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«Site specific ground motion simulation and seismic response analysis for microzonation of Kolkata Narayan Roy and R.B. Sahu* Civil Engineering ...»

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Geomechanics and Engineering, Vol. 4, No. 1 (2012) 1-18 1

Technical Note

Site specific ground motion simulation and seismic

response analysis for microzonation of Kolkata

Narayan Roy and R.B. Sahu*

Civil Engineering Department, Jadavpur University, Kolkata, India

(Received July 12, 2011, Revised December 5, 2011, December 6, 2012)

Abstract. The spatial variation of ground motion in Kolkata Metropolitan District (KMD) has been

estimated by generating synthetic ground motion considering the point source model coupled with site response analysis. The most vulnerable source was identified from regional seismotectonic map for an area of about 350 km radius around Kolkata. The rock level acceleration time histories at 121 borehole locations in Kolkata for the vulnerable source, Eocene Hinge Zone, due to maximum credible earthquake (MCE) moment magnitude 6.2 were generated by synthetic ground motion model. Soil investigation data of 121 boreholes were collected from the report of Soil Data Bank Project, Jadavpur University, Kolkata.

Surface level ground motion parameters were determined using SHAKE2000 software. The results are presented in the form of peak ground acceleration (PGA) at rock level and ground surface, amplification factor, and the response spectra at the ground surface for frequency 1.5 Hz, 3 Hz, 5 Hz and 10 Hz and 5% damping ratio. Site response study shows higher PGA in comparison with rock level acceleration.

Maximum amplification in some portion in KMD area is found to be as high as 3.0 times compared to rock level.

Keywords: synthetic ground motion; site response; time history; PGA; response spectra; amplification.

1. Introduction The Kolkata city, formerly known as Calcutta, covers an area of about 185 sq. km with population of about 4.5 million (Census of India 2011). More than 80% of the city has built up areas dotted with high rise residential buildings, congested business district, hospitals and schools, most of these without any proper town planning and are very old. Density of population in some parts of north Kolkata is over 100,000 per square kilometers (Nandy 2007).

Kolkata had suffered damage by both far and near source earthquakes in the past. Notable among them were 1897 Great Assam earthquake, 1906 and 1964 Kolkata earthquake. The Kolkata earthquake of 15 April, 1964 had an intensity VIII (MM scale) was felt over an area of 67,000 Km2. The earthquake was located over the Eocene Hinge Zone (SEISSAT 2000).

The Bureau of Indian Standard (IS: 1893, Part 1-2002) classified India into four seismic zones (zone II to zone V) (Fig. 1). Kolkata lies at the boundary of zone III and IV with a triangular zone of IV with its tip from north of Kolkata and spreading towards south covering the area of North and *Corresponding author, Professor, E-mail: rbsahu_1963@yahoo.co.in 2

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South 24 parganas and eastern part of Purba Medinipur district. However, the zones are not sufficient to predict the damage pattern within each zone in the event of an earthquake, as the damage will depend on the local geology, vicinity to active faults, geotechnical and geophysical properties of surface and subsurface strata, and topography.

Microzonation of Kolkata on the basis of PGA variation was presented by Mohanty and Walling (2008) using a quasi-probabilistic approach considering attenuation relationship of Toro et al. (1997).

They, however, did not consider local site effects. In this context, it may be noted that Kolkata lying on the alluvial Gangetic deposit has two distinct soil formations, namely, normal Kolkata deposit and river channel deposit. Normal Kolkata deposit consists of a thick soft compressible silty clay/ clayey silt down to a depth of about 14.0 m below existing ground surface followed by stiff/very stiff/hard/very hard clayey deposit with intermediate sand layers down to considerable depth of 40m. The river channel deposit existing along the old Adiganga channel consists of medium/ dense/very dense sandy deposit down to considerable depth. These soil characteristics along with phenomenal increase in industrial and commercial activities in Kolkata and adjoining areas necessitates microzonation considering local site effects in order to prevent huge loss of life and properties due to earthquake.

