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Bulletin of the Seismological Society of America; February 2006; v. 96; no. 1; p. 188-199; DOI: 10.1785/0120050037
© 2006 Seismological Society of America
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Article

Cumulative Attenuation along Source-to-Receiver Paths in Northwestern Turkey

D. Bindi1, S. Parolai2, H. Grosser2, C. Milkereit2 and S. Zünbül3

1 Istituto Nazionale di Geofisca e Vulcanologia
Via Bassini 15
20133 Milano, Italy
 (D.B.)

2 GeoForschungsZentrum Potsdam
Telegraferberg
14473 Potsdam, Germany
 (S.P., H.G., C.M.)

3 Ministry of Public Works and Settlement
General Directorate of Disaster Affairs
Earthquake Research Department
P.O. Box 763
Ankara, Turkey
 (S.Z.)


   Abstract
Top
 Abstract
Introduction
 Data
 Parametric Description:...
 Spatial Distribution of...
 Near-Surface Attenuation
 Discussions and Conclusions
 Appendix
 
The attenuation of shear waves propagating in the crust of northwestern Turkey has been investigated in the frequency range 1–10 Hz. A standard spectral inversion scheme is applied to a data set of 245 aftershocks (ML <4.5) of the 1999 Izmit earthquake. The obtained attenuation-with-distance curves have been described in terms of the t* cumulative attenuation parameter and its dependence on frequency and distance investigated. At 1 Hz, Q–1, evaluated by normalizing t* to the travel time, is generally larger than 0.025 for source-to-station distances smaller than 40 km, indicating the presence of a highly attenuating upper crust in the area. Over longer distances, Q–1 decreases, suggesting a decrease in the attenuation with depth. By contrast, the normalized t* computed for earthquakes recorded at stations having almost the same distance from the sources do not show a strong dependence on the backazimuth. These results suggest that the decrease of Q–1 with depth is more significant than its lateral variations. Regarding its frequency dependence, Q–1 almost linearly decreases with frequency.

Finally, the near-surface-attenuation parameter k is evaluated at 12 stations and the results discussed in terms of site, event, and propagation contributions. The event contribution is not negligible and shows a significant positive correlation with magnitude. The site term is smaller than 0.020 sec for rock or topographic sites, while it assumes values of 0.036 sec and 0.042 sec for two stations installed over thick soft sedimentary layers.


   Introduction
Top
 Abstract
 Introduction
Data
 Parametric Description:...
 Spatial Distribution of...
 Near-Surface Attenuation
 Discussions and Conclusions
 Appendix
 
The description and quantification of seismic-wave attenuation plays a fundamental role in many seismological analyses. Attenuation estimates allow the investigation of the structure and physical state of the Earth's interior, and provides corrections that permit a more accurate description of processes at the source. Moreover, studies on hazard assessment require a quantitative evaluation of seismic-wave attenuation in order to predict ground motion from earthquakes.

We present the results of a study on crustal attenuation performed in northwestern Turkey. Previous studies (Bindi et al., 2006, and the reference cited therein) were devoted to characterizing the attenuation in this region by estimating the average quality factor, Q(f), from the S-wave window or using the coda portion of the seismograms. In particular, Akinci, Del Pezzo, and lbanez (1995); Akinci, lbanez, et al. (1995); and Akinci et al. (2004) investigated the attenuation in the western portion of the North Anatolian fault (NAF), while Gündüz et al. (1998), Kaslilar-Özcan et al. (2002), and Horasan and Boztepe-Güney (2004) analyzed the attenuation properties of the Marmara region.

We analyze the aftershocks (ML <4.5) of the Izmit earthquake (Mw 7.4) that occurred on 17 August 1999. The attenuation-with-distance curves, obtained by applying the generalized inversion technique (Castro et al., 1990), are used to infer the frequency-dependent attenuation characteristics of S waves in the frequency range 1–10 Hz. We estimate the t* parameter (Kanamori, 1967) along each ray path and investigate its dependence on frequency, distance, and backazimuth. The spatial distribution of attenuation is also investigated, carrying out a two-dimensional tomographic inversion for a simplified model in which depth is not accounted for. Finally, the near-surface attenuation is evaluated by computing the k (kappa) parameter (Anderson and Hough, 1984). Following Purvance and Anderson (2003), the parameterization of k in terms of source, site, and propagation effects is investigated and the results compared with the area's geology, which is characterized by the presence of several alluvial basins (e.g., EERI, 2000) surrounded by complex topography.


