The 12th International Conference of International Association for Computer Methods and Advances in Geomechanics (IACMAG) 1-6 October, 2008 Goa, India

Site Response Studies for Seismic Hazard Analysis of Kolkata City

L. Govindaraju Assistant Professor, Department of Civil Engineering, Bangalore University, Bangalore (India) S. Bhattacharya, University Lecturer in Dynamics, University of Bristol (UK) & Academic visitor, University of Oxford (UK)

Keywords: Amplification, Microzonation, Shear wave velocity, Seismic response, Response spectra

ABSTRACT: Seismic microzonation for urban areas is the first step towards a seismic risk analysis and mitigation strategy. It is essential to obtain a proper understanding of the local subsurface conditions and to evaluate the ground shaking effects for the predicted earthquake. This paper presents a methodology to study the effects of local soil conditions on the propagation of seismic motion parameters for sites where no prior earthquake data is available. The methodology uses equivalent linear method for site response analysis and wavelet-based spectrum compatibility approach to generate synthetic earthquake motions. The Megacity of Kolkata (located between 22.6o N and 88.4o E), in India has been considered to explain the methodology. Typical results are presented. The results indicate the amplification of ground motion in the range of 4.46 to 4.82 with the fundamental period ranging from 0.81 to 1.17 s. Further, the maximum spectral accelerations vary in the range of 0.777 to 0.947 g.

1 Introduction

The city of Kolkata covers an area of 185 sq. km with a population about 13 million (as per census report 2001). More than 80% of the city has built-up areas dotted with high rise residential buildings, congested business districts, hospitals and schools. Most of these were constructed without any proper town planning and are very old. Density of population in some parts around north Kolkata is over 100,000 per square kilometres (Nandy, 2007). In the past, the city had suffered damage to building by both far and near source earthquakes mostly due to the associated Himalayan tectonics. Notable among them are 1897 Great Assam earthquake, 1906 Kolkata earthquake and 1964 Contai earthquake reaching maximum intensity of VIII (MM scale) and maximum magnitude 5.7. The vulnerability to earthquake damage in this region has increased many folds with time. The state of West Bengal has a recorded history of earthquake activity dating back to the past three centuries. Most of the earthquakes occur in Himalayan ranges in the northern part of the state or deep earthquakes within the Bengal fan. Several faults have been identified in this region out of which many show evidence of movement. There are earthquake fault lines barely 100 km from Kolkata (Murty, 2006). The seismic hazard zonation map published by the Bureau of Indian Standard (IS 1893 part I: 2002) has classified the whole Indian Territory in to four zones (zones II to V) and Kolkata falls in the boundary of zone III and IV. The experiences of seismic shaking in Kolkata during the Great Shillong Earthquake of 1897, the Assam Earthquake of 1950 and recent Sikkim Earthquake of February 14, 2006 are ample proof of the seismic vulnerability of the city. Such evidences are enough ground for concern regarding the earthquake preparedness of the city. Therefore, seismic microzonation of fast expanding mega city of Kolkata falling in seismic zone IV and V is essential towards retrofitting of old and vulnerable buildings and for earthquake resistant design of new constructions and installations.

2 Seismotectonic, physiographic and geologic setting of kolkata

Though Kolkata lies in Zone III, most parts of the adjoining North and South are in Zones IV and V. The presence of major north-south and east-west faults terminating in the Burdwan, Murshidabad and Birbhum districts in the city’s backyard (only ~150 km away) adds to the seismic risk of the city. Due to the lack of systematic recordings of earthquake events in the area over a reasonable period of time, a more rational and reliable seismotectonic map is not available. Physiographically, the area represents a typical deltaic flat

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Country. Several low lying depressions in the form of shallow lakes are seen in and around the area which represent river scars of past drainage channels in the area. Geologically, the area around Kolkata City forms a part of Bengal Basin and is underlain by Quaternary sediments of fluvio-deltaic origin consisting of a succession of clay, silt and sand of varying texture from fine to coarse grain size. There are certain long narrow zones with predominant sand representing old and abandoned river channels. Besides, there are many patches of land, which has been recently filled up for construction.

