ORI GIN AL PA PER

Deterministic seismic microzonation of Kolkata city

Amit Shiuly • J. P. Narayan

Received: 25 June 2011 / Accepted: 9 October 2011 / Published online: 26 October 2011� Springer Science+Business Media B.V. 2011

Abstracts This paper presents the deterministic seismic microzonation of densely pop-

ulated Kolkata city situated on the world’s largest delta island with very soft and thick soil

deposit in the surficial layers. A fourth-order accurate staggered-grid finite-difference

algorithm for SH-wave propagation simulation in visco-elastic medium is used for the

linear computation of ground motion amplifications in sedimentary deposit. Different maps

such as for fundamental frequency (F0), peak ground acceleration (PGA), peak ground

velocity, and peak ground displacement are developed for variety of end-users commu-

nities, including structural and geotechnical engineers for performance-based designs,

building officials, emergency managers, land-use planners, private businesses, and the

general public. The scenario of simulated amplification factors in the different frequency

bands revealed that the Kolkata city is very much prone to severe damage even during a

moderate earthquake and very selective damage may occur at some of the localities during

local and distant earthquakes. The deterministically predicted PGA at bedrock level is

0.0844 g and the maximum PGA predicted at the free surface is 0.6 g in Kolkata city due

to maximum credible earthquake (Mw = 5.4) associated with Eocene Hinge Zone at a

depth of 36 km. The seismic microzonation of Kolkata city reveals that the Nager Bazar

and Nimtala areas are the safest regions with earthquake point of view.

Keywords Seismic microzonation of Kolkata city � Ground motion amplification �Fundamental frequency and seismic hazard analysis

1 Introduction

Seismic microzonation is the subdivision of a large region into small sectors with similar

behavior of relevant seismic parameters. It is very much essential to predict the seismic

hazard at the micro-level for the mitigation of earthquake disaster and risk assessment. The

A. Shiuly � J. P. Narayan (&)Department of Earthquake Engineering, Indian Institute of Technology Roorkee,Roorkee 247667, Indiae-mail: jaypnfeq@iitr.ernet.in

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Nat Hazards (2012) 60:223–240DOI 10.1007/s11069-011-0004-5

earthquake ground motion characteristics mainly depend on three factors namely source,

path, and local site conditions. Local geology largely affect the ground motion charac-

teristics and play an important role in damage distribution during an earthquake (Oprsal

et al. 2005; Narayan 2005, 2010, 2011; Narayan and Singh 2006). Severe damage even at

large epicentral distances may occur due to the local site effects and double resonance

(double resonance is the resonance of body wave frequency with F0 of soil and then again

resonance with the natural frequency of structure) (Romo and Seed 1986; Narayan et al.

2002). The current practice of seismic microzonation in most of the countries is to predict

the bedrock motion using an empirical relationship or stochastic method and then transfer

the bedrock motion to the surface using the 1D S-wave response of a soil column.

Anbazhagan and Sitharam (2008) carried out seismic microzonation of Bangalore city,

situated in seismic zone II (IS:1893 (part 1):2002), and predicted maximum PGA of the

order of 0.48 g. The prediction of fundamental frequency (F0) of soil deposit is very

important task since earthquake engineers generally keep the natural frequency of struc-

tures below the F0 to avoid double resonance during an earthquake.

Kolkata Megacity, the study area and the capital of West Bengal, is situated in Bengal

basin in the eastern part of India (Fig. 1). Originally, Kolkata city has grown in north–south

direction along the eastern bank of Hoogly River. The Kolkata city lies on the border of

seismic zones III and IV as per the seismic zoning map of India incorporated in the Indian

standard criteria for earthquake resistant design of structures (IS:1893 (part 1):2002).

Therefore, on an average expected PGA during future earthquake in Kolkata city is

between 0.14 and 0.30 g. In Bengal basin, the range of sedimentary thickness from west to

east is 1.2–20 km (Nandy 2007). Three rivers namely Ganga, Brahmaputra, and Barak flow

in Bengal basin. Kolkata city being highly developed and an old city have many old

buildings, bridges, subways, multi storied buildings, huge shopping malls, etc. According

to the census report of 2001, the population of Kolkata city is about 13 millions. Very large

population of Kolkata city, huge man-made infrastructures, situated on very thick and soft

soil deposit, and use of filled swampy and marshy lands in an unplanned way (Nandy 2007)

call for the seismic microzonation of Kolkata city.

