Introduction

In a seismically active areas, investigation of soil-structure interaction due to strong earthquakes is one of the major tasks. For this purpose, three main factors primarily responsible for structural damage during an earthquake, e.g. earthquake source parameters, local soil conditions and structure features, are analyzed. Studies on structure damage due to earthquakes showed that in any given area, even if the structural quality is the same, the local soil attributes play an impactful role in the distribution of the structural damage during an earthquake (Yalçınkaya 2010). Therefore, local soil behavior under dynamic earthquake loading needs to be thoroughly examined and identified (Kramer 1996). The average S-wave velocity of the uppermost 30 m of a soil column (Vs30) values are mostly considered as the basis for soil identification, and are calculated by the following formula in accordance with the following expression: \( {Vs}_{30}=30/\sum \limits_i^N\Big({h}_i/{V}_i \)) where hi and Vi denote the thickness (in meters) and shear-wave velocity of the ith formation or layer, in a total of N, existing in the top 30 m (Kanlı et al. 2006).

Many studies have been carried out to estimate the dynamic properties of soils and soil characterization. Donohue et al. (2004) calculated Gmax values using multichannel analysis of surface waves (MASW) method for two soft clay sites in Ireland. Pamuk et al. (2017) determined the shear-wave velocity structure and predominant period features of Tınaztepe in İzmir using active–passive surface wave methods and single-station microtremor measurements. Carvalho et al. (2009) estimated P-wave velocity (Vp)-to-Vs ratios, Poisson’s ratio, and a subsoil classification based on geophysical and geotechnical parameters. Essien et al. (2014) determined Poisson’s ratio using P- and S-wave measurements. Akin and Sayil (2016) utilized active-source (MASW) and passive-source (single-station microtremor) surface wave methods for the soil dynamic characteristics in Trabzon, Turkey. Cavallaro et al. (2006) described and compared the results of in situ and laboratory investigations that were carried out in order to determine the soil dynamic characteristics in the city of Catania. Cavallaro et al. (2008) used borings and dynamic in situ tests for site characterization of the soil and deep site investigations in the city of Catania. Castelli et al. (2016a) carried out site investigation for determinig the soil geotechnical characteristics using down-hole (DH) tests, dilatometer tests (DMT), MASW and geotechnical borings in Sicily, Italy. Castelli et al. (2016b) obtained seismic microzoning maps of the city of Catania in terms of different peak ground acceleration at the surface and in terms of amplification ratios for given values of frequency. Ferraro et al. (2016) utilized boreholes, DH tests, seismic dilatometer Marchetti tests (SDMT) and an MASW method in the city centre of L’Aquila, Italy, for obtaining a detailed geotechnical model. Caruso et al. (2016) combined traditional tests for direct measurement of shear-wave velocity (DH, cross-hole and SDMT) with indirect and less expensive tests (MASW) for reliable geotechnical modeling used in seismic response analysis. Cavallaro et al. (2016) conducted in situ and laboratory investigations in order to obtain a geotechnical model for a realistic seismic response analysis. Cavallaro et al. (2017) utilized in situ investigations and laboratory tests for geotechnical dynamic characteristics of the foundation soil of the Augusta Hangar, Sicily, Italy. They provided a representative geotechnical model of the site where an important historical building is located.

The aim of this study is to determine soil dynamic properties of the Bornova Plain and its surroundings using geophysical and geotechnical data (Fig. 1a). At first, surface wave data were collected using MASW, ReMi and single station microtremor methods. The Vs30 values were calculated by inverting phase velocity data obtained from a combined survey of MASW and ReMi. Vs30 values ranging from 180 to 1400 m/s were used to prepare a soil classification map of the Bornova Plain according to the National Earthquake Hazard Reduction Program (NEHRP 1997) soil classification. Based on the one-dimensional (1D) velocity structure obtained from inversion of the phase velocity data, as mentioned above, we also created level maps of Vs values at depths of 5, 10, 20, 30, 40 and 50 m. Gmax values were calculated using Vs values at different depths and rendered two- and three-dimensionally up to a 50-m depth. In the next stage, single-station microtremor data was evaluated according to the Nakamura method (1989) and the predominant period of the soil was determined, which was in the range of 0.45–1.6 s, indicating the area exhibits lower predominant periods and less sediment thickness. In the next step, previous geotechnical studies [standard penetration test (SPT-N), Poisson ratio, groundwater level (GL)] carried out by Kıncal (2004) were examined. Next, risk maps were created using geotechnical and geophysical data for the study area. In the last stage, the predominant periods of the high-rise buildings in the study area and hospitals and educational institutions in the risky areas were calculated with the help of empirical relations between height (or number of floors) of buildings and the predominant period of the buildings; thusly, the resonance condition was investigated.

