavoidable approximations.
By taking into account that the number of accelerometric data available in Italy in not enough to allow an empirical approach providing reliable results at the scale of Seismic Microzonation, this approach has been recommended in IGSM.
In the IGSM, exemplar abacuses were provided to illustrate a possible procedure and hinting Italian Regional Authorities to formulate analogous tools specialized for each area. The exemplar IGSM abacuses were differentiated on the basis of the reference ground motion deduced from national seismic hazard maps, prevalent lithology and Vs profiles.
As input, each abacus requires the average shear wave velocity up to the bedrock (VsH) and provides as output the amplification factor in the form on integral parameters representative of amplification effects expected on a fixed spectral range. In this line, four Italian Regions (Lombardy, Latium, Emilia Romagna and Tuscany) realized (and implemented in the local regulatory codes) regionalized abacuses by following slightly different computational approaches. In all the cases, abacuses were built by considering seismo-stratigraphical and geotechnical data made available by local administrations. Main differences concern the choice of relevant input and output parameters and restrictions relative to their use by practitioners. In the following, the procedure adopted by the Tuscany Regional administration is illustrated in details. Similar approaches were used in the other cases.
2.2. Abacuses for Tuscany
Despite of the fact that Tuscany region (Figure 2) experienced strong earthquakes in the past (Mugello 1542, 1919; Garfagnana 1920; Valtiberina 1917 – Rovida et al., 2011) its territory is presently characterised by a relatively low seismicity level. This situation results in a lack of accelerometric data relative to earthquake registered in Tuscany and representative of the expected seismic hazard. This hampers any attempt to apply a purely empirical approach to define effective seismic abacuses valid for the study area. On the other hand, to this purpose, one can take advantage mainly of the huge amount of data geological, geotechnical and seismological data made available since 1996 within the VEL (Italian acronym for “Evaluation of Local Effects”) Project: (http://www.regione.toscana.it/-/banca-dati-vel) by the Tuscany Regional Administration. On its behalf, geological and geomorphological surveys (at the scale 1:2000) were carried out in 90 municipalities (including 207 settlements) of the Tuscany region (about 400 km2). In these
areas, about 1300 seismic lines (refraction surveys considering P and SH seismic phases) were performed along with about 570 geognostic drills and Down-hole seismic prospections (P and SH).
Figure 2. Geographical location of the Tuscany area and its Geological domains (GD). Geological information relative
to the whole Tuscany area was used to identify GDs (from Peruzzi et al., 2016b).
In this situation, the use of a numerical approach was mandatory. In particular, extensive numerical simulations were performed of the ground motion expected at the surface of seismic structures representative of seismo-stratigraphical configurations present in the Tuscany area. In line with similar analyses carried on in similar situations, 1D configurations were considered only: each subsoil configuration is considered to be a stack of uniform layers.
The procedure developed four main stages that will be illustrated in details in the following sections. 2.2.1. Parameterization
This part of the work was very troublesome and represented the most important step of the whole procedure. In this stage, main Litho-Stratigraphical Units (LSUs) have been defined. Each of them is considered to be representative of a specific geological body characterized by peculiar lithological/geotechnical properties. The basic rationale of this identification procedure is that each LSU is expected to show an internal variability lower than those observed between other LSUs. Typical LSUs are: “shallow covers”, “alluvial deposits”, “fine lacustrine deposits”, “coarse lacustrine deposits (gravels)”, “weathered bedrock” and “bedrock”. In this work, bedrock was defined according to geology and it has been generally assumed to correspond to an unweathered and lithified formation. The relevant Vs value was not considered. Just in some peculiar geological contexts, where depositional basins are very deep (they may reach thousands of meters of depth), the maximum depth of the top of bedrock was considered as the maximum depth at which Vs reaches 800 m/s, not considering its lithology.