This paper presents the seismic microzonation of Kolkata city. The ground motion at

bedrock and free surface has been simulated at different borehole (BH) locations using

Boore’s SMSIM program (Boore 2011) which is based on stochastic method (Boore 1983,

2003). Linear soil amplification has been computed separately using a 2D SH-wave

staggered-grid fourth-order finite-difference (FD) program developed by Narayan and

Kumar (2011) for viscoelastic media. The spectral amplification factors computed using

FD method are used as input to SMSIM program for the computation of ground motion at

the free surface and prediction of peak ground acceleration (PGA), peak ground velocity

(PGV), and peak ground displacement (PGD). Different maps such as for fundamental

frequency, PGA, PGV, and PGD are developed for a variety of end-users involved in

earthquake disaster mitigation and preparedness.

2 Seismotectonics of Bengal basin and adjoining region

The details of tectonic features present in the study area and its adjoining region are shown

in Fig. 2 (GSI 2000). The Bengal basin consists of three structural domains namely western

scrap zone, middle shelf zone, and eastern deeper basin part. The western part of the

Bengal basin is bounded by the basin margin fault zone to the west and north-west and by

the Eocene Hinge Zone (EHZ) to the east and south west and constitutes a broad shelf zone

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(Salt et al. 1986). In the shelf zone, the Tertiary sediment dips homogeneously toward the

southeast with a sudden flexure along the EHZ. The NE–SW trending curved EHZ is a very

prominent regional basement fault present in the study area. In fact, almost entire Kolkata

city is situated on EHZ. This hinge zone marks a zone of differential thickening and

subsidence rate of Oligocene and Miocene section (Salt et al. 1986). The Bengal basin

basement steeply plunges from 4 to 10 km or even further across the EHZ (Mukhopadhyay

and Dasgupta 1988). EHZ extends for at least 500 km from the Dauki fault on the north

and Kolkata on the south with probable extension into the Bay of Bengal having varying

width from 25 km in the north to 110 km in the central part and 35 km in the south. The

major fault systems in this region are Garhomoyana-Khandaghosh Fault (GKF), Jangipur-

Gaibandha Fault (GGF), Pingla Fault, Eocene Hinge Zone (EHZ), Debagram Bogra Fault

(DBF), Rajmahal Fault (RF), Dauki Fault (DF), Sylhet Fault (SF), Sainthia Bahmani Fault

(SBF), and Dhubri Fault (DHF).

During the last 350 years, Kolkata city has experienced the shaking at least 30 times

due to local and distant earthquakes (Mohanty and Walling 2008). The Calcutta earthquake

of September 29, 1906 (M = 5.0, epicenter just north of Kolkata city) caused major cracks

in large number of buildings in Kolkata city and intensity of the order of VI–VII was

assigned on Rossi-Forel scale (Middlemiss 1908). The intensity of shaking in Kolkata city

Fig. 1 Map showing the location of Kolkata city in India and important features, places, and BH locationsin Kolkata city

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during April 15, 1964 Calcutta earthquake (M = 5.2; epicentral distance 106 km) was of

the order of VII on MMI scale (Jhingran et al. 1969; GSI 2000). Both the local Calcutta

earthquakes of 1906 and 1964 occurred on EHZ. The great 1897 Assam earthquake

(M = 8.7; epicentral distance 470 km) developed shaking of the order of VIII on MMI

scale (Seeber and Armbruster 1981). The Bihar-Nepal earthquake of January 15, 1934

(M = 8.3, epicentral distance 480 km) caused considerable damage to the buildings and

developed intensity of the order of VI on MMI scale (Dunn et al. 1939). Srimangal

earthquake of July 8, 1918 (M = 7.6, epicentral distance 350 km) caused cracks in many

buildings of Kolkata city. The other large and distant earthquakes which were felt and

caused damage in Kolkata city are Dubhri earthquake of July 3, 1930 and earthquakes of

September 1, 1803, August 26, 1833, and December 31, 1881.

3 Seismic hazard analysis

Deterministic seismic hazard analysis (DSHA) has been carried out in order to find out the

Maximum Credible Earthquake (MCE) to be used for bedrock motion determination for

seismic microzonation of Kolkata city. A seismotectonic map (latitude 20�N–26�N and

Fig. 2 Shows the seismotectonic map for Kolkata city and surrounding region

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longitude 85�E–92�E), shown in Fig. 2, covering around 350 km radial distance from the

Kolkata city has been prepared from the seismotectonic atlas map of India (GSI 2000).

Earthquake data for the period 1762–2005 collected from India Meteorological Depart-

ment (IMD), New Delhi, are used for SHA. There are around 162 earthquakes which have

occurred in the area surrounding the Kolkata city in the above period, but mostly in the

Himalaya (Fig. 2). Figure 2 clearly depicts that even moderate earthquakes are not so

frequent in the vicinity of Kolkata city. Seven seismogenic sources have been identified in

the considered region for DSHA. The MCE for each seismogenic source is inferred from

the maximum magnitude earthquake occurred close to or on that seismogenic source.