In this work, an attempt has been made to generate site specific ground motion and local site effects for Kolkata Metropolitan District area (Fig. 3) which covers an area of 1250 sq. km. The analysis has been done using synthetic ground motion model by Boore (1983, 2003) for determination of rock level time history followed by ground response using SHAKE2000 considering 121 nos.

boreholes of depths 30 to 40 m below ground level. Further, it may be noted that most of the site effect studies in earthquake ground motion are based on the geotechnical properties in the upper 30 m (Finn 1991, Boore et al. 1993, Anderson et al. 1996). According to Borcherdt (1994), the upper 30-m soil column is considered to be responsible for site amplification. Here, the soil at a depth of 30 m below ground level is hard/very hard/very dense, so, rock level was considered to be at 30 m depth for synthetic ground motion generation. Due to unavailability of recorded ground motion data Site specific ground motion simulation and seismic response analysis for microzonation of Kolkata 3 in this region the method proposed by Boore (1983, 2003) [SMSIM-program] was applied to simulate the artificial ground motion using local seismotectonic parameters. The Boore’s model has been successfully used to generate ground motion in all over the world (Junn et al. 2002, Ebrahimian et al. 2008) as well as in India (Sitharam et al. 2006, Raghukanth et al. 2009). 1-D ground response analysis has been carried out using SHAKE2000, based on the SHAKE developed by Schnabel et al. (1972).

2. Geological and tectonic setting

The Kolkata metropolis the second largest urban agglomeration in India is bounded by latitudes 22°20'N-23°00'N and longitudes 88°04'E-88°33'E. Originally Kolkata grew in a north-south direction over the natural levee of the river Bhagirathi for over a length of 50 Km. But due to enormous population pressure it has encroached into the back swamp and marshy land to the east by way of filling up extensive areas, especially in the Salt Lake and subsequently Rajarhat areas as well in many other places in unplanned ways (Nandy 2007).

Kolkata is situated over the Bengal Basin which is a huge pericratonic Tertiary basin with enormous

thickness of fluvio-marine sediments. The Bengal Basin can be divided into three structural units:

the western most shelf or platform, the central hinge or shelf/slope break, known as Eocene Hinge Zone (EHZ), and deep basinal part in the east and southeast that open in the present Bay of Bengal.

Kolkata is located over the western part of hinge zone across which the sediment thickness significantly varies from shelf area in the west to deep basin area in the east. The hinge zone and the shelf area traverse by many faults, some of them seem to be tectonically active at present (Nandy 2007). The EHZ is about 25 Km wide that occurs at a depth of about 45000 m below Kolkata. The total sediment thickness below Kolkata is of the order of 7500 m above the crystalline basement; of these the top 350 to 450 m is Quaternary, followed by 4500-5500 m of Tertiary sediments, 500-700 m trap wash of Cretaceous Trap and 600-800 m Permocarboniferous Gondwana rocks.

The major fault systems of this region are Garhmoyna-Khandaghosh Fault (GKF), JangipurGaibandha Fault (JGF), Pingla fault, Eocene Hinge Zone (EHZ), and Debagram Bogra Fault (DBF).

The GKF extends along the basin margin that joins the Rajmahal Fault in the north. The NE trending JGF bifurcates from the GKF that separates the Rangpur-Malda saddle (a subsurface structural high joining the Indian Shield and the Shillong Plateau) from the Bogra Shelf. The shelf is limited to the southeast by the DBF. A similar section is present below the West Bengal part of the basin between GKF and the fault passing through Barrackpur and Krishnanagar (SEISSAT 2000).

Geological survey of India has complied all available geological, geophysical and seismological data for entire India and published a seismotectonic map of India in the year 2000. Seismotectonic atlas (SEISAT 2000) contains 43 maps in 42 sheets of 3o × 4o sizes with scale of 1:1 million, which also describes the tectonic frame work and seismicity. From the seismotechtonic atlas, a seismotectonic map of Kolkata covering a circular area of radius 350 Km (Fig. 2) has been prepared on ArcGis-9.2 showing faults, lineaments, shear zones and past earthquake events using seismotectonic atlas (SEISSAT 2000) of India and also earthquake data collected from IMD (Indian Meteorological Department, Delhi). Total 76 nos. of earthquake events have been collected for the period 1764 to

2009. The earthquake data was converted into moment magnitude (Mw) using the relation given by Idriss (1985). The seismotectonic map contains 41 nos. of faults, 19 nos. lineaments, 5 shear zones 4 Narayan Roy and R.B. Sahu Fig. 2 Seismotectonic map of Kolkata (from SEISSAT 2000) Site specific ground motion simulation and seismic response analysis for microzonation of Kolkata 5 and one hinge zone (EHZ).

