Site Response Estimation by Nakamura Method

MEMOIR OF THE GEOLOGICAL SOCIETY OF INDIA

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No. 77, 2011, ISBN: 978-81-907636-2-2, pp. 173-183

Site Response Estimation by Nakamura Method: Shillong City, Northeast India RAJIB BISWAS and SAURABH BARUAH* Geoscience Division, North East Institute of Science and Technology, Jorhat 785006, India Email: [email protected] Abstract The modified form of the Nakamura method, H/V ratio technique, is used to assess the site response through estimation of fundamental resonant frequency in the Shillong city, capital of the Meghalaya state in northeast India. Ambient noise measurements are carried out at 70 sites using three-component digital seismographs; the minimum duration of noise recording is about one hour at each site. It is observed that the fundamental resonant frequency for peak amplification in the Shillong city is in the range of 3 to 7 Hz. A good correlation is found to exist between the geology and the site response results. Key words: Ambient noise, H/V, site response, resonant frequency.

INTRODUCTION It has been observed that the damages caused by occurrence of earthquake not only depend on its magnitude and epicentral distance, but also on local site effects which are essentially frequency dependent caused by topography, sediment thickness, soil conditions and geology of the area. The reaction of the local geological conditions to the incoming seismic energy is known as the site response (Fernandez et al. 2000). For seismic hazard assessment, the site effect is typically represented by resonance frequency and the associated ground motion amplification. Several methods exist, such as array data analysis, Nakamura method of horizontal to vertical ratio H/V of ambient noise, site to reference spectral ratio and receiver function type analysis. Out of these, Nakamura method i.e. use of ambient noise records for determination of fundamental resonant frequency has recently gained world-wide acceptance because of quick data acquisition. The amount of amplification depends on several factors including layer thickness, degree of compaction and age (Siddiqqi et al. 2002). One of the many reasons for choosing ambient noise by several authors is that it allows the quick and reliable estimate of site characteristics of any type of an area. Apart from being a cost effective measure, it reduces time compared to estimating site characteristics from earthquakes which is always a time consuming as well as expensive process so far as the maintenance of equipment and man power is concerned.

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There are many instances of successful utilization of the H/V ratio estimate towards studying fundamental frequency from ambient vibrations in urban environments (Duval et al. 2001; Lebrun et al. 2001; Panou et al. 2005; Gueguen et al. 2000; Lombardo et al. 2007; Garcia-Jerez et al. 2007, Mundepi et al. 2009 & many others). The proximity of fundamental frequency of a site to the existing man-made structures causes damage of the later owing to resonance effects. Therefore, investigation of each site condition is an important step towards earthquake hazard mitigation. North East Region (NER) of India is one of the most tectonically active regions in the world. Two great earthquakes, one in 1897 and the other in 1950, had already ripped past through this region. One of the most striking features of the NER, India is that most of its cities and densely populated settlements are located in valley, sedimentary basins or hills etc. In this study, we try to examine site characteristics of the Shillong city area in terms of resonant frequency, site amplification etc using the H/V ratio methodology (Nakamura’s 1989 as modified by Bard, 1999).

Fig 1. Geological map of Shillong City.

GEOLOGY AND SEISMICITY OF SHILLONG The Shillong city, capital of the Meghalaya state, is situated in the almost elliptically shaped Shillong Plateau (henceforth abbreviated as SP). It covers an area of 6430 square