In order to find out the MCE for the seismic microzonation of Kolkata city, PGA has

been computed at the center of the study area using the attenuation relationship given by

Abrahamson and Litehiser (1989).

logðaÞ ¼ �0:62þ 0:177M � 0:982 logðr þ e0:284MÞ þ 0:132F � 0:0008Er ð1Þ

where ‘a’ is peak horizontal acceleration, ‘r’ is the closest distance (in km) from site to

the zone of energy release, ‘M’ is the magnitude, F is dummy variable that is ‘1’ for

reverse or reverse oblique fault otherwise ‘0’, and ‘E’ is a dummy variable that is ‘1’ for

interplate and ‘0’ for intraplate events. The closest distance from the site to the zone of

energy released is obtained using rupture width computed with the help of relation given

by Wells and Coppersmith (1994) and dip of the fault. The details are given in EQ:2008-

32 (2008). The magnitude scale is not given in IMD data. Therefore, it is considered as

surface wave magnitude, as discussed and suggested by Thingbaijam et al. (2008) and

Das et al. (2011). The PGA obtained from all the seven seismogenic sources considered

for DSHA are given in Table 1. Analysis of Table 1 reveals that MCE for the seismic

microzonation of Kolkata city is Ms = 5.2 on Eocene Hinge Zone with a focal depth of

36 km. In order to compute the ground motion at bedrock using stochastic method, the

required moment magnitude for MCE is obtained using the regression relation given by

Das et al. (2011).

4 Geotechnical data

Three rivers namely Ganga, Brahmaputra, and Barak flow in Bengal basin. This basin

covers an area of around 2,00,000 square km. Near the Indo-Bangladesh border, Farakka

Table 1 Estimated PGA fromdifferent seismogenic sources

S. no. Source Magnitude(Ms)

Distanceto zoneof energyrelease (Km)

PGA(g)

1 Eocene Hinge Zone 5.2 36 0.072

2 Debagram Bogra Fault 6.0 137 0.028

3 Garhomoyana-Khandaghosh Fault

5.0 55 0.045

4 Dauki Fault 8.7 387 0.031

5 Sylet Fault 7 290 0.021

6 Sainthia Bahmani Fault 5.5 182 0.018

7 Dhubri Fault 6.2 256 0.017

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barrage diverts the Ganga water to the river Hoogly to increase and maintain adequate

depths of flow of water for navigation and operation of Kolkata port. The sediment

transported by Ganga-Brahmaputra Rivers has formed world largest delta island in

southern part of Bengal basin. The study area is situated on the delta island. The geo-

technical data for the study area were collected from several agencies like C. E. Testing

Pvt. Ltd., S. Ghosh & Associates, etc. Geotechnical data from around 85 borehole locations

(BH) were obtained. The data include SPT values, density, plasticity index, finer content,

lithology, etc. Finally, data of only 44 borehole locations, shown in Fig. 1, were considered

for the seismic microzonation, keeping in view that data should be widely distributed in the

study area as well as at the same time to avoid the clustering of data points. The number of

sedimentary layers in upper 50-m deposit varies from 4 to 8 and surficial layers are very

young and soft. The correction factors namely overburden correction, dilatancy correction,

and correction for due to finer content are applied on N-values obtained from borehole

(Terzaghi and Peck 1967; Idriss and Boulanger 2004; Sitharam et al. 2007).

The corrected N-values have been used for obtaining the shear wave velocity in dif-

ferent layers up to a depth of 50 m using the formula Vs = 51.5N0.516 (Iyisan 1996). The

soil testing agencies have also provided the density of each sedimentary layer at BH

location. The deepest information from borehole (BH) data in the study area is only up to

50 m. At most of the BH locations, information up to only 30 m is available. Therefore,

where there is no information between 30 to 50 m depth, the known parameters of the

deepest layer of that BH location are extrapolated up to a depth of 50 m or are taken from

the nearby BH location. The shear wave velocity for layers deeper than 50 m and above the

bedrock was obtained as 740 and 1048.27 m/s up to a depth of 73 and 700 m, respectively,

by extrapolating the velocity structure at Bishnupur, which is very much near to Kolkata

city and falls on Gopali—Portcanning seismic refraction profile (Reddy et al. 1998). The

deepest two layers having thicknesses 23 m (50–73 m) and 627 m (73–700 m) are con-

sidered to be common for all the BH locations. The shear wave velocity of the order of

1,890.17 m/s for bedrock was also obtained from the same seismic refraction survey line

(Reddy et al. 1998).

5 Computation of spectral ground motion amplification

Ground motion amplification caused by sedimentary deposit in Kolkata city at each BH

location is computed separately in order to predict the free surface ground motion time

history using stochastic method (Boore 1983, 2003). The details of computation of spectral

ground motion amplification are given in the following subsections. Contours for various

maps are drawn with the help of ArcGIS packages.

