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

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Presentation on theme: "DOTTORATO DI RICERCA IN GEOFISICA XXIII CICLO Dottoranda: Sara Lovati Università degli Studi di Genova (Dipartimento per lo studio del Territorio e delle."— Presentation transcript:

1 DOTTORATO DI RICERCA IN GEOFISICA XXIII CICLO Dottoranda: Sara Lovati Università degli Studi di Genova (Dipartimento per lo studio del Territorio e delle sue Risorse) GROUND MOTION AMPLIFICATION INDUCED BY TOPOGRAPHIC IRREGULARITIES: RESULTS, OPEN 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

2 Summary 1 TOPOGRAPHIC EFFECTS: STATE OF THE ART Theory Analytical, numerical and experimental studies The Italian seismic rules for building (NTC 2008) 2 TECHNIQUES FOR SEISMIC SITE RESPONSE EVALUATION Experimental methods (reference and non reference site) Numerical simulations (BEM) 3 THE CASE STUDY OF NARNI RIDGE (CENTRAL ITALY) Seismic monitoring of the topography Results from recorded data Numerical simulations of the ridge Empirical 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

3 Seismic local site response A1 (ω) = S(ω) P(ω) r1(ω) t1(ω) A2 (ω) = S(ω) P(ω) r2(ω) t2(ω) A3 (ω) = S(ω) P(ω) r3(ω) t3(ω) For sites 1, 2 and 3 S(ω) and P(ω) are common F 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) = 1 For site 2 r2(ω) = 1 (rock) A3(ω)/A2(ω) = r3(ω) Sara Lovati, Ph.D. Thesis, final presentation Genova, 13 Aprile 2011

4 Topographic 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 f 0 = 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 Sara Lovati, Ph.D. Thesis, final presentation Genova, 13 Aprile 2011

5 Topographic amplification: analytical studies v= v 0 e[i ω (t+z/ β)] = v 0 eiKz ei ω t v= v 0 (eiKz +e-iKz)+ v 0 (eiKx +e-iKx) v= 2v 0 (cosKx + cosKz) On wedge sides, x = ±z v= 4v 0 cos Kx= 4v 0 cosKz On vertex, x=0 and z=0 v(0,0)= 4 v 0 (Sanchez-Sesma, 1990) Wedge-shaped homogeneous and elastic material φ = 90° For φ = 120° v (0,0)= 3 v 0 For φ = 180° v (0,0)= 2 v 0 For φ = 270° v (0,0)= 1.3 v 0 In general v (0,0)= v 0 360°/ φ According to the model the amplification at the top only depends on GEOMETRY Surface motion equations: 120° 270° Sara Lovati, Ph.D. Thesis, final presentation Genova, 13 Aprile 2011

6 Topographic amplifications: numerical studies Transfer functions Time – histories displacement EW section Example : 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 Sara Lovati, Ph.D. Thesis, final presentation Genova, 13 Aprile 2011

7 (from Buech et al., 2010) Little Red Hill (New Zeland) Calitri (Italy) (from Faccioli and Paolucci, 2005) 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) Sara Lovati, Ph.D. Thesis, final presentation Genova, 13 Aprile 2011

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

9 Results from worldwide experimental studies Amplification factors vs shape-ratio from experimental studies R 2 =0.87 Resonance frequency vs mountain width (left) and shape-ratio (right) as inferred from experimental (blue) and analytical studies (red) Sara Lovati, Ph.D. Thesis, final presentation Genova, 13 Aprile 2011

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

11 Techniques 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 TECHNIQUEREFERENCE SITE TECHNIQUE A ik (f)A ij (f) A ij (f) = S i ( f ) *P ij (f)*G j (f)*I j (f) A ij H(f) / A ij V(f) A ij (f) = S i ( f ) *P ij (f)*G j (f)*I j (f) A ij (f) / A ik (f)= [S i ( f )*P ij (f)*G j (f)*I j (f)]/[S i ( f )*P ik (f)*G k (f)*I k (f)] G j (f) / G k (f) A ii V(f) A ii H(f) Example of directional SSR at Narni site

12 (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) P - SV K 1(x) 2(y) 3(z) SV-x direction I 1 (f)=1; I 2 (f)= I 3 (f)= 0 H 11 (f)= O 1 (f)/I 1 (f) H 21 (f)= O 2 (f)/I 1 (f) H 31 (f)= O 3 (f)/I 1 (f) H 12 (f)= O 1 (f)/I 2 (f) H 22 (f)= O 2 (f)/I 2 (f) H 32 (f)= O 3 (f)/I 2 (f) P-z direction I 3 (f)=1; I 1 (f)=I 2 (f)=0 H 13 (f)= O 1 (f)/I 3 (f) H 23 (f)= O 2 (f)/I 3 (f) H 33 (f)= O 3 (f)/I 3 (f) SV- y (SH) direction I 2 (f)=1; I 1 (f)= I 3 (f)= 0 Sara Lovati, Ph.D. Thesis, final presentation Genova, 13 Aprile 2011

13 THE CASE STUDY OF NARNI RIDGE (CENTRAL ITALY) Sara Lovati, Ph.D. Thesis, final presentation Genova, 13 Aprile 2011

14 Morphological setting (A-T3/T4) SE NW 1300 m 450 m 870 m 22° 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 Sara Lovati, Ph.D. Thesis, final presentation Genova, 13 Aprile 2011

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

16 Data set and processing 702 selected events (about 10.000 waveforms) 642 from April 2009 LAquila 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 M L up to 3.6) Far field data set (LAquila sequence, M L 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) Sara Lovati, Ph.D. Thesis, final presentation Genova, 13 Aprile 2011

