DOTTORATO DI RICERCA IN GEOFISICA XXIII CICLO Dottoranda: Sara Lovati Università degli Studi di Genova (Dipartimento per lo studio del Territorio e delle

DOTTORATO DI RICERCA IN GEOFISICADOTTORATO DI RICERCA IN GEOFISICA

XXIII CICLOXXIII CICLO

Dottoranda: Sara Lovati

Università degli Studi di Genova(Dipartimento per lo studio del Territorio e delle sue Risorse)

GROUND MOTION AMPLIFICATION INDUCED BY GROUND MOTION AMPLIFICATION INDUCED BY TOPOGRAPHIC IRREGULARITIES: RESULTS, TOPOGRAPHIC IRREGULARITIES: RESULTS, OPEN ISSUES AND FUTURE DEVELOPMENTSOPEN ISSUES AND FUTURE DEVELOPMENTS

Tutor interno: Prof. Claudio Eva (Università di Genova – Dip.Te.Ris.)

Tutor esterno: Dr. Marco Massa (INGV MI-PV)

Istituto Nazionale di Geofisica e Vulcanologia

Genova, 13 Aprile 2011

Summary Summary

1 TOPOGRAPHIC EFFECTS: STATE OF THE ARTTheoryAnalytical, numerical and experimental studiesThe Italian seismic rules for building (NTC 2008)

2 TECHNIQUES FOR SEISMIC SITE RESPONSE EVALUATIONExperimental methods (reference and non reference site)Numerical simulations (BEM)

3 THE CASE STUDY OF NARNI RIDGE (CENTRAL ITALY)Seismic monitoring of the topographyResults from recorded dataNumerical simulations of the ridgeEmpirical topographic site correction coefficients

4 CONSEQUENCE OF TOPOGRAPHIC EFFECTS ON GMPE PREDICTION AND ITALIAN SEISMIC CODE FOR BUILDING

Examples from some Italian morphologies

Sara Lovati, Ph.D. Thesis, final presentation Genova, 13 Aprile 2011

Seismic local site responseSeismic local site response

A1 (A1 (ωω)) = S( = S(ωω) P() P(ωω) r1() r1(ωω) t1() t1(ωω) )

A2 (A2 (ωω) = S() = S(ωω) P() P(ωω) r2() r2(ωω) t2() t2(ωω) )

A3 (A3 (ωω) = S() = S(ωω) P() P(ωω) r3() r3(ωω) t3() t3(ωω) )

For sites 1, 2 and 3 S(ω) and P(ω) are commonF

For sites 1 and 2 r1(ω) = r2(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)

For sites 3 and 2 r3(ω) ≠ r2(ω) (different lithology)For sites 3 and 2 t1(ω) = t2(ω) = 1For site 2 r2(ω) = 1 (rock)

A3(ω)/A2(ω) = r3(ω)


Topographic amplification: causesTopographic amplification: causes

Boore (1972): 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;

Bard (1982): the variations of seismic motion, relatively 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.

Shape ratio H/L; Vertex angle

f0= n β/L

Resonance frequency

(from Geli et al., 1988)

(from Lanzo and Sivestri, 1999)

H

L

n is a coefficient that depends on Poisson ratio, type and velocity of incident waves


Topographic amplification:Topographic amplification: analytical studiesanalytical studies

v= v0e[i ω (t+z/ β)] = v0 eiKz ei ω t

v= v0 (eiKz +e-iKz)+ v0 (eiKx +e-iKx)

v= 2v0 (cosKx + cosKz)

On wedge sides, x = ±z v= 4v0 cos Kx= 4v0 cosKz

On vertex, x=0 and z=0 v(0,0)= 4 v0

(Sanchez-Sesma, 1990)

