Seismic Microzonation"

Individual Training Course "Seismic Microzonation" Report Author: Evgheni Isicico Institute of Geophysics and Geology Moldavian Academy of Sciences December 2004 CONTENT 1. Local Site Conditions Modeling on Seismic Microzonation Stage ..........................................3 1.1. Introduction.........................................................................................................................3 1.2. Geometrical parameters of model.......................................................................................5 1.3. Elastic properties of model ...............................................................................................10 1.4. Comparison of amplitude-frequency characteristics of modeling and real sites ..............13 1.5. Comparison of modeling characteristics and H/V ratio of microtremors spectra ............15 1.6. Amplification properties of sites.......................................................................................17 1.7. Resonance properties of sites............................................................................................20 Conclusion ...............................................................................................................................23 2. Study trips ...............................................................................................................................24 2.1. Study trip to Kochi............................................................................................................24 2.2. Study trip to Fukuoka, Kyushu University .......................................................................25 2.3. Study trip to Hokkaido......................................................................................................26 2.4. Study trip to Kobe.............................................................................................................34 2.5. Study trip to Yokohama, Kanagawa University ...............................................................35 3. Lecture course in BRI, IISEE and other activity in Tsukuba..................................................41 Gratitude......................................................................................................................................42 2 1. LOCAL SITE CONDITIONS MODELING ON SEISMIC MICROZONATION STAGE 1.1. Introduction The basic task of modeling in our case is the reception of continuous 3D geophysical model of site conditions for territory Kishinev city. For this purpose it is necessary to know geometrical parameters of distribution of separate layers and their elastic properties, both on the area, and with depth. The modeling usually is the compelled measure, owing to our fragmentary knowledge of geological medium and is based therefore, as a rule, on averaging of geometrical and elastic parameters of layers. In other words, inevitable attributes of modeling are simplification of geological layers and averaging of elastic properties in them. It is natural, that the degree of a divergence of real seismic effects observable on a surface, from simulated entirely is explained by differences of local site conditions from averaged, used in model. For last 50 years on Kishinev city sizeable volume of the geological, hydro-geological and geophysical data was assembled. It has allowed to create continuous simplified 3D-model of site conditions for territory of city. And then to use it as a basis for creation of a map of seismic microzonation. All cartographic drawing and calculations are executed with use of GIS technologies The territory of city represents a valley with hilly slopes. The absolute heights from sea level change within the limits of 30-230 m (fig. 1.1). The area of city is equal 122,3 sq. km. In fig. 1.2 points of seismic observation (21 - earthquakes, 45 - special explosions, 143 – microtremors) and 2673 boreholes (from which 1045 - geotechnic, 118 – seismic borehole logging) are shown. 3 Fig. 1.1. Relief of Ground Surface Fig. 1.2. Map of observation data 4 1.2. Geometrical parameters of model The geometrical parameters of model were determined with the help of maps of various layers and geological cross-sections made on the data of drilling 2673 boreholes. In figures 1.3 and 1.4 the geological cross-sections on lines IV and XI are shown, giving representation about a geological structure of territory. Fig. 1.3. Geological cross-section (line VI) Fig. 1.4. Geological cross-section (line IX) 5 The bedrock of cross-sections is limestone of Neogene age, above lay hard clay of Neogene age, on them lay quaternary deposits consisting from loam, loamy sand, sand, clay and uppermost is the thin layer of soil. Quaternary deposits can be damped by ground water. Vertical scale in figures in 