Northern Aegean Earthquake (Mw=6.9): Observations at three seismic downhole arrays in Istanbul

Soil Dynamics and Earthquake Engineering 77 (2015) 321–336

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Soil Dynamics and Earthquake Engineering journal homepage: www.elsevier.com/locate/soildyn

Northern Aegean Earthquake (Mw ¼ 6.9): Observations at three seismic downhole arrays in Istanbul S.U. Dikmen n, A. Edincliler, A. Pinar Bogazici University, Kandilli Observatory and Earthquake Research Institute, Department of Earthquake Engineering, Cengelkoy, 34684 Istanbul, Turkey

art ic l e i nf o

a b s t r a c t

Article history: Received 19 September 2014 Received in revised form 31 March 2015 Accepted 14 June 2015

The Northern Aegean Earthquake (Mw ¼6.9\ML ¼ 6.5) took place on May 24, 2014 between the islands of Gokceada, Turkey and Samothraki, Greece. The tremors were felt as far as in Istanbul, about 300 km on the East – Northeast (ENE) side of the epicenter. Kandilli Observatory and Earthquake Research Institute (KOERI) of Bogazici University, Turkey operate three downhole arrays in Istanbul, namely Atakoy (ATK), Fatih (FTH) and Zeytinburnu (ZYT) arrays. In this study, waveforms and site response observed at the KOERI operated seismic downhole arrays during the May 2014 Northern Aegean Earthquake (NAE2014) are analyzed in detail and presented. Evaluation of the acceleration records have shown low amplitude but long period and long duration motions at Istanbul. Furthermore, the analyses of the recordings suggest that Vs30 alone may not be a sufficient parameter for the characterization of site amplification. & 2015 Elsevier Ltd. All rights reserved.

Keywords: Seismic downhole arrays Northern Aegean Earthquake Site response Surface waves Site amplification Long period waves

1. Introduction On May 24, 2014, a magnitude Mw ¼6.9\ML ¼6.5 earthquake took place in the Northern Aegean Sea between the islands of Gokceada, Turkey and Samothraki, Greece. Partly because the hypocenter of the earthquake was about 80 km away from any major settlement the damage was quite limited without any casualties. However, it was widely felt in the coastal areas of the Northern Aegean Sea and the Marmara region of Turkey. The tremors were felt as far as in Istanbul, about 300 km on the East – Northeast (ENE) of the epicenter [12]. The earthquakes which have not caused a major destruction of the built environment usually attract less attention. However, the features of the soil strata above the bedrock level have major effect on the values and characteristics of site response which has great significance especially in design of the structures. These strata, even in the simplest formations, exhibit a complex behavior especially under seismic loading. Thus understanding the soil behavior under seismic conditions has also great value for structural design. Starting as early as late 1950s, seismic downhole arrays have emerged as field laboratories to observe, understand and evaluate the complex dynamic behavior of soils [41]. Each and every single downhole array having different soil characteristics and being

located in a different physical environment provides valuable data for further understanding of the subsoil behavior and site response. In this respect, Kandilli Observatory and Earthquake Research Institute of Bogazici University (KOERI) deployed three seismic downhole arrays in Istanbul through several different partnerships and project funding, details of which will be presented below. The objective of this study is to analyze and present the site response observed at the KOERI operated Seismic Downhole Arrays during the May 2014 Northern Aegean Earthquake (NAE2014). The significance of NAE2014 is twofold. Hence, on one hand it is the highest magnitude earthquake that took place in local distances after the deployment of KOERI arrays. The peak surface accelerations produced by this event at these arrays were higher than the recordings made in the past. On the other hand, it is the long period characteristics of NAE2014 observed in the recordings. In the following sections, first a presentation of the NAE2014 will be made. It will then be followed by a description of the KOERI downhole arrays. After these explanatory subsections, a comparative analysis of the findings and observations at the arrays will be presented.

2. May 2014 North Aegean Earthquake n

Corresponding author. Tel.: þ 90 216 516 3219. E-mail address: [email protected] (S.U. Dikmen).

http://dx.doi.org/10.1016/j.soildyn.2015.06.008 0267-7261/& 2015 Elsevier Ltd. All rights reserved.

The Northern Aegean Earthquake, NAE2014 took place on May 24, 2014 at UTC 09:25:01. The epicenter of the earthquake was on

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the west end of the North Anatolian Fault Zone (NAFZ) and 20– 25 km deep in the sea between the Aegean islands of Gokceada, Turkey and Samothraki, Greece [2,37,38]. The earthquake was felt widely both in Greece and Turkey. On the Turkish side, it was felt primarily in Canakkale, Balikesir and Edirne. As stated before, the tremors were also felt as far as in Istanbul. The peak accelerations were higher on the southern coast of the West side of Istanbul, as demonstrated in the distribution map prepared by KOERI (Fig. 1). The map was prepared using the data recorded by the rapid response network in Istanbul, operated by KOERI [12].