   Data
Top
 Abstract
 Introduction
 Data
Parametric Description:...
 Spatial Distribution of...
 Near-Surface Attenuation
 Discussions and Conclusions
 Appendix
 
We analyze 245 aftershocks following the 1999 Izmit earthquake, recorded at the Sapanca-Bolu (SABO) and German Task Force (GTF) networks (Grosser et al., 1998; Baumbach et al., 2003). The seismological equipment used in this study consists of Mark L4-3D 1-Hz geophones, 24- bit digitizers with a sampling rate of 100 samples per second, and GPS timing. The analyzed earthquakes have magnitudes, ML, from 1 to 4.5, the hypocentral distances ranging between 10 and 142 km, with most events having a source depth of 5–15 km. We analyze the spectra of S-wave windows 5 sec wide. Each window is cosine tapered (5%) and Fourier transformed. Instrumental corrections are applied and the spectral amplitudes are smoothed using the Konno–Ohmachi window, b = 20 (Konno and Ohmachi, 1998). With respect to the pre-event noise, the selected S windows have signal-to- noise ratios (S/N) greater than 3 over the frequency band between 1 and 10 Hz. Finally, the two horizontal (east–west and north–south) and the vertical (Z) spectra are vectorially summed.

The spectra are collected from 23 stations (Table 1). Figure 1 shows the station locations and the earthquake epicenters. The earthquakes are grouped into three different sets: set s1 consists of the earthquakes that occurred between Izmit Bay and Sapanca Lake, including the Gölcük and Sapanca segments of the NAF (EERI, 2000); set s2 mainly includes the earthquakes that occured south of the Adapazari basin, including the Sakarya segment of the NAF; and set s3 is the earthquakes that occurred close to the Düzce basin, including part of the Karadere segment of the NAF. A sketch of some of the alluvial basins is shown in Figure 1 (EERI, 2000).


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  		Table 1 Stations Used in This Study
 

Figure 001
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  		Figure 1. Study area. Top panel: a sketch of the main faults (black lines) and some segments (thick lines) of the North Anatolian fault are shown. Bottom panel: triangles and circles are for stations and events, respectively. Some alluvial basins are shown as areas filled with linear vertical pattern.

 


   Parametric Description: Attenuation along Single Paths
Top
 Abstract
 Introduction
 Data
 Parametric Description:...
Spatial Distribution of...
 Near-Surface Attenuation
 Discussions and Conclusions
 Appendix
 
Following Bindi et al. (2006), the attenuation-with- distance curves A(f,r) of S waves are obtained by applying the generalized spectral inversion, GIT (e.g., Castro et al., 1990) to the velocity spectra. The curves A(f,r) can be parameterized in term of attenuation along a single path as (Sanders, 1993)


Formula 001

(1)
where G(r) is the geometrical spreading, tij is the travel time relevant to event i recorded at station j, and Qij is the apparent quality factor along the ray path. The cumulative attenuation along the path ij can be described by introducing the t* operator (Kanamori, 1967; Cormier, 1982; among others):


Formula 002

(2)
In this work, we processed t* as a frequency-dependent quantity, and therefore equation (1) can be rewritten as


Formula 003

(3)
We evaluated the attenuation along all ray paths, assuming G(r) = 10r–1 for r less than 60 km and G(r) = 10r–1.2 for r ≥ 60 km, following Bindi et al. (2006). Figures 2, 3, and 4 show the results for three selected stations. Following Haberland and Rietbrock (2001), the t* is normalized to the travel time, to represent the average attenuation Q–1 along each ray path, being dependent upon the backazimuth, frequency (top panels), and event location (middle and bottom panels).


