estimation of site effects in the eastern part of romania on the basis

EARTH PHYSICS

ESTIMATION OF SITE EFFECTS IN THE EASTERN PART OF ROMANIA ON THE BASIS OF H/V RATIOS OF S AND CODA WAVES GENERATED

BY VRANCEA INTERMEDIATE-DEPTH EARTHQUAKES

B. GRECU, V. RAILEANU, A. BALA, D. TATARU

National Institute for Earth Physics, P.O.BOX MG-2, Bucharest, Romania E-mail: [email protected], [email protected], [email protected], [email protected]

Received October 30, 2009

The purpose of our study is to investigate the ground motion characteristics in 15 sites located in the Eastern part of Romania by applying the H/V spectral ratio method to the data (S and coda waves) recorded during the CALIXTO’99 tomography experiment. The results show no significant differences as regarding the resonant frequencies of the spectral ratios computed for the two types of waves, while the level of the amplification for S-wave is slightly higher than for coda waves. Only for two sites, located on thick Quaternary deposits, the amplification obtained from S-wave is larger by a factor of 2 than the amplification obtained from coda waves at low frequencies. In the studied locations the ground motion amplification varies by a factor of 2 to nearly 6 over the frequency range of 0.5 to 10-12 Hz.

Key words: S and coda waves, spectral ratios, site effects, Vrancea

1. INTRODUCTION

The amplification of ground motion from earthquakes due to the local geology has long been accepted in the earthquake related community. Two classical examples of such amplifications are Mexico City and San Francisco. During the 1985 Guerrero Michoachan earthquake, seismic waves were strongly amplified in the soft clay basin beneath Mexico City more than three hundred kilometers away from the epicenter. This led to a high death toll and large economic losses in the city. In San Francisco, during the 1989 Loma Prieta earthquake, the level of intensity increased by 2 units in areas with unconsolidated sediments. Amplifications due to soft soils have been also present during the recent destructive earthquakes (Northridge 1994, Kobe 1995, Armenia 1999, Colombia 1999, Turkey 1999, etc.). Consequently, site amplification has become an important factor in seismic hazard assessment.

The fundamental phenomenon responsible for the amplification of seismic motion over soft sediments is the trapping of seismic waves due to the impedance contrast between sediments and underlying bedrock. The interference of these Rom. Journ. Phys., Vol. 56, Nos. 3–4, P. 563–577, Bucharest, 2011

B. Grecu, V. Raileanu, A. Bala, D. Tataru 2 564

trapped waves leads to resonances, the shape and the frequency of which are related with geometrical and mechanical characteristics of the structure.

Various empirical methods using seismic data for site effects estimations have been proposed in the last decades. These methods can be separated in two main categories: methods that calculate the site response with respect to a reference site located on a hard rock and methods that don’t need a reference site, the so called non-reference techniques. For the first ones, if two sites have similar source and path effects, and if the reference site has a negligible site response, then by dividing the spectrum of the recorded earthquake motion in a site by that of the reference site one can estimate the site response [3]. These approaches identify the fundamental resonant frequency and are considered to be the most reliable [4], [1], [9]. In practice it is not easy to find a good reference site, thus alternative techniques have been developed and applied to site response studies. One of these methods uses spectral ratio between the horizontal to vertical (H/V) spectra of the S-wave window for each site. This method is based on the so called receiver-function technique [12] applied to studies of the upper mantle and crust using teleseismic records, which assumes that the local site conditions are relatively transparent to the motion that appears on the vertical component. Many authors [13], [11] agree that the H/V ratio from earthquake data provides a reliable estimation of the site response.

Nakamura [17] assumed that using only one recording station the site response can be estimated from the horizontal-to-vertical ratio of the microtremors. Although the results of this technique are controversial regarding the amplification level, it was largely used in many sites all over the world by different authors [14], [5], [10], [16]. The main results of these studies confirm that this technique gives a reliable estimation of the fundamental resonance frequencies of soft deposits. Recently, within the European project SESAME (http://sesame-fp5.obs.ujf-grenoble.fr), it has been shown that the H/V technique from ambient noise recordings proved to be a very useful tool for microzonation and site response studies, although the measurements and analysis should be performed with caution.

