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CORSO DI AGGIORNAMENTO PER GEOLOGICORSO DI AGGIORNAMENTO PER GEOLOGI18 18 --

19 Ottobre 201119 Ottobre 2011

Marco Massa (*)

Università degli Studi di Genova(Dip.Te.Ris., Geofisica)

Gli effetti di amplificazione topografica: Gli effetti di amplificazione topografica: aspetti teorici e casi studioaspetti teorici e casi studio

Istituto Nazionale di Geofisica e Vulcanologia

STUDI DI MICROZONAZIONE SISMICA: STUDI DI MICROZONAZIONE SISMICA: TEORIA E APPLICAZIONI TEORIA E APPLICAZIONI

(*) Istituto Nazionale di Geofisica e Vulcanologia, Milano

Regione autonoma Valle D’Aosta

marco.massa@mi.ingv.it Regione Valle D’Aosta, 18 ottobre 2011

Summary Summary

TOPOGRAPHIC EFFECTS: STATE OF THE ARTPrimary topographic effectsInduced (or secondary) topographic effectsThe Italian seismic rules for building (NTC 2008)

A DETAILED CASE STUDY : NARNI (CENTRAL ITALY)Seismic monitoring of the topographyResults from recorded dataComparison with numerical simulationsEvaluation of NTC 2008

EXAMPLES FROM OTHERS ITALIAN SITES

Seismic local site responseSeismic local site response

A1 (A1 (ωω))

= S(= S(ωω) P() P(ωω) ) s1(s1(ωω) t1() t1(ωω) )

A2 (A2 (ωω) = S() = S(ωω) P() P(ωω) ) s2(s2(ωω) t2() t2(ωω) )

For sites 1, 2 (and

also site 3)S(ω)

and P(ω)

are common

For sites 1 and 2 s1(ω) = s2(ω) (same lithology)For sites 1 and 2 t1(ω) ≠

t2(ω)For site 2 t2(ω) = 1 (flat surface)

A1(ω)/A2(ω) = t1(ω)

factors

modify

a seismic

signal

in amplitude, duration

and frequency

content

during

propagation

the source

the recording

Primary Topographic effects: causesPrimary Topographic effects: causes

1) The free oscillation of the topography can have significant effects on seismic waves when the incident wavelengths are comparable to the size of the topographic features

and the

topographic slopes are relatively steep;

= n V/LResonance frequency

(from Geli et al., 1988)

L = width of the topography

V = shear wave velocity

fundamental mode

2) The variations of seismic motion, related to isolated reliefs, are due to different physical phenomena such as the focusing of seismic waves

near the

crest, because of the reflection on free surface and/or the interaction between incident and diffraction waves.

Homogeneous body

Geometrical factors : SR and WIDTHGeometrical factors : SR and WIDTH

1) The shape ratio (SR) H/L

2) The total width

(from Lanzo and Sivestri, 1999)

Geometrical factors : vertex angleGeometrical factors : vertex angle

(Sanchez-Sesma, 1990)

Homogeneous

and elasticmaterial φ

= 90°

e[i ω

(t+z/ β)] = v0

+e-iKz)+ v0

+e-iKx)

v= 2v0

(cosKx

+ cosKz)

On vertex, x=0 and z=0

v(0,0)= 4 v0

Surface

motion

equations:

= 120°

v (0,0)= 3 v0For

= 180°

v (0,0)= 2 v0

= 270°

v (0,0)= 1.3 v0

In general v (0,0)= v0

360°/ φ

Effects induced by topography : subEffects induced by topography : sub--blocksblocks

Marzorati et al., 2010

Effects induced by topography : landslidesEffects induced by topography : landslides

(Burianek et al., 2009)

Stable

area Unstable

Frequency

[Hz] Frequency

Time domain : crest vs. baseTime domain : crest vs. base

Transversal section of Narni ridge, Transversal section of Narni ridge, NRN4 (top), NRN1 (base)NRN4 (top), NRN1 (base)

(%) = 118

Time domain : NS component vs. EWTime domain : NS component vs. EW

NarniNarni, station at the top, station at the top

Mw 4.2, R 5.5 kmMw 4.2, R 5.5 km

No No filteredfiltered

datadata

Other stations Other stations on topographieson topographies

PGA (%) = 103

Frequency domain : polarizationFrequency domain : polarization

MATLAB Handle Graphics

TopographyTopography

Polarization Polarization effects in EW effects in EW directiondirection

H/V difference = 4H/V difference = 4

Absence of Absence of topographytopography

H/V difference = 0.3H/V difference = 0.3

H/V=0.3

Frequency domain : different phasesFrequency domain : different phases

Agreement between different phasesAgreement between different phases

Italian seismic rules for buildings Italian seismic rules for buildings (NTC 2008, D.M. 14/01/2008)(NTC 2008, D.M. 14/01/2008)