2.1. Tectonics The North Anatolian Fault Zone (NAFZ) is one of the largest plate-bounding transform faults that separate the Anatolian and Eurasian plates. It extends for about 1600 km between the Eastern Anatolia and the Northern Aegean (Fig. 2). The Anatolian block (i.e. south side of the fault line) is moving westward at a rate of 25 mm/yr relative to Eurasia. Recalling the recent history about the North Anatolian Fault, a series of large earthquakes started in 1939 near Erzincan and propagated westward toward the Istanbul-Marmara region located in Northwestern Turkey where two major earthquakes, namely Izmit and Duzce, occurred in 1999. Thus, the NAFZ experienced an exceptional seismic moment release cycle rupturing the entire 1600 km long fault zone except two segments; one beneath the Marmara Sea and the other further in the west beneath the Northern Aegean Sea [13]. The May 24th, 2014 event filled one of those seismic gaps leaving only the Marmara faults unruptured. The location of the NAFZ and the seismic moment release phase associated with major earthquakes is demonstrated in Fig. 2. The rupture extent and the date of the events are also shown in the figure.

2.2. Source region A prominent feature of this earthquake is the widespread distribution of its aftershocks. The location carried out by the National Earthquake Monitoring Center (UDIM) of KOERI portrays lateral variation between longitudes 25.01E and 26.21E. The aftershock distribution associated with the main shock and the focal mechanism of some selected aftershocks together with that of the main shock is illustrated in Fig. 3. The focal mechanism of the main shock shows predominantly right-lateral strike-slip faulting with strike 701 from North. Whereas, the focal mechanism of the aftershocks reveal ruptures both on strike-slip and normal faults (Fig. 3). Assuming that all the aftershocks occurred along the ruptured fault plane, it will correspond to approximately 120 km of fault rupture length. Alternatively, the rupture length can be estimated by the following equation proposed by Wells and Coppersmith [39]: Log ðLÞ ¼ ðM w  5:16 70:13Þ=1:12 70:08

ð1Þ

where Mw and L are the moment magnitude and the rupture length, respectively. Using the USGS estimated seismic moment magnitude of Mw ¼6.9, the equation will yield a rupture length between 35 and 60 km. Thus indicating that, the estimated rupture length is shorter than the rupture length derived from the aftershock distribution [12]. Accelerograms of the recordings made at Gokceada station (Station no. 1711) of Disaster Emergency Management Presidency of Prime Ministry, Republic of Turkey (AFAD) are presented in Fig. 4. The station coordinates are 40.19082N–25.90783E. Whereas, the epicenter coordinates of NAE2014 as per AFAD are 40.21080N–25.30730E. Also, as per AFAD the peak accelerations recorded at this station are 0.177, 0.171 and 0.131 g for EW, NS and UD components, respectively [2]. The duration of the record is about 95 s.

Fig. 1. Istanbul peak acceleration distribution map of NAE2014 [12].

Fig. 2. Major earthquakes on the NAFZ [37].

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3. Downhole arrays operated by KOERI KOERI operates three seismic downhole arrays on the West side of Istanbul (Fig. 5). They are namely Atakoy (ATK), Fatih (FTH) and Zeytinburnu (ZYT) seismic downhole arrays, named after their districts. ATK and FTH arrays are located approximately 2 km inland from the shore of the Marmara Sea, while the ZYT is only about 400 m inland. The longest linear distance between the two arrays is about 9 km between ATK and FTH. At this point, it should be noted that all three downhole arrays are immediately North of the present seismic gap where the location of the impending earthquake expected. Comprehensive technical information regarding these arrays can be found in an earlier publication by Kurtulus et al. [26] and

the references cited therein. This particular study being the latest of a series of publications reporting the subsoil structures of these arrays is selected to be used in this study. However, it should also be noted that the differences existing between the used profiles and those in the earlier publications are minimal and will not have a major impact on the ground response analysis and the scope of this study. A brief summary about the soil profiles will be presented below. 3.1. Atakoy downhole array (ATK) Atakoy downhole array was deployed in 2005 as part of a research project of KOERI together with German Research Center for Geosciences (GFZ). The project was also financially supported

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by Istanbul Metropolitan Municipality (IMM). The array is located in a residential neighborhood in the West side of Istanbul at about 15 km from the Historic Peninsula. The ATK array has instrumentation at the ground surface and at 25, 50, 70 and 140 m depths. The downholes are placed in the corners of a 3.0  3.0 m2 area in plane. The surface instrument is located in the center of this square area. The array is at about 15.0 m above the sea level [25,26,31]. The cross section of the subsoil is presented graphically in Fig. 6 (adapted from Fig. 2 of [26]). The average shear wave velocity, Vaverage, of a cross section can be calculated using H total =V average ¼ SH i =V i

ð2Þ

where Hi are the thicknesses of the individual layers and Htotal is the total thickness under consideration. Using the thicknesses and the shear wave velocities of the individual layers, the shear wave velocity of the top 30 m, Vs30, the average shear wave velocity to the bedrock level, Vavg and the average shear wave velocity of the profile (i.e. between the surface and the deepest sensor), Vp, are calculated as 280, 430 and 465 m/s respectively. Consequently, the soil group of the site can be designated as Site Class D per NEHRP [4], Class C per Eurocode 8-1 [14] and Z3 per Turkish Earthquake Code – 2007 [36].