Wedge-shaped homogeneous and elastic material φ = 90°

For φ = 120° v (0,0)= 3 v0

For φ = 180° v (0,0)= 2 v0

For φ = 270° v (0,0)= 1.3 v0

In general v (0,0)= v0 360°/ φ

According to the model the amplification at the top only depends on GEOMETRY

Surface motion equations:

120°

270°


Topographic amplifications: numerical studies Topographic amplifications: numerical studies

Transfer functions

Time – histories displacement EW sectionExample : Civita di Bagnoregio (Italy)

(from Paolucci, 2002)

2D numerical investigation based on Spectral Elements Method

Input: Ricker type wavelet (2 Hz)

In-plane (SV) and anti-plane (SH) solutions


(from Buech et al., 2010)

Little Red Hill (New Zeland) Calitri (Italy)

(from Faccioli and Paolucci, 2005)

Topographic amplifications: experimental studies Topographic amplifications: experimental studies

Evidence of damages at the top of the morphology after the 1980 Irpinia (Italy), Mw 6.9, earthquake (EMS 1998 scale)


At the top of the mountain the ground motion is amplified with respect to the base;

At the base of the relief the ground motion is alternatively amplified and de-amplified;

The spectral amplitude at the top of the mountain shows a maximum for wavelengths with dimension comparable to the mountain width (resonance frequency of the hill);

In the case of 2D ridge, the mountain undergoes larger amplification for motion perpendicular to the ridge axis;

Topographical amplification is lower for incident P waves with respect to incident S waves;

In particular the amplification is lower for incident SV waves (in-plane motion) with respect to SH ones (out of plane motion);

The amplification at the top of the mountain generally increases if the shape-ratio H/L increases.

The maximum topographical amplification appears in the case of vertical incidence.

What is well knownWhat is well known


Results from worldwide experimental studiesResults from worldwide experimental studies

Amplification factors vs shape-ratio from experimental studies

R2=0.87

Resonance frequency vs mountain width (left) and shape-ratio (right) as inferred from experimental (blue) and analytical studies (red)


Italian seismic rules for buildings (NTC 2008)Italian seismic rules for buildings (NTC 2008)

Topographic categories and related corrective coefficients

NTC 2008 design response spectra calculated for Narni site (A soil category and T3/T4 topographic category)



Techniques for site response evaluationTechniques for site response evaluation

F

Horizontal to Vertical Spectral Ratio (Lermo and Chavez-Garcia, 1993)

Standard Spectral Ratio (Borcherdt, 1970)

Directional Analysis

NON REFERENCE SITE TECHNIQUE REFERENCE SITE TECHNIQUE

Aik(f) Aij(f)

Aij(f) = Si( f ) *Pij(f)*Gj(f)*Ij(f)

AijH(f) / AijV(f)

Aij(f) = Si( f ) *Pij(f)*Gj(f)*Ij(f)

Aij(f) / Aik(f)=

[Si( f )*Pij(f)*Gj(f)*Ij(f)]/[Si( f )*Pik(f)*Gk(f)*Ik(f)]

Gj(f) / Gk(f)

Aii V(f)

Aii H(f)

Example of directional SSR at Narni site

(from Paolucci,1999)

Once obtained H it is possible to compute at a generic K site H/V and SSR applying the transfer functions to spectra of real seismograms.

• The site is excited by an impulse (displacement);

• Evaluation of s(t) at K receiver at the top;

• FFT to pass in frequency domain;

• Ratio of spectra (K / base) to obtain H

Boundary Elements Method (BEM)Boundary Elements Method (BEM)

)(

)()(

)()()(

)()()(

)()()(

)(

)()(

3

2

1

333231

232221

131211

3

2

1

fI

fIfI

fHfHfH

fHfHfH

fHfHfH

fO

fOfO

kkk

kkk

kkk

k

k

k

P - SVP - SV

KK

1(x)2(y)

3(z)

SV-x direction I1(f)=1; I2(f)= I3(f)= 0

H11(f)= O1(f)/I1(f)

H21(f)= O2(f)/I1(f)