In Romania, the studies of the 30 August 1986 (Mw = 7.2) and 30 May 1990 (Mw = 6.9) earthquakes have shown the important role played by the local and regional conditions in the distribution of the Vrancea intermediate depth earthquake effects [15]. Taking into account the simplicity of the H/V ratio method and its large usage in different parts of the world, this method was also applied in Romania using different data sets, such as noise data, earthquake data from strong and moderate Vrancea events and explosion data. Thus, [2] applied Nakamura’s technique in order to characterize the local response in 16 sites within Bucharest area. They showed that, except for one site, all the H/V ratios indicate a relatively stable peak in the period domain from1 to 2 s. This peak was interpreted as a large and stable resonance of the soil. [29] came to the same result by analyzing the seismic noise recorded in 20 sites in Bucharest. [22], [23], and [24], [25] studied

3 Vrancea intermediate-depth earthquakes

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site effects in several sites based on the spectral ratios derived from data collected during Vrancea 99 and Vrancea 2001 refraction experiments [8], [21]. The results show that the amplitudes and frequencies of the spectral ratios depend not only on the distance to the source and the energy released, but also on the local geological conditions.

2. DATA AND METHOD USED

In the present work we concentrate on evaluating site effects at several seismic stations by applying the H/V method to earthquake data recorded during the Carpathian Arc Lithosphere X-Tomography (CALIXTO) experiment. The CALIXTO experiment was performed in Romania in 1999 in the framework of the Collaborative Research Centre 461 programme of the University of Karlsruhe "Strong Earthquakes: A Challenge for Geosciences and Civil Engineering" [28]. 120 stations were deployed around Vrancea region for more than 6 months. The stations, consisting in different types of dataloggers (PDAS, REFTEK, GURALP, MARS) with both short-period (90) and broadband (30) velocity transducers were running continuously with a resolution of about 16 to 24 bit at 50 samples/sec. During the experiment operation (May to November 1999) 77 local earthquakes with magnitude 3 ≤ Mw ≤ 4.2 have been recorded. The CALIXTO experiment provided a unique data set for seismic wave attenuation studies. [19] and [20] showed that the attenuation of the seismic waves traveling from Vrancea sources toward Transylvanian basin is much more important at high-frequencies (> 1 Hz) than at low-frequencies (< 1 Hz).

Four our purpose we selected only 15 CALIXTO (Table 1, Figure 1) stations installed in the eastern part of Romania. For the selection of the stations, we looked first at the waveforms at each station and selected only those stations that recorded at least 5 earthquakes, and then we analyzed the quality of the data. In this matter, we computed the Fast Fourier Transform (FFT) of the S-wave window and the FFT of the noise window before P arrival and selected only the stations and earthquakes for which the signal-to-noise ratio was greater than a factor of 2 (see Table 1, Table 2).

From the geological point of view, the stations are located on Quaternary and Neogene sediments. The Quaternary is represented by alluvium (Holocene), loess and loess-like deposits (upper and lower Pleistocene), and sands and gravels (upper Pleistocene). The thickness of the Quaternary deposits increases from meters-tens of meters to almost 2000 m for the stations located in the Focsani Basin. Marl deposits, clay and sand deposits (Pliocene), and sands and clays (Miocene) represent the Neogene. The thickness of the Neogene sediments varies from several hundred of meters to several thousands of meters.