5.1 Salient features of the used computer program

A computer program developed by Narayan and Kumar (2011) which is based on stag-

gered-grid finite-difference approximation of viscoelastic SH-wave equation for hetero-

geneous anelastic medium is used for the computation of spectral amplification factors.

This FD program is an upgradation of the program developed by Narayan and Kumar

(2008). The FD algorithm is second-order accurate in time fourth-order accurate in space

(Levander 1988; Moczo et al. 2000; Narayan and Kumar 2008). Both the sponge boundary

(Israeli and Orszag 1981) and A1 absorbing boundary (Clayton and Engquist 1977) con-

ditions are implemented on the model edges to avoid the edge reflections. In order to avoid

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thickness discrepancy of the first soil layer, which causes increase of value of numerically

computed fundamental frequency, VGR-stress imaging technique developed by Narayan

and Kumar (2008) is used. Variable grid size in the vertical direction is used in order to

accommodate the thickness of different soil layers and to reduce the requirement of

computational memory and time (Narayan and Kumar 2008).

5.2 Development of 1D FD models

In this section, first, details of development of 1D model for borehole location No. 4

(BH4) are given for the computation of spectral soil amplification of ground motion.

There are six sedimentary layers above the bedrock at BH4. The shear wave velocity,

density, thicknesses, and quality factors for different layers are given in Table 2. The

quality factor for layers having S-wave velocity between 175 to 610 m/s was computed

using the relation Q = 0.08Vs ? 6.99 (Iyisan 1996) and it was 10% of Vs, where Vs was

larger than 610 m/s. The shear wave velocity, density, and quality factor for bedrock were

1,890.17 m/s, 2.34 g/cc, and 189.01, respectively. In order to incorporate the different

layers with their exact thicknesses in FD-grid as well as to avoid grid dispersion and to

reduce the requirement of computational memory and time, variable grid size in vertical

direction was used. The grid size in X-direction was taken as 1.5 m. In case of 1D model

for BH4, the grid sizes in vertical direction were taken as from top to bottom 2.1, 1.95, 2,

2, 4.6, and 10.45 m, respectively. The vertical grid size in bedrock was taken as 10.45 m.

Ricker wavelet with a dominant frequency 4.0 Hz and frequency bandwidth 0–10 Hz is

used as a source excitation function. The time step is taken as 0.0005 s keeping in view

the stability criteria.

5.3 Computation of spectral amplification of ground motion

Seismic responses of 1D model corresponding to BH4 were computed with and without

considering the sedimentary layers above the bedrock using a line source at a depth of

1.0 km. The time domain responses without and with sediment layers are shown in Fig. 3a.

The maximum amplitude in case of model having sediment layers is about 4.4 times larger

than that without sediment layers. The spectral amplification caused by sediments above

the basement was computed just by taking the ratio of spectra shown in Fig. 3b. Figure 3c

shows the spectral amplification factors in a frequency bandwidth of 0.25–10 Hz. The

fundamental frequency and the corresponding spectral amplification factor were obtained

as 1.73 and 7.0, respectively. The obtained average amplification factors (AAF) in the

frequency band of 0.25–10.0, 0.25–1.0, 1.0–2.0, 2.0–5.0, and 5.0–10.0 Hz are 4.28, 3.08,

5.69, 4.81, and 3.82, respectively. Further, AAF is highly variable from band to band and

Table 2 Shows the thickness,S-wave velocity, quality factor,and density of different sedi-mentary layers as well as bedrockfor 1D model corresponding toBH4 location

Layers Thickness (m) Vs (m/s) Quality factor Density (g/cc)

1 4.2 223.46 24.9 1.81

2 7.8 297.76 30.8 1.58

3 18 368.10 36.4 1.90

4 20 399.48 38.9 1.95

5 23 704.00 70.4 2.02

6 627 1,048.27 104.8 2.03

Bedrock – 1,890.01 189.0 2.34

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highest (5.69) is in frequency band of 1.0–2.0 Hz. In a similar way, 1D models for all the

BH locations were developed and then seismic responses were computed for the prediction

of spectral amplification factors and fundamental frequency.

5.4 Fundamental frequency

The variation of fundamental frequency of sediment deposit in Kolkata city is given in

Fig. 4 and Table 3. The smallest value of F0 of the order of 0.8 Hz is obtained in Bar-

anagar area (BH33 and BH40), very near to the Hoogly River in north-western part of the

study area. F0 of the order 1.6 Hz is obtained at BH44, BH43, and BH39, very near the

Hoogly River in the south-western part of study area, in Nagar Bazar area (BH29), and

Kolkata airport region (BH41). Small patches of low F0 are also present sporadically in the

study area. For example, F0 of the order of 1.2 Hz at Tiljala road (BH17) and 1.33 Hz in IB

Salt Lake area (BH32 and BH34). But, the large F0 (1.73 Hz) obtained in Nimlata region

(BH31) may be due to very thin surficial soft soil layer. Largest value of F0 2.53 Hz was

obtained in Manohar Pukur road area (BH19). This may be due to higher S-wave velocity

in the surficial soil layers. At most of the BH locations, F0 is between 1.73 and 1.86 Hz.

