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Site effects assessment through historical

and instrumental data integration

Il contributo del dato storico e la sua integrazione con il

dato strumentale nella stima degli effetti di sito

M. Stella GIaMMarInaro, DoMenIco BarBa, DavID cIonco, antonIo contIno, Paola vallone Dipartimento di Geologia e Geodesia - Università Palermo. Via Archirafi, 20- 90123 Palermo. E-mail: giammar@unipa.it

ABSTRACT

This work illustrates the historical data contribution to the analysis of the microtremor survey results carried out in May 2003 in the city of Palermo by the Istituto nazionale di Geofisica e vulcanologia and the Dipartimento di Geologia e Geodesia – University of Palermo.

The microtremor survey has been carried out within the project SeSaMe (Site EffectS Assessment using AMbient Excitations) funded by the european commission, devoted to test the effectiveness of the seismic noise, and in particular of nakamura technique, in the site effects assessment. The nakamura technique singles out zones characterized by a near-surface geology suitable to produce site effects, by exploiting the microtremors horizontal-to-vertical spectral ratio features.

The measurements carried out in Palermo generally confirmed the nakamura method effectiveness. However for two measurement points the microtremor spectral ratio exhibited anomalous features, here explained by an integrated analysis of spectral and historical data.

keywoRdS: Site Effects, GIS, Microtremors, Spectral Analysis, Stratigraphic Model, Historic Cartography.

RiASSunTo

nel presente lavoro viene illustrato il contributo fornito da dati storici alla lettura e comprensione dei risultati forniti dalla campagna di acquisizione di microtremori condotta nell’area urbana palermitana nel maggio 2003 dall’Istituto nazionale di Geofisica e vulcanologia e dal Dipartimento di Geologia e Geodesia dell’Università di Palermo. tale campagna è stata eseguita nell’ambito di un Progetto della comunità europea, denominato SeSaMe (Site EffectS Assessment using AMbient Excitations ), finalizzato a verificare l’efficacia, ai fini della valutazione della risposta di sito, del rumore sismico ambientale ed in particolare della tecnica di nakamura che individua le zone caratterizzate da una geologia di superficie favorevole ad effetti di sito dalle caratteristiche esibite dal rapporto tra gli spettri delle componenti orizzontali e verticale dei microtremori. le misure effettuate nell’area palermitana in generale hanno confermato l’efficacia del metodo di nakamura. In corrispondenza a due punti di acquisizione tuttavia il rapporto spettrale sui microtremori esibiva caratteristiche anomale che vengono giustificate attraverso un’analisi integrata di dati spettrali e di dati storici.

TeRMini CHiAVe: Effetti Di Sito, GIS, Microtremori, Aanalisi Spettrale, Modello Stratigrafico, Cartografia Storica.

inTRoduCTion

as the advanced technologies in data acquisition and processing allow nowadays high resolution levels, the data acquired with old technologies could appear obsolete. However these data are relevant for territorial studies, as they allow extending the observation time scale; for instance historic data about past earthquakes are essential to outline the seismic history of an area.

The surface geology variability can be responsible for significant variations of ground motion and for a consequent diversified damage distribution. The local increases in amplitude and time duration of the seismic signal are commonly known as “Site effects”.

The studies on site effects started in the first ‘900 and have increased in the recent past, especially after the heavy damage in Mexico city 1985 earthquake that produced the major damage in an area 300 km far from the epicenter, where

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several skyscrapers were founded on marshy lake sediments related to an old dried lake.

The site effects definition is a relevant issue in urban areas both for their high vulnerability and for the frequent occurrence, inside them, of favorable conditions to site effects generation. Urban areas are characterized by a complicated surface geology and often by the presence of thick alluvial and fill deposits, characterized by low values of vS30, defined as: vS30 = 30/Σ (Hi/vi) where the sum is made on the layers of the first 30 m under ground level, Hi and vi are respectively thickness and S-wave velocity of the i-layer.

The new legislation on antiseismic planning adopts a soils classification based on vS30. low values of vS30 point out the occurrence of favorable conditions to seismic signal amplification (Scec 2000).

Therefore the knowledge of the subsoil geology and of the elastic and anelastic soils properties is a necessary starting point for site effects prevision.