3.2. Fatih downhole array (FTH) The Fatih Downhole Array is located within the Fatih Mosque Complex in the Historic Peninsula of Istanbul. The array was deployed through a research project funded by the Scientific and Technological Research Council of Turkey (TUBITAK) and Bogazici University. The array was deployed in 2010 and consists of 3 downholes. The array has instrumentation at 23, 60 and 136 m depths as well as one on the ground surface. The array is at about 68.0 m above the sea level [25,26]. The shear wave velocity of the top 30 m, Vs30 is around 350 m/s. The average shear wave velocity for the whole profile excluding and including the bedrock portion are 460 and 610 m/s (namely Vavg and Vp) per Eq. (2) respectively. The cross section of the subsoil is demonstrated graphically in Fig. 6 (adapted from Fig. 2 of [26]). In this respect, the soil group of the site can be designated as Site Class D per NEHRP, Class C per Eurocode 8-1 and Z3 per Turkish Earthquake Code – 2007. 3.3. Zeytinburnu downhole array (ZYT) The Zeytinburnu Downhole Array is located within premises of the Zeytinburnu Municipality's complex. The financial support for

Fig. 5. Locations of the Downhole Arrays Operated by KOERI (Courtesy of Google Earth).

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4. Downhole array recordings and discussions Classical seismic signal analysis tools and techniques are utilized in the analysis and interpretation of the data. Hence, to have an initial opinion about the data, the first step is to select the time window of the earthquake. As it was mentioned earlier the recordings are transmitted to KOERI on real time basis via ADSL. Therefore there exists sufficient amount of continuous data for signal-to-noise ratio (S/N) calculations both before and after the earthquake. Despite the 300 km distance between the source and the receivers, the acceleration recordings were of good quality with high signal to noise ratio owing to the sensors deployed in the engineering bedrock at depths ranging more than 100 m from the earth surface (Fig. 7a). Consequently, analyzing the recordings at the bedrock level, a 600 s data window has been identified and extracted with S/N41. This duration is significantly longer than that of the record at Gokceada Station, as presented above. The causes of this long duration will be treated in the following sub-sections. 4.1. Waveforms After the selection of the data window, the raw acceleration data integrated to obtain the velocity and displacement time histories of the record (Fig. 7). Thus another distinct feature of the records is observed from the velocity time history (Fig. 7b). In the first 30–40 s duration after the onset of the P waves, high frequency waveforms are dominating, followed by relatively lower frequencies but larger amplitude S waves. A long period phase is observed immediately after the S-waves with periods ranging

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the array was provided through a research project funded by The Scientific and Technological Research Council of Turkey (TUBITAK) and Bogazici University. The array was deployed in 2010 and consists of 3 downholes. The sensors are placed at the ground surface and at 30, 57 and 288 m depths. The top of the array is at about 22.0 m above the sea level [25,26]. The cross section of the subsoil is presented graphically in Fig. 6 (adapted from Fig. 2 of [26]). As can be seen from the figure the shear wave velocity of the top 30 m, Vs30 is around 200 m/s. The average shear wave velocity for the whole profile both excluding and including the bedrock portion is approximately 290 m/s again per Eq. (2). Considering these values, the soil group of the site can be designated as Site Class E per NEHRP, Class D per Eurocode 8-1 and Z4 per Turkish Earthquake Code – 2007. The top 30 m layer at both the ATK and ZYT arrays is fill material above a weathered limestone layer. The FTH array has a clay layer at the top. On the other hand, when the ground water levels are considered, the ZYT array has the highest water table depth, while ATK has the lowest (Fig. 6). Yet it should be noted that these were the measurements made at the time of deployment and of course they are subject to seasonal variations. The depths of the deepest downholes were selected based on the shear wave velocity. Rocks having a shear wave velocity of 750–1500 m/s are generally classified as engineering bedrock. In this respect, in all three cases the deepest sensors are placed at the engineering bedrock level. All instrumentation used in the downhole arrays are Kinemetrics make. The sensors in the downholes are HypoSensor type and the surface sensors are EpiSensor type. All are triaxial force balance accelerometers. The recordings are continuously made at a rate of 200 Hz. The data recorded by all the sensors in the arrays are transferred to KOERI on real time basis through ADSL connection. Further details about the subsoil conditions, as well as funding and partner institutions of the KOERI downhole arrays can be found in other publications [25,26,31].