SITE RESPONSE ESTIMATION BY NAKAMURA METHOD: SHILLONG CITY, NORTHEAST INDIA

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kilometer, with an average elevation of 1000 m and approximate population of 2, 70,000. The SP with an Archaean gneissic basement and late Cretaceous-Tertiary sediments along its southern margin is bounded by the Brahmaputra river/ fault to the north and by the Dauki fault in to the south (Rao et al. 2008, Kayal et al. 2006). The general rock type in the study area consists of Shillong series of parametamorphites, which include mostly quartzites and sandstones followed by schist, phyllites, slates etc. The base of Shillong series is marked by a conglomerate bed containing cobbles and boulders of earlier rocks, i.e., Archaean crystalline, which formed the basement over which the Shillong series of rocks were originally laid down as sedimentary deposits in Precambrian, probably in shallow marine conditions (Mitra et al. 1980; Sar et al. 1973). The Shillong groups of rocks are intruded by epidiorite rocks, known as Khasi Greenstone as outlined in Fig 2. The Khasi Greenstone is a group of basic intrusives in the form of linear to curvilinear occurring as concordant and discordant bodies within the Shillong group of rocks, and suffered metamorphism (Srinivasan et al. 1996). These rocks are widely weathered and the degree of weathering is found to be more in the topographic depressions than in other areas. The metabasic rocks are more prone to weathering than the quartzite rocks. In low lying areas, valley fill sediments are also prominent. Numerous lineaments trend in NE-SW, N-S and E-W directions in the area (Chattopadhay and Hasmi, 1984). The SP within which our study area falls is regarded as one of the most seismically active regions in India. The Plateau is separated out from the peninsular shield and moved to the east by about 300 km along the Dauki fault (Evans, 1964). The area is surrounded by active faults, the Dhubri fault to the west, Oldham / Brahmaputra fault to the north, Dauki fault/Dapsi thrust to the south and the Kopili fault to the northeast (Kayal et al. 2006), (Fig.1). Further, the Plateau is dissected by several small and large faults and lineaments. Among them, mention may be made of the active faults/lineaments, like the Chedrang fault, Dudhnoi fault and the Barapani lineament/shear zone (Fig.1). Northern end of the SP was the source area for the 1897 great Shillong earthquake Ms 8.7, that caused severe damages and more than 1500 casualties (Oldham, 1899). Over the past hundred years, there are instrumental records of 20 large earthquakes (M> 7.0) in the region (Kayal, 2008). Mention may be made about the significant earthquake of June 1, 1969 with a magnitude of 5.0 within an epicentral distance of 20 km from the Shillong city (Gupta et al. 1980). With the installation of local digital seismic networks by the then Regional Research Laboratory-Jorhat (RRL-J, now named as North East Institute of Science & Technology, NEIST- Jorhat); National Geophysical Research Institute (NGRI) and by the India Meteorological Department (IMD), there has been a tremendous improvement in recording and locating the lower magnitude earthquakes (Kayal et al. 2006). During the past few months, there have been a few felt earthquakes in the Shillong city that occurred within 100-150 km radius. This populous city of Shillong is not far from the source zone of the great earthquake of 1897 (MS 8.7). Intense seismicity beneath the Plateau is caused by pop-up tectonics of the Plateau between the Dapsi thrust and the Oldham/Brahmaputra fault (Kayal et al. 2006). Further, far east of the Plateau, the Kopili


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LATITUDE

fault is the most active that caused two recent felt earthquakes (Mw 5.1 and 6.2) in August and September, 2009 respectively (Kayal et al. 2010).

LONGITUDE

Fig.2. Tectonic map showing the major features around Shillong Plateau (as modified from Baruah and Hazarika, 2008). The inset map shows the study region. BS –Barapani Shear Zone; SF- Samin Fault; DT- Dapsi Thrust; DuF- Dudhnoi Fault; OF- Oldham Fault; CF- Chedrang Fault; BL- Bomdila Lineament

DATA ACQUISITION AND PROCESSING Ambient noise survey was made in 70 different sites in the Shillong city area (Fig. 3). The area of observation was divided into 500m X 500m meshes. Since, the city is well developed and populous, there are certain constraints in ambient noise recording in urban environments. To ensure reliable noise recording, we followed the guidelines proposed by Koller et al. (2004) in the framework of SESAME. Besides, quiet environment and good weather condition had been our prime requisite for the data acquisition. The minimum duration of ambient noise recording was kept around one hour. During this survey, we used


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Teledyne Geotech-1S triaxial velocity sensors equipped with 24 bit Reftek 72A-08 digitizer. The data are digitized at 100 samples per second and recording time was maintained by Reftek GPS clock. The locations of the sites were determined by the built-in GPS. Data from each site are processed using LGIT Software, SESARRAY package. Since similar sensors are used for all the three components, no instrumental correction has been applied. The processing schemes are listed below: i)

Determination of stable windows in the 30-40s range, using an antitrigger with 1-s STA , a 30s –LTA and STA/LTA ratio threshold of 0.25 and 2.50 as minimum and maximum respectively.

ii) For each window, a 5 % cosine taper is applied on both sides of the window signal of the Vertical (V), North-South (NS) and East-West (EW) components. iii) For each window, an FFT is applied to the signal of the three components to obtain the three spectral amplitudes, to which a Konno and Ohmachi smoothing factor(1998) is applied with a bandwidth of 40Hz and followed by an arithmetical average.

Fig.3. Location of ambient noise recording sites (filled circle) along with borehole locations (filled triangles). AB and CD are the two profiles as selected in conformity with the borehole locations.

iv) Subsequently, spectral ratios (NS/V, EW/V), and average (NS, EW)/V are computed.