25 times stretches concerning horizontal scale. We can know about behavior of layers within city’s territory from map of top bedrock surface (fig. 1.5) and map of top hard clay surface (fig. 1.6). Fig. 1.5. Map of top bedrock surface (depth from ground surface) 6 Fig. 1.6. Map of hard clay surface (thickness of quaternary deposits) The map of water level (fig. 1.7) gives one more boundary of layers – between dry and damped quaternary deposits. Fig. 1.7. Map of water level 7 The belonging to this or that geotechnical zone defines a degree of damping quaternary deposits. The map of geotechnic zonation is shown in fig. 1.8. We can specify three basic zones: I - low sites of river valleys, II - terraced and watershed sites, III - steep slopes and landslide sites. Fig. 1.8. Map of geotechnic zonation The spatial crossings of each of four maps have allowed to receive contours model polygons with fixed geometrical and elastic parameters of site conditions. For example, polygon {geotechnical zone - II / water level = 10-15 m / top of clay = 25-30 m / top of limestone = 60-70 m} uniquely define depths of each of layers of a modeling site, and also their elastic properties. The procedure of allocation of model polygons was carried out with use GIS (fig.1.9). Total within the limits of city more 4300 polygons were allocated, that corresponds to 424 combinations of mapped parameters. 8 Fig. 1.9. Map of model polygons with fixed properties of site conditions 9 1.3. Elastic properties of model The basic parameters determining elastic properties, are density and velocity of waves in layers. We have more than 1000 boreholes with measurements of density and 118 boreholes with measurements of velocity. It allows to define territorial and deep changes of the specified parameters for each layer of deposits. After the analysis of distribution of density in layers it is possible to tell, most significant is the dependence of density with depth. On its background the territorial variations of density are insignificant. Distributions of density has large dispersion and low coefficient of correlation. The steady tendency of increase of density with humidifying quaternary deposits is observed. As an example in a fig. 1.10 the correlations of density with depth and their approximation for various layers on the data seismic borehole logging is shown. Coefficient of correlation for various layers changes within the limits of 0,68 - 0,77. From figure it is visible, that quaternary deposits it is statistically difficult to separate by density. Quaternary and Neogene clay obviously differ by average meanings of density. 2.2 2.0 loam loamy san sand 1.8 clay(Q) clay(N) loam loamy san 1.6 Density, g/cm^3 sand clay (Q) clay (N) 1.4 1.2 0102030405060 Depth, m Fig. 1.10. Correlations of density with depth and their approximation for various layers 10 The analysis of wave velocities is best for considering depending from density of layers. In this case dependencies have more universal and stable character. They are less sensitive to a type of layers. The correlation of velocities of S-waves with density of deposits of various composition is represented in a fig. 1.11. Owing to large dispersion of the data, the coefficients of correlation are insignificant and make 0,5 - 0,65 for various layers. The tendency in distribution of the data however is clearly traced. The average meanings for various layers differ insignificantly, that specifies an opportunity of forecasting of average velocities of S- waves on the basis of the generalized dependence from density of various layers. 600 500 loam 400 loamy san sand с clay(Q) clay(N) 300 loam elocity, m/ V loamy san sand 200 clay(Q) clay(N) 100 0 1.4 1.5 1.6 1.7 1.8 1.9 2.0 2.1 2.2 2.3 Density, g/сm^3 Fig. 1.11. Correlation’s of velocity of S-waves with density and their approximation for various layers Following the specified principles of modeling and knowledge of geometry and elastic properties of layers in territory of Kishinev, as geophysical model for cities we used a 5-layers medium. It is shown in the table 1 with the indication of range for properties in layers. 11 Table 1 Type Age Thickness Density Velocity Soil Q4 0.5 1.5 120 Dry deposits Q1-4 0-20 1.52-1.78 140-230 Humid deposits Q1-4 0-22 1.71-2.03 200-350 Clay N1 0-200 1.86-2.17 260-450 Limestone N1 2.45 1300 Layers 2 - 4 have not only variable thickness, but also variable elastic properties dependent from depth. Depending from local conditions any of these layers can be absent in some sites. Thus, it is possible to consider creation of model completed. It has the fixed geometrical parameters (fig. 1.9) and certain elastic properties for various layers. 12 1.4. Comparison of amplitude-frequency characteristics of modeling and real

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