Gökçe Tönük

Source and Site Factors in Microzonation

The comprehensive calculation of site specific earthquake characteristics on the ground surface for microzonation requires input acceleration time histories compatible with the regional earthquake hazard representing source factors. Real and simulated acceleration records were used as input for site response analyses to evaluate the reliability and to observe the induced variability. The second component of site specific calculations is site characterisation and establishment of representative soil profiles down to engineering bedrock that represent the site factors. Grid systems with 250m × 250m cells were adopted to define the site conditions in terms of representative soil profiles for each cell. The third component of the site specific calculations is the analytical procedure used for site response analysis and the interpretation scheme of the calculated site specific parameters. Parametric studies were conducted for the probabilistic assessment of microzonation with respect to ground shaking intensity and liquefaction susceptibility as well as for vulnerability assessment based on acceleration response spectra, peak ground accelerations, and cyclic shear stress variation with depth, calculated by site response analyses for each cell.

Microzonation for Urban Planning

Microzonation is identification of areas having different earthquake hazard potentials and will primarily serve for urban planning and land use management. The two principal factors controlling earthquake loss are site response and structural features. The seismic microzonation maps would indicate the distribution of site response with respect to ground shaking intensity, liquefaction and landslide susceptibility; thus providing an input for urban planning and earthquake mitigation priorities at an urban scale. It is also possible to estimate building damage and causalities based on microzonation maps used as an input to earthquake risk scenarios. These estimates may be very approximate based on the accuracy of the input data and methods of analysis. However, they can also be more realistic when more comprehensive data and more sophisticated analysis methods are implemented. The first stage in an earthquake risk scenario is the estimation of the earthquake hazard on the ground surface based on seismic microzonation maps for detailed assessment of earthquake response on the ground surface. The seismic microzonation maps, in other words earthquake hazard maps, are prepared using estimated earthquake characteristics on the ground surface based on local site conditions and input earthquake characteristics. The estimation of damage and causalities for buildings, lifelines and transportation networks is the second stage and is considered as earthquake damage scenarios. In order to assess the effects of earthquakes in urban areas, it would be necessary to compile historical earthquakes, geological, geotechnical and seismological data, to evaluate the probabilistic and deterministic earthquake hazard, to determine the variation of earthquake characteristics with respect to local site conditions, and microzonation and damage maps that need to be drafted utilizing GIS software packages. Within the contents of this chapter, the first stage of earthquake risk scenarios, microzonation – probabilistic earthquake hazard scenario will be presented based on some case studies.

Site specific response analysis for performance based design earthquake characteristics

During strong earthquakes, seismic waves travelling towards the ground surface alter the engineering characteristics of the soil layers and consequently the characteristics of travelling seismic waves also change with respect to their frequency and amplitude contents. In assessing the site-specific design earthquake characteristics in seismically active zones for performance levels of Collapse Prevention, Life Safety, and Immediate Occupancy that may correspond to 72, 475 and 2475 year return period earthquakes, detailed site characterization and site response analyses may be required. This process may be conducted in two consecutive statistically independent stages. The first stage involves the seismic hazard study to assess the design earthquake characteristics on rock outcrop for selected exceedance levels and the second stage involves detailed site characterization and site response analyses to estimate design earthquake characteristics on the ground surface. The uncertainties arising from the source characteristics need to be taken into account by using a representative number of strong motion acceleration records for site response analyses recorded in locations that are compatible with the seismic hazard with respect to fault mechanism, earthquake magnitude, and source distance. In addition, the strong motion acceleration records should be compatible with respect to peak acceleration and acceleration response spectra levels estimated by the probabilistic or deterministic seismic hazard study. One approach is to use the uniform acceleration hazard spectra and another option is to adopt conditional mean spectrum on rock outcrop estimated in the first stage from the earthquake hazard study for scaling input motions for site response analysis. It was observed that the scaling methodology adopted may play an important role in the calculated earthquake characteristics on the ground surface. A semi empirical procedure was proposed to determine the site specific design earthquake characteristics on the ground surface. A parametric study was conducted to demonstrate the applicability of the proposed methodology based on one dimensional site response analyses using Shake91 and DeepSoil site response codes to evaluate design earthquake characteristics on the ground surface.

Microzonation for Earthquake Scenarios

Seismic microzonation involves generation of seismic hazard maps with respect to estimated ground motion characteristics on engineering bedrock outcrop based on a regional seismic hazard study compatible with the scale of the microzonation. A grid system is implemented dividing the investigation area into cells according to the availability of geological, geophysical and geotechnical data. Site characterizations are performed based on available borings and other relevant information by defining representative soil profiles for each cell with shear wave velocities extending down to the engineering bedrock. 1D site response analyses are conducted to estimate site specific earthquake ground motion characteristics on the ground surface for each representative soil profile to estimate elastic response spectrum based on calculated acceleration time histories. Average of spectral accelerations between 0.1 and 1 s periods of elastic acceleration response spectrum are calculated as one of the two parameters representing earthquake shaking intensity on the ground surface. Site specific peak spectral accelerations corresponding to 0.2 s period are also calculated as the second microzonation parameter using the empirical amplification relationships proposed by Borcherdt (1994) based on equivalent shear wave velocities for the top 30 m of the soil profiles. Superposition of these two parameters is assumed to represent overall effect of site conditions and is adopted as the criteria for the microzonation with respect to ground shaking intensity. Recently, an extensive site investigation study was carried out on the European side of Istanbul as the first phase of the large-scale microzonation project for the Istanbul Metropolitan Municipality. A detailed microzonation with respect to earthquake ground shaking intensity is carried out for the Zeytinburnu town in Istanbul using part of these recently compiled soil data and the regional probabilistic seismic hazard scenario proposed by Erdik et al. (2004). The microzonation maps are compared with the previously generated Zeytinburnu microzonation maps for the European Union Framework FP6 LessLoss Project (Ansal et al., 2007a) and for the Zeytinburnu Pilot Microzonation Project (Ansal et al., 2005; Kilic et al., 2006; Ozaydin et al., 2004) where microzonation maps were produced with limited number of site investigations and site response analyses using more approximate microzonation procedures.

Effects of Stress Reduction Factors on Liquefaction Analysis

Two variables are essential for the assessment of liquefaction susceptibility of sandy soil layers; the seismic demand expressed in terms of cyclic stress ratio, CSR; and the capacity of the soil layers to resist liquefaction, expressed in terms of cyclic resistance ratio, CRR. The variation of the safety factors (CRR/CSR) with depth were determined for set of soil profiles where CSRs were calculated using stress reduction factors proposed in the literature and CRRs based on SPT blow counts. In addition, CSRs were also determined based on site response analyses. A total stress approach (Shake91) and an effective stress approach (Cyclic1D) were used for site response analysis. Safety factors for effective stress analysis were calculated using the CSRs and CRRs both based on the calculated excess pore water pressures for the analyzed soil profiles. A relationship between CSR and number of cycles with respect to excess pore water pressure ratio, r u was derived based on previously conducted cyclic laboratory test results. In the second stage of the study, the effects of input motion on liquefaction susceptibility were evaluated. Site response analyses were conducted using 24 previously recorded acceleration time histories that are compatible with the earthquake hazard for the investigated site scaled to the same PGA. All the results are compared and the differences are discussed in terms of the final assessment of the liquefaction susceptibility.