Figure 002
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  		Figure 2. Station 007. Top panels: Q–1 (i.e., t* normalized to travel time) versus backazimuth at 1 Hz (left) and 10 Hz (right). Distance r from 10 to 38 km, black filled circles; 38 < r < 60 km, gray squares; 60 < r < 80 km, empty triangles. Middle panel: Q–1 shown as a circle located in correspondence of the earthquake epicenter and filled in accordance with a gray palette table. Bottom panel: projection of the Q–1 values over a west–east vertical plane located at the latitude marked by the black cross in the middle panel.

 

Figure 003
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  		Figure 3. The same as Figure 2 but for station 026.

 

Figure 004
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  		Figure 4. The same as Figure 2 but for station 039.

 

The frequency dependency of the attenuation parameter is clear: Q–1 decreases by about a factor of 10 for frequencies increasing from 1 to 10 Hz. In the polar diagrams of Figures 2–4, the dependence of Q–1 on distance is also evident: at each frequency, the highest values correspond to the shortest distances (black filled circle, 10 ≤ r ≤ 38 km), while the lowest values correspond to the longest paths (empty triangles, 60 ≤ r ≤ 80 km). At 1 Hz, Q–1 for 10 ≤ r ≤ 38 km is generally between 0.04 and 0.08 (i.e., 12.5 < Q < 25), while for 60 ≤ r ≤ 80 km, Q–1 is generally between 0.001 and 0.04 (25 < Q < 1000). The attenuation along each ray path does not show a strong dependence on the backazimuth, and a comparison between the maps showing Q–1 as a function of the event location (middle and bottom panels of Figs. 2–4) shows that the highest attenuation values correspond to the closest earthquakes. The decrease of Q–1 with distance is particularly evident by comparing the projection of Q–1 on a vertical plane for different stations (e.g., station 007 and 026, bottom panels). Comparing Q–1 evaluated at several stations surrounding the two main clusters of events (e.g., sets s2 and s3 of Fig. 1), the decrease in Q–1 with source-to-station distance appears to dominate the variations related to different directions of propagation. Figure 5, top panel, shows Q–1 at 1 Hz relevant to earthquakes belonging to set s3 and recorded at stations 039, 026, and 033, which have different backazimuths, but almost the same distance from the selected earthquakes. The values of Q–1 range from 0.02 to 0.05 sec, regardless of the azimuth. The bottom panel of Figure 5 shows Q–1 at 1 Hz against distance for data sets s1, s2, and s3.


Figure 005
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  		Figure 5. Top panel: Q–1 at 1 Hz for earthquakes in set s3 of Figure 1 and recorded at stations 033 (stars), 026 (inverse triangles), and 039 (diamonds). Bottom panel: Q–1 against distance at 1 Hz; triangle, circle, and cross symbols represent earthquakes belonging to sets s1, s2, and s3, respectively.

 

The behavior of Q–1 with distance is similar for the three sets, providing a further indication that lateral contrasts of attenuation play a minor role with respect to source-to- station distance (and therefore the vertical variation of Q–1) in determining the average attenuation along each single ray path. The dispersion in the attenuation values at distances in the range 60–80 km is worth attention. This feature could be ascribed to the secondary arrivals whose energy contributes to the direct S waves, leading to an apparent attenuation lower than the actual value for of the direct waves, consistent with the results of Bindi et al. (2006).


   Spatial Distribution of Attenuation
Top
 Abstract
 Introduction
 Data
 Parametric Description:...
 Spatial Distribution of...
Near-Surface Attenuation
 Discussions and Conclusions
 Appendix
 
The analysis of parameter t* has indicated that seismic attenuation in northwestern Turkey is frequency dependent and varies mainly with depth. Since most of the earthquakes are shallow (hypocentral depths less than 15 km) and clustered into two main groups (sets s2 and s3 in Fig. 1), and with most of the hypocentral distances within 60 km, a three- dimensional tomographic inversion could provide an accurate reconstruction only for the upper crust in the Adapazari and Düzce regions. Therefore, in order to infer some characteristics of the attenuation in northwestern Turkey within the range 1–10 Hz, we invert the cumulative attenuation along each ray path by using a simplified approach where the t* values are spread throughout the straight line connecting each epicenter to the recording station. A fully three- dimensional inversion, however, will be the goal of future studies after obtaining a larger data set. The inversion performed in this study aims simply to check whether the spatial distribution of attenuation correlates with the main geologic features of the region. From equations (1) and (2), the anelastic attenuation can be expressed in terms of quality factor and velocity as