The definition of the mechanical properties and geometry of the geological formations, essential to elaborate a realistic soil model, is a difficult task if the area is characterized by a variable surface geology.

The new technologies, in particular the GIS, successfully support this task and allow to perform surface geology modeling with levels of reliability and spatial resolution adequate to elaborate local zonation maps based on the expected site effects.

In order to exploit the obtained maps for territorial planning, management and safeguard, instrumental data check is necessary. Instrumental acquisition in urban areas, characterized by high geological variability over short distances, requires the installation of a local seismic network with several stations, capable to adequately sample the shaking, that has relative high costs.

These topics encouraged the development of new methodologies for experimental assessments of site effects.

SeSAMe PRoJeCT

The SeSaMe project (Seismic effectS assessment using aMbient excitations. Scientific supervisor P. Y. BarD, Web site: http://sesame-fp5.obs.ujf-grenoble.fr/index.htm) is a project funded by the european Union to evaluate the reliability of some ambient noise measurement techniques in predicting properties of ground motion during earthquakes. among them the H/v technique, advertised by nakamura. according to nakamura (1989) the vertical component of the ambient noise recordings is not amplified passing from the bedrock to the surface. Thus the seismic ambient noise data must be processed using the horizontal to vertical spectral ratio (HvnSr) technique.

according to nakamura:

HvnSr exhibits a spectral peak in presence of soft soils;

the peak frequency (fp) is the resonance frequency of soft sediments; the maximum amplitude (a) is the local amplification factor; the local site can be characterized by fp and a quantities.

The HvnSr method works sharing the recorded signal in time windows; for each window the horizontal-to-vertical spectral ratio is calculated and then the average value of the obtained spectral ratios is determined.

The final evaluation of the H/v spectral ratio depends on the windows dimensions and number, and on the software used to isolate the frequency band of interest and to clean the signal.

The SeSaMe project aims at investigating the reliability of the H/v technique and proposing user guidelines and processing software for this one.

In the context of SeSaMe project, microtremors surveys have been carried out in order to verify the nakamura method effectiveness in site effects definition.

Palermo was selected by the project as test-site for a microtremors acquisition survey both for its near-surface geology suitable for site effects and for the availability of detailed stratigraphic and geotechnical datasets elaborated for Palermo in the frame of Project MIUr 7t7 - cluster 29 and managed by city-GIS, a GIS designed to assess natural hazards in the urban area (Giammarinaro & Maiorana, 2001). The high reliability level of the elaborated datasets allows the measurement points optimization.

THe PALeRMo MiCRoTReMoRS MeASuReMenT SuRVey

The microtremors measurement survey, lasted 5 days, was carried out in May 2003. Six seismological stations were used, equipped with lennartz le3D-5s tri-axial velocity sensor and lennartz - Marslite digital recorders.

noise measurements were performed at 90 different sites of Palermo. The minimum recording time 30 minutes. The figure 1a shows the position of measurement sites, along 5 profiles; crossing the Kemonia and Papireto river beds

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and the oreto river valley.

The acquired dataset has been processed using the horizontal-to-vertical noise spectral ratio technique (HvnSr). along each profile when the transition stiff to soft soils is crossed a spectral peak appears in HvnSr, mostly in frequency band 1 to 2 Hz and exceeding a factor of 3 in amplitude (cultrera et al., 2004, cara et al., 2008). The peak disappears as soon as each profile, concluding the river crossing, finds the soft to stiff soils transition.

In this work the results obtained along the aa’ profile, crossing the low oreto river valley (fig. 1b), are discussed. By means of an integrated analysis of spectral and historical data two apparently anomalous H/v spectral ratios are explained.

ReSuLTS oF SeiSMiC AMBienT noiSe MeASuReMenTS in THe Low oReTo RiVeR

VALLey

along the aa’ profile, about 650 m far from the oreto outfall, 12 microtremors measurements have been acquired (fig. 1b). Their mutual distance ranges from 60 m to 141 m, with the lower distances close to the geological transition generated by the river.

The table below describes the soils and the H/v spectral ratio characteristics for the different measurement points. as the table shows, the profile can be subdivided in 3 portions.

