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Advances and problems in understanding the seismic response of potentially unstable slopes

Advances and problems in understanding the seismic response of potentially unstable slopes. Vincenzo Del Gaudio 1 and Janusz Wasowski 2. 報告 者：林子翔 指導 教授：李錫堤 報告 日期： 01/06. 1. Dipartimento di Geologia e Geofisica, Universita` di Bari, Bari, Italy.

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Advances and problems in understanding the seismic response of potentially unstable slopes

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  1. Advances and problems in understanding the seismic response of potentially unstable slopes Vincenzo Del Gaudio1 and Janusz Wasowski2 報告者：林子翔 指導教授：李錫堤 報告日期：01/06 1. Dipartimento di Geologia e Geofisica, Universita` di Bari, Bari, Italy. 2. Istituto di Ricerca per la Protezione Idrogeologica, Consiglio Nazionale delle Ricerche, Bari, Italy.

  2. Seismic response Seismograph Site effect Source effect Path effect Picture from： 大地地理雜誌

  3. Site effects • September 19, 1985 • May 20 & November 15,1986 • October 17, 1989

  4. Research motive • Why are we studying the site effects on unstable slopes? • What conditions will the site effects happen? • What will happen cause by site effects? • Our comprehension from previously studies in this problem • The expect of improvement and breakthrough through this study

  5. Research motive Accelerometric monitoring of Caramanico landslide-prone slopes. The influence of site effects on landslide triggering during earthquakes has been inferred in several studies, but its evaluation is made difficult by the complexity of factors controlling the dynamic response of potentially unstable slopes and also by the lack of local ground motion instrumental observations. Considering the above, a local permanent network of accelerometricstations was sited in2002 on unstable slopes in a mountainous area of central Italy, around the town of Caramanico.

  6. ROMA Study Area Orfento and Orta river valleys • high relief • active river erosion • strong permeability contrasts between different lithologies • abundant rainfall • close to active seismogenicstructures • a case triggered by an event that was quite far away (with epicentral distance more than 100 km)

  7. Maiella Mts. Alto hill Orta river Caramanico Terme Orfento river Study Area Mt. Morrone CaramanicoTerme Legend Lm = limestones - Miocene; Mp = marly mudstones, Early Pliocene; Bq= carbonate megabreccias – Quaternary (?); Sh = soils (colluvial materials, landslide deposits, water-laid and eluvial sediments, artificial ground – Holocene); b = carbonate brecciaz (Quaternary); 1 = overthrust front of the Morrone Mt; 2 = faults; 3 = steep scarp of the megabrecciacaprock; 4 = lithological limit; 5 = spring.

  8. 1 km Study Area Accelerometric monitoring of a landslide-prone slopes at Caramanico Terme (Central Italy) CAR3 CAR1 CAR4 CAR5 CAR2 2004.12.03 2005.11.11 2002.10.10 2006.03.27 2002.11.06

  9. CAR4 CAR2 CAR5 CAR1 CAR3 Study Area accelerometricnetwork = accelerometricstations. = microseismic noise measurements.

  10. Study Area Geologicprofiles Car1 Car5 Car2 Car3 Car4

  11. Data Acquisition Weak Ground motion data Period N. ev. N. record M>4 & N. rec >1 Max Mag Max Dist. 2002-2008 82 152 2 5.7 161 km Molise 2002 mainshock

  12. Data Acquisition L’Aquila earthquake of 6 April 2009 (MW = 6.3)

  13. Data Acquisition L’Aquila earthquake of 6 April 2009 (MW = 6.3) Period N. ev. N. record M>4 & N. rec >1 Max Mag Max Dist. 2002-2008 82 152 2 5.7 161 km 2009-2010 119 332 14 6.3 149 km Molise 2002 mainshock L’Aquila 2009 mainshock Up Up North North East East CAR1 recordings

  14. Weak motion data2002-2008

  15. Data Analysis CAR1vs CAR2 CAR2 (landslide site) CAR1 (substratum outcrop) Amplification (16 events) PHA(gal) Ia(m/s) Average 1.36 1.88 Min 0.83 1.06 Max 1.83 3.62

  16. Data Analysis CAR3vs CAR4 CAR4 (reference) CAR3 (on breccias) Amplification (18 events) PHA(gal) Ia(m/s) Average 1.02 0.65 Min 0.30 0.20 Max 4.75 1.68

  17. Research Method Calculate the Ia Arias(Arias, 1970),所定義AI之公式如下： 其中g為重力加速度（m/sec2）；Td為時間（recording duration）（sec）；a(t)為測站接收到地震所產生的加速度值（m/sec2），AI單位為（m/s）。 (corrected for the instrument response with20 Hz cut-off frequency)

  18. Research Method Polar diagrams and Calculate the ellipticityratio ellipticity ratio： the ratio between maximum and minimum of Ia values measured along horizontal directions at different azimuths. Using E-W and N-S components of the recordings to calculate the Arias Intensity on accelerogramsrotated at 10° azimuth intervals. Polar diagrams show the directional variations of AI, normalized by its maximum value. Diagrams are given for a representative sample of events differing for magnitude, distance and back-azimuth, together with the average normalized AI values (NAIav), calculated in different directions for all the recorded events.