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Fig. 7. Time histories of FTH 136 m sensor, EW direction. a. Acceleration b. Velocity c. Displacement

between 10 and 20 s. This long period package of waveforms lasts about 100 s (Fig. 7b). When the displacement time history is considered, it can easily be seen that the periods of the displacement cycles gradually decrease by time following the onset of the surface waves (Fig. 7c). This reflects the dispersive properties of the waveforms. The first cycle of this group corresponds to the surface (Love) waves traveling with higher velocity followed by shorter period surface waves traveling with lower velocity. This is an indication of the waves traveling in different media, i.e. consisting relatively harder and softer materials. Taking into account the long durations of the NAE2014 induced motions observed at Istanbul, it is important to have a better insight of the causes of the phenomenon. Since, low frequency, large amplitude and long duration waves affect the long period structures, namely the high-rise buildings, suspension bridges, oil tanks and even some base isolated structures [20]. In the past effects of this phenomenon on the long period structures has been observed during the 1985 Michoacan (Mw ¼ 8.3, also known as Mexico City Earthquake) and 2003 Tokachi-oki (Mw ¼ 8.3, also known as Hokkaido Earthquake) Earthquakes [20]. In the case of NAE2014, a high-rise building over 250 m height monitored by KOERI in Istanbul had horizontal motion measured for a duration over 15 min [35]. Before discussing further the causes and mechanisms of the low frequency, large amplitude and long duration waves produced at the arrays by NAE2014, it will be worthwhile to view some of the recordings around Istanbul. Fig. 8 illustrates such long period and long duration waveforms acquired at the strong motion stations deployed at Büyükada, Heybeliada and Burgazada islands,

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Fig. 8. Velocity time histories (EW direction) of the NAE2014 at Buyukada (BUYA), Burgazada (BRGA) and Heybeliada (HYBA).

namely the Prince Islands. Prince Islands are a set of small islands located at about 20 km away from the arrays in the SE direction in the Marmara Sea and about 5 km offshore of the South coast of the Anatolian side of Istanbul. Waves with similar characteristics were also observed at the distant Earthquake Early Warning seismic network stations of KOERI situated mostly around the Eastern Marmara Sea [18]. Likewise, the seismic network within the Marmaray Submerged Tunnel has also shown such behavior [11]. It is known that with the help of the path effects, namely the crustal features along which the waves propagate, long period ground motions at distant sedimentary basins are produced as a result of large subduction zone and/or moderate to large shallow earthquakes [3]. Subsequently, at stations far from the source, as in this case, surface waves with longer duration than those recorded near source and near fault. Hence, the shallow depth of the NAE2014 event can be stated as an effective parameter also in generating large S-wave amplitudes (Fig. 3). The attenuation of the seismic waves strongly depends on the quality factor, Q of the medium through which seismic waves propagate. It is a well-known fact that the Q value of the P waves is roughly twice of the S-waves which in turn suggest that the Swaves lose their frequency content with distance two times faster than the P-waves. Because of such facts, the high-frequency content of the P-waves is higher. Furthermore, when seismic waves are traveling through a deep basin, the waves get reflected from the edges of the basin and possibly enhance the ground motion at longer periods. Yoshimoto and Takemura [40] examined the predominant period of the longperiod ground motions excited in the Kanto Basin, Japan and reported a distinct correlation between the basin depth and the period of the waves. Moreover, long-period seismic amplification in the Kanto Basin estimated from ambient seismic field showed similar results. Based on the measured velocities, researchers reported that the basin can enhance the ground motion amplitude and duration at periods between 2 and 5–7 s [40]. Denolle et al. [10] proposed a linear relationship between ground motion and basin depth at periods of 2–10 s that could be used to represent 3D basin effects in ground motion prediction equations. They also find that the strength of basin seismic amplification depends strongly on the direction of the seismic wave propagation. Marmara Sea between the arrays and the epicenter of the NAE2014 has three deep basins, namely Tekirdag (1133 m), Central

(1268 m) and Cınarcık (1270 m). The basins are believed to be filled with basinal sediments over 2 km thickness [29]. Another important feature of the Marmara Sea is that the Northern shelf of Marmara, shortest path to arrays, is much narrower as compared to the Southern shelf. Yet another reason for the large amplitudes of the S waves is the rupture propagation pattern associated with the co-seismic ruptures. Stations located fault-parallel or fault-normal are sites where large shear waves are observed. Such an example is the November 9, 2011 largest aftershock of the Van earthquake that caused widespread damage in Van where extremely large PGA values were recorded [1,5]. An alternative approach to investigate the topic is plotting the spectrogram of the acceleration time history, namely the short time Fourier spectrum of the signal. The spectrogram is calculated using 10.24 s window lengths with 50% overlap (Fig. 9). The arguments made about occurrence and magnitude of the periods of the wave form can be more explicitly seen from this figure. One may easily notice from the figure the fact that the seismic waveforms with frequency higher than 5 Hz attenuates rapidly and disappear around 75–80 s range of the horizontal axis; whereas, the waveform content with periods larger than 1 s can be traced as far as 250 s limit. 4.2. Determination of the analysis window Husid diagrams demonstrate the measure of the energy buildup of a ground motion, namely the percent build-up of the Arias intensity. Hence, when the Husid diagram of the 136 m EW of FTH array component is analyzed, it can be seen that the duration between the 5% and 95% values, which is also known as TrifunacBrady duration, is around 150 s (Fig. 10). It is also recommended that in the analyses of the seismic records window length should be selected on the base of energy criteria. Thus including part of the signal containing 80% or 90% of the energy is normally deemed sufficient [30]. Yet, it is recommended that a few seconds of the data prevailing the P-waves kept also [30]. Consequently, in this study, in lieu of the above discussions and taking into consideration the upward wave propagation and other arrays, in the analyses 200 s duration records are used. This also corresponds to the use of data with maximum signal to noise ratio at or above 4–5 at the FTH and ZYT