17 16 th December 2000, Mw 4.2 Narni earthquake Sara Lovati, Ph.D. Thesis, final presentation Genova, 13 Aprile 2011 R 5.5 km Est of Narni (depth 9.8 Km) NRN – 10s of S phase

18 HVSR results on S-phase 27 events R-epi < 30 Km, 1.5 M L 3.6 Sub set of events with source to site azimuths between 60° and 120° NR10 NRN1 NRN7 Sara Lovati, Ph.D. Thesis, final presentation Genova, 13 Aprile 2011

19 HVSR results on coda 27 events R-epi < 30 Km, 1.5 M L 3.6 NR10 NRN1 NRN7 Sub set of events with source to site azimuths between 60° and 120° Sara Lovati, Ph.D. Thesis, final presentation Genova, 13 Aprile 2011

20 SSR results : dependence on source-site direction R-epi < 30 Km, 1.5 M L 3.6 azimuths between 60° and 120° NRN4 NRN7 NRN2 Sara Lovati, Ph.D. Thesis, final presentation Genova, 13 Aprile 2011

21 SSR results: dependence on epicentral distance R-epi < 30 Km60 < R-epi < 80 Km NRN7 NRN2 NRN7/NRN2 Sara Lovati, Ph.D. Thesis, final presentation Genova, 13 Aprile 2011

22 SSR results: dependence on phase events R-epi < 30 Km, 1.5 M L 3.6 NRN4 NRN7 NRN2 Sara Lovati, Ph.D. Thesis, final presentation Genova, 13 Aprile 2011 coda S-phase

23 SSR results: components NRN4 NRN7 NRN2 HorizontalVertical Sara Lovati, Ph.D. Thesis, final presentation Genova, 13 Aprile 2011

24 HVSR vs SSR : LAquila aftershocks 4.7 M L 5.3 NRN1NRN7 NRN2 NRN1 HVSR SSR basetopmiddle top Sara Lovati, Ph.D. Thesis, final presentation Genova, 13 Aprile 2011

25 NR10 NRN4 Soil 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 Sara Lovati, Ph.D. Thesis, final presentation Genova, 13 Aprile 2011

26 Considerations about experimental results For 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 LAquila 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. Sara Lovati, Ph.D. Thesis, final presentation Genova, 13 Aprile 2011

27 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/m 3 θ = 0.37 Vs = 1000 m/s Vp = 2210 m/s Input at bedrock: Ricker wavelet (fc=3Hz) Investigated frequencies 1 - 8 Hz Transverse section P1 (NRN7) e P2 (NRN4) : DEM resolution 20 m 2D and 3D modelling: input parameters Sara Lovati, Ph.D. Thesis, final presentation Genova, 13 Aprile 2011

28 27 eventi M L <3.7 - Re < 30 Km SSR H/V 2D results: NRN7 (T247) Sara Lovati, Ph.D. Thesis, final presentation Genova, 13 Aprile 2011

29 2D results : NRN4 (T268) 27 eventi M L <3.7 - Re < 30 Km SSR H/V 27 eventi M L <3.7 - Re < 30 Km Sara Lovati, Ph.D. Thesis, final presentation Genova, 13 Aprile 2011

30 27 eventi M L <3.7 - Re < 30 Km SSR H/V 27 eventi M L <3.7 - Re < 30 Km 3D results: NRN7 (T549) and NRN4 (T122) SSR H/V 27 eventi M L <3.7 - Re < 30 Km Sara Lovati, Ph.D. Thesis, final presentation Genova, 13 Aprile 2011

31 1 Hz 3 Hz 4 Hz 5 Hz 3D results for the whole ridge 3D results for the whole ridge Sara Lovati, Ph.D. Thesis, final presentation Genova, 13 Aprile 2011 1 2 Freq.=1 Hz Freq.=4 Hz Freq.=3 Hz Freq.=5 Hz

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

33 Empirical 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 Sara Lovati, Ph.D. Thesis, final presentation Genova, 13 Aprile 2011 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 2 St corrective coefficients for NRN7

34 Application of empirical corrective coefficients Sara Lovati, Ph.D. Thesis, final presentation Genova, 13 Aprile 2011 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.

35 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 sites Sara Lovati, Ph.D. Thesis, final presentation Genova, 13 Aprile 2011 Example for Narni

36 Aulla (AUL) - A T1 (?) Sara Lovati, Ph.D. Thesis, final presentation Genova, 13 Aprile 2011 PGA (%) = 63

37 Sara Lovati, Ph.D. Thesis, final presentation Genova, 13 Aprile 2011 Castelvecchio Subequo (CA03, CA02) : A PGA (%) = 47

38 Sara Lovati, Ph.D. Thesis, final presentation Genova, 13 Aprile 2011 Sellano Est (SELE) : B / T2 PGA (%) = 31

39 Final 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, 2008 The 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 MODELS The 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. Sara Lovati, Ph.D. Thesis, final presentation Genova, 13 Aprile 2011

40 …thanks for your attention…

41 EXTRA SLIDE

42 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. SSR 1. For 2D configuration the SSR 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: 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: Sara Lovati, Ph.D. Thesis, final presentation Genova, 13 Aprile 2011 HVSR and SSR

43 Sara Lovati, Ph.D. Thesis, final presentation Genova, 13 Aprile 2011 Normalized design spectra (NTC 2008) NARNIAULLACASTELVECCHIO 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


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