Formula 004

(4)
The source-to-receiver path rij is approximated as a straight line and the traveled medium is approximated as a plane. Discretizing the plane into rectangular pixels with index k having constant Qkvk, equation (4) becomes


Formula 005

(5)
where {gamma}k = 1/Qkvk, and lijk is the length of the ray ij in each pixel k, normalized such that rij = {sum}klijk, where rij is the hypocentral distance. For all stations and sources, and for a given frequency f, equation (5) describes a tomographic problem where the unknowns {gamma}k must be reconstructed from their projections along some straight lines. The projections are the attenuation curves corrected for the geometrical spreading, and the coefficients of the projection matrix are equal to –{pi}f times the length lijk. Several techniques can be exploited to find approximate solutions of (5). We used an iterative method known as the row action maximum likelihood algorithm, RAMLA (Brown and De Pierro, 1996; Bindi and Caponnetto, 2001), which has the advantage of furnishing nonnegative solutions without any additional constraints, under some quite general conditions. After some tests, we set the number of RAMLA iterations to 10, and the iterative scheme is initialized with a uniform solution equal to {gamma}k = 0.01.

Figure 6, top-left panel, shows the station locations (triangles), epicenters (black filled circles), and ray-path coverage relevant to the data set considered for the tomographic inversion. The top-right panel shows the parameterization and number of rays crossing each pixel k. Each pixel is 0.15 and 0.075 degrees wide in longitude and latitude, respectively. The pixels in region s2 (see Fig. 1) have the highest sampling, while some pixels at the edges are not sampled at all (cross symbol). The tomographic reconstruction will not be shown for unsampled pixels.


Figure 006
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  		Figure 6. Top panels: left, stations (triangles), earthquakes (black filled circles), and path coverage relevant to the two-dimensional attenuation tomography; right, number of rays crossing each pixel, given in logarithmic scale. Middle panels: left, location (star) of the input-source function for point-spread analysis, as well as the actual rays sampling the pixel where the point source is located; right, symbols correspond to the different amplitude range of the restored source function, being the true amplitude of the point-input function equal to 0.02. Bottom panels: the same as in the middle panels but for a different location of the input function.

 

The middle and bottom panels show the results of the resolution estimate in terms of the point-spread function (e.g., Cong and Mitchell, 1998). Synthetic attenuation values are computed by solving equation (5) in the forward direction by applying the matrix for the actual source and station geometry to a point-input function that assumes a value of 0.02 in a single pixel and zero elsewhere. In particular, we show the results relevant to an input function located in region s2 (middle panels) and in region s3 (bottom panels). The middle- and bottom-left panels show the rays that sampled the selected pixel (star). The right panels show the results of the tomographic inversion. In both cases, the reconstruction of the point-input function is satisfactory. At the edge of the considered region, a few artifacts with amplitudes between 5% and 50% of the input-point function appear.

Figure 7 shows the results of the tomographic inversion for frequencies 1 and 10 Hz. Before discussing the results obtained with real data, we recall that the values of {gamma} in Figure 7 do not represent the attenuation averaged over a certain fixed depth interval, since the sampling of actual rays is not uniform with depth. Therefore, if a shallow high Q–1 zone exists in a region crossed by only long (and hence deep) rays, this anomaly cannot be imaged in the restored model. On the contrary, if the crust in a certain area is mainly sampled by rays propagating mostly at shallow depth, the resulting values of {gamma} will be determined by the t* values corresponding to these rays. Consequently, if the Q–1 factor for a certain area varies (decreases) with depth but the rays sample mostly the uppermost layers, the value of {gamma} inside the pixels will be dominated by the high Q–1 of the uppermost layers. The lower Q–1 values (sampled by a few deep rays) will be mapped either at the edge of the investigated area (where the solution is less constrained) or in zones sampled exclusively by these rays.