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arrays. Afterwards, the selected raw data have been baseline corrected and 0.01–48 Hz bandpass filtered. The filtering has been applied to high frequencies and high periods to conserve the characteristics of the surface waves. 4.3. Ground motion records The acceleration time histories recorded are presented in Figs. 11, 12 and 13 for ATK, FTH and ZYT arrays, respectively. The figures for FTH and ZYT, as anticipated, demonstrate an increase in the acceleration amplitudes, namely amplification through the profile from the engineering bedrock towards the ground surface. However, in case of the ATK array though the amplification between the array bottom and the surface is quite apparent, the time histories of the sensors at 23, 50 and 70 do not seem to exhibit an amplification. This issue will be treated while discussing the maximum values below. To complement the discussion about the waveforms and the phases in the previous sub-sections, the velocity and displacement time histories of the FTH array for all sensor depths and directions are presented in Figs. 14 and 15. Corresponding time histories for the other arrays are not shown due to space limitations, but they also present the same behavior as the FTH array. The only difference that observed is the presence of higher frequencies, especially in the velocity time histories, towards the surface. These higher frequencies are mostly due to the fact that the arrays are located in an urban environment. Possibly, they are resulting from the seismic noise induced by urban activities such as traffic and as well as the result of the soil structure interaction of nearby buildings. In case of the ATK array, there is a busy highway located about 300 m away from the array. Whereas at the ZYT array, there are two buildings on each side of the array in the East-West direction. There is also a parking lot of the municipality right on the south of the array. In case of the FTH array, there is the Fatih mosque about 100 m (center of the dome to array distance)

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from the array. On the other hand, as can be seen from Figs. 14 and 15, the waveforms of both the velocity and displacements towards the surface demonstrate similar behavior. This also indicates the strong presence of the long period surface waves. The increasing trend of the accelerations is also apparent when the maximums are considered at each sensor level (Fig. 16). Yet a drop can be observed in the ATK values right above the 70 m array. A similar behavior at the ATK site was also observed earlier by Kurtulus [25]. However, in that study the drop was around the 23 m sensor and the earthquakes considered had produced lower acceleration values than the NAE2014, somewhat a half of what is observed in this case. Kurtulus attributed the drop to the weathered limestone layer at that level. Yet, the starting point of the deamplification being around the 70 m sensor suggests that attributing the drop to the behavior of the limestone layer may not provide sufficient evidence in this case. On the other hand a close look at the SPT values presented by Parolai et al. (Fig. 4 of [31]) indicates the presence of low SPT interlayers between the 23 and 70 m sensors. Authors suggest that these interlayers are possibly acting as filter layers and are instrumental in the deamplification in that section of the profile. An important value in the geotechnical earthquake engineering is the amplification ratio of the accelerations from bedrock to surface level. In Table 1, the ratios calculated by dividing the peak surface accelerations to the peak accelerations of the deepest sensor are demonstrated. Depending on the array location and the direction of the recording the ratios observed are as high as 5.3. But calculating the soil-to-rock amplification ratio using the downhole array recordings is somewhat misleading. In case of a surface array with soil surface and nearby rock outcrop recording, the amplification ratio will be 2As/2Ab, considering the incident wave reflection at the surface. In this representation As and Ab are the motion recordings at soil surface and rock outcrop, respectively. However in the case of a downhole array, calculating the amplification ratio using the surface and bedrock level values should yield a ratio twice that of the surface array if there were no reflection of the up-going incident waves. But, obviously the recording at the deepest sensor will be affected by the downward incident waves reflected from the upper layers. Hence in this case the ratio will be 2Aa/(Ab þBb), Bb representing the reflected portion of the incident wave at the bedrock level [21–23,32–34,16,17]. This means that an adjustment is needed to determine the amplification ratio more accurately. However, though there are several approaches proposed, a unified procedure is not available yet. An extensive treatment of the topic can be found in a study published by Heloise et al. [16,17]. One of the proposed approaches for predicting surface amplification is the method proposed by Kokusho and Sato [21,22]. They proposed an equation for amplification with two parameters, namely the average shear wave velocity of the profile above bedrock and the shear wave velocity of the base rock. Later, Kokusho studying the recordings of a large number of surface and downhole arrays from eight earthquakes suggested that the amplification ratio at a given site can be empirically expressed as 2As =2Ab ¼ 0:345 þ0:634V sb =V