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In order to ensure whether an H/V peak is of natural or anthropic origin, each peak is tested by Randomdec method (Huang and Yeh, 1999; Dunand et al. 2002; Guillier et al. 2007). As observed, the origin of the H/V peak is ascertained to be of anthropic origin if the critical damping is found below 5%; otherwise it is considered to be generated by natural origin wherein we get a critical damping above 5%. The frequency, pertaining to the low damping, as affirmed by the Randomdec technique has to be of industrial origin which is not considered for further analysis and interpretation.

RESULTS AND DISCUSSION Following the guidelines of SESAME (2004) and after testing through the Randomdec Method, the H/V ratios are evaluated for the selected 70 sites. Mainly, two different ranges of fundamental frequencies are observed; the first category of frequency is found within the range of 1 to 3 Hz, and the other remains above 3 Hz. For example, the lower category of frequency is displayed in Fig 4(a). Here, the fundamental resonant frequency is observed to be 2.2 Hz. Similarly, Fig 4(b) represents the higher category of frequency which is above 3 Hz. The resonant frequencies which are in the higher category of frequency might be caused by the harder soil strata beneath the surface. These results are complemented with the local geology/borehole data.

(a)

(b) SSSS SS. 30


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Fig.4. (a) & (b) Lower and higher fundamental frequencies observed from H/V ratio. The grey vertical line represents the peak of the H/V ratio corresponding to the resonant frequency. The dashed line indicates the standard deviation whereas the average H/V ratio is indicated by the solid line.

The contour distribution of fundamental frequencies estimated from H/V ratio is given in Fig 5(a). It is observed that some sites in some pocket areas show peak values at higher

(b)

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Fig.5. (a) Contour plot of resonance frequency from H/V ratio estimate, (b) Contour plot of amplification from H/V ratio estimate.

Fig.6. (a) Fundamental frequencies shown against reconstructed geological cross-section for Profile AB. The bold vertical lines in Fig. (b) represent the location of the boreholes. The nos appearing in Fig. (a) correspond to the location of the ambient noise location points in conformity with the borehole sites along the Profile AB. Along the horizontal axis of Fig. (b) distances between the ambient vibration recording sites inclusive of the boreholes are taken. The thicknesses of the strata as shown by the legends are plotted along the vertical axis.


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frequencies of 5 to 7 Hz; these sites are composed of compact rocks like greenstones, quartzites etc. Some sites in the centre of the city, on the other hand, exhibit lower fundamental frequency in the range of 1 to 2 Hz; these sites are marked by the presence of weathered soil cover as detailed in the geological map of Fig 2. The contour plot of amplifications corresponding to the fundamental frequencies is exhibited in Fig 5(b). There are ensembles of litholog information at 14 sites which are correlated with the location of ambient noise sites. Two profiles AB and CD are illustrated in Fig 3; the profile AB comprises of three boreholes and the CD encompasses six boreholes. The geological cross-sections and the observed peak frequencies along the profile AB are shown in Fig 6. We may divide the resonance frequencies into three classes; the first ranges from 1 to 3.5 Hz; the second 3.5 to 5.5 Hz and the third from 5.5 to 7.5 Hz. (Fig. 6). In the northwestern part of the profile an average resonance frequency is 5 Hz observed, which may be attributed to the presence of hard rocks in the region. Towards the south eastern edge of the profile, lower resonant frequency is prominent; where comparatively low density weathered soil is dominant. On the other hand, a wide range of resonance frequencies is contemplated along the profile CD. The sites 69, 56 and 39 show a higher mode of fundamental frequencies and the sites 60, 61and 62 reveal lower resonant frequencies. Thickening of weathered layer could be a possible explanation for the lower resonant frequencies. The good correlation between the H/V results and litho-logs complements our results.

CONCLUSIONS The fundamental resonance frequency for Shillong city is found within the range of 3 to 7 Hz. Major parts of the area show higher fundamental frequency which leads to an implication that there exists in the basement rock. Wide variation of resonance frequencies at short distances indicate a lateral heterogeneity prevailing in the surface of the region. A good correlation is observed between the resonant frequencies and the lithologs.

ACKNOWLEDGEMENTS We express our sincere gratitude to Dr. P. G. Rao, Director for his constant encouragement and permission to publish the work. We are thankful to Prof. H.K. Gupta, Chairman, Research Council, NEIST-Jorhat for his kind interest. Prof J.R. Kayal is also acknowledged for his valuable suggestions and kind help. The first author is thankful to CSIR, New Delhi for providing the NET-JRF. We are also grateful to MoES, Delhi for their financial support. References BARD, P.Y. (1999) Microtremor measurements: a tool for site affects estimation? Proc. 2nd Internat. Symp. Effect of Surface Geology on Seismic Motion. Yokohama, Japan. pp.1251-1279.

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