Fig. 1
figure 1

a) Representation of a site location map of study area. b The geological map of study area together with geomorphology (modified from Kıncal 2004; Uzel et al. 2012)

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Geology of the study area and its surroundings

There are three tectonic belts in West Anatolia, around Izmir. These belts are from the east to west; the Menderes Massif, the Izmir–Ankara Zone and the Karaburun Belt (Bozkurt and Oberhänsli 2001). The Menderes Massif consists of metamorphic rocks that reach to the early Eocene at their uppermost levels. Located upon the Menderes Massif, the Izmir–Ankara Zone is represented by sedimentary rocks settled on Campanian–Danian-aged flysch facies and a mafic volcanic intercalated matrix and a unit formed of limestone blocks longer than 20 km, swimming in the matrix stretching in a wide region from Manisa to Seferihisar. This unit is called the Bornova Mélange; its limestone blocks and mega-blocks were carried to a sedimentation environment during the precipitation of its matrix and, consequently, complex contact structures, which exhibit soft sediment deformations, were formed around the blocks. This generalized stratigraphy was obtained by putting together the measured sections of the limestone mega blocks, which is similar to the carbonate pile that outcrops on the Karaburun Peninsula (Erdoğan 1990). The foundation is formed by Upper Cretaceous and old Bornova Mélange. Older limestone mega-olistoliths compared to the mélange matrix are located in the matrix of Bornova Mélange in a random order. The aforementioned limestone is known as Işıklar limestone in Altındağ and its vicinity (Özer and İrtem 1982). Bornova Mélange consists of platform-type limestone which swims inside the matrix which is created by the alternation of sandstone/shale chalk and diabase blocks and pebble lens/channel fillings (Erdoğan 1990). Neogene-aged lacustrine sediments come upon Bornova Mélange with angular unconformity. Yamanlar ebonites cover the existing units unconformably as well. A typical Bornova Mélange unit was segregated by the precipitated flysch facies between the Upper Cretaceous and Paleocene; its matrix consists of alternating sandstone–shale , pebble lenses and limestone olistoliths (in various ages and sizes) that are older than the matrix in which they are located. The Bornova melange is in the oldest unit position in the study area. The aforementioned sedimentary rocks are pebbles, argillaceous limestone and silicified limestone. Yamanlar ebonites are represented by andesitic–dacitic massif lava, tuff, auto-combined andesite and agglomerates within the field of the study. Ebonites cover the regional Neogene sediment rocks unconformably. Two faults with directions of NE-SW and E-W are discovered in Izmir and its vicinity. It has been established that the regular fault following an E-W direction is crossing the other fault which is diagonally following a NE-SW direction (Kıncal 2004). Fundamentally being developed upon the same terrestrial fillings, today’s alluvial plains near Izmir vary regarding their geomorphologic formations. Deltas of the inner bay shores, Balçova and Alsancak in the south and Karşıyaka in the north are basic delta plains developed before mountain streams. However, the Gediz Delta is a vast and complex geomorphologic formation which collects water from the majority of the streams in the Western Anatolia and is shaped by the Gediz River’s alluvions. Despite originating from the shore, the Bornova Plain in the east is not a typical delta plain, primarily due to the lack of a large river carrying its water to the sea originating from Bornova. In reality, the mountain stream watershed which descends to Bornova is very close to the plain (Kayan 2000). The geology of study area is shown in Fig. 1b.

Geophysical survey

A geophysical site characterization was conducted as part of the Bornova Plain analysis. MASW and ReMi (64 sites), ReMi and single-station microtremor (137 sites) measurements were carried out in the study area (Fig. 1b).

MASW method

MASW is a method of estimating the shear-wave velocity profile from surface waves. It uses the dispersive properties of Rayleigh waves for imaging the subsurface layers. In the MASW method, surface waves can be easily generated by an impact source (sledge hammer, etc.; Park et al. 1999).

