Elsevier

Engineering Geology

Volume 151, 29 November 2012, Pages 64-81
Engineering Geology

Local site characterization and seismic zonation study by utilizing active and passive surface wave methods: A case study for the northern side of Ankara, Turkey

https://doi.org/10.1016/j.enggeo.2012.09.002Get rights and content

Abstract

This study encompasses dynamic soil characterization and site classification zonation mapping of the Plio-Quaternary and especially Quaternary alluvial sediments based on the current seismic codes. The study area is located in the Çubuk district and its surrounding area that is situated towards the north of Ankara, the capital of Turkey. The project site is located in a region which has the potential of being seriously affected by probable seismic events occurring along the Çubuk Fault Zone. The Çubuk Fault Zone is thought to be a continuation of the Dodurga Fault Zone and a sub-fault belt of the North Anatolian Fault System that is one of the most prominent fault systems in Turkey with a significant earthquake potential. Sediment characteristics were determined and soil profiles were characterized by surface wave methods at different locations. Non-invasive seismic methods were used to obtain a shear wave velocity profile at 41 sites and two measurements were taken at each site for passive and active surface wave methods. The Multichannel Analysis of Surface Wave Method (MASW) and the Microtremor Array Method (MAM), which gave comparable results, were used as active and passive surface wave methods, respectively. By combining these two techniques, the shear wave velocity profile of the site was obtained. Additionally, the MAM survey alone was implemented at 10 of the sites, especially to get information on the deeper parts of the soil profiles. The geological characteristics of these sedimentary units were compared with the geological and geotechnical boring and seismic site characterization studies to classify the soil deposits. Therefore, the average shear wave velocity results and the variable characteristics of the seismic data along the soil profiles were correlated with these boring studies. This was performed to develop site categories which took site conditions into account according to the design codes of IBC (International Building Code) 2006 and TSC (Turkish Seismic Code) 1998. Consequently, the regional site classification map of the north of the Çubuk basin considering site classes was assessed based on the average shear wave velocity (Vs30) results in IBC 2006 and the shear wave velocity data and thickness of the surface layer based on the Vs values in the TSC.

Highlights

► Plio-Quaternary sediments of northern Ankara were assessed by surface wave methods. ► Passive (MAM) and active (MASW) surface wave methods gave compatible results. ► Combination of two techniques was utilized to characterize local site conditions. ► The seismic studies were also correlated and complemented by the boring studies. ► IBC 2006 and TSC 1998 based site classification maps were produced and discussed.

Introduction

The local soft and unconsolidated sedimentary deposits have an influence on the ground motion characteristics when the generated seismic waves propagate through different geological layers towards the ground surface and these deposits cause different effects on the earthquake motion at the surface even in the case of moderate earthquakes (Oliveira, 2004, Pitilakis, 2004). Nearly all recent destructive earthquakes, such as the 1985 Michoacan event, in Mexico; the 1988 Spitak event, in Armenia; the 1989 Loma Prieta and the 1994 Northridge events in USA; the 1995 Kobe event in Japan; the 1999 Kocaeli event in Turkey; and the 2010 Christchurch event in New Zealand have clearly showed that local soil conditions can have a significant influence on the ground motion and on the damage pattern. To map the geographic variations of the sites located in the sediments in terms of earthquake response and to predict the variations in ground motion due to differences in site geology, necessary parameters should be assigned to each key site. Several studies (Midorikawa, 1987, Borcherdt, 1994, Joyner et al., 1994, Anderson et al., 1996, Boore and Atkinson, 2007) showed that shear wave velocity is a key parameter to be utilized to simply and quickly evaluate the ground motion characteristic parameters such as the fundamental frequency of the soil profile and the amplification ratio. The amplification of ground motions at a site is significantly affected by the natural period of a site by considering both characteristic shear wave velocity and thickness of soil depth (Rodriguez-Marek et al., 2001). Rather than the determination of a soil type and estimation of the site amplification and fundamental period, the shear-wave velocity is a basic input parameter to identify the stiffness of the sediment and might be a useful parameter to characterize local sediment conditions quantitatively for calculating site response (Park and Elrick, 1998, Holzer et al., 2005, Koçkar et al., 2010). Therefore, adequate number of shear-wave velocity measurements at the key sites is utilized to characterize the local geologic conditions over the area in a seismic zonation study and to identify the local site response.

At present, site characterization based on the effects of the soil types related with ground motion response is often reduced to the specification of a single number, i.e., Vs30, average shear wave velocity of the uppermost 30 m of the soil profile (Borcherdt, 1994). These results are used in some well-known building codes (IBC 2006, UBC97, EC8 and NEHRP) to classify the site and to compute the expected site amplification characteristics corresponding to the spectral period of the soil profile (IBC 2006; International Code Council (ICC), 2006)). Also, Vs30 is used as a site classification parameter in the next generation attenuation model along with the engineering bedrock depth (Abrahamson and Silva, 2008). Therefore, Vs30 is a key parameter in earthquake hazard and/or risk related studies. Rather than generalization of the Vs values to a depth, the shear wave velocity data along with the depth of the surface layer of the soil deposits is another approach to characterize the sites (TSC 1998, the Turkish Seismic Code (TSC); Ministry of Public Works and Settlement Government of Republic of Turkey, 1998).