Figure 007
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  		Figure 7. Q–1 values resulting from the two-dimensional tomographic inversion at 1 Hz and 10 Hz.

 

The values of {gamma} depicted in Figure 7 confirm that a high attenuation is present in the area, in particular in association with the alluvival basins and in the Izmit Bay region. At 1 Hz, values of Qk down to 125/vk are obtained inside the Adapazari basin, and down to 100/vk and 85/vk inside the Düzce basin and Izmit Bay, respectively. These results also outline the existence of lateral variations of attenuation in the area. Assuming the seismic velocity to be frequency independent, Qk increases with frequency in all basins and reaches the value of about 330/vk at 10 Hz. It is worth noting that high attenuation affects the region surrounding the Izmit Bay at all frequencies. High attenuation for the regions at the borders, as well as the high contrast affecting the maps in Figure 7, might have been emphasized by the simplified model adopted, as previously explained.


   Near-Surface Attenuation
Top
 Abstract
 Introduction
 Data
 Parametric Description:...
 Spatial Distribution of...
 Near-Surface Attenuation
Discussions and Conclusions
 Appendix
 
In this section, the spectral amplitudes at frequencies higher than 10 Hz are analyzed to investigate the effect of local geology on seismic-wave attenuation by computing the k parameter (Anderson and Hough, 1984). The k parameter is calculated as the high-frequency asymptote of the acceleration spectrum A(f):


Formula 006

(6)
where A0 depends upon the source, epicentral distance, and other factors. Since both source (Papageorgiou and Aki, 1983) and propagation effects (Hanks, 1982) can cause deviations from the flat high-frequency acceleration spectra predicted by the Brune model (Brune, 1970), the interpretation of k is usually based on empirical models.

Recently, Purvance and Anderson (2003) showed that


Formula 007

(7)
is an effective parametrization of k. In equation (7), Formula (r) describes the distance dependence of k while ksite and kevent are the parameters of the model that capture the dependence of k on the specific recording site and earthquake, respectively. Several empirical observations showed that ksite is generally greater for sites on soft sediments than for sites on rock. It is interpreted as a measure of the attenuation due to the propagation through the shallow subsurface geology; Formula (r) shows the tendency to increase with increasing distance, and it can be considered as a regional effect due to lateral propagation; and kevent is mainly a source effect, as shown by Purvance and Anderson (2003), who analyzed strong- motion records obtained by the Guerrero Array (Mexico). They observed that kevent varies systematically with focal mechanism, being lower for normal faulting than for thrust faulting.

Since site amplification, within the frequency range considered for evaluating k, could affect the slope of the spectrum decay (Parolai and Bindi, 2004), we discarded stations that exhibited peaks of amplification in the high- frequency range, following the results of Parolai et al. (2004). Examples of stations excluded due to amplification peaks at frequencies higher than 10 Hz are stations 004 and 008 in Parolai et al. (2004, fig. 5, p. 1102). Eventually, the analysis was performed on a subset of 12 stations. We evaluated the high-frequency decay for 312 recordings from 60 earthquakes triggered by at least 3 of the 12 selected stations.

From equation (7), a linear system is obtained by considering all the kij values evaluated for the jth earthquake recorded at the ith station (e.g., Purvance and Anderson, 2003). A nonparametric description is used for Formula (rk) (Anderson, 1991) by discretizing the distance range from 0 to 110 km into 22 intervals 5 km wide (k = 0, ... , 22). The system is solved constraining Formula (rk) to be a smooth function of r, and setting equal to zero the site term Formula for two a priori assumed reference stations (039 and 002) installed on rock sites. These stations exhibited almost flat high-frequency spectra and a site response less than 2 in the range 1–20 Hz (Parolai et al., 2004, fig. 7, p. 1105). Moreover, these stations allow k to be constrained over the whole analyzed distance range. The system is solved performing 50 iterations of the Paige and Saunders (1982) least squares algorithm (LSQR). The bootstrap technique (Efron, 1979) is used for an assessment of the model parameter uncertainties, with the standard error of Formula , and Formula (rk) computed from 200 replications. The two horizontal components are not composed but used together in the inversion. In Figure 8, the high-frequency spectral fitting for an earthquake recorded at six stations is shown as an example.