3. Ground motion simulation

3.1 Synthetic ground motion model

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3.2 Input parameters Raghukanth and Somala (2009) estimated the stress drop for Bengal Basin-Shillong Plateau earthquakes that occurred in the region to be 156 to 258 bars. In this study a stress drop of 250 bars was used.

In the path properties the whole-path attenuation is governed by the frequency dependent quality factor Q. For the Bengal basin-Shillong plateau region the Q was estimated as 224f 0.93 (Raghukanth and Somala 2009).

Geometrical spreading factor (GSP) is 1/r for r 100 km and equal to 1/10 r for r 100 km (Singh et al. 1999), where, r is the hypocentral distance.

Distance dependent duration, which is a function of the path as well as the source, is given by T = 1/fc + br (9) Where, the first part 1/fc is the source duration and the second part is a distance dependent term that accounts for dispersion. The value of b was taken to be 0.05 (Herrmann 1985, Boore and Atkinson 1987, Boore 1996).

The regional velocity model in the Kolkata region for equivalent rock-type sites with density used in this study was taken from Gopali-Port Canning Deep Seismic Sounding (DSS) profile near Kolkata city (Prasad et al. 2005) and is given in Table 1. From this data average shear wave velocity was considered to be 870 m/s. Site amplification [A( f )] at the assumed engineering rock level was determined using quarter-wavelength method of Boore and Joyner (1997) and is given in Table 2.

Chandler et al. (2006) compiled a database of kappa factors estimated for various parts of the world. They proposed an empirical equation for estimating the kappa factors from the average shear wave velocity in the top 30 meters of the soil (Vs,30) as

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PGA at rock level considered to be at a depth of 30 m below existing ground level were calculated using synthetic ground motion model for four major sources around Kolkata and is given in Table 4 along with the corresponding source to site distance, hypocentral distance and earthquake magnitude.

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Fig. 3 Borehole locations with most vulnerable source for Kolkata Site specific ground motion simulation and seismic response analysis for microzonation of Kolkata 9

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From Table 4 it is seen that Maximum PGA value is 0.141g due to Eocene Hinge with a earthquake magnitude of 6.2 Mw. For other three sources PGA values are 0.005 (F2 GarhmoynaKhandaghosh Fault), 0.027 (F3) and 0.037 (F6) respectively. So, Eocene Hinge Zone (EHZ) is the most vulnerable source with maximum credible earthquake magnitude of 6.2 Mw for Kolkata. Rock level PGA and time histories was, then, generated at 121 borehole locations in KMD area (Fig. 3) considering EHZ as the vulnerable source. This rock level PGA was used to develop a contour map using Geostatistical Analyst Wizard ArcGis9.2 (Fig. 4).

4. Site specific response

4.1 Local soil condition In this study, detailed soil investigation data as obtained from the report of Soil Data Bank Project (1986) of 121 boreholes at various locations of Kolkata (Fig. 3) were used for ground response analysis.

Typical bore log of normal Kolkata deposit and river channel deposit are shown in Fig. 5 and Fig. 6 respectively. Subsurface profile information including unit weight, ground water level, SPT values etc. at different borehole locations were used for the ground response analysis using SHAKE2000.

4.2 Input parameters

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Hinge Zone with MCE 6.2 Mw were used as input ground motion in SHAKE2000 for evaluating peak acceleration values and acceleration time histories at the top of each sub layer. A typical input motion for the borehole at 22 CIT Road site is shown in Fig. 7. The dynamic properties such as Site specific ground motion simulation and seismic response analysis for microzonation of Kolkata 11 modulus reduction and damping ratio were taken from the database of material properties provided with SHAKE2000 (Seed and Idriss 1970, Schnabel 1973, Seed et al. 1986, Sun et al. 1988, Vucetic and Dobry 1991).

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