MASW measurements were conducted at 64 sites. The MASW system consisted of a 24-channel Geode seismograph with 24 4.5-Hz geophones. The seismic waves were generated by impulses from a hydraulic sledgehammer (100 lb) with three shots. The data processing contains three steps. The first is the preparation of a multichannel record, the second is dispersion-curve analysis and the third is inversion (using a least-squares approach).

The ReMi method

The ReMi process, developed by Louie (2001), has been widely used to determine shear-wave velocity profiles using ambient noise recordings. This array-analysis technique finds average surface-wave velocity over the length of a refraction array. Also, 64 ReMi measurements were carried out at the same locations as the MASW measurements. For the ReMi measurements, eight records were recorded at each site. The array lengths were 60 m and 120 m. In the ReMi interpretation and analysis, firstly, p–f transformation, which is the basis of velocity spectral analysis, takes a recorded section of multiple seismograms, with seismogram amplitudes relative to distance and time (x–t), and converts it to amplitudes relative to the ray parameter, p (the inverse of apparent velocity). Secondly, the Rayleigh phase-velocity dispersion is selected. This analysis only adds a spectral power ratio calculation for the spectral normalization of noise records. The final step is shear-wave velocity modeling. The modeling iterates on phase velocity at each frequency.

Dispersion curves obtained by active (MASW) and passive (ReMi) surface wave methods were combined to enlarge the analyzable frequency range of dispersion and improve the modal identity of the dispersion trends. High-resolution Vs profiles were obtained by inverting the dispersion curve, and S-wave velocities were obtained from the combined dispersion curves using the damped least-squares method (Levenberg 1944; Marquardt 1963; Fig. 2).

Fig. 2
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Examples of Vs-depth cross-sections were obtained by inverting the combined dispersion curve

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Combined dispersion curves were used in the study to increase the depth of the research and to identify the velocity differences that occur within the soil in detail. According to Vs-depth cross sections obtained from each measurement site, sudden velocity differences are observed in a lateral and vertical direction within the soil. These changes need to be considered for soil dynamic analysis studies. Based on the Vs30 distribution map which is made for soil type identification, it has been observed that Vs30 values vary between 180 and 1400 m/s (Fig. 3).

Fig. 3
figure 3

Vs30 distribution values overlaid on the 3D topographic map of Bornova Plain and its surroundings

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When these velocity changes and the geological structure of the area are considered together, andesites and Miocene pyroclastics north of the area of study and Miocene-aged limestone in the south of area of the study are observed to have higher Vs values compared to the rest of the study area, and the threshold value of the bedrock’s Vs30 values are also observed to be higher than 760 m/s. In spite of this, Vs30 values, especially in the areas nearby the sea, are confirmed to change between 100 and 300 m/s, and Vs30 values along the Bornova Plain are confirmed to be lower than 500 m/s (Fig. 3). When evaluating the NEHRP distribution map (Fig. 4), B, C and F soil types are seen. We also created level maps of Vs values at depths of 5, 10, 20, 30, 40 and 50 m (Fig. 5).

Fig. 4
figure 4

Soil classification map of the study area according to NEHRP standards

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Fig. 5
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Average Vs distribution maps of Bornova Plain and its surroundings down to depths of 5, 10, 20, 30, 40 and 50 m depth

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Single-station microtremor method [Nakamura method (1989)]

The microtremor method is widely used for assessing the effect of soil conditions on earthquake shaking. The horizontal-to-vertical (H/V) spectral ratio was first introduced by Nogoshi and Igarashi (1970). The H/V method is convenient and inexpensive for soil investigations. It was developed by Nakamura (1989), who demonstrated that the ratio between horizontal and vertical ambient noise records related to the fundamental frequency and amplification of the soil beneath the site. Microtremor observations were carried out at 137 sites in the study area. All microtremor measurements were taken with the Guralp Systems CMG-6TD seismometer with a sampling rate of 100 Hz. At each location, recording duration was approximately 30 min. To remove intensive artificial disturbance, all signals were band pass-filtered in a band pass of 0.05–20 Hz. Then, they were divided into 81.92-s-long windows and tapered individually using Konno-Ohmachi smoothing. For each window, the amplitude spectra of the three components were computed using a fast Fourier transform (FFT) algorithm. As a result, the average spectral ratio of H/V was thus calculated (Fig. 6). Predominant period values were determined using the H/V spectral ratio and they have been mapped (Fig. 7). Microtremor measurements were processed and interpreted using the GEOPSY software package (www.geopsy.org).