Table 1

CALIXTO stations used in the study

Nr. Station code Locality Latitude

(N) Longitude

(E) Number of

earthquakes used 1 A02 Hadîmbu 47.011 27.431 5

2 A03 Dumeşti 46.847 27.288 10

3 A04 Odobeşti 46.687 27.147 11

4 A06 Dumbrava 46.315 26.888 8

5 A07 Caşin 46.198 26.743 11

6 B09 Greabănu 45.375 26.980 9

7 E03 Pralea 46.104 26.831 12

8 E06 Răcoasă 45.989 26.876 7

9 E23 Cândeşti 45.521 27.051 6

10 E25 Cernăteşti 45.327 26.738 8

11 F04 Zabodeni 46.747 27.660 11

12 F05 Dealu Mori 46.301 27.300 12

13 F06 Pogana 46.312 27.579 14

14 F09 Vârlezi 45.881 27.858 14

15 F10 Scânteieşti 45.676 28.005 13

The method we use in our study is in fact a combination of the Langston’s

[12] receiver function technique for determining the velocity structure of the crust from the horizontal to vertical spectral ratio of teleseismic P waves, and the Nakamura’s method [17] who used this ratio on recordings of ambient noise.

This technique is based on the assumption that the spectrum of the horizontal components can be divided by the spectrum of the vertical component and the site effect could be estimated as

( )

( )( )

iji

ij

H fS f

V f= (1)

where Si(f) is the site effect determined after dividing the spectrum of the horizontal components, Hij(f), by the spectrum of the vertical one, Vij(f), for the j-th earthquake at a given i-th station.


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Table 2

Intermediate-depth Vrancea earthquakes considered in the study

Nr. Origin time Latitude (N) Longitude (E) Depth (km) Mw

1 990525 09:35:53 45.59 26.49 113 3.9

2 990611 08:20:04 45.74 26.66 109 3.6

3 990620 00:09:05 45.57 26.53 131 3.6

4 990622 08:02:05 45.66 26.47 149 3.7

5 990629 20:04:06 45.59 26.54 131 4.2

6 991307 13:10:57 45.70 26.49 132 4.0

7 990715 02:52:03 45.74 26.85 116 3.5

8 990715 07:36:23 45.58 26.49 135 3.7

9 990722 10:07:50 45.45 26.33 137 3.7

10 990108 05:56:49 45.50 26.60 124 3.5

11 990807 02:25:45 45.57 26.37 159 3.9

12 990808 07:16:16 45.72 26.71 131 3.7

13 990914 23:48:09 45.56 26.52 121 3.5

14 991012 23:48:31 45.66 26.41 150 3.7

[14] were the first to apply this method on S-wave windows of earthquakes

recordings for three sites in Mexico City. Their results showed a good fit in both, the frequencies and the amplitudes of resonant peaks, with those obtained using standard spectral ratios [3]. Other studies [11], [26], [27], [30] outlined that the H/V ratio shape exhibits a very good experimental stability and it is well correlated with surface geology, and much less sensitive to the source and path effects. [5] also applied this technique in their systematic comparisons, and found that the method reproduces very well the shape of the site response, but underestimates the amplification level.

The earthquake data used in this study consisted of 10 seconds windows of S-wave and 20 seconds windows of coda waves following the S-wave train. The S-wave window contains the strongest part of the seismic recording, while the coda wave has the advantage that its spectral shape is supposed to be independent of source and receiver location and of source orientation [18], because this part of the signal is dominated by backscattered waves from crustal heterogeneities.


Fig. 1 – Distribution of CALIXTO stations – reversed triangles (normal triangles – stations used in this study). The epicenters of the earthquakes are represented by circles and are scaled with magnitude

The H/V ratios were computed using standard procedure. For each event, the seismograms were rotated to their back-azimuths to obtain the radial (R) and transverse (T) components. Then, the data were processed by removing the linear trend and tapering by a 5 % cosine taper. Fourier amplitude spectra for both horizontal and vertical components were computed by FFT and smoothed using an arithmetic algorithm. The resultant horizontal component was calculated as the quadratic mean of the two horizontal components (H = ((R2+T2)/2)1/2) (http://sesame-fp5.obs.ujf-grenoble.fr/Delivrables/D09-03_Texte.pdf). The H/V ratio was computed by dividing the spectrum of the horizontal component (H) to the spectrum of the vertical component. Finally, a mean H/V curve with its standard deviations was computed for all earthquakes at each station.