Geological information relative to the whole Tuscany area was used to identify typical Litho-Stratigraphical Configurations (LSCs). Each of them is a stack of LSUs that are typical of a geological domain (GD), i.e., a domain characterized by similar tectonic/depositional history. As a whole, 13 of such domains were identified for the Tuscany area (Figure 2). For each domain, a set of LSUs was identified. What is typical of each LSC in the GD of concern is the typology and the stratigraphic sequence of the relevant LSUs. An example concerning the Lunigiana GD is shown in Figure 3.
Figure 3. Example of the LSCs in Lunigiana area. (a): main litho-stratigraphical configurations (LSC); (b) Vs mean
values (squares) for the considered Litho-Stratigraphic Units (LSUs) and respective standard deviation (red bars) (from Peruzzi et al., 2016b)
By considering available data, seismological/geotechnical properties were attributed to each LSU in the domain of concern. Mean, standard deviation, maximum and minimum values of the relevant parameters (Vs, density, thickness) where assessed. When possible, depth-gradients for these parameters in each LSU were assessed. Dynamical soil properties (shear modulus reduction curves and damping ratio curves as a
function of shear strain) where assessed from laboratory samples relative to the LSU of concern. Due to the huge amount of data available, the use of data from the literature was very limited.
For each LSC, a definition of the LSU representative of the local seismic bedrock is provided. It represents the layer where the reference input motion will be applied in simulations: the eventual expected amplification will be defined with respect to this reference ground motion. This last was defined by considering the Italian Reference Seismic Hazard Map (http://esse1-gis.mi.ingv.it/) relative to the ground motion expected to be exceeded with a probability equal to 10% in 50 years. In the present analysis, the hazard values have been discretized into four levels of hazard. For each level, a group of seven accelerometric records representative of the respective seismic hazard where provided for each municipality by the Tuscany Regional Administration. In particular, we will refer to accelerograms of groups 1, 2, 3 and 4 for hazard levels respectively corresponding to four different levels of the PGA value characterized by an exceedance probability of 10% in 50 years (Table 1). The relevant data are available in http://www.regione.toscana.it/-/accelerogrammi-di-riferimento-per-la-pianificazione
Table 1: PGA value ranges (characterized by an exceedance probability of 10% in 50 years) for each accelerometric
records group, corresponding to the four different hazard levels considered in the abacuses Accelerometric records
group PGA (Italian Reference Seismic Hazard Map, INGV 2004
Group 1 PGA > 0.175g
Group 2 0.150g < PGA ≤ 0.175g
Group 3 0.125g < PGA ≤ 0.150g
Group 4 PGA ≤ 0.125g
Thus, for each LSC one or more groups of accelerograms were attributed to each LSC and GD, depending on the seismic hazard levels of the respective GD.
2.2.2. Numerical Simulations
Numerical simulations were performed by the use of a linear equivalent procedure (Kramer, 1996). In particular, the free software STRATA was used on purpose (Rathje and Kottke, 2013). With respect to numerical codes used few years ago like SHAKE (Shnabel et al., 1972) or EEERA (Bardet et al., 2000), STRATA also allows to randomly vary subsoil properties (in a way determined by the user) and input motion by extracting response functions accounting for the relevant uncertainty in the input parameters.
Stochastic properties of the random generators responsible for the simulations were set to reproduce variability observed in the experimental data concerning Vs and bulk density profiles (including non monotonic patterns). In particular, variability of modulus reduction curves and damping was reproduced by following Darendeli (2001): information available in Tuscany were used to check feasibility of this parameterization. In any case, each simulation was checked on its turn and unrealistic realization of the random process were eliminated manually. In order to fix the effective strain ratio (Kramer, 1996), the sample magnitude values associated to the input accelerograms has been considered. We took as reference the magnitude M75 corresponding to the 75th percentile of the magnitude sample and computed
the effective strain ratio as equal to 0.1(M75-1) as suggested by Idriss and Sun (1992).
By considering seven input motions relative to specific hazard level, LSC and GD, a range of 350-700 simulations were carried on for each LSC and any single GD. Correspondingly, a set of acceleration response spectra (Sa) and amplification functions was obtained.