In the first one, where measurement points 31 – 44 fall, the vS30 is prevalently controlled by the calcarenites, that is in good agreement with the corresponding H/v spectral ratio, characterized by a wide band maximum with amplitude ≤ 3.

In the second portion, where measurement points 45 – 48 fall, the H/v spectral ratio exhibits a marked narrow peak, whose maximum, with amplitude > 6, falls in the range 1.4 - 1.6 Hz (fig. 1d, fig. 1e).

The position and spatial density of the stratigraphic dataset elements allow well constraining the soils underlying measurement points 45 and 46, constituted by alluvial deposits thicker than 40 m. Therefore the vS30 is controlled by the alluvial deposits, that is in good agreement with the corresponding H/v spectral ratio, characterized by a narrow marked peak with maximum ≥ 6 around 1.4 Hz. (fig. 1d).

Fig. 1 - a) ambient noise recordings sites (Palermo survey); b) ambient noise recordings sites in the study area; c) Horizontal-to-vertical spectral ratio obtained in measurement point n. 41; d) Horizontal-to-Horizontal-to-vertical spectral ratio obtained in measurement point n. 45; e) Horizontal-to-vertical spectral ratio obtained in measurement point n. 47; f ) Horizontal-to-vertical spectral ratio obtained in measurement point n. 49

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173

Measurement

points H/V spectral ratio characteristics Description of the first 30 m soils, obtained by the stratigraphic dataset

39 – 40 - 41 Almost uniform trend with maximum value about 2 in the 1 - 3 Hz band (see fig. 1c) Sea deposits with thickness 5 - 8 m, laying over nodular calcarenites with sandy layers 42 - 43 - 44 Almost uniform trend, similar to the previous points, with maximum value about 3 in the

1 - 3 Hz band

Alluvial deposits with thickness 3 - 6 m, laying over nodular calcarenites with sandy layers

45 - 46 Occurrence of a narrow peak with maximum value 6, around 1.5 Hz (see fig. 1d) Alluvial deposits laying under fills with thickness of 8 m 47 - 48 Occurrence of a narrow peak with maximum value 6 - 7, around 1.5 Hz (see fig. 1e) Alternation of calcarenites and sands laying under fills with thickness of 3 - 8 m 49 - 50 Analogous trend to 39 - 41 points (see fig. 1f) Nodular calcarenites with sandy layers laying under fills with thickness of 3 m

The stratigraphic dataset elements n. 1467 and 1468, the nearest to the measurement points 47 and 48 and far from them about 30 m, describe the first 30 m soils as alternation of calcarenites and sands, laying under fills with thickness <5 m. Therefore the vS30 is mainly controlled by the calcarenites. The spectral ratio, still characterized by the occurrence of the narrow marked peak, appears therefore anomalous.

In the third portion, where measurement points 49 and 50 fall, the first 30 m soils are again constituted by calcarenites, with 20 m thickness, in the nodular facies with sandy layers, over which about 5 m of stiff calcarenites lie. The spectral ratio, coherently, exhibits the same characteristics of the first portion, with the occurrence of the wide band maximum with amplitude ≤ 3 (fig. 1f ).

along the entire aa’ profile the spectral ratio characteristics are in good agreement with the underlying soils characteristics, except for measurement points n. 47 and 48.

The lack of stratigraphic data just around these two points, together with the corresponding spectral ratio characteristics, suggested performing further analyses in order to achieve a better definition of the underlying soils.

THe HiSToRiC CARTogRAPHy

The microtremors measurement profile is located in an area that suffered quite a lot of transformations in the time. Some are ascribable to the oreto river evolution, widely documented in the cartographic production on Palermo started since the end of XvI century.

The more ancient maps, mainly focused on a true depiction of the city to the detriment of an accurate representation of the extra urban areas, are affected by errors and exhibit a low level of reliability due to the distortions, especially in the boundary portions of the map, generated by the use of non-zenithal cartographic projections.

even with these limitations the ancient maps allow to single out many aspects of the evolution experimented by the oreto river in the investigated area.

comparing a 1902 map (Blandi, 1999) with the Palermo 1640 map (Dufour, 1992), notable for the use of the orthogonal projection and for the detail level, a relevant downstream displacement of the confluence of the river branches flowing under the bridges called Ponte dell’ammiraglio and Ponte delle teste emerges. In the 1640 map the Ponte delle teste is located nearby the river confluence point, close to a fluvial island. Strong erosion phenomena downstream the confluence and depositional ones upstream it, can explain its downstream displacement, clearly evident in 1902 map.