“Le suesposte categorie topografiche si riferiscono a configurazioni geometriche prevalentemente bidimensionali, creste o dorsali allungate, e devono essere considerate nella definizione dell’azione sismica se di altezza maggiore di 30 m”

“ ... per configurazioni superficiali semplici si può adottare la seguente classificazione”

Techniques for Techniques for experimental siteexperimental site

response evaluationresponse evaluation

Horizontal to Vertical Spectral Ratio

(Lermo and Chavez-Garcia, 1993)

Standard Spectral Ratio (Borcherdt, 1970)

Directional Analysis

NON REFERENCE SITE TECHNIQUE REFERENCE SITE TECHNIQUE

(f) Aij

(f) = Si

( f ) *Pij

(f)*Gj

(f)*Ij

H(f) / Aij

(f) = Si

( f ) *Pij

(f)*Gj

(f)*Ij

(f) / Aik

( f )*Pij

(f)*Gj

(f)*Ij

(f)]/[Si

( f )*Pik

(f)*Gk

(f)*Ik

(f) / Gk

Example of directional SSR at Narni

THE CASE STUDY OF NARNI RIDGE (A-T3)

Morphological Morphological settingsettingScarpata Sud-Ovest

NWScarpata Nord-Ovest

Lato Nord

Scarpata Est

1300 m

870 m22°

Azimuth dorsale: N 31°

max ∆H ~ 220 m

Geological setting and temporary Geological setting and temporary velocimetricvelocimetric

networknetwork

Sensors

: velocimeters

Lennartz

LE3D-5s (flat instrumental response 0.2-40 Hz)

Recording systems: 24 bits Reftek

130/01 and 20 bits Lennarts

Mars-Lite

Geology : Massive limestone

Network

: 10 surveyed sites from March to September 2009

3 white triangles : base6 black triangles : crest1 grey triangle : middle1 white square : DPC station

Data set and processingData set and processing

events

(about 10.000 waveforms)642

from April 2009 L’Aquila sequence

Mean removal and baseline correction;Butterworth filter 0.2 Hz -

25 Hz;FFT on different windows (S-phase and coda);Smoothing (Konno Omachi, b=20);rotations of NS component (0°

175°, step 5°)

AnalysesHVSR

(Single

station

spectral analysis )SSR

(Standard

spectral ratio )

Local Magnitude :

1.5 ÷

5.3Epicentral

distance :

100 km

The target event: 16The target event: 16thth

December 2000, Mw December 2000, Mw 4.24.2

R 5.5 km Est

of Narni(depth 9.8 Km)

NRN station –

10s of S phase

NRN2 (base)

NRN1 (middle)

Microtremor survey (results on EW component)Microtremor survey (results on EW component)

Top Top Top

NonNon--reference site technique (HVSR)reference site technique (HVSR)

SSR SSR ::

dependence on dependence on source to sitesource to site

directiondirection

All azimuths

Km, 1.5≤M≤3.6) azimuths between 60°

and 120°

SSR SSR ::

dependence on dependence on epicentralepicentral

distancedistance

< 30 Km

NRN7/NRN2 NRN7/NRN2

60 < R

< 80 Km

SSR SSR ::

dependence on phasedependence on phase

events R

< 30 Km, 1.5 ≤

S-phase

SSR SSR ::

dependence on componentsdependence on components

NRN2 Horizontal Vertical

Wrong evaluations : HVSR vs. SSRWrong evaluations : HVSR vs. SSR

NRN1NRN7

base topmiddle

middle top

An example from An example from LL’’AquilaAquila

aftershocks 4.7 aftershocks 4.7 ≤≤

ML ML ≤≤

5.35.3

SSR between top and base

not polarized

SSR between two stations at the top

not polarized

Noise measurement in the tower (2nd floor)

Wrong evaluations : soilWrong evaluations : soil--structure interactionstructure interaction

Considerations about experimental resultsConsiderations about experimental results

For the

stations installed at the top

the amplification effects involve frequencies between 3 Hz and 5 Hz,

with an amplification factor up to 9

(with respect to the reference station);

The peak between 3 and 5 Hz shows a clear polarization effect: the highest amplifications are detected for direction perpendicular to the main axis of the ridge;

The amplification factor increases with increasing difference of quota

between top and bottom;

The amplification factor increases with respect to the source to site direction, showing the highest values for direction

perpendicular to the main elongation of the ridge;

The same result is obtained if different phases of signal

are considered (noise or coda);

Amplification peak at frequencies between 4 Hz and 10 Hz are detected also on vertical component;

results generally well agree, in terms of frequencies, with

those obtained from SSRs, even if the case of L’Aquila highlights the possibility of wrong interpretations in absence of SSR;

Noise measurements, being a fast and cheap tool often used in site response analyses, appear to be suitable

but only for very preliminary considerations.

