Local Site Effects Evaluation for Atbara Area Using Microtremor Measurements

Transcription

1 American Journal of Earth Sciences 2015; 2(5): Published online September 7, 2015 ( Local Site Effects Evaluation for Atbara Area Using Microtremor Measurements Khalda Y. Ibrahim 1, Abu Elela A. Mohamed 2, Insaf S. Babiker 3, Abdel Hafiz G. Elmula 3 1 Remote Sensing and Seismology Authority, National Center for Research, Khartoum, Sudan 2 National Institute of Astronomy and Geophysics, Cairo, Egypt 3 Department of Geology, Faculty of science, University of Khartoum, Sudan address khalda7@hotmail.com (K. Y. Ibrahim), insafsanhouri@gmail.com (I. S. Babiker) To cite this article Khalda Y. Ibrahim, Abu Elela A. Mohamed, Insaf S. Babiker, Abdel Hafiz G. Elmula. Local Site Effects Evaluation for Atbara Area Using Microtremor Measurements. American Journal of Earth Sciences. Vol. 2, No. 5, 2015, pp Abstract The use of microtremor data to estimate site effects is very common in the world, nowadays. In the present research, the recorded ambient noise in Atbara area was used to compute the H/V spectral ratio and then to evaluate the local site effect of the area. Atbara city is situated on the superficial deposits that are able to amplify the seismic waves. A number of 83 microtremor measurements were obtained at the ground surface in Atbara area, using short period stations (SARA SL07). The processing of the data has been done by recent software using Nakamura techniques. The obtained H/V spectral ratio results illustrated that the fundamental frequency value ranges from 0.72 to 3.3 Hz, while the amplification factor value ranges between 2.1 and 9.6. From these parameters a vulnerability index was calculated which ranges from 1.76 to 128, the fundamental frequency, amplification factor and vulnerability index models in two and three dimensions were obtained. The final results showed that weak points are occurring within the Nile silts and clayey cover in Atbara region. Keywords Atbara, Microtremor, H/V Spectral Ratio, Fundamental Frequency, Amplification Factor, Vulnerability Index 1. Introduction Sudan is categorized as being comparatively of moderate seismic activity. However the recent increase in earthquake activity in Sudan warrants researchers to study seismic hazards, earthquake source parameters, focal mechanism and to estimate the local site effects especially in the urban areas which are located predominantly on soft soil layers. For global and national settlement and rehabilitation, earthquake hazard evaluation of large cities is necessary to mitigate actual damage due to seismic activity. Seismic hazard assessment include: determining seismic sources, and attenuation forms in the area, studying the microzonation, estimating the local site effects for urban area and identifying the structural weaknesses. This paper focuses on the evaluation of local site effects and vulnerability index for Atbara area, the study of the local site effects is one of the most important goals of engineering seismology. Numerous techniques can be used to evaluate the site effects but the famous one is the one that uses earthquake data. Many researchers used aftershocks, seismic noise and microtremor data to determine the site effects, while others used numerical and empirical relationships to evaluate the site response. Various authors studied the site effects at different region around the world such as Nakamura (1989), Nakamura (1996), Field and Jacob (1993), Duval et al. (1994), Duval et al. (1995) and Nakamura (2000). Many authors used different types of data, multiple techniques and recent software such as Moustafa (2002),El-Shahat (2003), Walling et al (2009), Woolery et al (2009), Gosar et al (2010), Zandieh et al (2011), Kockar et al (2012), Iyisan et al (2013), Choobbasti et al (2013) and Rezaei et al (2013). Recently, in Sudan many studies were performed in the field of seismic hazard assessment such as Eltahir (2001) who studied the site response in and around Khartoum basin and Warage (2008) who studied the local site effects in western Khartoum. The main objectives of these investigations in Atbara area were to record microtremor at many locations in and around the town, analyze and interpret them by the Nakamura method in order to determine the fundamental

