Micro Seismic Hazard Zonation Workshop/Training on Earthquake - PowerPoint PPT Presentation

Micro Seismic Hazard Zonation Workshop/Training on Earthquake Vulnerability and Multi-Hazard Risk Assessment: Geospatial Tools for Rehabilitation and Reconstruction Efforts Siefko Slob INTERNATIONAL INSTITUTE FOR GEO-INFORMATION SCIENCE AND EARTH OBSERVATION

Approach to use GRA in GIS for seismic microzonation purposes � Seismic microzonation based on spatial variation of ground ground response � Ground response is mainly dependent on: � Soil model or overburden thickness – LATERAL variation � Maximum shear strength of soil units (G max ) or Seismic velocity of the soil units (V s ) – VERTICAL variation � Therefore, if the spatial variation of ground properties can be modeled, the spatial variation in ground response can also be calculated 2

How to model the spatial variation in ground properties? � Create a model of the subsurface � True 3D soil model � Boundary layer model � Overburden thickness model � Acquire and model soil properties � Lateral – may assume more or less homogeneous properties for same soil unit � Vertical – Generally large difference between different units 3

What parameter do we use for the microzonation? � We want to spatially model the spatial hazard for different building types � Therefore, model spatial variation of � Amplification (qualitative), or � Spectral acceleration (quantitative) For different frequency ranges (i.e. corresponding to natural frequencies of specific building categories) 4

Seismic microzonation: scope � Seismic hazard analysis first step towards earthquake risk reduction strategy in earthquake-prone areas � Particularly for “smaller” cities in developing countries risk reduction is important � Challenge: to develop simple method for microzonation on basis of limited information 5

How do we do microzonation? � Calculation of spatial distribution of seismic response � Identification of areas where amplification of seismic signal results in unacceptably high acceleration levels � Knowledge of subsurface characteristics essential. e.g.: � Overburden thickness � Soil profile � Material properties of soil layers 6

How can we do microzonation? � Standard approach: � Classify into areas with homogeneous subsurface conditions � Calculate seismic response (with e.g. SHAKE) for typical soil profile � Disadvantage: � Large generalisation beforehand � Discrepancies at borders of “homogeneous” areas 7

Improved method (1) 1. Create a continuous (3D) ground model in GIS using any available surface and subsurface information: Boreholes, SPT’s � Geophysical profiles (VES, refraction) � (Engineering) Geological maps � Any other a-priori geological knowledge of the � “model builder” 8

Improved method (2) 2. Calculate seismic response for every surface point on the continuous layer model (using SHAKE) 3. Visualise spatial variation of seismic response over the entire modeled area using GIS 4. Classify seismic response analyses into areas with different hazard levels (e.g. exceeding design acceleration levels) 9

Spectral acceleration as classification criterion � Before, Peak Ground Acceleration (PGA) was considered as main criterion � More important to know the acceleration or amplification for different frequencies � Energy-content of seismic signal is frequency- dependent. � This approach allows for: � Visualisation of spatial variation of the acceleration for different frequencies, i.e. corresponding to natural frequencies of typical buildings. � In this way, hazard maps can be made specific for different building types, e.g. high-rise buildings v.s. low-rise buildings. 10

Case study 1 – Armenia, Colombia Earthquake 28 Jan. � 1999, M 6.1 1,100 persons killed, � 4,800 injured 45,000 houses � damaged and destroyed Approach: boundary � layer model 11

Modelling of the seismic response of this earthquake 1. Creation of a (semi-) 3D model of the subsurface using: Geotechnical maps � Borehole information � Seismic surveys (refraction and resistivity) � 2. Calculation of the seismic response of the ground at regular grid intervals (15x15 m) 12

Step 1: Creation of subsurface boundary layer model in GIS � Import data � Overburden thickness map � Borehole data � Geophysical profiles (resistivity) � Geological surface map � Laboratory tests for soil properties � Interpolation of input data 13

(Semi-) 3D grondmodel � How can we create a 3D model in a 2D GIS? � By modelling the boundaries of the different geotechnical units as surfaces (done in Ilwis) � I.a.w: interpolating the surfaces as DEM’s � This only works well with relatively simple geology (no faults, folds, etc.) 14