ð3Þ

where Vsb is the shear wave velocity of the bedrock layer [23]. The equation proposed was a slight modification of the earlier Kokusho and Sato approach, namely an adjustment of the coefficients. The amplification values calculated by Eq. (3) for ATK, FTH and ZYT are presented also in Table 1. Another important observation that is worth to mention is the difference between the bedrock level accelerations of the ATK and ZYT arrays. The accelerations at the bedrock level (140 m) for ATK are 0.38% and 0.45% g for the EW and NS directions respectively. But the accelerations at the bedrock level (288 m)

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ZYT are 0.22% and 0.31% g for EW and NS directions, respectively (Fig. 16). Considering the distance between the two arrays which is approximately 5.0 km and their distance from the source which is about 300 km, one would expect that the maximum acceleration values for both the arrays at the bedrock level would be reasonably similar. However, there is almost a factor of 1.7 and 1.5 between the corresponding values, ATK values being higher. This might be due to several reasons. One reason is due to the properties of the surface layers above the bedrock layer. The reflection coefficient for the ATK

array considering the bedrock properties and the layer just above the bedrock is calculated as 0.30. Whereas the same value for the ZYT array is 0.54. Thus the portion of the up-going wave reflected downwards is higher at the ZYT array. In such case one would expect that the accelerations would higher at the base rock of ZYT. But the case is the contrary. So, another possibility is the characteristics of the wave propagation within the engineering bedrock layer. A study made by Parolai et al. [31] suggests that the engineering bedrock at ATK

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extends to 230 m depth, but the sensor was placed at 140 m based on previous engineering bedrock estimation. However, Fig. 4 of the same study indicates the presence of a stratum below this engineering bedrock at 230 m with lower shear wave velocity. This implies that the Paleozoic bedrock layer is possibly at around 330 m depth and as suggested by Dalgic overlaid by Oligocene and Miocene sediments in the northern coast of the Marmara Sea [8,9]. Thus, the almost twofold amplification at ATK bedrock possibly

reflects an amplification between the Paleozoic bedrock at 330 m and the engineering bedrock at 140 m. Similar results regarding the depth of the Paleozoic bedrock and amplification at Tuzla (located at East side of Istanbul) are observed in the borehole drilled for a study made by Kartal et al. [19]. Hence, knowledge about the depth of the Paleozoic bedrock and the strata above the Paleozoic bedrock to the surface also play an important role on the amplification of motion.

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Fig. 16. Variation of maximum accelerations with depth. (a). Atakoy. (ATK). (b). Fatih (FTH). (c). Zeytinburnu (ZYT).

Table 1 Ratio of peak accelerations.

ATK FTH ZYT

2Aa/(Ab þ Bb) from recordings

Per Kokusho's equation, 2Aa/2Ab

EW 2.7 5.2 5.3

Using Vavg 1.6 1.9 2.3

NS 1.9 3.2 5.1

UD 1.5 3.9 2.5

To complement this topic, it also worth comparing the peak accelerations at FTH and ATK arrays. The bedrock layer starts at about 120 m and 80 m depth for ATK and FTH respectively. On the

other hand, the average shear wave velocity for the soil layers, in both cases is reasonably close, i.e. 430 and 460 m/s. Whereas Vs30 values are 280 and 350 m/s for ATK and FTH respectively. In this case, considering only the Vs30 values one would expect that the magnification of accelerations from bedrock to surface should be higher for FTH. Yet the result is the opposite. Again there might be several reasons of this result. One reason might be the existence of a filter layer in the profile at ATK as noted earlier. The other reason might be that the FTH array is located on top of a hill while ATK is at a flat array. Thus possibly the reflections of waves are increasing the PGA amplitudes [15]. Another reason for the increase at FTH can also be due to the Fatih Mosque, 100 m away. This point will be further elaborated while discussing the spectra.

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Considering all the findings and discussions in the foregoing paragraphs, as a final remark, it can be said that the Vs30 may not be a sufficient parameter to characterize surface motion. 4.4. Particle motion In Figs. 17 and 18 particle motions at the bedrock and surface levels are given respectively. Especially at the bedrock level, the EW–NS particle motion graphs indicate that the displacements with the largest amplitudes are in the NNW and SSE direction. That is an inclination of about 20–251 counterclockwise from the N–S direction. On the other hand as mentioned above, the rupture faulting of NAE2014 had been considered to have an azimuth of 701 (Fig. 19). This directional behavior preserves itself throughout the soil profiles. Thus both results confirm each other mutually, namely the direction of particle motion of the Love waves is perpendicular to the azimuth of faulting as it should be. The focal mechanism solution of the main shock portrayed in Fig. 3 shows predominantly strike-slip mechanism. Taking into account the elongation of the aftershocks one may easily depict