Fig. 3 a–c Show the seismic responses, spectral amplitudes, and amplification factors, respectively

230 Nat Hazards (2012) 60:223–240

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5.5 Spectral amplification factors

The amplification factors at resonance frequency are also given in Table 3. Largest spectral

amplification at F0 of the order of 13–15 is obtained very near the Hoogly River (BH40,

BH33, BH44, BH43, and BH39) except in Nimtala area (BH31). Small patches of large

spectral amplification at F0 are also observed in Salt Lake area (BH32, BH34, and BH35)

and on Tiljala road area (BH17). Very low spectral amplification of the order of 3.6–3.9 is

obtained in Nager Bazar area (BH29) and Nimtala area (BH31). This may be due to the

presence of thick low velocity layer at some depth with considerable impedance contract at

both the upper and lower interfaces (Kumar and Narayan 2008). In rest of the study area,

spectral amplifications at F0 are between 4 and 8.

Shiuly (2011) computed the average amplification factor (AAF) in the frequency range

of 0.25–10 Hz for all the 44 BH locations (result not shown here). Very high AAF of the

order of 5–7 was obtained in south-western part of study area, very near the Hoogly River

(BH39, BH43, and BH44). Small patches of large AAF were also observed at BH32 (Salt

Lake area), BH10 (Rash Bihari Avenue), and BH2 (Upendra Banerjee Road area). Shiuly

(2011) also reported very low AAF (\3) in Nager Bazar area (BH29) and Tiljala road area

(BH17). Reported AAF of the order of 3–5 at most of the BH locations reveals that Kolkata

city may suffer severe damage even during a moderate earthquake.

Fig. 4 Shows the fundamental frequency of sedimentary deposit in Kolkata city at different BH locations

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Table 3 Gives the borehole (BH) locations, and associated F0, amplification factors at F0, PGA, PGV, andPGD

BHno.

Long. Lat. Locality name Fo

(Hz)Amp.at F0

PGA(g)

PGV(cm/s)

PGD(cm)

BH1 22.5413�N 88.3679�E 91, Park Street, Kolkata 1.73 6.07 0.49 6.57 0.64

BH2 22.5752�N 88.3886�E 33/1A, Upendra Banerjee Road 1.83 6.25 0.37 6.97 0.52

BH3 22.5134�N 88.3559�E 89, Southern Avenue 1.73 7.00 0.32 5.37 0.70

BH4 22.5237�N 88.3808�E 42/197/1, Bediadanga 2nd Lane 1.73 7.00 0.25 7.21 0.69

BH5 22.5182�N 88.3269�E 15B/1A Raja Santosh Road 1.73 7.21 0.39 6.94 0.54

BH6 22.5772�N 88.4313�E DN-1, Sector-5, Salt Lake, Kol 1.87 6.54 0.38 6.42 0.68

BH7 22.5442�N 88.3533�E 3A Upper, Wood St. Kol. 1.73 8.89 0.43 6.25 0.59

BH8 22.5775�N 88.3810�E 1, Radhagovinda Nath Sarani,Tollygunge

1.73 6.46 0.40 5.83 0.65

BH9 22.5333�N 88.3569�E Ballygunge Circular Road,Kolkata

1.73 7.51 0.46 5.84 0.65

BH10 22.5176�N 88.3568�E 121, Rash Bihari Avenue, Kolkata 1.73 6.70 0.59 9.65 0.66

BH11 22.5176�N 88.3642�E 68, Hindusthan Park, Kolkata 1.87 6.08 0.51 7.10 0.66

BH12 22.5300�N 88.3084�E K.K.Roy Choudhury Rd, Behala,Kolkata

1.87 6.61 0.56 6.93 0.67

BH13 22.4806�N 88.3300�E 8/5/1, Alipore Road, Kolkata 1.73 5.95 0.50 7.05 0.62

BH14 22.4964�N 88.3195�E 31, James Long Sarani, Behala,Kolkata

1.60 6.31 0.26 6.38 0.63

BH15 22.5144�N 88.3252�E 53 Haraprasad Shastri Sarani,New Alipore

1.73 8.28 0.43 6.03 0.57

BH16 22.5683�N 88.3510�E 18, British India Street, Kolkata 1.87 5.91 0.57 7.42 0.64