H31(f)= O3(f)/I1(f)

H12(f)= O1(f)/I2(f)

H22(f)= O2(f)/I2(f)

H32(f)= O3(f)/I2(f)

P-z direction I3(f)=1; I1(f)=I2(f)=0

H13(f)= O1(f)/I3(f)

H23(f)= O2(f)/I3(f)

H33(f)= O3(f)/I3(f)

SV- y (SH) direction I2(f)=1; I1(f)= I3(f)= 0


THE CASE STUDY OF NARNI RIDGE (CENTRAL ITALY)


Morphological setting (A-T3/T4) Morphological setting (A-T3/T4)

SE

NW

1300 m

450 m

870 m22°

35°

Scarpata Nord-Ovest

Lato Nord

Scarpata Est

INGV-DPC agreement 2007-2009 S4 Project task 4 “Identification of anomalous sites and records”

Seismic monitoring: March - September 2009

Site response estimation by experimental and numerical approaches


Geological setting and temporary velocimetric networkGeological setting and temporary velocimetric network

Site investigations

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

REFERENCE SITE


Data set and processingData set and processing

702 selected events (about 10.000 waveforms)

642 from April 2009 L’Aquila sequence

Local Magnitude range : 1.5 ÷ 5.3

Epicentral distance range : 5 ÷ 100 km

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°)

Selected sub sets Near field data set (R < 30 km and ML up to 3.6)

Far field data set (L’Aquila sequence, ML up to 5.3)

Source to site direction selected from near field data

Influence of different phases (S and coda)

Analyses on different components of motion

Analyses Single statio spectral analysis (HVSR, Lermo and Chavez Garcia, 1993)

Standard spectral ratio (SSR, Borcherdt, 1970)


1616thth December 2000, Mw 4.2 Narni earthquake December 2000, Mw 4.2 Narni earthquake


R 5.5 km Est of Narni (depth 9.8 Km)

NRN – 10s of S phase

HVSR results on S-phaseHVSR results on S-phase

27 events R-epi < 30 Km, 1.5 ≤ ML ≤ 3.6

Sub set of events with source to site azimuths between 60° and 120°

NR10

NRN1

NRN7


HVSR results on codaHVSR results on coda

27 events R-epi < 30 Km, 1.5 ≤ ML ≤ 3.6NR10

NRN1

NRN7

Sub set of events with source to site azimuths between 60° and 120°


SSR results : dependence on source-site directionSSR results : dependence on source-site direction

R-epi < 30 Km, 1.5 ≤ ML ≤ 3.6 azimuths between 60° and 120°

NRN4

NRN7

NRN2


SSR results: dependence on epicentral distanceSSR results: dependence on epicentral distance

R-epi < 30 Km 60 < R-epi < 80 Km

NRN7

NRN2

NRN7/NRN2 NRN7/NRN2


SSR results: dependence on phaseSSR results: dependence on phase

events R-epi < 30 Km, 1.5 ≤ ML ≤ 3.6

NRN4

NRN7

NRN2


coda

coda

S-phase

S-phase

SSR results: componentsSSR results: components

NRN4

NRN7

NRN2

Horizontal Vertical


HVSR vs SSR : L’Aquila aftershocks 4.7 HVSR vs SSR : L’Aquila aftershocks 4.7 ≤ M≤ MLL ≤ 5.3 ≤ 5.3

NRN1NRN7

NRN2

NRN1

NRN1

HVSR

SSR

base topmiddle

middle top


NR10

NRN4

Soil structure interactionSoil structure interaction

NRN4 in the basement Noise 24 h: spectrograms

SSR between top and base

not polarized

SSR between two stations at the top

not polarized

Noise measurement inside the tower


Considerations about experimental resultsConsiderations about experimental resultsFor all stations installed at the top of the ridge the amplification effects mainly involve frequencies between 3 Hz and 5 Hz with an amplification level up to 9 with respect to the reference station (NRN2);

