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DOTTORATO DI RICERCA IN GEOFISICA XXIII CICLO

Università degli Studi di Genova (Dipartimento per lo studio del Territorio e delle sue Risorse). Istituto Nazionale di Geofisica e Vulcanologia. DOTTORATO DI RICERCA IN GEOFISICA XXIII CICLO.

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DOTTORATO DI RICERCA IN GEOFISICA XXIII CICLO

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  1. Università degli Studi di Genova (Dipartimento per lo studio del Territorio e delle sue Risorse) Istituto Nazionale di Geofisica e Vulcanologia DOTTORATO DI RICERCA IN GEOFISICA XXIII CICLO GROUND MOTION AMPLIFICATION INDUCED BY TOPOGRAPHIC IRREGULARITIES: RESULTS, OPEN ISSUES AND FUTURE DEVELOPMENTS Dottoranda: Sara Lovati Tutor interno: Prof. Claudio Eva (Università di Genova – Dip.Te.Ris.) Tutor esterno: Dr. Marco Massa (INGV MI-PV) Genova, 13 Aprile 2011

  2. Summary 1TOPOGRAPHIC EFFECTS: STATE OF THE ART Theory Analytical, numerical and experimental studies The Italian seismic rules for building (NTC 2008) 2TECHNIQUES 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 4CONSEQUENCE 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 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) 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) 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. L Resonance frequency H f0= n β/L (from Geli et al., 1988) n is a coefficient that depends on Poisson ratio, type and velocity of incident waves (from Lanzo and Sivestri, 1999) Shape ratio H/L; Vertex angle Sara Lovati, Ph.D. Thesis, final presentation Genova, 13 Aprile 2011

  5. Topographic amplification:analytical studies Wedge-shaped homogeneous and elastic material φ = 90° Surface motion equations: v= v0e[i ω (t+z/ β)] = v0 eiKz ei ω t (Sanchez-Sesma, 1990) 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 In general v (0,0)= v0 360°/ φ 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 120° 270° According to the model the amplification at the top only depends on GEOMETRY Sara Lovati, Ph.D. Thesis, final presentation Genova, 13 Aprile 2011

  6. Topographic amplifications: numerical studies Time – histories displacement EW section Example : Civita di Bagnoregio (Italy) (from Paolucci, 2002) Transfer functions 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. Topographic amplifications: experimental studies Little Red Hill (New Zeland) Calitri (Italy) (from Buech et al., 2010) (from Faccioli and Paolucci, 2005) 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. What is well known 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 amplificationfor 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. Sara Lovati, Ph.D. Thesis, final presentation Genova, 13 Aprile 2011

  9. Results from worldwide experimental studies Resonance frequency vs mountain width (left) and shape-ratio (right) as inferred from experimental (blue) and analytical studies (red) Amplification factors vs shape-ratio from experimental studies R2=0.87 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 Horizontal to Vertical Spectral Ratio (Lermo and Chavez-Garcia, 1993) Standard Spectral Ratio (Borcherdt, 1970) Directional Analysis 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) Aij(f) = Si( f ) *Pij(f)*Gj(f)*Ij(f) AijH(f) / AijV(f) F Aii H(f) Aik(f) Aij(f) Aii V(f) Example of directional SSR at Narni site NON REFERENCE SITE TECHNIQUE REFERENCE SITE TECHNIQUE Sara Lovati, Ph.D. Thesis, final presentation Genova, 13 Aprile 2011

  12. K 3(z) 2(y) 1(x) P - SV Boundary Elements Method (BEM) • 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 (from Paolucci,1999) P-z direction I3(f)=1;I1(f)=I2(f)=0 SV- y (SH) direction I2(f)=1;I1(f)= I3(f)= 0 SV-x direction I1(f)=1;I2(f)= I3(f)= 0 H13(f)= O1(f)/I3(f) H23(f)= O2(f)/I3(f) H33(f)= O3(f)/I3(f) 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) Once obtained Hit is possible to compute at a genericK siteH/V and SSR applying the transfer functions to spectra of real seismograms. 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) Scarpata Est Scarpata Nord-Ovest SE 870 m 22° 35° 1300 m 450 m NW Lato Nord • INGV-DPC agreement 2007-2009 • S4 Projecttask 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 REFERENCE SITE Geology : Massive limestone Network : 10 surveyed sites from March to September 2009 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 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 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) Sara Lovati, Ph.D. Thesis, final presentation Genova, 13 Aprile 2011

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

  18. HVSR results on S-phase NR10 NRN1 27 events R-epi < 30 Km, 1.5 ≤ ML≤ 3.6 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

  19. HVSR results on coda NR10 27 events R-epi < 30 Km, 1.5 ≤ ML≤ 3.6 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 NRN2 azimuths between 60° and 120° R-epi < 30 Km, 1.5 ≤ ML≤ 3.6 NRN7 NRN4 Sara Lovati, Ph.D. Thesis, final presentation Genova, 13 Aprile 2011

  21. SSR results: dependence on epicentral distance NRN2 NRN7 R-epi < 30 Km 60 < 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 NRN2 events R-epi < 30 Km, 1.5 ≤ ML≤ 3.6 NRN7 S-phase S-phase NRN4 coda coda Sara Lovati, Ph.D. Thesis, final presentation Genova, 13 Aprile 2011

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

  24. HVSR vs SSR : L’Aquila aftershocks 4.7 ≤ ML ≤ 5.3 HVSR base middle top NRN1 SSR NRN2 middle NRN7 NRN1 top NRN1 Sara Lovati, Ph.D. Thesis, final presentation Genova, 13 Aprile 2011

  25. Soil structure interaction NRN4 in the basement Noise 24 h: spectrograms NR10 NRN4 Noise measurement inside the tower SSR between two stations at the top SSR between top and base not polarized not polarized 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 levelup 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. Sara Lovati, Ph.D. Thesis, final presentation Genova, 13 Aprile 2011

  27. 2D and 3D modelling: input parameters 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 = 0.37 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 Sara Lovati, Ph.D. Thesis, final presentation Genova, 13 Aprile 2011

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

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

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

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

  32. Considerations about numerical simulations • 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: • the peak around 1Hz reflects the fundamental frequency of the ridge and the peak around 4 Hz represents the first higher mode of vibration • 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). 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 1 Log10 ( Y ) = a + ( b*M ) + ( c * Log ( R ) ) + σ a, b, c coefficients for NRN2 (59 near field earthquakes) 2 Log10 ( Y ) = a + ( b*M ) + ( c * Log ( R ) ) + ( St * 1 ) + σ St corrective coefficients forNRN7 Sara Lovati, Ph.D. Thesis, final presentation Genova, 13 Aprile 2011

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

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

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

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

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

  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. HVSR and SSR 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 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

  43. Normalized design spectra (NTC 2008) NARNI AULLA CASTELVECCHIO S. 20090924161457 Mw 4.2, R 48 Km 20001216073107 Mw 4.2, R 5 Km 20081223152421 Mw 5.4, R 47 Km 20081223215825 Mw 4.9, R 48 Km SELLANO EST ARQUATA T. LAURIA 19980403072636 Mw 5.1, R 35 Km 19980909112800 Mw 5.6, R 10 Km 19790919213537 Mw 5.8, R 22 Km 19980321164509 Mw 5.0, R 7 Km 19800524201605 Mw 4.3, R 15 Km 19900116001941 Mw 3.7, R 5 Km Sara Lovati, Ph.D. Thesis, final presentation Genova, 13 Aprile 2011

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