Fig. 6
figure 6

Examples of H/V spectral ratio for the study area (dashed lines demonstrate the standard deviation)

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Fig. 7
figure 7

Predominant period distribution values overlaid on the 3D topographic map of Bornova Plain and its surroundings

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The predominant period values vary between 0.45 and 1.6 s where the regions have thick soil layers from rivers in the Bornova Plain, and decreases gradually from the bay. Decreasing periods are correlated with the increasing topography in the north and south parts of the study area and indicate a different geological unit (Fig. 7). Towards the eastern part of the study area, the predominant period values shift towards lower values. When maps of the predominant period and Vs30 are compared, they match up to higher Vs values where the period values are observed to be less than 1 s. The period values are observed to be higher than 1 s and Vs30 values are observed to be much lower than 500 m/s, especially in the areas which are Quaternary-aged and mostly consist of soil layers that are thicker than 30 m.

Dynamic properties of the study area

Gmax studies; For geotechnical investigations, measurement of the small strain shear modulus, Gmax, of a soil is very important parameter. Gmax can be calculated from the shear-wave velocity using the following equation:

$$ {\mathrm{G}}_{\mathrm{max}}=\uprho .{\mathrm{Vs}}^{2\ast }100 $$
(1)

where Gmax = shear modulus (kg/cm2), Vs = shear-wave velocity (m/s) and ρ = density (gr/cm3). The density values used in modeling were calculated using S and P-velocity values and formulas in Table 1.

Table 1 Relationship between Vs-Vp and density [P velocity values were calculated using the equation Vp = Vs*1.74 (Catchings 1999)]
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Figure 8a, b shows that Vs10 and Gmax values at 10 m depth. In addition, the calculated Gmax values was drawn three-dimensionally until 50 m of depth with 3D Vs model (Fig. 9a, b). Examining the Gmax distribution indicates the Gmax values in the Bornova Plain are lower than 5000 kg/cm2. As for the north and south parts of the study area, the values are greater than 10,000 kg/cm2.

Fig. 8
figure 8

a Geological map; b Vs10; c Gmax distribution maps at 10 m overlaid on the 3D topographic map of Bornova Plain and its surroundings

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Fig. 9
figure 9

a 3D Vs distribution map; b 3D distribution of Gmax of Bornova Plain and its surroundings up to a 50-m depth [the red rectangle shows that area damaged during the 2003 Urla earthquake (Mw = 5.8)]

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SPT-N studies were conducted on the 422 bores in the study area (Kıncal 2004). When the SPT-N30 distribution map was examined for 10 m of depth, it was observed that these values vary between 0 and 60. Especially in the waterfront, these values are less than 20 and they increase towards the east (Fig. 10b).

Fig. 10
figure 10

a Fault zones in the study area with A-A’, B-B’ and C-C’ cross sections; b SPT-N30; c Poisson ratio; d groundwater level distribution maps at a 10 m overlaid on the 3D topographic map of Bornova Plain and its surroundings

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Poisson ratio studies: In the S- and P-seismic refraction studies of Kıncal (2004) carried out on the 29 sites of the Bornova Plain, the Poisson ratio has been calculated for 10 m of depth (Fig. 10c). Poisson’s ratio (υ) can be calculated using Eq. 2;

$$ v=\left(\frac{Vp^2-2{Vs}^2}{2\left({Vp}^2-{Vs}^2\right)}\right) $$
(2)

where Vp is P-wave velocity and Vs is S-wave velocity.

In the distribution map of the Poisson ratio at 10 m of depth, it is observed that the areas with the values between 0.3 and 0.5 probably have a loose soil layer. The areas with values lower than 0.3 comprise hard and firm soil.

GL studies; As a result of the measurements carried out in the 422 boreholes, the GL map was prepared by Kıncal (2004; Fig. 10d). Examining the GL distribution shows the GL increases from west to east.