BH17 22.5342�N 88.3745�E 72, Tiljala Road, Kolkata 1.20 14.38 0.29 5.32 0.67

BH18 22.5332�N 88.3460�E 18B, Ashutosh Mukherjee Rd,Kolkata

1.73 6.85 0.57 8.28 0.62

BH19 22.5218�N 88.3492�E 14/1B, Manohar Pukur Road, Kol. 2.53 7.15 0.30 7.90 0.70

BH20 22.5020�N 88.3565�E 375, Prince Anwar Shah Road,Kolkata

1.73 6.96 0.49 6.28 0.66

BH21 22.5822�N 88.3948�E 41 & 46, Canal Circular Rd. 1.73 6.04 0.55 7.48 0.61

BH22 22.5416�N 88.3494�E 20, Lee Road, Kolkata 1.73 7.34 0.40 5.54 0.69

BH23 22.5191�N 88.3400�E 20/1, Chetla Road, Kolkata 1.73 5.88 0.46 6.16 0.62

BH24 22.5654�N 88.3988�E 134B, Beliaghata Road, Kolkata 1.60 7.40 0.59 7.29 0.66

BH25 22.5387�N 88.3978�E E.M.Bypass, Kolkata 1.73 7.58 0.44 6.43 0.65

BH26 22.5437�N 88.3812�E 1/3, Mahendra Roy Lane, Kolkata 1.73 8.06 0.33 6.03 0.62

BH27 22.5041�N 88.3317�E 45, Buroshibtala Main Road,Behala, Kolkata

1.60 8.18 0.51 7.09 0.65

BH28 22.5634�N 88.3534�E Prop.Topsel Toyota at Kolkata 1.73 5.92 0.58 8.42 0.70

BH29 22.6413�N 88.3821�E PROP. M.B.D.Airport Hotel atKolkata

1.60 3.92 0.12 3.35 0.27

BH30 22.5376�N 88.4144�E 83/3 & 84/3 AT Topsia Road,Kolkata

1.73 8.03 0.37 5.59 0.68

BH31 22.5946�N 88.3549�E Nimtala Burning Ghat 1.73 3.64 0.25 4.66 0.23

BH32 22.5727�N 88.4144�E IB Salt Lake 1.33 14.83 0.57 9.43 0.87

BH33 22.6404�N 88.372�E Baranagar Isi 0.80 13.77 0.50 7.45 1.02

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6 Ground motion synthesis

Ground motion is simulated using the Boore’s SMSIM program (Boore 2011), which is

based on the stochastic method (Boore 1983, 2003). In the stochastic point source mod-

eling, it is assumed that the source is concentrated at a point and that the acceleration time

series generated at a site carry both deterministic and random aspects of ground motion

shaking. The deterministic aspects are specified by the average Fourier spectrum, typically

as a function of magnitude and distance.

6.1 Ground motion simulation at bedrock level

Ground motion at bedrock level has been simulated using the SMSIM program corre-

sponding to MCE (Mw = 5.4) obtained from the DSHA. The deterministic point source

spectrum at a site Y(M0, R, f) takes into consideration of point source spectrum E(M0, f),path effects P(R, f), and site effects G (f).

Y M0; R; fð Þ ¼ E M0; fð ÞP R; fð ÞG fð Þ ð2Þ

where R is the distance in km from source to site and M0 is the moment released in dyne-

cm. The details of point source spectrum computation (including corner frequency, radi-

ation pattern, partitioning of S-wave energy, and free surface effect) are given in Boore

(1983, 2003). x2-model for displacement source spectrum is used (Brune 1970, 1971;

Atkinson and Boore 1995). Atkinson and Boore (1995) approach was used for path effect

computation for which the frequency-dependent quality factor was obtained using the

relation Q = 680*f0.38, because of non-availability of Q for the study area (Boore 2003).

Although this relation for Q is applicable for United States of America, but it seems

reliable to some extent since Q of sedimentary deposit (700 m) in Kolkata city is being

taken into consideration during the computation of soil amplification separately and the

variation of Q in crustal mass at global level is not so high. Similarly, due to non-

availability of stress drop for the EHZ and the surrounding area, the stress drop of 100 bars

Table 3 continued

BHno.

Long. Lat. Locality name Fo

(Hz)Amp.at F0

PGA(g)

PGV(cm/s)

PGD(cm)