2 American Journal of Earth Sciences 2015; 2(5): frequency (Fo) and the amplification factor (Ao) and to compute the vulnerability index (Kg) which shows vulnerable zones in and around the investigated area. Atbara city is one of the largest cities in Sudan. It is called the town of fire and iron. Many critical constructions are found in and around Atbara city which is located at the confluence of the Nile River and Atbara River on the sedimentary basin called Atbara basin, where the sedimentary soils are creating geotechnical problems and may amplify the seismic waves. Explosions from cement plant activities, heavy traffic and other human activities may affect the region seismically; therefore the study of site effect and the dynamic soil properties may illustrate the tendency of earth structures to failure. Seisan software version which was used to convert the data format from Seisan to mseed. 2. The Study Area The study area is located between latitudes 17 30` and 18 00` N and longitudes 33 50` and 34 30` E in the northern part of Sudan. The area is crossed by the River Nile and the River Atbara where the surface is generally flat in the area south and north of the River Atbara, while the western part of the study area is surrounded by Bayuda desert. The area belongs to the desert-semi desert climatic regime. The geological set up of Atbara area consists of the superficial deposits, Umm Ruwaba Formation, Cianozoic volcanic (Basalt), Hudi Chert Formation, Nubian Sandstone Formation and Precambrian basement Rocks. 3. Data Collection Ambient noise measurements were performed in the basin of Atbara during July The location of all sites is shown in figure (1). The Global Positioning System (GPS) was used to determine the location and elevation of the measurement sites. The ambient noise vibrations were recorded using laptop and SARA Sl07 equipment with seismometers consisting of three component 4.5Hz geophones. Eighty three measurements of ambient noise were collected in and around Atbara city. The sampling rate was 100 Hz with a recording duration of 30 minutes. Some signals, were affected by transients, thus 16 records were rejected. Finally, 67 measurements of ambient noise were used for the H/V processing. The measurements were taken during the day, the distance between sites is about one Kilometer due to environmental constraints. It is well known that the problem of high noise is expected if the measurements were taken in densely populated areas because in this case the signal will be dominated by transient noise from traffic or industrial activity. To avoid such problems, measurements were taken early in the morning or late in the evening. 4. Methodology In the present study the microtremor technique was used to illustrate the local site effects in Atbara area, the processing of data has been done using two softwares the first one is Fig. 1. The location of noise measurement sites plotted on the geological map of the study area (GRAS, 2004). The second software is the Geopsy software version , which is designed to process and calculate the spectral ratio of the horizontal to vertical components from the ambient seismic noise records. In order to exclude dominated transient signals an STA/LTA window range was used to reject parts of the amplitudes higher than the ambient seismic noise. After excluding these transient signals the remaining part of the record is divided into windows their length is suitable to the frequency range. For example, for the frequency range 0.3 to 20 Hz, a 35 s time window was used, for the range 0.4 to 20 Hz a 25s time window was used -this was the mostly used window size- and in few cases for the frequency ranges 0.5 to 20 Hz a 20 s time window was used. For a similar study performed earlier it was found that a 20 s long windows of ambient seismic noise records are suitable for a reliable evaluation of the site effect study, (Atakan et al. 2003; Atakan et al. 2004; Bard et al.2004). For the present dataset the average number of windows ranges from 10 to 15. All data were checked for industrial and natural origin by checking the damping at defined frequency. The vulnerability index values were calculated in the current study from the microtremor measurements using the following equation: Kg= A 2 / F Nakamura (1996) Where Kg is the vulnerability index, A is the amplification factor and F is the fundamental frequency. The measurement points were scattered in an acceptable grid model, which was considered to accomplish good quality contour maps (using Surfer software version 7) that

3 136 Khalda Y. Ibrahim et al.: Local Site Effects Evaluation for Atbara Area Using Microtremor Measurements are able to display the distribution of the fundamental frequency, amplification factor and vulnerability. 5. Results

4 American Journal of Earth Sciences 2015; 2(5): Fig. 2. Examples of the H /V spectral ratio at different sites in the study area. The results show large variability in the values of H/V spectral ratio. Based on the frequency of the H/V peak, the measurement sites were classified into 4 classes of frequency: 1. H/V spectral ratios with a frequency range from 0.72 Hz to Hz 2. H/V spectral ratios with a frequency range from Hz to 2.01 Hz 3. H/V spectral ratios with a frequency range from 2.01 Hz to Hz 4. H/V spectral ratios with a frequency range from to 3.3 Hz. In the present study most of H/V spectral ratios have one single clear peak; the explanations for the H / V results are obvious, because the spectral ratio which has only one clear peak is interpreted to be the fundamental frequency. The interpretation of sites which have single peaks with low frequencies is that they are located on thick sediment layers. In other cases, where a clear peak at high frequency is observed, thinning of the sedimentary layer under the site or the presence of an artificial noise source close to the site could be the reason. For the site effect evaluation sites were classified into four groups depending on the geology beneath each site as follows: i) Group one is underlain by recent Alluvium, wadi deposits and older Alluvium (Quaternary sediments). The majority of H/V spectral ratios calculated for this group have a clear peak at frequency range from 0.72 to 3.3 Hz with an amplification value ranging from 2.2 to 9.6. The interpretation of the difference in frequency and amplification may be due to the reflection of waves through the unconsolidated sedimentary rock underlying the surface or to the thickness of the soft soil layers at the surface. ii) Group two consists of many sites underlain by Tertiary sedimentary rocks (undifferentiated sandstone). The fundamental frequency ranges from 1.3 to 2.4 Hz with an amplification ranging from 2.4 to 5.3 iii) Group three are underlain by Cretaceous sedimentary rocks (fluviatile sandstone, lacustrine siltstone and mudstone). The fundamental frequency ranges from 1.2 to 2.6 Hz with an amplification ranging from 2.9 to 3.8 iv) Group four is underlain by three different types of rocks. The H/V spectral ratio calculated for sites laying on quartzite had a fundamental frequency equals 1.5 Hz and an amplification value of about 4.2. The noise measurements taken in the weathered granitic rocks had shown frequency ranges from 1 to 1.2 Hz and the amplification ranging from 2.5 to 3.9. Another measurement was taken in the ultra basic rocks, the frequency was found to equal 0.81 Hz with amplification around 5. Figures (2. a, b, c, d, e and f) illustrate the H/V spectral ratios measured on quartzite, ultra basic rocks, weathered granite, Cretaceous, Quaternary and Tertiary rocks, respectively. The obtained results from the H/V spectral ratio curves show that the fundamental frequency value ranges from 0.72 to 3.3 Hz (figure 3), while amplification factor at the measured sites of the study area ranges between 2.1 and

