Aslı Kurtuluş

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.

Site response from Istanbul vertical arrays and strong motion network

In the framework of Istanbul Microzonation Project for the European side, the investigated region was divided by a grid system of 250m×250m and site investigations were performed for each cell based on borings and in-situ seismic wave velocity measurements for defining representative soil profiles with shear wave velocity values extending down to the engineering bedrock. Geological and geotechnical laboratory and field testing data with measured seismic wave velocities enabled to determine the engineering properties of the soil and rock layers encountered in all the cells. There have been limited number of earthquakes within 100km range of Istanbul with local magnitude in the range of ML=4-5 and few more distant and more stronger earthquakes that were recorded by the existing three vertical arrays as well as by the Istanbul Rapid Response Network (IRRN) strong motion stations. Even though the maximum PGA were similar, the observed spectral response were different indicating the importance of the distance and source magnitude concerning the frequency content and predominant soil period ranges. Even though the level of ground shaking intensity is relatively low, efforts were made to evaluate the variation of the recorded accelerations with depth in vertical arrays located at Ataköy, Zeytinburnu and Fatih. Attempts were also made to model the recorded acceleration time histories at the triggered IRRN stations using the acceleration records obtained at the bedrock level from the vertical array stations in the case of the recent 19.5.2011 Mw=5.7 Kütahya earthquake that took place approximately 185km away. 1 Professor, Dept. of Civil Engineering, Ozyegin University, Istanbul, Turkey, e-mail:atilla.ansal@ozyegin.edu.tr 2 Assoc. Professor, Dept. of Civil Engineering, Ozyegin University, Istanbul, Turkey 3 Research Assoc. Dr., Bogazici Uni. Kandilli Observatory and Earthquake Research Institute, Istanbul, Turkey Ansal A, Kurtulus A, Tönük G. Site Response From Istanbul Vertical Arrays And Strong Motion Network. Proceedings of the 10 National Conference in Earthquake Engineering, Earthquake Engineering Research Institute, Anchorage, AK, 2014 Site Response From Istanbul Vertical Arrays And Strong Motion Network A.Ansal , A.Kurtulus , and G.Tönük ABSTRACT In the framework of Istanbul Microzonation Project for the European side, the investigated region was divided by a grid system of 250m×250m and site investigations were performed for each cell based on borings and in-situ seismic wave velocity measurements for defining representative soil profiles with shear wave velocity values extending down to the engineering bedrock. Geological and geotechnical laboratory and field testing data with measured seismic wave velocities enabled to determine the engineering properties of the soil and rock layers encountered in all the cells. There have been limited number of earthquakes within 100km range of Istanbul with local magnitude in the range of ML=4-5 and few more distant and more stronger earthquakes that were recorded by the existing three vertical arrays as well as by the Istanbul Rapid Response Network (IRRN) strong motion stations. Even though the maximum PGA were similar, the observed spectral response were different indicating the importance of the distance and source magnitude concerning the frequency content and predominant soil period ranges. Even though the level of ground shaking intensity is relatively low, efforts were made to evaluate the variation of the recorded accelerations with depth in vertical arrays located at Ataköy, Zeytinburnu and Fatih. Attempts were also made to model the recorded acceleration time histories at the triggered IRRN stations using the acceleration records obtained at the bedrock level from the vertical array stations in the case of the recent 19.5.2011 Mw=5.7 Kütahya earthquake that took place approximately 185km away.In the framework of Istanbul Microzonation Project for the European side, the investigated region was divided by a grid system of 250m×250m and site investigations were performed for each cell based on borings and in-situ seismic wave velocity measurements for defining representative soil profiles with shear wave velocity values extending down to the engineering bedrock. Geological and geotechnical laboratory and field testing data with measured seismic wave velocities enabled to determine the engineering properties of the soil and rock layers encountered in all the cells. There have been limited number of earthquakes within 100km range of Istanbul with local magnitude in the range of ML=4-5 and few more distant and more stronger earthquakes that were recorded by the existing three vertical arrays as well as by the Istanbul Rapid Response Network (IRRN) strong motion stations. Even though the maximum PGA were similar, the observed spectral response were different indicating the importance of the distance