BH34 22.5878�N 88.4448�E Substation New Town 1.60 11.06 0.32 5.85 0.78

BH35 22.5801�N 88.4096�E Sector 3 FC 1.73 10.29 0.32 5.7 0.56

BH36 22.5705�N 88.4216�E Nicopark 1.87 6.38 0.60 8.73 0.65

BH37 22.5729�N 88.3641�E Sealdah, Ultadanga 1.73 7.63 0.46 5.78 0.66

BH38 22.4521�N 88.3046�E Diamond Park, Joka, Kolkata 1.73 8.30 0.57 7.66 0.68

BH39 22.5230�N 88.2870�E Garden Reach 1.60 15.63 0.30 9.4 1.14

BH40 22.6550�N 88.3870�E 4M. M. Feedar Road, Kolkata 0.80 14.50 0.19 5.68 0.75

BH41 22.6150�N 88.3940�E 8, Mall Road, Kolkata 1.60 6.69 0.14 6.18 0.62

BH42 22.5174�N 88.4051�E E.M.Bypass, Plot No.3794 1.87 6.86 0.48 6.5 0.68

BH43 22.5174�N 88.4051�E 16/1A Gardenrich Road 1.62 13.34 0.19 5.68 0.75

BH44 22.5174�N 88.4051�E 20/2 Hestings Sarani 1.60 13.05 0.20 5.55 0.71

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was used (Boore 2003). The corner frequency fc is computed automatically using stress

drop and moment magnitude (MW = 5.4) by SMSIM program using Brune’s relation

(1970, 1971). A filter based on diminution parameter, which is a function of magnitude, is

used to restrict high frequency component. This is also taken care of by the SMSIM

program. In this case, the considered value of fmax is very high (50 Hz).

The acceleration, velocity, and displacement time histories were computed at bedrock

level. The average density and S-wave velocity for bedrock were taken as 2.8 g/cc and

3.0 km/s, respectively. The ground motion simulated at the bedrock level for the moment

magnitude 5.4 and focal depth 36 km and epicentral distance 0 km (applicable for all the

BH locations) is shown in Fig. 5. Epicentral distance as 0 km is taken since Kolkata city

more or less lies on EHZ whose width is around 25 km, locally. The obtained PGA value at

the bedrock level is 0.084 g.

6.2 Ground motion simulation at free surface

The spectral amplification factors computed for sedimentary layers in the previous section

are used as input to SMSIM program to obtain the ground motion at the free surface. The

ground motion simulated at the free surface for BH4 location using moment magnitude 5.4

and focal depth of 36 km is shown in Fig. 6. The obtained PGA value at the free surface is

0.25 g, which is larger than 2.96 times to that at bedrock level. Similarly, the free surface

Fig. 5 Deterministically predicted acceleration, velocity, and displacement time histories at bedrock levelcorresponding to MCE (MW = 5.4) on EHZ at a depth of 36 km

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ground motion time history was computed for all the 44 BH locations using the respective

soil spectral amplification factors as input to SMSIM program.

7 Results

In the current seismic codes, there is a discrepancy concerning seismic demand and seismic

capacity of structure. The performance-based design obeys more general design philosophy

to achieve a stated performance level when it is subjected to seismic hazard. Displacement-

based design can be considered a subset of performance-based design. In displacement-

based design, displacement or drift is one of the performance criteria of structural and

non-structural systems. Therefore, maximum values of the velocity and displacement from

the time histories are also picked up in spite of maximum value of acceleration time history

for the development of maps for PGA, PGV, and PGD (Table 3). Contours of PGA, PGV,

and PGD at free surface are drawn with the help of ArcGIS packages.

7.1 Peak ground acceleration

The computed free surface PGA at different BH locations for the considered MCE

(Mw = 5.4) is given in Table 3 and Fig. 7. Lowest PGA of the order of 0.12 g is obtained in

Nager Bazar area (BH29) due to the presence of thick low velocity layer at some depth. This

effect is also inferred in Nimtala area (BH31). Largest PGA of the order of 0.6 g is observed in

Nicco Park area (BH36). Larger PGA ([0.55 g) is obtained in Rash Bihari Avenue (BH10),

Fig. 6 Deterministically predicted free surface acceleration, velocity, and displacement time histories atBH4 location corresponding to MCE (MW = 5.4) on EHZ at a depth of 36 km

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British India Street (BH16, BH28), Ashutosh Mukherjee road (BH18), Canal Circular road

(BH21), Beliaghata road (BH24), Salt Lake area (BH32), and in south Behala region (BH38).

In these regions, larger amplification in high frequency band is responsible for the obtained

higher PGA. The lower PGA (\0.3 g) is obtained in localities close to Hoogly River (BH40,

BH33, BH31, BH43, BH44, and BH39), in Kalighat area (BH19), and in Tiljala Road area

(BH4, BH17), although in these localities, there is high amplification but in lower frequency

range. At most of the BH locations, PGA is between 0.3–0.55 g.

7.2 Peak ground velocity

The variation of PGV at free surface is given in Table 3 and shown in Fig. 8. The largest

(9.65 cm/s) and lowest (3.35 cm/s) PGV were obtained in Rash Bihari Avenue area

(BH10) and in Nager Bazar area (BH29), respectively. The larger PGV ([9 cm/s) is

obtained in Rash Bihari Avenue area (BH10), IB block Salt Lake area (BH32), in South

Behala (BH28), and Gardenreach area (BH39). The larger PGV is obtained at those BH

locations where there is large amplification in the middle of the considered frequency

range. The low velocity effect is also observed at Nimtala area (BH31), where PGV is only

4.6 cm/s. At most of the BH locations, PGV is on an average between 6 and 7 cm/s.