In 1935 (Blandi, 1999) intense anthropic interventions modified the oreto river course, rectifying its terminal track. The river today is a single artificial channel.

The natural evolution of the river and the anthropic interventions could have generated areas characterized by low values of vS30. The H/v spectral ratio of the seismic ambient noise acquired in points falling inside these areas could show analogous characteristics than those relative to points located over remarkable thicknesses of alluvial deposits.

Therefore a detailed study has been carried out in order to verify the spatial relations existing between: the pertinence areas of measurement points n. 47 and 48 and the areas interested by the oreto river before that the anthropic interventions determined the exclusion of the branch crossing the Ponte dell’ammiraglio from the river flow.

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Fig. 2 - a) Geo-referred points displayed on 1902 Palermo map; b) Boreholes, ambient noise recordings sites and 1902 oreto river course displayed on the 1994 official regional cartography 1:10 000 scale.

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175

THe AnCienT MAPS geo - ReFeRRing

exploiting the section 595050, scale 1: 10000, of the official technical regional cartography (1994), geo-referred with 1 m accuracy, the 1902 map has been geo-referred by single out a set of points present in both the map (coincidence points).

The chosen points (fig. 2a) mainly fall inside the historic centre. 13 points have been used to geo – refer the 1902 map; 4 coincidence points (falling both inside and outside the historic centre) have been used as validation points in order to assess, at least empirically, the geo-referring error.

on the geo-referred 1902 map the geographic coordinates of some points falling inside the river course have been taken and transferred in the 1994 map, thus obtaining the ancient river course. This is reported in fig. 2b, where in grey color is marked the uncertainty area deriving from the geo-referring error. In the figure are also displayed the microtremors measurement points and the boreholes falling inside the investigated area.

looking at the fig. 2b it is possible to notice that the measurement point n. 47 falls inside the zone interested in the past by the river branch crossed by Ponte ammiraglio and the measurement point n. 48 falls at the edge of this zone.

In order to achieve a stratigraphic check for the obtained results, a further data search has been carried out, thus finding a borehole executed by Palermo Municipality in the frame of a waters channeling pipe realization. The borehole, 10 m deep, displayed as Sn in figure 3a, intercepts fill deposits (5.70 m thick) overlaying silty alluvial deposits.

exploiting these new data it is possible to suppose that

the right river side falls between the measurement points n. 48 and 49, which coherently exhibit in the microtremors spectral ratio respectively the occurrence and the disappearance of the narrow marked peak in the 1 - 2 Hz band;

the soils underlaying the measurement point n. 48 are fills and alluvial deposits;

the boreholes 1467, 1468 and 1469, intercepting the Pleistocene sands and calcarenites, responsible for the anomalous interpretation of the spectral ratio obtained for the measurements points n. 47 and 48, fall outside the right river side.

In fig. 3a the probable trend of the right river side in extraordinary flood flow conditions is reported with a dash mark; the trend appears coherent with the meander morphology of the river.

exploiting the stratigraphic dataset and using the historic cartography a geological section has been elaborated on the measurement points profile aa’.

Fig. 3 - a) 1902 oreto river right side near measurement points n. 47 and 48;

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Fig. 3b shows the section, the used stratigraphic dataset elements and the microtremors measurement points.

The reading of the section shows that the vS30 is controlled in the first segment (as far as measurement point n. 44) by the nodular calcarenites with sandy layers, in the second segment (from measurement point n. 45 to 48) by the alluvial deposits, in the third segment by the nodular calcarenites with sandy layers again. That validates the spatial evolution of the microtremors spectral ratio characteristics, confirming the nakamura method effectiveness in single out the presence of thick soft soils suitable to produce strong site effects.

ConCLuSionS

By investigating a single case study, the relevance of the territorial historical data in site effects assessment has been illustrated.