and source magnitude concerning the frequency content and predominant soil period ranges. Even though the level of ground shaking intensity is relatively low, efforts were made to evaluate the variation of the recorded accelerations with depth in vertical arrays located at Ataköy, Zeytinburnu and Fatih. Attempts were also made to model the recorded acceleration time histories at the triggered IRRN stations using the acceleration records obtained at the bedrock level from the vertical array stations in the case of the recent 19.5.2011 Mw=5.7 Kütahya earthquake that took place approximately 185km away. Introduction The first stage of the Istanbul Microzonation Project involved detailed microzonation studies that were conducted on the European side of the city [1]. The investigated region was divided by a grid system into cells of 250m×250m and detailed site investigations were conducted in each cell based on borings and in-situ measured seismic wave velocities for defining representative soil profiles with shear wave velocity values extending down to the engineering bedrock [2]. The Istanbul Rapid Response Network (IRRN) composed of strong motion stations distributed more or less evenly with the metropolitan city of Istanbul. 55 of these strong motion stations are located within the area where detailed microzonation study was conducted [3]. In 1 Professor, Dept. of Civil Engineering, Ozyegin University, Istanbul, Turkey, e-mail:atilla.ansal@ozyegin.edu.tr 2 Assoc. Professor, Dept. of Civil Engineering, Ozyegin University, Istanbul, Turkey 3 Research Assoc. Dr., Bogazici Uni. Kandilli Observatory and Earthquake Research Institute, Istanbul, Turkey Ansal A, Kurtulus A, Tönük G. Site Response From Istanbul Vertical Arrays And Strong Motion Network. Proceedings of the 10 National Conference in Earthquake Engineering, Earthquake Engineering Research Institute, Anchorage, AK, 2014 addition three vertical arrays were installed in the same area at Ataköy, Zeytinburnu and Fatih extending all the way down to the engineering bedrock (Vs > 750m/s) as in Figure 1 [4,5,6,7,8]. There have been limited number of earthquakes within 100km range of Istanbul with local magnitude in the range of ML=4-5 and few more distant and more stronger earthquakes that were recorded by the existing three vertical arrays as well as by the Istanbul Rapid Response Network (IRRN) strong motion stations. The stations that recorded the evaluated two events Cınarcık and Kütahya earthquakes are also shown in Figure 1. Figure 1. The distribution of IRRN strong motion stations and the locations of the three vertical arrays within the detailed microzonation area on the European side of Istanbul. Çınarcık and Kütahya Earthquakes During the recent years, Istanbul has experienced limited amount of minor earthquakes. Among these, the highest peak ground accelerations were produced by the ML=4.8 Çınarcık earthquake that took place on 12/03/2008. It was a shallow event with focal depth about 11km, generated by strike-slip movement of one segment of the North Anatolian Fault in the Marmara Region. On the other hand, the Mw=5.7 Kutahya Earthquake of 19.5.2011 were the strongest but distant earthquake recorded by the three vertical arrays. The epicenter distances for these two events were significantly different, Cinarcık epicenter was about 43km while the Kutahya epicenter was about 205km away. As a result as shown in Figure 2, the predominant periods based on acceleration spectra for these two events recorded by the Atakoy vertical array were significantly different due to the differences in the triggering dominant wave frequencies. These two events, even though the PGA at the bedrock level was very similar, in the range of 7mg, indicate that the frequency content and predominant site periods can be significantly different even at the same level of ground shaking and thus is controlled mainly by the source distance and magnitude rather than local site conditions. The effect of distance has introduced differences not in the recorded peak ground accelerations but rather in the frequency content of the acceleration time history and thus in elastic acceleration response spectra as shown in Figure 2. This observation indicates the importance of distance resulting in filtering the higher frequency content of the earthquake ground motion. Thus the definition of predominant periods based on near field small earthquakes and noise measurements may be questionable with respect to the definition of predominant soil periods. The predominant soil periods are very dependent on input ground motion characteristics and thus the use of noise measurements may not always give reliable results with respect to the effects of local site conditions. Figure 2. Acceleration response spectra at different depths for (a) ML=4.8 Çınarcık Earthquake of 12.3.2008 (b) Mw=5.7 Kütahya Earthquake of 19.3.2011 at Ataköy Vertical array. The Mw=5.7 Kütahya Earthquake of 19.5.2011 was the first event that is recorded by all three vertical arrays. Even though the meas