Predominant period values increase, as expected, on areas that equaled the bay coast and in middle parts of sections in A-A’ and B-B’ cross-sections. In C-C’, predominant period values are higher than in other sections. In the A-A’ section; lower Vs10 (100–200 m/s) and lower SPT-N30 (<30) values are obtained in middle parts of the A-A’ cross-section due to the alluvial unit. GL values change from 1 to 8 m in the A-A’ cross-section. Poisson ratio values range from 0.15 to 0.42 in the A-A’. Lower GL values are observed in middle parts of the section, while lower Poisson ratio values are observed in the southern part of the A-A’ cross-section. In the B-B’ cross-section, Vs10 changes from 250 to 1300 m/s, while SPT-N30 values are between 30 and 50 m. GL values are between 1 and 20 m in the B-B’, and Poisson ratio values range from 0.18 to 0.40 in the B-B’. Higher GL and lower Poisson ratio values are obtained in 1–3 km of the B-B’ cross-section. In the C-C’ cross-section, Vs10 changes from 150 to 550 m/s, while SPT-N30 values are between 3 and 60 m. GL values are between 1 and 20 m in the C-C’ cross-section, and Poisson ratio values range from 0.15 to 0.42. Higher GL and lower Poisson ratio values are obtained in the east portion of the C-C’ cross-section. Therefore, it is clearly seen that these values are generally compatible with each other (Fig. 11).

Fig. 11
figure 11

a Geological cross sections; b predominant period; c Vs10; d groundwater level; e Poisson ratio; f SPT-N30 at 10-m depth on A-A’, B-B’ and C-C’ cross sections

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Soil-structure resonance studies

The main goal of resonance study is to determine the expected resonance phenomena using the empirical relationship between the fundamental period of buildings and their height (or floor number) during future earthquakes. We have calculated predominant periods of nine high-rise buildings using empirical formulas in the study area. There are nine buildings whose heights range from 68 to 216 m (i.e. from 17 to 50 floors), as listed in Table 2. The years of construction are from 2009 to 2017. The tallest building is the T8 and its height is 216 m, while the shortest building is T2 and its height is 68 m. Regions where high-rise buildings exist have very low Vs values. Moreover, the GL in this region is quite shallow (Figs. 8 and 10). If the dominant period of the building and the soil predominant period values are close to each other, soil-structure resonance may occur. The predominant periods calculated by the height-based area of Gallipoli et al. (2010; T = 0.016 H; T = predominant building period, H = height of building) varies between 1.1 and 3.5 s, while these values calculated by the floor-based area of Navarro and Oliveira (2008; T = 0.049 N; T = predominant period of building, F = number of floors) change from 0.8 to 2.5 s. We calculated the average of the predominant periods of the high-rise buildings obtained from different formulas in this study (Table 2). In addition, predominant soil period values obtained from microtremor measurements change from 1.0 to 1.6 s in the high-rise building area. Figure 13 shows the predominant period values of the buildings and the soil. We determined buildings T2, T5, T7 and T8 to potentially possess soil structure resonance. Because periods of T4, T6 and T9 are higher than the soil periods, these buildings are not significantly affected by resonance.

Table 2 Comparison of the high-rise buildings periods and soil predominant periods
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Risk maps of the study area

Risk map I

This map was formed by superimposing the area with Vs30 values lower than 760 m/s and with soil predominant period values greater than 1 s (Fig. 12). The Urla earthquake on 10 April 2003 with a magnitude of Mw = 5.8 partly damaged buildings with 8–9 floors in the Bornova Plain (Kıncal 2004; Fig. 12). Figure 12 also shows the high-rise buildings (towers) and health and education institutions. The dominant periods and the soil predominant period values, calculated with the help of empirical relations, of the education institutions, hospitals and towers risk map I are compared in Fig. 13. The education institutions and the hospitals generally have less than 10 floors. Therefore, it was not predicted that most of these buildings would be affected by the resonance in the area with soil predominant period values greater than 1 s. But this is not the same for towers. Because the average periods of these buildings that vary between 17 and 50 floors is between 0.95 and 2.95 s. The buildings that have the same or similar color with the soil predominant period have resonance risk (Fig. 13). The area including the buildings with eight or nine floors that were damaged in the Urla earthquake are completely in risk map I. In this area, the soil predominant period values vary between 0.8 and 1.4 s. The dominant period values of the buildings with eight to nine floors are between 0.4 and 0.5 s with the Navarro and Oliveira (2008) relationship. Then, it is possible to say that these buildings were not exposed to the resonance effect during the Urla earthquake.