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the nodal plane striking ENE as the fault plane. Combining this fact with the particle motion diagrams can be utilized as a tool to retrieve the phase type of the large amplitude waveforms observed at Figs. 11–15. The particle motion constructed from the NS and EW components is sensitive to the horizontal component of the particle vibrations. The particle motion of the SHwaves and the Love waves is perpendicular to the direction of the traveling waves. As it was mentioned in the previous sections, the large amplitudes emerge after the onset of the S-waves. Thus, it can be stated that the large deviations observed in the NS–EW particle motions corresponds to the Love waves with particle motions in NNW–SSE direction which is perpendicular to the ENE–WSW direction of the fault plane (Figs. 17 and 18). On the other hand, graphs demonstrating the NS–UD motion clearly indicate the effect of the Rayleigh waves in the displacement time history. This effect is very consistent throughout the whole array cross sections. An assessment of the UD-NS and UDEW particle motions orientations together yield the traveling direction of the Rayleigh waves. The approximate EW/NS ratio of 0.4 observed at ATK station corresponds to ESE–WSW direction

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Fig. 17. Particle motions graphs at bedrock level. (a). ATK (140m) (b). FTH (136 m) (c). ZYT (288 m).

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Fig. 19. Directional relationship of the fault line azimuth and the seismic downholes at Istanbul (Courtesy of Google Earth). (a). ATK (b). FTH (c). ZYT.

(about 201 bias from EW) which is the expected route of the Rayleigh waves. Smaller and larger EW/NS ratios might be due to the bias of the sensor from the true North.

The 2551 backazimuth of the ATK downhole array with respect to the epicenter of NAEQ and the focal mechanism parameters of the ruptured right-lateral strike-slip fault plane (N70E or N250E)

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point out that the borehole instruments are located closely to the nodal plane. The radiation pattern of a strike-slip fault predicts small amplitudes for the P- and large amplitudes for the S-waves as were the observations [3] 4.5. Displacements and shear strains The shear strain time histories are calculated by dividing the relative horizontal displacement between the two consecutive sensors to the height between them. Indeed, this is a rather crude approximation for the calculation of the shear strains but it will provide an indication of the average values in the strata. The maximum values of the shear strains are given in Table 2. The maximum value observed is about 6.2  10  5. While the minimum value of the maximums is about 8.0  10  6. Hence the shear strains calculated can be considered as very small strains, except the maximum value being at the moderately large strain level. However the moderately large strain value was quite spontaneous in the strain time history. Thus, it can be stated that the soil layers behaved as a quasi – elastic material. 4.6. Fourier spectral ratios Both response spectrum and Fourier amplitude spectrum find use in the investigation of dynamic response of soil sediments and design of structures. While Fourier amplitude spectrum is generally preferred by researchers in earth sciences, the response spectrum is favored especially by the structural engineers concerned in the super structure design. Hence the Fourier spectral ratio between the surface motion and the motion at the bedrock level is a commonly used tool to compute the site response, especially when the response assumed to remain in the linear range. Hence it is quite customary to report these curves. However, it should be kept in mind that the use of spectral ratios calculated using the downhole arrays have limitations and may produce misleading results [21–23,32–34,16,17]. The reason for the error contained in these curves is due to the contribution of the reflected waves, namely the down-going waves, to the up-going incident waves. This phenomenon was briefly explained in Section 4.1. In other words the spectral ratio curves calculated from the observed records does not correspond to the correct transfer function curve. Yet on the other hand generating the correct transfer function from the data recorded at downhole arrays is a very complex topic and valuable studies can be found in the literature. However, the algorithms proposed have either Table 2 Maximum shear strains.