3. RESULTS

Following the procedure described above we obtained for each station two H/V curves, one for S waves and one for coda waves. The results are shown in Figure 2. From their analysis, we can observe the significant differences of the


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H/V curves from one station to another which suggests the influence of the local conditions on the seismic ground motion. One can also notice that the shape of the curves is similar for S and coda waves, and the frequencies where the resonances occur are common in both types of ratios.

As regarding the amplitude of the H/V ratios, we should mention that the ground motion is not amplified when the amplitude is close to unity. When the amplitude of the curves is greater than one, we can talk about the amplification of the ground motion, the level of amplification depending on the local conditions. We should also notice in Figure 2 that the level of amplification for S waves is slightly larger than for coda waves, except for the stations B09 and E23 for which it is much larger. These stations are located on the thick sedimentary deposits of the Focsani Basin and this suggests that the seismic energy is much stronger amplified for S-waves than for coda waves within thick Quaternary deposits (~ 2000 m, Focsani Basin) on which both stations are installed. To exemplify this phenomenon we present in Figure 3 the Fourier spectra computed for the three directions of the ground motion (NS, EW, Z) recorded at station B09 during the June 29, 1999 event (Mw = 4.2). As it can be seen, the spectral amplitudes of the horizontal components for S-wave are larger than those for coda wave, while the level of amplification of the vertical component is the same for both types of waves.

For the same stations as those used in this study, [6] and [7] analyzed the H/V ratios obtained from seismic noise.

Station A02 Station A03


Station A04 Station A06

Station A07 Station B09

Station E03 Station E06


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Station E23 Station E25

Station F04 Station F05

Station F06 Station F09


Station F10

Fig. 2 – H/V spectral ratios (thick line – average, thin line – ± standard deviation)

The results were characterized by the absence of the spectral peaks, except for the station E03 for which a well defined peak is observed around 3 Hz and for the stations A06 and F05 for which a less pronounced peak is observed around 2 Hz, and 2.5 Hz respectively (Figure 4). The H/V ratios computed for the station E03 from S and coda waves show also clear peaks around 3 Hz with amplitudes greater than 4. This station for which the H/V ratios obtained from different data are very similar having a well defined spectral peak, demonstrates the typical influence of the soft surface deposits on seismic ground motion. For station A06, the peak obtained from noise data could be identified on the H/V ratios from earthquake data, but with an amplitude smaller than 3. Two more peaks, one of which at around 5 Hz and having amplitude greater than 3, are also present in the S-wave and coda H/V ratios. In case of the station F05, the H/V ratios from S and coda waves show a larger peak between 2.5 and 3 Hz with an amplitude smaller than 4.

From Figure 2 we should also notice that for several stations the level of amplification is roughly constant on a wide frequencies band. Thus, for station F04 we can see that the amplitudes vary around 2 with no clear peaks for frequencies greater than 1 Hz. For station A02 we notice amplitudes greater than 3.5 for frequencies bellow 1.7 Hz, while for frequencies higher than this value we can observe a slight decrease of the level of the amplification. The station A03 has a relatively constant amplitude around 3 up to 5 Hz, while for frequencies larger than this value we observe peaks with amplitudes larger than 3.5. For two stations (A07 and E06) the configuration of the spectral ratios is very similar. Thus, we notice a decrease of the amplification at lower frequencies, then relatively constant amplitude at intermediate frequencies and an increase of the amplification towards higher frequencies (> 4.5 Hz). This suggests the same geological conditions at both sites. For station E25 the difference between the amplification of the H/V ratios