20 ))((2

0max exp))((21)( ttfp

pttfAtf

Method:

boundary elements

Codes:2D : HYBRID (Kamalian

al., 2003)

3D : BEMSA (Sohrabi

et al., 2009)

Domain:

elastic, homogeneous and isotropic

Input parameters:γ

= 23.5 KN/m3

= 0.37Vs

= 1000 m/s

Vp = 2210 m/s

Input at bedrock:

Ricker wavelet (fc=3Hz)

Investigated frequencies 1 -

Transverse sections : P1

(NRN7),P2 (NRN4). DEM

resolution 20 m

2D and 3D 2D and 3D modellingmodelling: input parameters: input parameters

P P --

1(x)2(y)

results: results: NRN7NRN7

results : results : NRN4NRN4

SSRNRN4

results: results: NRN4NRN4

At least in terms of frequencies, 2D and 3D analyses agree

with the experimental ones: a constant amplification between 4 and 5 Hz

was found.

As highlighted in many studies, the models are not able to reproduce the amplification estimated by the experimental analyses: an underestimation of a factor up to 3

was found;

Having the Narni

ridge a clear 2D configuration, the 3D model does not improve the final results;

Considerations about numerical simulationsConsiderations about numerical simulations

Considering the discrepancies, in term of amplification factors,

between experimental results and modeling, it is possible to suppose that the observed amplification between 3 and 5 Hz derives from a coupling of different factors (e.g. topography, rock weathering and structural anisotropies).

Characterization

of anomalies

the single station

Calculation of normalized residual (SA, 5%) between observed and

predicted values

Reference Italian model : Bindi

et al., 2010 (M >4; R<200; rock site –

NTC A category)

Evaluation of Italian predictive modelsEvaluation of Italian predictive models

H (WE) VH (NS)

Topography corrective coefficientsTopography corrective coefficientsCalibration of empirical predictive models, in terms of SA (5%) up to 1s, for NRN2 (base) and NRN7 (top) stations.

a, b, c coefficients for NRN2

(59 near field earthquakes)Log10 ( Y ) = a + ( b*M ) + ( c * Log ( R ) ) + σ

Log10 ( Y ) = a + ( b*M ) + ( c * Log ( R ) ) + ( St * 1 ) + σ

2St corrective coefficients for

St corrective coefficientsEW component at NRN7

EvaluationEvaluation

of NTC 2008 of NTC 2008 --

ShapeShape

Normalized

) design response spectrum

A-T3 (in black) Weak

motions

experiment(NTC for

A T3, in grey)

16/12/2000 Mw

4.2, R 5.5

Italian predictive model (Bindi et al., 2010, in black), for an event of Mw 5 at 5 km, as inferred from the deaggregation

analysis (SA at 0.2 s).

of NTC 2008 of NTC 2008 --

AmplitudeAmplitude

(Barani et al., 2009),

Grey line is NTC for A-T3

Grey squares are SA (for A T1) corrected for the St topography coefficients

(OPCM 3519, 28/04/2006) 10% of probability (T 475 yr)

exceede

in the next

PGA (on rock)

between

0.150 and 0.175 g (http://esse1-gis.mi.ingv.it/)

ExampleExample

of of otherother

ItalianItalian

sitessites

Aulla, AAulla, A

PGA (%) = 159

Sellano, BSellano, B

PGA (%) = 32

Lauria,Lauria,

A A --

1σ2σ

PGA (%) = 69

CastelvecchioCastelvecchio

Subequo, A Subequo, A --

PGA (%) = 126

CA03 (crest), CA02 (base)CA03 (crest), CA02 (base)

of NTC in of NTC in AmplitudeAmplitude

forfor

CA03CA03

Final commentsFinal comments

Considering the experimental evidences (also reported in bibliography), a no negligible amplification of ground motion in presence of a particular topography exists.

All investigated site

are characterized by:

strong polarization effects

for directions perpendicular to the main elongation of the topography;2)

overestimation with respect to the ground motion predictions

for “normal”

rock site;3)

amplification involving only a narrow range of frequency.

NTC (D.M. 14/01/2008)The St corrective coefficient

(ranging from 1 to 1.4) proposed by NTC08 is period independent

and seems not to be useful to well predict the real amplifications.

A simple shift

(in amplitude) of the design response spectrum for A-T1

site leads to

underestimate

the frequencies of interest.

SEISMIC INPUT Positive residuals between recorded data at the top of topographies and predicted ones, highlight high frequency amplifications (> 1 Hz) also for hard-rock site (A category): these

recordings

don’t

seismic

input for

analyses.

da NTC 2008 , cap 3.2.2 (Azione sismica)

“per condizioni topografiche complesse è necessario predisporre specifiche analisi di risposta sismica locale;

... per configurazioni superficiali semplici si può adottare la seguente classificazione ...”

.... ma esistono nella realtà configurazioni semplici ?

…thanks for your attention…

Presentazione di PowerPoint · March to September 2009. marco.massa@mi.ingv.it Regione Valle...

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