Geotechnical units mapped at the surface 15

Isopach map of the top Ash layer 16

Representation of the surface of the top of the basement unit 17

Profile through the 3D model (1) 18

Profile through the 3D model (2) 19

Step 2: Calculation of seismic response using SHAKE � SHAKE is 1D ground response analysis: for level or gently sloping sites with parallel material boundaries � GIS raster data of 4-layer model; for every pixel a ground profile can be obtained � Ground profile exported and seismic response calculated using SHAKE � Calculated results subsequently visualised in the GIS 20

Ground response is calculated for every pixel � Fourier amplitude spectrum: Frequency- dependency of amplification ON Soil profile - A N. Brasilia - Layer 1 - CCA nalysis No. 1 - Profile No. 1 LA EW 0.050 0.050 0.045 0.045 Layer No. 1 Layer No. 4 0.040 0.040 Amplitude 0.035 0.035 Fourier Amplitude Spectrum Fourier Amplitude Spectrum 0.030 0.030 0.025 0.025 0.020 0.020 0.015 0.015 0.010 0.010 0.005 0.005 0.000 0.000 0 0 2 2 4 4 6 6 8 8 Frequency (Hz) Frequency (Hz) Frequency 21

Seismic response analysis using the external software ‘Shake’ � Input for the program � Stratigraphy (in this case a simple 4 layer model) � Thickness of every layer � For every unit geotechnical parameters (assumed constant): � Density � Shear wave velocity � Earthquake signal: � Accelerogram that was recorded with an accelerograph in rock 5 km SE of Armenia 22

Accelerogram of the Quindío quake used as input signal Accelerogram CCALA EW Acceleration (cm/sec 2 ) 600 400 200 0 -200 -400 -600 0 5 10 15 20 25 30 35 40 Time (sec) 23

Important is the possible amplification of the input signal due to resonance of the soil layers Peak Acceleration (g) 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 0 Ash Ash -20 Depth (ft) -40 Residual soil Residual soil -60 Saprolite Saprolite -80 Base lahar lahar/pf /pf Base -100 CCALA EW - Profile N. Brasilia 24

Amlification (resonance) is dependent on frequency N. Brasilia - Layer 1 - CCALA EW 0.050 0.045 Layer No. 1 0.040 0.035 Fourier Amplitude Spectrum 0.030 Fourier spectrum of the 0.025 0.020 output signal at the surface 0.015 0.010 0.005 0.000 0 2 4 6 8 Frequency (Hz) ON Soil profile - Analysis No. 1 - Profile No. 1 0.050 0.045 Layer No. 4 0.040 Largest resonance Fourier spectrum of the 0.035 Fourier Amplitude Spectrum 0.030 occurs at 3Hz input signal in the 0.025 0.020 basement 0.015 0.010 0.005 0.000 0 2 4 6 8 25 Frequency (Hz)

Respons spectrum: accelerations for different frequencies CCALA EW - Profile N. Brasilia 6 5 Spectral Acceleration (g) 4 Natural frequencies Acceleration of 2g(!) of low-rise buildings at 3Hz 3 2 1 0 0.01 0.1 1 10 Period (sec) 26

Seismic hazard zonation � Map the differences in accelaration for a particular frequency, characteristic to a the natural frequency of certain buildings Type of object or structure Natural frequency (Hz) One-story buildings 10 3-4 story buildings 2 Tall buildings 0.5 – 1.0 High-rise buildings 0.17 27

Practical realisation � Special software developed as interface between the 3D ground model of Ilwis and the seismic response program ‘SHAKE’ � For GRID cells of 15x15m a stratigraphic column is created on the basis of the 3D GIS model which is exported as an ASCII point table: � (X,Y,thick1,thick2,thick3) � The output of the SHAKE analysis (the spectral acceleration) is subsequently added as an extra attribute to the ASCII table: � (X,Y,thick1,thick2,thick3,Accel10Hz,Accel5Hz) 28

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Three-dimensional Effects � Three-dimensional modeling of topographic effects – Brasilia area � Using DIANA: a real 3D finite element method � Limited number of elements: only a small model of the topography could be build � Surface accelerations are up to 2 to 4 times the base level (input) accelerations 33

Surface acceleration as result of 3D modeling: red is high; blue is low acceleration 34

Surface acceleration as result of 3D modeling: maximum acceleration ≈ 3 m/s2 which is about 3 to 4 times the maximum in the base level signal 35

Case study 2 – San Jose, Costa Rica � San Jose valley in earthquake-prone area � Early 1990’s several large earthquakes in Costa Rica � Seismic hazard zonation carried out, traditional approach � Test case for improved method � Approach: overburden thickness model 36