Since the expected outcomes will not be used for anti-seismic design but for relative evaluations only, by following indications provided in IGSM (WGSM, 2008) the expected amplification effects have been summarized by an integral spectral parameter (the Amplification Factor or AF) defined as
[1]
where T is the period of the considered spectral ordinate and Sb is the acceleration response spectrum of the input motion at an outcropping bedrock. In particular, two values of AF were computed corresponding to short (0.1 - 0.5 s) and long (0.5 - 1.0 s) periods.
2.2.3. Identification of characteristic parameters to identify representative AF values
The outcome of numerical simulations is a large number of AF values relative to specific LTCs, divided according to geographic area and the relevant seismic hazard (and consequently the reference set of accelerograms used as input motion). For each GD and hazard level, AF values have been grouped as a function of a set of parameters representative of the specific litho-stratigraphical configuration. These parameters should be representative of physical process responsible for amplification phenomena but also cost-effective and easy-to-apply in the context of professional activity. In principle, seismo-stratigraphical amplification is controlled by the seismic impedance contrast (relative to S waves) between the sedimentary cover and the bedrock; the depth of this bedrock controls the period of vibrations mostly affected by amplification (e.g., Kramer, 1996).
By strongly simplifying the problem, one can fix the Vs value in the bedrock and estimate the contrast by considering the average Vs profile in the sedimentary cover down to the bedrock at the depth h (Vsh): the larger VsH is, the lower is the expected amplification effect. The deeper is the seismic bedrock, the smaller is the frequency f0 of amplified spectral ordinates: f0 ≈ VsH /4h (Kramer, 1996). Thus, in this simplified view, a couple of parameters (h and VsH) seems enough to capture the essence of stratigraphical amplification phenomena. This choice is in line with most recent suggestions provided by empirical analyses (see above section 2.1).
A simple way to evaluate the effect of the thickness h of the sedimentary cover is by measuring directly the resonance frequency f0. Single station three-directional ambient vibration measurements are very effective on purpose (see, e.g., Bonnefoy-Claudet et al., 2006) at least when a significant impedance contrast exits at the top of the bedrock. A large number of experimental tools are available for the field in-situ estimate of
VsH when h is relatively small (<30 m): both active and passive seismic techniques are available on purpose
and are currently applied by practitioners (e.g. Kramer, 1996; Foti et al., 2011). However, these techniques become increasingly less effective and troublesome when the depth of the bedrock increases. Thus, for deeper sedimentary covers, Vs30 can reveal to be an effective proxy. Fast and cheap procedures have been recently proposed to estimate this parameter from passive seismic surveys (Albarello and Gargani, 2010; Castellaro and Mulargia, 2009).
As a result of the above considerations, the following parameterization has been adopted to estimate AF: o shallow bedrock (h≤30 m): f0 and VsH are jointly considered
o deep bedrock (h>30 m): f0 and Vs30 are jointly considered
The distinction between the two situations is determined in advance based on available geological/geophysical data provided in the level I of Microzonation analysis.
In order to simply the following analysis, the above parameters have been discretized as reported in Table 2.
2.2.4. Statistical analysis
For each LTC relative to the pre-determined GD and level of seismic hazard, an abacus was realized where the population of the respective AF values have been grouped by considering the entries in Table 2.
In order to provide a reasonable conservative estimate of AF for each entry, the 75th percentile of the relevant AF population was considered as representative. In order to allow the AF calculation also in the case that f0 cannot be estimated (e.g., ambient vibration measurements do not allow the identification of any resonance frequency), in the last column (the green one) reports the 75th percentile of the population relative to the corresponding velocity value.