The amplification peak frequencies between 4 and 5 Hz shows a clear polarization effects: the highest amplification factors are detected for direction of motion in the range 80° and 100°;

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;

Considering near field data amplification peaks not probably due to the site are detected (peak around 2 Hz): considering far field data this effect disappears, highlighting the site response between 3 and 5 Hz;

The same results is obtained if different phase of signals are considered: analyses on coda less undergo local effects not reflecting the site response (the peak around 2 Hz disappears);

Amplification peak at frequencies between 4 Hz and 10 Hz are detected also on vertical component both considering near and far field data sets;

HVSRs results generally well agree, in terms frequencies, to those obtained from SSRs, even if the example of L’Aquila highlights the possibility of wrong interpretations (in terms of amplified frequency) in absence of corresponding SSR results;

In urban areas, 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 (BEM)

Codes:2D : HYBRID (Kamalian et al., 2003) 3D : BEMSA (Sohrabi et al., 2009)

Domain: elastic, homogeneous and isotropic

Input parameters:γ = 23.5 KN/m3

θ = 0.37Vs = 1000 m/sVp = 2210 m/s

Input at bedrock: Ricker wavelet (fc=3Hz)

Investigated frequencies 1 - 8 HzTransverse section P1 (NRN7) e P2 (NRN4) : DEM resolution 20 m

2D and 3D modelling: input parameters2D and 3D modelling: input parameters


27 eventi ML<3.7 - Re < 30 Km


SSR

H/V

2D results: NRN7 (T247)2D results: NRN7 (T247)


2D results : NRN4 (T268) 2D results : NRN4 (T268)


SSR

H/V




SSR

H/V


3D results: NRN7 (T549) and NRN4 (T122)3D results: NRN7 (T549) and NRN4 (T122)

SSR

H/V




1 Hz1 Hz 3 Hz3 Hz

4 Hz4 Hz 5 Hz5 Hz

3D results for the whole ridge3D results for the whole ridge


1

2

Freq.=1 Hz

Freq.=4 Hz

Freq.=3 Hz

Freq.=5 Hz

In general, at least in terms of amplified frequencies, both 2D and 3D analyses well agree with those obtained from the experimental ones: a constant amplification between 4 and 5 Hz was found. For profile P2 (NRN4), where the ridge shows the highest transversal width, the peak slightly moves to lower frequencies;

Both 2D and 3D models also produce a slight amplification peak around 1 Hz. This result, also considering the experimental evidences, leads to a double interpretation: 1) the peak around 1Hz reflects the fundamental frequency of the ridge and the peak around 4 Hz represents the first higher mode of vibration 2) the peak around 4 Hz represents the fundamental frequency of the ridge while the peak around 1 Hz is due to other causes;

As highlighted in many studies, also for Narni, the models are not able to well reproduce the amplification estimated by the experimental SSRs: an underestimation of a factor 2 was obtained;

Having the Narni ridge a clear 2D configuration, the 3D model does not improve the final results: while for HVSRs, 2D and 3D results well agree, for SSRs the 3D model is not able to isolate particular peaks;

The missing improvement of results from 2D to 3D homogeneous models of Narni, suggests that the observed amplification between 3 and 5 Hz could not be due to topography effects only, but to a coupling between topography effects and other causes (e.g. rock weathering or structural anisotrophies).

Considerations about numerical simulationsConsiderations about numerical simulations


Empirical topography corrective coefficientEmpirical topography corrective coefficient

Calibration of empirical predictive models, in terms of PGA and SA (5%) ordinates up to 1s, for the reference station NRN2 and those located at the top.