3 Study area and geotechnical investigations

Originally the city of 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 in to the back swamp and marshy land to the east by way of filling up extensive areas, especially in the Salt Lake and Rajarhat areas as well in many other places in unplanned ways (Nandy, 2007). The dredged silty soil between Howrah Bridge and Vivekananda Bridge from river Ganges at a depth of about 10 m to 12 m was used to reclaim the entire land. Figure 1 shows the location map of the study area. In the present study, geotechnical bore hole data from more than 100 locations were obtained from various construction organisations and research institutions in and around Kolkata. Table 2 shows the important locations and sites from which the geotechnical data was collected. Based on the similar sub soil conditions, the four locations are categorised as Type 1, Type 2, Type 3 and Type 4. Figures 2 to 5 illustrate typical soil profiles and the corresponding average Standard Penetration (SPT) ‘N’ values for Type-1 to Type-4 locations respectively.

Howrah

KOLKATA

Bay of Bengal Hoogli River

Hoog

li Rive

r

Salt Lake City

Rajarhat(Reclaimed

NGlacier

EIIMS

Land)

Figure 1. Location map of study area

The sub soil explorations at various locations provide a wide range of geotechnical properties. The average unit weight of soil layers vary from 16 to 19 kN/m3 and plasticity index from 10% to 58%. It is found that the average annual ground water levels for Type 1 and Type 2 are 0.95 m and 2.5 m respectively from the ground surface. However, for Type 3 and Type 4, the ground water levels vary from 1 to 3 m due to seasonal variation. 4 Seismic response analysis

In the present study the parameters of interest in the seismic response analysis involves the fundamental frequency of the soil layers, amplification factor of ground motion parameters and response spectra. These parameters are evaluated based on Equivalent Linear Approach using the computer program SHAKE2000 (Idriss and Sun, 2004). The input parameter for the model such as shear wave velocity of each soil layer has been obtained from the empirical relation proposed by Japanese Road Association (Lee, 1992). To account for soil behaviour under irregular cyclic loading, the dynamic properties of soils such as modulus reduction and damping vs. shear strain curves proposed by Vucetic and Dobry (1991) were used based on plasticity characteristics of respective soil layers in Type-1 to Type-4 soil profiles.

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4.1 Selection of peak ground acceleration and input earthquake motion

Appropriate rock motions (either natural or synthetic acceleration time histories) are selected to represent the design rock motion for the site. The rock motion should be associated with the specific seismotectonic structures, source areas or provinces that would cause most severe ground motion at the site under currently known tectonic framework. In seismically active regions it may be possible to reduce earthquake damage by conducting detailed specific prediction of seismic ground motion. With the knowledge of earthquake source mechanisms and path effects, detailed ground motion at any specific site of interest may be determined without waiting for the earthquakes to occur. This approach is of great importance, especially in the regions where ground motion records of engineering interest are totally absent. In the absence of recorded motions, artificial motions can be generated.

Table 2. Geotechnical investigations in the study area

Figure 2 . Generalised soil profile and SPT ‘N’ variation for category Type 1

4.2 Peak ground acceleration

According to seismic hazard map of India, Bureau of Indian Standards (BIS), the city of Kolkata, lies in moderate seismic zone (Zone III) with a zone factor 0.16. In this area, no seismic recording stations were established and consequently no records of strong ground motion are available. Therefore, the design ground motion parameter for the study area has been obtained from Global Seismic Hazard Assessment Programme (GSHAP) map which is based on 10% probability of exceedance in 50 years. Hence the corresponding maximum peak ground acceleration (PGA) of 1.6 m/s2 (0.163 g) has been selected for Kolkata.