As one can expect, some cells are empty, so not all Vs30- f0 combinations are expected exist due to the peculiarities of each LSC of the area. Furthermore, it is possible to identify some VS30 - f0 combinations that are more common or other that are less common. An example of this preliminary abacus is reported in Table 3. Classes f0<1 1.5 1≤f0<2 2.5 2≤f0<3 3.5 3≤f0<4 4.5 4≤f0<5 5.5 5≤f0<6 6.5 6≤f0<7 7.5 7≤f0<8 fre qu en cy (H z) ≥ 8 Vs<200 300 200≤Vs<400 500 400≤Vs<600 700 600≤Vs<800 ve lo cit y ( m /s ) Vs≥800
Table 3: It is about the area of Amiata Mountain (an inactive volcano in the southern Tuscany), for litho-stratigraphic
situations where bedrock is not within 30 meters and seismic hazard is represented by accelerograms of group 3. The first table reports AF values calculated for period between 0.1 and 0.5 seconds, the second table for period between 0.5 and 1.0 seconds. In the third one, the level population relative to each cell is reported (modified from Peruzzi et al., 2016b)
Amiata Mountain (bedrock > 30 m) accelerograms: group 3 AF (0.1 < T < 0.5 s) f0 (Hz) <1 1.5 2.5 3.5 4.5 5.5 6.5 7.5 ≥ 8 75° perc. <200 1.3 1.7 0.0 0.0 0.0 0.0 0.0 0.0 0.0 1.6 300 1.5 1.6 2.1 2.6 2.4 2.2 2.1 2.4 2.6 1.9 500 1.3 1.6 2.0 2.1 2.0 1.9 2.4 2.2 2.1 2.0 700 0.0 0.0 2.1 1.8 1.7 1.6 0.0 0.0 1.9 1.8 V s3 0 ( m /s ) ≥800 AF (0.5 < T < 1 s) f0 (Hz) <1 1.5 2.5 3.5 4.5 5.5 6.5 7.5 ≥ 8 75° perc. <200 1.5 2.6 0.0 0.0 0.0 0.0 0.0 0.0 0.0 2.5 300 1.8 2.5 2.2 2.0 2.5 2.8 2.3 1.9 2.3 2.3 500 1.5 2.3 1.8 1.5 1.4 1.2 1.9 1.9 1.5 1.7 700 0.0 0.0 1.7 1.4 1.2 1.2 0.0 0.0 1.2 1.4 V s3 0 ( m /s ) ≥800 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
population inside cells
f0 (Hz) <1 1.5 2.5 3.5 4.5 5.5 6.5 7.5 ≥ 8 <200 7 14 300 267 883 551 133 28 24 34 10 30 500 131 132 370 294 95 15 15 14 243 700 7 42 21 7 7 V s3 0 ( m /s ) ≥800
After this analysis, “local” abacuses were built for each GDs in figure 1. This result in a huge amount of abacuses since each GD has at least two abacuses (AF for 0.1 s < T < 0.5 s and for 0.5 s < T < 1.0), which can double where the depth of the bedrock varies from less than 30 meters (abacus with Vsh) to more than 30 meters (abacus with Vs30). Furthermore, abacuses for each area can be multiplied according to the number of LSCs and the different seismic levels that characterize the relevant GD. This results into an un-manageable situation, were the advantage of a simplified approach is completely frustrated. In order to cope with this problem and also to make statistically more robust each abacus, the “local” abacuses have been compared and, when possible, joined.
To this purpose, abacuses relative to LSCs included in the same GD were compared at first. When absolute differences in the AF values for all the cells were lower than 0.2, the two abacuses were merged. The new abacuses obtained for each GD were compared with those in the other GDs. In this case also, when for all the cells differences were lower than 0.2, the respective abacuses were merged. At the end of this process, five group of abacuses were identified, each corresponding to a specific Macro-Area or MA (Figure 4).
Figure 4. Tuscany Region Macro-areas (MAs). Each MA has its own group of abacuses, which reflects MA’s seismic
hazard and geological peculiarities (from Peruzzi et al., 2016b)
1. Appennine Tuscany, characterized by inter-mountains basins; 2. Central Tuscany;