Example for EW component recorded at NRN7


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

1

2St corrective coefficients for NRN7

Application of empirical corrective coefficientsApplication of empirical corrective coefficients


Deaggregation results for Narni in terms of SA at the period of 0.2 s, obtained from the seismic hazard map of Italy http://esse1.mi.ingv.it

Predictive model of Bindi et al., 2010 (blue), supposing an earthquake of M5 at 5 km of distance from Narni, as inferred from the deaggregation analysis. Red is NTC 2008 design spectrum

Grey squares are SA corrected for St

This image is a courtesy of Barani S.

Characterization of anomalies for the single station

Selection of stations at the top of topography from ITACA (http://itaca.mi.ingv.it)

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

Reference GMPEs : Bindi et al., 2010 (M >4; R<200; EC8)

Correction for inter-event variability (if possible)

Considerations for other Italian sitesConsiderations for other Italian sites


Example for Narni

Aulla (AUL) - A T1 (?)Aulla (AUL) - A T1 (?)


∆ PGA (%) = 63


Castelvecchio Subequo (CA03, CA02) : A Castelvecchio Subequo (CA03, CA02) : A

∆ PGA (%) = 47


Sellano Est (SELE) : B / T2Sellano Est (SELE) : B / T2

∆ PGA (%) = 31

Final commentsFinal comments

Considering the results obtained both for Narni ridge, but also considering many experimental evidences reported in bibliography, a relevant effect of the morphology on ground shaking is observed, in particular for stations installed at the top of the topography.

NTC, 2008The St corrective coefficients (ranging from 1 to 1.4) proposed by NTC08 is period independent and seems not to be useful to well predict amplification at the top of topography. A simple shift (in amplitude) of the design response spectrum for A-T1 site leads to underestimate the frequencies of interest.

SEISMIC INPUT Residuals between recorded data at the top of a topography and predicted ones by Italian GMPEs, calibrated by Bindi et al. (2010), highlight amplifications at high frequencies for sites classified in A soil category: these recordings could not be used as seismic input.

PREDICTIVE MODELSThe inclusion of topographic effect in the calibration of empirical ground motion predictive models could be useful to reduce the inter-station variability for site characterized by particular morphology, leading to a possible decrease of the epistemic uncertainties in seismic hazard estimation.


…thanks for your attention…

EXTRA SLIDE

Once the transfer functions are available, it is possible easily to obtain the site response, at receiver points by having some real seismogram at reference stations.

1. For 2D configuration the SSRSSR would define as follows:

If the cross-coupling H12 term would be disregarded then the SSR would be simply equal with H11.

2. For 2D configuration the H/V would define as follows:

)()()()(/)()()()()(/)()( 22212121211121 fIfHfIfHfIfHfIfHfOfOfHV kkkkkkk

If the cross-coupling terms H12 and H21 are disregarded, and the ratio between the spectrum of the horizontal to vertical component of the reference site is supposed to be unit, as the classical definition of a reference site, then the ratio would be as follows:

)(/)()()(/)()()( 2211222111 fHfHfIfHfIfHfHV kkkkk

)(/)()()()()(/)()( 121211111 fIfIfHfIfHfIfOfSSR kkkkkk


HVSR and SSR HVSR and SSR


Normalized design spectra (NTC 2008)Normalized design spectra (NTC 2008)

NARNI AULLA CASTELVECCHIO S.

SELLANO EST ARQUATA T. LAURIA

19790919213537 Mw 5.8, R 22 Km

19800524201605 Mw 4.3, R 15 Km

19980403072636 Mw 5.1, R 35 Km

20081223152421 Mw 5.4, R 47 Km

20081223215825 Mw 4.9, R 48 Km

19980909112800 Mw 5.6, R 10 Km

19900116001941 Mw 3.7, R 5 Km19980321164509 Mw 5.0, R 7 Km

20001216073107 Mw 4.2, R 5 Km 20090924161457 Mw 4.2, R 48 Km

Documents

DOTTORATO DI RICERCA IN GEOFISICA XXIII CICLO Dottoranda: Sara Lovati Università degli Studi di Genova (Dipartimento per lo studio del Territorio e delle