573

from S and coda waves is very small. Thus we can observe a relatively constant amplification over a wide frequency band. However, for the S wave H/V ratio we can notice a spectral peak around 5.8 Hz, the amplitude of which is slightly over 3.1. The H/V ratio for the station A04 shows an increase of the amplification towards higher frequencies. The level of amplification reaches almost 5 for S-wave and 3.5 for coda waves around the frequency of 6.9 Hz. Large amplifications at higher frequencies can be also observed for stations F06, F09 and F10. For the first one, the H/V curve reaches its maximum of 4 around 5.4 Hz for S wave, while for coda waves the maximum is 2.7 around 5.7 Hz. For this station, one can also notice large amplifications at low frequencies (< 1.6 Hz) probably due to the very thick (hundred of meters) Neogene sediments. For station F09, we can identify three spectral peaks around the frequencies 4.3, 6.4 and 12.1 Hz with amplitude greater then 4 in case of S-wave. For coda waves, the peaks at same frequencies have the amplitude between 3.3 and 3.5. Finally, the station F10 shows a well defined spectral peak between 6.8 and 9 Hz. For S wave this peak reaches its maximum of 5.4 around 7.8 Hz, while for coda waves this peak is nearly 3 around 7.6 Hz. For both curves, a secondary well defined peak can be observed between 4.2 and 5.4 Hz, but with amplitudes smaller then 3.5.

Fig. 3 – Comparison of amplitude Fourier spectra obtained for June 29, 1999 event (Mw = 4.2).


Fig. 4 – H/V spectral ratios obtained from noise data (only the mean curves are represented).

4. CONCLUSIONS

The large number of seismic stations (120) installed in Romania during a period of 6 months between May and November 1999 offered a high quality data set for different seismological studies like: seismic tomography of Vrancea area, seismic wave attenuation, influence of local conditions on the ground motion, etc.

In this study, in order to estimate the site effects, we applied a widely used method, the H/V spectral ratio. This approach outlines the resonant frequencies where the amplification of the ground motion occurs. The data used (S and coda waves) for computing the H/V ratios came from 14 small Vrancea intermediate-depth earthquakes (Mw = 3.5 – 4.2) that were recorded at 15 stations located in the Eastern part of Romania.

The results emphasize that the H/V spectral ratio method is able to identify the resonant frequencies irrespective of the data used (S, coda wave) and the shape of the curves is in general very similar for the both type of waves. As regarding the level of the amplification, this is slightly higher for S-wave than for coda waves. Two stations out of 15 show amplifications larger then a factor of 2 for S-wave. These stations (B09, E23) are located on the thick Quaternary deposits of the Focsani basin and the energy of the S-wave is much stronger amplified than the energy of coda waves.


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The results obtained from the analysis of the noise data show that only for three stations (A06, E03, F05) the H/V ratios exhibit a spectral peak which can be identified also in the H/V ratios obtained from S and coda waves. In this case, we can say that there is an important impedance contrast between a surface layer and the structure underneath and this dominates the local response. For the other stations such a contrast does not exist in the local structure and the H/V ratios from noise data show no clear peaks.

By dividing the horizontal spectrum to the vertical spectrum we consider that the propagation path effects and the source effects are eliminated, so the results show also the influence of the local geological structure on the configuration of the H/V curves. This is suggested by the fact that there are clear differences between the H/V ratios from one station to another. Nevertheless, a common characteristic to all H/V ratios is that the level of amplification is larger than 1 in the frequency domain 0.5 – 12.5 Hz, and this confirms once more the fact that the sedimentary deposits amplify the ground motion during an earthquake. However, for a better understanding of the results, detailed investigations regarding the correlation with the information of the structure of the sedimentary package in the studied area should be performed. Acknowledgments. This work was partially supported by CNCSIS grant no. 48/2007 and CNMP project no. 32157/2008, financed by the Ministry of Education, Research, Youth and Sports.

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estimation of site effects in the eastern part of romania on the basis

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