Figure 008
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  		Figure 8. Log–lin acceleration spectra of an M 3.4 aftershock (gray line), the relative spectral amplitude of pre-event noise (dotted line), and the result of the high- frequency fall-off fitting. In each panel, the value of k, the station code, and the hypocentral distance are also shown.

 

Figure 9 shows the results of the regression (7). The kevent term ranges from –0.014 sec to 0.019 sec. An analysis we performed (but not reported here) shows that positive and negative values do not correlate with earthquake location. The high scatter in the distribution of kevent against magnitude (Fig. 9, top-left panel) implies a small linear correlation of R = 0.30, but a null hypothesis of no correlation between kevent and magnitude can be rejected at a 0.98 level of confidence by performing a Student's t-test.


Figure 009
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  		Figure 9. Top panels: kevent against magnitude (left) and Figure 009(r) against hypocentral distance (right). Bottom panels: ksite for each analyzed station (left) and mean ±1 standard deviation of the ambient noise horizontal-to-vertical (H/V) ratio for stations 013 and 029 (right panels).

 

Table 1 and Figure 9 (bottom-left panel) show the values of ksite. The highest values are found for stations installed in areas underlaid by deep sedimentary cover, such as station 013 (ksite = 0.036 sec) and station 029 (ksite = 0.042 sec). Rock sites show ksite values less than 0.010 sec. The dependence of k on distance shown in Figure 9 is consistent with the decrease in attenuation with depth observed in the previous sections. For distances smaller than 30 km, Formula (rk) increases with distance at a rate of nearly 0.0007 sec/km while, for longer ray paths, its dependency on distance weakens. Finally, for distances larger than 80 km, Formula (rk) slightly decreases with distance.


   Discussions and Conclusions
Top
 Abstract
 Introduction
 Data
 Parametric Description:...
 Spatial Distribution of...
 Near-Surface Attenuation
 Discussions and Conclusions
Appendix
 
The attenuation of seismic waves propagating in the crust of northwestern Turkey has been investigated by applying the generalized inversion scheme to a data set of aftershocks of the 1999 Izmit earthquake. The estimated attenuation-distance curves have been described in terms of the t* attenuation parameter and its dependency on frequence and distance has been investigated.

At 1 Hz, we found values of Q–1 =t*/T, where T is the travel time, generally greater than 0.025 (i.e., Q < 40) for distance up to 40 km, indicating that the seismic waves undergo a strong attenuation in the upper crust. The decrease of Q–1 with distance suggests a diminishing of the attenuation with depth. Moreover, the attenuation-versus-distance curves estimated from the inversion show a bump for distances between 40 and 60 km, suggesting that secondary arrivals significantly contribute to the spectral amplitudes in this distance range. This issue should be accounted for in the description of attenuation that is adopted in hazard- oriented studies. Attenuation is strongly frequency dependent in the range between 1 to 10 Hz, and it decreases almost linearly with frequency. In the case of using a simplified two-dimensional tomographic inversion, high attenuations are concentrated in the main alluvial basins, such as the Adapazari (region s2) and Düzce (region s3) basins, where rays spent a large portion of their travel times, and in the region surrounding the Izmit Bay. Coinciding with the sedimentary basins, strong vertical variations in attenuation must be expected.

These results are in agreement with the conclusions of previous studies that showed that several regions of the analyzed area exhibit low and strongly frequency-dependent Q values. Examples are Qs = 46.59f0.67 for the Bursa region in the distance range 5–60 km, and in the frequency range 0.5–25 Hz (Akyol et al., 2002); Qc = 50.7f1.01 in western Anatolia considering a lapse time of 30 sec (Akinci et al., 1994); and Qc = 41f1.08 and Qs = 50f1.09 for the Marmara region in the distance range 20–110 km and the frequency range 1.5–24 Hz (Gündüz et al., 1998). Recently, Horasan and Boztepe-Güney (2004) found regional differences in the attenuation evaluated for five regions in the Sea of Marmara. Their estimated Qs values range from 13f1.22 to 94f0.83.