The scope of this study is to mainly investigate the geological, seismic and geotechnical site characteristics, and to perform site classification of the Upper Pliocene to Pleistocene fluvial clastics and Quaternary alluvial and terrace deposits (henceforth named as Plio-Quaternary sediments in its entirety) located at the Çubuk district and its close vicinity which is situated towards the north of Ankara. In order to determine the shear wave velocity (low strain level) for a depth of 30 m from the surface, or its generalization to different depths, mostly in-situ seismic methods are utilized to derive the shear-wave velocity as a function of depth (Kramer, 1996). Non-invasive seismic methods, such as the Multichannel Analysis of Surface Wave Method (MASW) and the Microtremor Array Method (MAM) were used as active and passive surface wave methods, respectively, in order to obtain a shear wave velocity profile of the subsurface at 51 sites. A combined usage of the active and passive surface wave methods was adopted in order to meet the requirements of preserving high resolution at shallow depths while also extending the Vs measurements to greater depths. The relationships between the geologic units and their shear wave velocity results, vertical variations with respect to the sediment type and Vs with the information collected from the boreholes and average shear wave velocity for the upper 30 m of the soil profile were determined and site classes based on the seismic codes (IBC 2006 and TSC 1998) were utilized in this zonation study.

Section snippets

Location of the study area

The study area covers the Çubuk district and its close vicinity, mainly the northern part of the Çubuk plain which is situated approximately 40 km north of Ankara. The location of the study area is given by Fig. 1. The study area is approximately 120 km2 in size and the area lies within the major growth potential of one of the major municipalities of the city of Ankara and has been moderately populated with mostly residential structures, a considerable amount of small to large industrial

Geology of the study area

The Çubuk basin extends nearly 20 km from northwest to southeast. The study area covers about the 14 km northern portion of the Çubuk basin. Approximately 60 km2 of the study area includes the Quaternary deposits (Fig. 2). The width of the plain increases towards the south and reaches up to 4 km at the Esenboğa county and the airport. The Çubuk river drains (or traverses the basin) from south to north. There are some main tributaries joining the Çubuk river from the east and the west (Fig. 2).

The

Seismotectonics

On a regional scale, some of the recent (after year 1900) seismic activities that took place around the study area mostly along the Ulusu, Çerkeş-Kurşunlu, Tosya and Dodurga Fault Zones in the İsmetpaşa-Kargı section of the North Anatolian Fault Zone (NAFZ) are: the 26.11.1943 Tosya Earthquake (ML = 7.3), the 01.02.1944 Bolu-Gerede Earthquake (ML = 7.3), the 13.08.1951 Kurşunlu Earthquake (ML = 6.9), the 07.09.1953 Çerkeş Earthquake (MW = 6.0) and the 06.06.2000 Orta Earthquakes (MW = 6.0 and the four

Active and passive surface wave survey

Surface waves are generated by two ways; active and passive sources. Active source means that seismic energy is intentionally generated at a specific location relative to the geophone spread and the recording begins when the source energy is imparted into the ground. MASW (Park et al., 1999) and Spectral Analysis of Surface Wave (SASW) (Nazarian, 1984, Stokoe et al., 1994) methods are classified as active surface methods. On the other hand, in passive surface wave surveying such as MAM (Okada,

Surface wave survey results

To characterize the sedimentary units and to discriminate the sediment type, the vertical and lateral variation of the shear wave velocity profiles along four sections were generated. The trends of these sections can be seen in Fig. 6. In creating four 2D Vs models in Fig. 7, a digital elevation map of the study area was produced from 1:25,000 topographic maps of the General Command of Mapping. Upper surface limit of the models was adjusted with respect to the topography and it was exaggerated

The seismic codes used in this study

The average shear wave velocity results and the variation of the shear wave velocity along the soil profiles with boring data (if available) were used in order to classify site categories which take site conditions into account according to the design codes of IBC 2006 and TSC 1998. Although IBC 2006 reflects the basic knowledge and technology of the present time, having an acceptable level of accuracy and compatibility among the seismic codes in terms of the tools used for the seismic design

Discussion

The geological, geophysical and geotechnical properties of the Neotectonic units, namely, Upper Pliocene to Pleistocene fluvial and Quaternary alluvial and terrace sedimentary (Plio-Quaternary) deposits in the Çubuk basin were investigated for site characterization. These sediments are deposited in and along a fault controlled depression. The depositional setting of the area which took its final form due to the fault activities in the area was quantitatively analyzed and the results revealed

Conclusions

Seismic zonation studies in the Upper Pliocene to Pleistocene fluvial sediments and especially Quaternary alluvial deposits of the Çubuk district and its close vicinity were implemented by the non-invasive surface wave methods. By this study, sediment conditions were determined and the variations of the velocity throughout the soil profiles were characterized by the Microtremor Array Method and the Multi-Spectral Analysis of Surface Wave Method at 51 locations. These passive and active surface

Acknowledgments

The authors would like to acknowledge the financial support provided by the Middle East Technical University Research Fund Project (BAP-2007-03-09-05). The authors would like to especially thank Kaya Engineering, Consulting, Commitment and Trading Inc. and Mr. Mete Mirzaoğlu who provided assistance and conducted the field tests implemented in this study. The authors would like to thank Mr. Selim Cambazoğlu, Miss Ayten Koç and Mr. Evrim Sopacı for their help during this study. The senior author

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