a. ATK Layer surface – 25 m 25–50 m 50–70 m 70–140 m

EW

NS

0.000014 0.000023 0.000021 0.000020

0.000008 0.000019 0.000012 0.000028

b. FTH Layer Surface – 23 m 23–60 m 60–136 m

EW 0.000019 0.000013 0.000014

NS 0.000023 0.000018 0.000014

c. ZYT Layer Surface – 30 m 30–57 m 57–288 m

EW 0.000023 0.000036 0.000022

NS 0.000017 0.000042 0.000025

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limitations due to their empirical nature or too theoretical to apply in practice. Thus, in this study the discussion will be limited to the raw spectral ratios. The Fourier spectral ratio for each array and each direction were calculated and presented in Fig. 20. The figures contain both the unsmoothed and smoothened data. The smoothing was performed by Konno–Ohmachi procedure (b¼ 40) [24]. The selection of the smoothing method is considered to be somewhat subjective as well as its effectiveness and accuracy. Yet a study made by Boore indicates that the Konno–Ohmachi method performs better at the low range as compared to the triangular smoothing approach [7]. From the figures, it can be seen that the highest amplifications for the horizontal directions of the ATK, FTH and ZYT arrays occur at about 1.0, 1.8 and 0.7 Hz, respectively. Fig. 16 portrays the PGA values recorded at different levels in the ATK, FTH and ZYT boreholes including the surface records where the amplitude ratio between the horizontal components at the surface and at the bottom of the boreholes are several times larger than the ratios of the vertical components. Thus, confirming the fact that the vertical components are not affected so much as the horizontal ones do from the amplification observed at basins, which constitute the basis of the H/V ratio method known as Nakamura method, which is a practical and convenient method to determine resonant frequencies when its limitations are observed [27,28]. However, this is not the case observed for the spectral ratios calculated by dividing the surface amplitude spectra to the ones acquired at the bottom of the boreholes (Fig. 20). The 3component data from the ATK borehole depicts no amplification in the frequency range 0.1–0.5 Hz. But, for the frequencies beyond the 0.5 Hz significant amplification is observed both at the vertical and the horizontal components. Similar, features are observed at the FTH and ZYT boreholes (Fig. 20). 4.7. Response spectra The response spectra of the earthquakes are regarded as an important design tool especially by the structural engineers. The design codes specify response spectra calculated for 5% critical damping. The 5% critical damping velocity response spectra of the NAE2014 recorded at each array are calculated and presented demonstrated in Fig. 21. Since the curves plotted are produced using the pseudo acceleration spectra the zero period values are slightly different than the measured values. In case of ATK and ZYT arrays the major part of the amplifications seems to occur between the sensor at the engineering bedrock level and the sensor at around the 60–70 m level, excluding the top 30 m layer of the ZYT. The amplification in the layers above seems limited at almost all periods. However, on the other hand in the FTH array the amplifications seem to be more evenly distributed between the layers. The spectral ratios (surface/bedrock) are plotted for all three directions for comparative analysis (Fig. 22). Again 5% damping pseudo acceleration spectra are used. Yet again as in the case of Fourier Spectral ratios, the response spectral ratios should be considered cautiously. Due to the reflections of the incident waves the result can be misleading. Yet on the other hand, a study made by Safak concluded that the response spectral ratios for frequencies less than 5 Hz yield reliable results [33]. Amongst the three, ATK array happens to have the lowest spectral ratios. For the ATK array, the peak occurs at around 1.0 s for all three directions. In case of the FTH array the peak occurs at around 0.5 s for both horizontal directions. This result compares well with the values observed in an earlier study on Fatih Mosque, which is less than 100 m from this array [6]. But also indicates the

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possibility of strong soil structure effects. Hence, the surface and near surface measurements seem to be affected by the existence of the Fatih mosque resulting in high amplification at site. In turn this also another reason that Vs30 alone may be not be sufficient for site characterization. On the other hand, the peak amplification at ZYT array occurs at around 1.5–1.7 s for the horizontal components. Both ZYT and FTH amplification factors are considerably higher than those of the ATK values in all three directions. This is a rather common phenomenon for the cases where major changes take place in the shear wave velocities between the layers, namely where high velocity contrast between the bedrock and the overlaying sediments exist. Thus again it can be stated that the use of Vs30 may not be a sufficient parameter for site characterization especially for the cases where the thickness of the overlaying layers are considerably deeper and also where high impedance contrast exists.

5. Conclusion In this study, the site response observed at the KOERI operated seismic downhole arrays in Istanbul during the May 24, 2014

Northern Aegean Earthquake (NAE2014) are analyzed in detail and presented. NAE2014 was significant due to its waveform characteristics occurring around Istanbul and also being the highest magnitude earthquake within local distances after the deployment of the KOERI downhole arrays. Though the significant duration of strong motion records of NAE2014 were around 90 s near the source, the duration observed at the downholes were longer than 600 s where the amplitudes are at S/N 41 levels. Also, a distinct phase is observed immediately after the S-waves with long periods ranging between 10–20 s. This long period package of wave forms last about 100 s. The waveforms with periods larger than 1.0 s can be traced about 250 s after the P-wave arrivals. The cause of these long period waveforms are attributed to the basin effects taking place due to the three deep basins in the Marmara Sea. The waveforms observed at different levels in the boreholes as anticipated demonstrate increase in the acceleration amplitudes, reflecting amplification through the profile from engineering bedrock towards the ground surface. In addition, our observations provide evidences for amplification occurring within the engineering bedrock layer which alternatively may suggest that the Paleozoic bedrock layer at ATK is possibly much deeper than the

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Fig. 22. Spectral ratios (surface /bedrock, 5% critical damping).

engineering bedrock observed at 140 m depth by drilling. The amplification of maximum displacements towards the surface is minimal because of the dominant long period (larger than 10 s) surface waves. The results obtained from this study indicate that Vs30 may not be a sufficient parameter, or possibly even a misleading factor in site characterization. Especially for the cases such as where the thickness of the overlaying layers above engineering bedrock are

considerably deeper and high impedance contrast exists between the upper layers and the engineering bedrock.

Acknowledgments: Authors wish to acknowledge the financial support provided for the deployment of downhole arrays Granted by the Scientific

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