7.3 Peak ground displacement

The obtained free surface PGD at different BH locations are given in Table 3 and Fig. 9.

Large displacement (&0.75 cm) is obtained in the northwest of Kolkata city (Baranagar area,

Fig. 7 Shows the variation offree surface peak groundacceleration (PGA) in theKolkata city

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BH33 and BH40), southwest of the city (Gardenreach, BH39 and BH43), and in Salt Lake

area (BH32, BH34). The large PGD is obtained at those locations where there is large spectral

amplification in low frequency range (\2.0 Hz). In these regions, there is more possibility of

damage of high-rise buildings during distant earthquakes. Low PGD (\0.3 cm) is obtained in

Nager Bazar area (BH29) and Nimtala area (BH31) due to low velocity layer effect. At most

of the BH locations, the observed PGD is between 0.6 and 0.7 cm.

On the basis of analysis of Table 3 and Figs. 7, 8, 9, it can be inferred that the range of

PGA variation (0.12–0.6 g) and PGD variation (0.23–1.14 cm) is comparable but much

larger than the range of variation of PGV (3.35–9.65 cm/s). All the three predicted

parameters are very low in Nager Bazaar area (BH29) and Nimtala area (BH31). PGA is

mainly controlled by larger amplification in high frequency range ([7 Hz), PGV is con-

trolled by larger amplification in middle frequency range (2–5 Hz), and PGD is controlled

by larger amplification in low frequency range (\2 Hz). It can be concluded that larger

PGA obtained at most of BH locations in Kolkata city for MCE may be due to computed

linear response of soil, large amplification due to thick and soft soil deposit, and relatively

larger amplification in high frequency range. But, at the same time on some of the BH

locations, PGA is lesser than 0.2 g because of lesser amplification in high frequency range

and large PGD due to larger amplification in low frequency range.

8 Discussion and conclusions

The scenario of simulated amplification factors in the different frequency bands revealed

that the Kolkata city is very much prone to severe damage even during a moderate

Fig. 8 Shows the variation offree surface peak ground velocity(PGV) in the Kolkata city

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earthquake. It has already been reflected in terms of unexpected intensity of the order of

VII during April 15, 1964 Kolkata earthquake (M = 5.2 and epicentral distance 106 km)

and intensity VIII during Assam earthquake of 1897 (M = 8.7 and epicentral distance

470 km) (Jhingran et al. 1969; Seeber and Armbruster 1981; GSI 2000). Unexpected

damage to multi-story buildings may occur during distant and large future earthquakes due

to double resonance effects as was observed in Ahmedabad city during Bhuj (INDIA)

earthquake of 2001 (Narayan et al. 2002) and in Mexico city during Mexico earthquake of

1985 (Romo and Seed 1986). 5–20 story buildings in Baranagar, Gardenreach, and Salt

Lake areas may suffer unexpected damage. In contrast to this, 1–2 story buildings may

suffer more damage in Rash Bihari Avenue area during local earthquakes. Very large

amplification in low frequency range in Kolkata city (Vaccari et al. 2011) and in Roorkee

city (Singh 2009), both situated in Ganga basin, has been reported.

The deterministically predicted ground motion at bedrock level due to MCE (Mw = 5.4)

on EHZ and at a depth of 36 km is 0.0844 g. The range of PGA variation in Kolkata city is

0.12–0.6 g. Large PGA greater than 0.55 g is obtained in Rash Bihari Avenue, British

India Street, Ashutosh Mukherjee road, Canal Circular road, Beliaghata road, Nicco Park

area, and in South Behala region. It seems that the Nager Bazar area and Nimtala area are

the safest region with earthquake point of view since in these areas PGA, PGV, and PGD

are very low. This may be due to the presence of thick low velocity sedimentary layer at

deeper horizon with a considerable impedance contrast on both the upper and lower

interfaces. Similar, low velocity effect on ground motion amplification was also reported

by Kumar and Narayan (2008). Finally, Kolkata city situated on the world’s largest delta

Fig. 9 Shows the variation of free surface peak ground displacement (PGD) in the Kolkata city

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island and being the main socio-political and economic nerve in the eastern part of India

needs special attention to be given by planners, engineers, and decision makers for

earthquake disaster preparedness.

Acknowledgments Authors are grateful to two unknown reviewers for their valuable comments andsuggestions, which lead to a great improvement in the original manuscript. Second author is also thankful tothe Ministry of Earth Sciences (MoES), New Delhi, for the financial assistance through Grant Number MES-484-EQD.

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