In particular some anomalies observed in microtremors spectral ratios acquired in the low oreto river valley (Palermo) have been explained by means of an integrated analysis of historical, stratigraphic and experimental data.

nowadays advanced methodologies in data acquisition and processing are available. However the today’s data, even if surely more detailed, can hide or be deficient in essential information, present instead in data acquired through earlier technologies, affected by errors and often considered obsolete.

Sometimes these data can provide a simple and correct interpretation of experimental data previously considered inconsistent, that occurred in the illustrated case.

The past scientists, among which G. G. Gemmellaro is to be mentioned, didn’t have advanced instrumentations and calculators and software, but perhaps this means poorness forced them to develop a greater intelligence. They are responsible indeed of the knowledge of the great global laws ruling a complex system as our earth is.

ACknowLedgMenTS

The authors want to thank Dr vinti (lavori Pubblici, comune di Palermo) for providing the stratigraphic data essential to explain the experimental evidences illustrated in the work.

This work has been funded by the MIUr (60% 2003 and 2004 M.S. Giammarinaro).

ReFeRenCeS

Blandi G. (1999) - Palermo storia dello sviluppo urbanistico della città dalle origini all’età contemporanea. edizioni axon Sicilia, 179.

cara F., cultrera G., azzara r.M., De rubeis v., Di Giulio G., Giammarinaro M.S., tosi P., vallone P. and rovelli a. (2007) - Microtremor measurements in the City of Palermo, Italy: Analysis of the correlation with local geology and damage. accepted by Bull. Seism. Soc. am..

cultrera G., azzara r.M., cara F., D’anna r., Di Giulio G., Giammarinaro M.S., Passafiume G., rovelli a., vallone P. (2004) - Microtremor measurements in Palermo, Italy: a comparison with macroseismic intensità and earthquake ground motion. Proceedings of the 13th World conference on earthquake engineering vancouver, canada, Paper no 915. Dufour l. (1992) – Atlante storico della Sicilia. Le città costiere nella cartografia manoscritta 1500-1823- arnaldo lombardi ed., 83

carta n.35.

Giammarinaro M.S. & Maiorana S. (2001) - City-GIS: Una applicazione GIS dedicata ai sistemi urbani. Geologi di Sicilia 3, 21-25.

nakamura Y. (1989) - A method for dynamic characteristics estimation of subsurface using microtremor on the ground surface. Qr of rtrI, 30, 25-33.

oddone e. (1915) – Gli elementi fisici del grande terremoto Marsicano Fucense del 13 gennaio 1915. Societa tipografica Modenese – Modena.

Scec Phase III report (2000) - Bull. Seism.Soc.am.,61- totally dedicated volume. SeSaMe Project; see Web site: http://sesame-fp5.obs.ujf-grenoble.fr, 2001.

SeSaMe Project – Deliverable D08.02 – Final report on Measurement Guidelines – WP02 H/v technique: experimental conditions, available from the web site: http://sesame-fp5.obs.ujf-grenoble.fr/Delivrables/D08-02_texte.pdf, 2003. SeSaMe Project – Deliverable D09.03 – report on the Multiplatform H/v Processing Software J-Sesame - WP03 H/v

technique: Data Processing, available from the web site: http://sesame-fp5.obs.ujf-grenoble.fr/Delivrables/D09-03_texte. pdf, 2003.

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177 SeSaMe Project – Deliverable D16.04 – report of the WP04 H/v technique: empirical evaluation, available from the web site:

http://sesame-fp5.obs.ujf-grenoble.fr/Delivrables/D16-04.pdf, 2004.

SeSaMe Project – Deliverable D23.12 – Guidelines for the Implementation of the H/v Spectral ratio technique on ambient vibrations Measurements, Processing and Interpretation, available from the web site: http://sesame-fp5.obs.ujf-grenoble.fr/ Papers/Hv_User_Guidelines.pdf, 2004.

Figure

Fig. 1 - a) ambient noise recordings sites (Palermo survey); b) ambient noise recordings sites in the study area; c) Horizontal-to- Horizontal-to-vertical spectral ratio obtained in measurement point n
Fig. 2 - a) Geo-referred points displayed on 1902 Palermo map; b) Boreholes, ambient noise recordings sites and 1902 oreto river  course displayed on the 1994 official regional cartography 1:10 000 scale.
Fig. 3 - a) 1902 oreto river right side near measurement points n. 47 and 48;

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