  19. Discussion and Conclusions Preliminarily study amplification： • The dynamic response of Car2 compare with Car1,an average relative amplification by a factor of 2.2 in total shaking energy. • At the site CAR2 constantly high ellipticity values were found, with all the Ia maxima oriented within anarrow azimuth interval around the local maximum slope direction. Site Effects： Support： Preliminarymeasurements of S-wave velocity with the techniqueof refraction microtremor analysis [Louie, 2001] gavevalues of 300–600 m/s for the landslide material and1000–1500 m/s for the mudstone. At CAR3 a contributionto amplification likely derives from both topographic effectand impedance contrast between the carbonate brecciasandunderlying limestones. Hypothesis： The relative amplification at CAR2 was attributed to impedance contrast between the colluvial (landslide) deposits and the underlying mudstone.

  20. Data Analysis Polar diagrams of normalized Arias intensity Max,0.90；min,0.83 Max,0.99,260° Min,0.55,170° Max,0.96,290° min,0.33,200° Max,0.89；min,0.59

  21. Research Method site effect evaluations site effect evaluations byreference site methodand non-reference sitemethod HSS=Horizontal Spectrum ofSedimentary site Reference site method HSR=Horizontal Spectrumof Reference site HNS=Horizontal Noise of Sedimentary site HNR=Horizontal Noise of Reference site Borcherdt(1970) HNS=Horizontal Noise of Sedimentary site Non-reference sitemethod VNS=Vertical Noise of Sedimentary site HSS=Horizontal Spectrum ofSedimentary site VSS=Vertical Spectrum ofSedimentary site Nakamura(1989) Lermo and Chávez-García (1993)

  22. Data Analysis Polar diagrams of HVSR CAR1 CAR2 Horizontal - to -Vertical Spectral Ratio (HVSR) from seismic “weak motion” data until 2008 (15 events) (14 events) CAR3 CAR4 (19 events) (7 events)

  23. Discussion and Conclusions Directivity Site Effects： Directivity： Given the occurrence of directivity in landslides, fault zones and fault-bounded slopes, both with or without the presence of ground motion amplification This phenomenon can be masked by directivity related to source effects, and hence its recognition requires the analysis of several recordings of events with sources located at different station-epicentre back-azimuths and having different focal mechanisms.

  24. strong motion data2009L’Aquila earthquake

  25. Data Analysis Preliminarily study • Site response relative amplification: soft soil sites

  26. Data Analysis PHA amplification vs event magnitude

  27. Data Analysis Arias intensity amplification vs event magnitude 3.1 2.5 22.6

  28. Data Analysis ELLIPTICITY VS EVENT MAGNITUDE& ELLIPTICITY VS EPICENTRAL DISTANCE Source Effects

  29. Data Analysis Arias intensity amplification vs epicentre back-azimuth

  30. Discussion and Conclusions HVSRResults CAR2

  31. Discussion and Conclusions CAR2: comparison HVSR (Horizontal-to-Vertical Spectral Ratios) HVNR (Horizontal-to-Vertical Noise Ratios) SSR (Standard Spectral Ratios)

  32. Discussion and Conclusions • There is evidence that seismic ground motion on slopes covered by thick colluviaor by deep-seated landslides can be considerably amplified and that in some cases this amplification can have a pronounced directional character with maxima oriented along potential sliding directions. • The causes of the directivity phenomena are still unclear: possibly a combination of topographic, lithological and structural factors acts to re-distribute shaking energy, focusing it on site-specific directions.

  33. Discussion and Conclusions • Limiting the observations to few nearby and small magnitude events can cause an underestimate of site response amplification. • The wide range of shaking energy amplification (1.4–36.4) observed for different events relatively to a reference station on rock highlights the difficulty in quantifying local amplification factors.

  34. The End

  35. 波的傳導，根據Snell’s Law，在不同介質中情況 此為討論建立於平行層狀構造上，但事實上地震波所經過的地球內部應是呈現球狀層狀構造。 i V V’ I’ V’>V

  36. 回顧研究方向性文獻 場址效應： 方向性(directivity)： 提到方向性與地形無關： Vidale et al.(1991) at Los Angeles Bonamassa and Vidale (1991) at California (認為是非均質地質條件造成) 提到方向性與地形有關： Squdich et al.(1996) at California(沿著最大坡面有方向性) 與山崩構造有關： Xu et al.(1996)(透過模型發現沿著山崩滑脫面S波產生偏振現象) 其他： Rial(1996)(波被滯留在山波內低速物質而產生放大) Vahdani and Wikstrom(2002)(地震波通過基盤時產生傾斜) Gallipoli and Mucciarelli(2007)(HVSR峰值方向與滑移方向相同)

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