5 138 Khalda Y. Ibrahim et al.: Local Site Effects Evaluation for Atbara Area Using Microtremor Measurements 9.6 and the vulnerability index ranges from 1.76 to 128 as illustrated in figure (4). The result of the microtremor measurements were displayed in two and three dimensional models. Figures (5a and b) show the fundamental frequency distribution in two and three dimensions, respectively. Obviously, the red color delineates high values of fundamental frequency located in the western and southwestern part of the study area reaching 3.3 Hz while the light blue color represents the low values of fundamental frequency in the central part of the study area reaching 0.72 Hz. Fig. 3. The distribution of fundamental frequency values in the study area. Fig. 4. The distribution of amplification factor and vulnerability index values in the study area.

6 American Journal of Earth Sciences 2015; 2(5): Fig. 5a. Distribution of the fundamental frequency (f o) in the study area in two dimensions (2D). Fig. 5b. Distribution of the fundamental frequency (fo) in the study area in three dimensions (3D). Fig. 6a. Distribution of the amplification factor (A o) in the study area in two dimensions (2D).

7 140 Khalda Y. Ibrahim et al.: Local Site Effects Evaluation for Atbara Area Using Microtremor Measurements Fig. 6b. Distribution of the amplification factor (A o) in the study area in three dimensions (3D). Fig. 7a. Distribution of the vulnerability index in the study area (Kg) in three dimensions (2D). Fig. 7b. Distribution of the vulnerability index in the study area (Kg) in three dimensions (3D). Figures (6 a and b) represent the spatial distribution of the amplification factor in two and three dimensions. The red color reveals the high values of amplification factor reaching 9.6 while the light yellow color illustrates the low values. Also figures (7 a and b) illustrate the vulnerability index in two and three dimensions. The dark blue color demonstrates the high values of vulnerability index reaching 128 while the light blue color indicates the low values of the vulnerability index being 1.76.

8 American Journal of Earth Sciences 2015; 2(5): Conclusion The study was carried out by measuring the microtremor at 67 sites scattered in Atbara region. The fundamental frequency and amplification factor were obtained and the vulnerability index was calculated based on these parameters. The majority of H/V spectral ratios have one single clear peak laying between 0.72 and 2.0 Hz reflecting the effect of sediment thickness. The final results of this research indicated that the area extending along the main path of the Nile River and Atbara River which is characterized by superficial deposits (Quaternary sediments) revealed weakness points that may amplify the ground motion. So these results should be taken into consideration by the designers and planners before establishment of new urban areas, high towers, industrial, governmental and strategic buildings or any other engineering construction. This contribution presents a fast technique to study the site effect using ambient noise measurements; it is relatively low cost, of limited requisites in equipment and personnel. Nakamura s technique is an effective method that can be used to reliably evaluate site effect in urban areas. Acknowledgement The authors wish to express their deep gratitude to the National Center for Research in Sudan and the National Research Institute of Astronomy and Geophysics in Egypt for the support and training. References [1] Atakan K., Bard, P. Y., Moreno, B., Roquette, P., Tento, A., J-SESAME User manual. SESAME Project WP03 H/V Technique: data processing, Department of Earth Science, University of Bergen, 12 pp. [2] Atakan, K., Bard, P. Y., Kind, F., Moreno, B., Roquette, P., and Tento A., JSESAME A standardized software solution for the H/V Spectral Ratio technique: Proceedings of the 13th World Conference on the Earthquake Engineering, v. 2270, p. 37. [3] Bard, P. 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