The Q–1 values, estimated by normalizing the cumulative attenuation parameter t* to the travel time, show a stronger decrease with distance than a dependence on backazimuth. These results also suggest that, in the analyzed area, the depth variations in attenuation could be larger than the lateral ones. The almost linear decrease of t* with frequency is also in agreement with the results of Bindi et al. (2006).

The near-surface attenuation parameter k has been evaluated, and its dependence on source, site, and distance was investigated. Most of the stations show values of ksite smaller than 0.01 sec. Only for stations 029 and 013 is ksite greater than 0.030 sec. Station 029 was installed close to the Adapazari basin, near the fault of the 1967 Mudurnu earthquake (Ms 7.1). Goto and Sawada (2004) proposed a three-dimensional velocity model for the Adapazari basin, consisting of three soil layers and two rock mediums. According to this model (Goto and Sawada, 2004, fig. 4, p. 6), the sediments below station 029 should be between 200 and 400 m thick, with a shear velocity of the order of 200 m/sec. These values are also in agreement with the peak values in the H/V ratios shown in Figure 9. Assuming that ksite = 0.042 sec at station 029, as determined by the attenuation inside the soft sedimentary cover, a quality factor Q ~25 is found. Station 013 is installed over the thick sedimentary fill of the Yalova area (southern side of the Izmit Bay). The ksite = 0.036 sec for this station is higher than the values found for rock sites. The estimated ksite is consistent, although lower, with the average value of 0.056 sec found by Durukal and Catalyurekli (2004) in northwestern Turkey for the sites belonging to class D of the National Earthquake Hazards Reduction Program (NEHRP) soil classification (Boore and Joyner, 1997). The same authors found an average value of 0.041 sec for sites corresponding to NEHRP class C.

The high attenuation found in northwestern Turkey can have a strong implication on hazard assessments for the area. Recently, Erdik et al. (2004) observed that the amplitudes of the recorded ground motions in the Marmara region induced by the Izmit earthquake were lower than those predicted using standard attenuation relations. They suggested that this discrepancy could be related to a source effect. Since the high attenuation found might partially explain the observed discrepancies, the calibration of scaling laws for this region is a matter that deserves future attention.

Following Purvance and Anderson (2003), we introduce a term related to the seismic source for describing the observed k values. We found that kevent correlates with magnitude, in agreement with the results of Purvance and Anderson. In the same region we analyzed, Durukal and Catalyurekli (2004) found a correlation between magnitude and k values, although they calculated the latter by averaging, for each event, the values relevant to different stations without isolating the site and propagation contributions. Their average k values showed an increase with magnitude at a rate of 0.0038. The best least squares fit of the results in Figure 9 is kevent = 0.0051 ML – 0.014.

Finally, the agreement of the overall tomographic features with the surface geology should stimulate the development of future studies devoted to achieving a more detailed image of the attenuation properties in the region that properly take into account the depth-dependent structure of the region. Such a goal motivates efforts to enlarge the data set, hence making it suitable to perform a three-dimensional tomographic inversion.


   Appendix
Top
 Abstract
 Introduction
 Data
 Parametric Description:...
 Spatial Distribution of...
 Near-Surface Attenuation
 Discussions and Conclusions
 Appendix
 
The authors express their gratitude to the Hannover Rückversicherung AG for their significant financial support of the field mission. Helpful suggestions were provided by F. Scherbaum. We are thankful to R. Milkereit for drawing some figures using the Generic Mapping Tools software (Wessel and Smith, 2000). K. Fleming kindly improved our English. Comments from two anonymous reviewers also improved the manuscript. Part of this work was conducted during the visits of D. Bindi at the GeoForschungsZentrum Potsdam that were partially funded by the GFZ-Potsdam.

Manuscript received March 4, 2005

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