Presentazione di PowerPoint · March to September 2009. [email protected] Regione Valle...
Transcript of Presentazione di PowerPoint · March to September 2009. [email protected] Regione Valle...
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
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Summary Summary
1
TOPOGRAPHIC EFFECTS: STATE OF THE ARTPrimary topographic effectsInduced (or secondary) topographic effectsThe Italian seismic rules for building (NTC 2008)
2
A DETAILED CASE STUDY : NARNI (CENTRAL ITALY)Seismic monitoring of the topographyResults from recorded dataComparison with numerical simulationsEvaluation of NTC 2008
3
EXAMPLES FROM OTHERS ITALIAN SITES
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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
F
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)
All
factors
able
to
modify
a seismic
signal
in amplitude, duration
and frequency
content
during
its
propagation
from
the source
to
the recording
site
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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;
f0
= n V/LResonance frequency
(from Geli et al., 1988)
L = width of the topography
V = shear wave velocity
n = 1
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
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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)
H
L
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Geometrical factors : vertex angleGeometrical factors : vertex angle
(Sanchez-Sesma, 1990)
Homogeneous
and elasticmaterial φ
= 90°
v= v0
e[i ω
(t+z/ β)] = v0
eiKz
ei
ω
t
v= v0
(eiKz
+e-iKz)+ v0
(eiKx
+e-iKx)
v= 2v0
(cosKx
+ cosKz)
On vertex, x=0 and z=0
v(0,0)= 4 v0
Surface
motion
equations:
For
φ
= 120°
v (0,0)= 3 v0For
φ
= 180°
v (0,0)= 2 v0
For
φ
= 270°
v (0,0)= 1.3 v0
In general v (0,0)= v0
360°/ φ
120°
270°
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Effects induced by topography : subEffects induced by topography : sub--blocksblocks
Marzorati et al., 2010
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Effects induced by topography : landslidesEffects induced by topography : landslides
(Burianek et al., 2009)
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Stable
area Unstable
area
Frequency
[Hz] Frequency
[Hz]
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)
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∆
PGV
(%) = 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
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∆
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
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∆
H/V=0.3
∆
H/V=4
Frequency domain : different phasesFrequency domain : different phases
Agreement between different phasesAgreement between different phases
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Italian seismic rules for buildings Italian seismic rules for buildings (NTC 2008, D.M. 14/01/2008)(NTC 2008, D.M. 14/01/2008)
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“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
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)
Aij
H(f) / Aij
V(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
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THE CASE STUDY OF NARNI RIDGE (A-T3)
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Morphological Morphological settingsettingScarpata Sud-Ovest
SE
NWScarpata Nord-Ovest
Lato Nord
Lato Nord
Scarpata Est
1300 m
450 m
870 m22°
35°
Azimuth dorsale: N 31°
W -
max ∆H ~ 220 m
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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
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3 white triangles : base6 black triangles : crest1 grey triangle : middle1 white square : DPC station
Data set and processingData set and processing
702
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 )
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Local Magnitude :
1.5 ÷
5.3Epicentral
distance :
5 ÷
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
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321
5
4
NRN2 (base)
NRN1 (middle)
Microtremor survey (results on EW component)Microtremor survey (results on EW component)
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Top
Top
Top Top Top
Top
NonNon--reference site technique (HVSR)reference site technique (HVSR)
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NRN1
NRN2
NRN7
SSR SSR ::
dependence on dependence on source to sitesource to site
directiondirection
All azimuths
(R<30
Km, 1.5≤M≤3.6) azimuths between 60°
and 120°
NRN4
NRN7
NRN2
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SSR SSR ::
dependence on dependence on epicentralepicentral
distancedistance
R
< 30 Km
NRN7
NRN2
NRN7/NRN2 NRN7/NRN2
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60 < R
< 80 Km
SSR SSR ::
dependence on phasedependence on phase
events R
< 30 Km, 1.5 ≤
ML
≤
3.6
NRN4
NRN7
NRN2
coda
coda
S-phase
S-phase
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SSR SSR ::
dependence on componentsdependence on components
NRN4
NRN7
NRN2 Horizontal Vertical
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Wrong evaluations : HVSR vs. SSRWrong evaluations : HVSR vs. SSR
NRN1NRN7
NRN2
NRN1
NRN1
HVSR
SSR
base topmiddle
middle top
An example from An example from LL’’AquilaAquila
aftershocks 4.7 aftershocks 4.7 ≤≤
ML ML ≤≤
5.35.3
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NR10
NRN4
NRN4
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
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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;
HVSR
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.
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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/s
Vp = 2210 m/s
Input at bedrock:
Ricker wavelet (fc=3Hz)
Investigated frequencies 1 -
8 Hz
Transverse sections : P1
(NRN7),P2 (NRN4). DEM
resolution 20 m
2D and 3D 2D and 3D modellingmodelling: input parameters: input parameters
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P P --
SVSV
KK
1(x)2(y)
3(z)
2D2D
results: results: NRN7NRN7
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SSR
H/V
NRN4
NRN7
NRN2
2D2D
results : results : NRN4NRN4
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H/V
SSRNRN4
NRN7
NRN2
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
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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
for
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
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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
1
2St corrective coefficients for
NRN7
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St corrective coefficientsEW component at NRN7
EvaluationEvaluation
of NTC 2008 of NTC 2008 --
ShapeShape
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Normalized
(to
ag
) design response spectrum
for
Narni
A-T3 (in black) Weak
motions
from
Narni
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).
EvaluationEvaluation
of NTC 2008 of NTC 2008 --
AmplitudeAmplitude
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(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)
to
exceede
in the next
50 yr
PGA (on rock)
between
0.150 and 0.175 g (http://esse1-gis.mi.ingv.it/)
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ExampleExample
of of otherother
ItalianItalian
sitessites
Sellano, BSellano, B
--
T2T2
∆
PGA (%) = 32
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Lauria,Lauria,
A A --
T2T2
H V
1σ2σ
1σ2σ
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∆
PGA (%) = 69
CastelvecchioCastelvecchio
Subequo, A Subequo, A --
T3T3
∆
PGA (%) = 126
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CA03 (crest), CA02 (base)CA03 (crest), CA02 (base)
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EvaluationEvaluation
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:
1)
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
have
to
be
used
as
seismic
input for
other
analyses.
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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 ...”
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.... ma esistono nella realtà configurazioni semplici ?
…thanks for your attention…