Fig. 12
figure 12

Risk maps of the study area

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Fig. 13
figure 13

Distribution of expected resonance effect in the buildings in Risk-1 area on soil predominant period map. The circle shows building periods. Colour harmony signifies probable resonance phenomena

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Risk map II

Risk map II was prepared using the GL, Gmax, Poisson ratio and SPT-N30 obtained for the 10-m depth of this study and the study of Kıncal (2004). The considered values for each of the data layers in risk map II are presented below; the places where SPT-N30 ≤ 30; 0.3 ≤ Poisson ratio ≤ 0.5; Gmax ≤ 3000 kg/cm2 and GL ≤ 10 values superimposed are determined as the risk areas. Risk map II is included in risk map I. Also, some of the buildings damaged during the Urla earthquake are included in risk map II.

Conclusion

This study shows the Vs and predominant period characteristics of different soil units in Bornova Plain. The Vs profiles were determined by using an MASW and ReMi combined survey at 64 sites. Also, predominant periods were determined using Nakamura’s method at 137 sites. The predominant period in the range is 0.45–1.6 s, which indicates the area exhibits lower predominant periods and comprises less sediment thickness (Figs. 5 and 7). The Vs30 values which change from 180 to 1400 m/s were used to create a soil classification map of the Bornova Plain according to NEHRP (Fig. 3).

Soil classification results show that most parts of the region, located in alluvial basin, have low shear-wave velocity values. These values are within the range of 180–400 m/s and thus classified the C and D categories according to NEHRP, generally. Some parts located on the north and the south part of the study area have better soil conditions and have comparatively high shear wave velocities in the range of 500–1400 m/s. Vs30 and soil classification maps were compared with predominant distribution associated with the earthquake. According to results of comparing these parameters, in general, it is noticed that there is a correlation between the Vs30 values and the predominant distribution of the region (Figs. 3 and 4).

According to Vs30 values, the bedrock is mostly dominant in the regions south of the bay and north of the bay due to Vs30 values being higher than 760 m/s. These regions are classified as B-type soil according to NEHRP regulations. C and F types of soil are dominant in these regions according to NEHRP regulations in other parts of the study area.

Based on the Vs level maps created for depths up to 50 m, the soil thickness in the regions where C and F types are dominant is more than 30 m (Fig. 5). The predominant period values being higher than 1 s in these regions supports the notion that the soil thickness of these regions is more than 30 m.

Distinctive soil bedrock models are suggested for soil deformation analysis in regions where the soil thickness is more than 30 m and where there are sudden Vs differences in lateral and vertical directions. Therefore, 2D or 3D soil engineering bedrock models should be established for these regions.

Risk map- I almost covers the Bornova Plain. It can be seen that there are many education institutions and hospitals within the risk map 1 area. All of the buildings partially damaged in the earthquake are located in the risk map. The existing Turkish earthquake regulations CODE TE (2007) for buildings to be built in this area are insufficient. Therefore, detailed studies are required to calculate the in situ design spectrum. Resonance risk exists for buildings with periods varying from 1 to 1.6 s within this area. Seismic impedance must be determined from the seismic bedrock (Vs > 3000 m/s) to the surface in order to calculate the earthquake effect on the surface within this area. Risk map II is within the risk map I and some of the buildings damaged by the earthquake are located within this area.

The number of floors of educational institutions and hospitals is less than 10 (dominant period values of the buildings between 0.4 and 0.5 s) in the risky areas. Therefore, it cannot be said that these structures in risky areas carry a resonance risk. However, some of the high-rise buildings carry a resonance risk (Fig. 13). The resonance effect must be taken into account when designing the buildings to be built in this area. Therefore, it is proposed that the predominant period map be used in the design of new buildings.

The GL in the study area is close to the surface, ranging from 1 to 24 m, and is especially shallow in the western part of the study area. Since these areas have risk of liquefaction during an earthquake, these areas should be examined in detail. The Poisson ratio, which indicates the water saturation level of soil, varies from 0.3 to 0.5, especially in soft soil. The Poisson ratio varies from 0.3 to 0.5 where the GL is 10 m or lower in the study area. The Gmax values at a 10-m depth are less than 4000 kg/cm2, in accordance with these values.