Location Site (s) Category

Salt lake City SOMOCO – premised, Block EP & GP and State Health System Development Project (SHSDP)

Type 1

New Town Rajarhat (Annandalok) Type 2

Close to river Ganges EIIMS Type 3

Howrah & Remaining parts of Kolkata

Garfasafuipara, Jangalpur,High Court, Marquis Street , P.K.Guha Road, Bhairab Dutta Lane, Balai Mistry Lane, Girish Ghose Lane, Narasingha Dutta Road, G.T.Road (Liluah),

Basirhat (Town Hall)

Type 4

2.9 m

12.6 m

17.8 m

37.1 m

46.0 m

60.0 m

Brownish silty clay mixed with brick pieces (Fill) Soft silty clay with decomposed vegetation (CH) Stiff bluish gray silty clay with calcareous nodules (CH-CI) Dense yellowish brown fine silty sand with mica (SM/SM-SP) Stiff brownish gray silty clay with mica and fine sand (CI-CH) Very dense gray silty fine sand with mica (SM/SP)

60

50

40

30

20

10

00 20 40 60 80 100

SPT- N

Dep

th (m

)

Type1

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4.3 Generation of input earthquake motion using wavelet based method

In our study, a wavelet-based procedure has been used for the generation of spectrum-compatible time-histories. The wavelet-based procedure uses the decomposition of recorded accelerogram in to desired number of time-histories with non-overlapping frequency contents, and then each of the time-history has been suitably scaled for matching the response spectrum of the revised accelerogram with the specified design spectrum. WAVGEN is one such computer program developed by Mukherjee and Gupta (2002) and is used in this study.

Figure 3. Generalised soil profile and SPT ‘N’ variation for category Type 2

Figure 4. Generalised soil profile and SPT ‘N’ variation for category Type 3

Silty clay with grass roots (CH) Loose brownish gay sandy silt mixed with mica (ML)

Medium dense light gray fine sand mixed with mica and varying percentages of fines (SP-SM) Medium dense / dense light gray sand mixed with mica and varying percentages of fines (SP-SM)

0.9 m

1.6 m

9.8 m

36.0 m

40

35

30

25

20

15

10

5

00 10 20 30 40 50 60

SPT - N

Dep

th (

m)

Type 2

4.0 m

37.0 m

41.0 m

10 m

Fill consisting of heterogeneous material with silty soil (CI) Medium dense bluish gray fine sandy silt (CI) Stiff to hard bluish gray clayey silt mixed with high percentage of Kankar modules

Medium dense bluish grey fine sandy silt

40

30

20

10

00 10 20 30 40 50 60 70 80

SPT - N

Dep

th (

m)

Type 3

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Figure 5 Generalised soil profile and SPT ‘N’ variation for category Type 4

Presently, as no records of strong ground motion are available in the study area, the strong motion records available in the literature are used. Based on the above, five earthquakes of magnitude (M) in the range of 6.0 to 6.9 recorded at rock sites (site class ‘A’ as per USGS classification for which shear wave velocity, Vs > 750 m/s) were selected from the literature for target response spectra matching. The selected earthquakes represent nearly the similar magnitude of 6 to 6.5 for city of Kolkata and surroundings (as per IS: 1893-2002). Table 4 illustrates five earthquake records and their characteristic obtained from the database compiled by the pacific Earthquake Engineering Research Center (PEER), University of California at Berkeley. Figure 6 shows the time history of selected strong motions. Based on the desired PGA (0.163g) as per GSHAP and target response spectra at 5% damping for rock or hard soil according to IS 1893 (part I): 2002, the program WAVGEN was used to generate spectrum-compatible time-histories. The typical response spectrum of selected earthquake motion [Northridge (ST:127 Lake Hughes #9)] before and after matching the target spectra is shown in figure 7. Figure 8 shows the spectral matched time history of accelerations which are the input earthquake motions for the site response analysis. The peak accelerations (amax) of spectral matched records have been indicated in the figure 8 for respective strong motions.

Table 4. Characteristics of selected earthquake records

Earthquake Northridge SanFernando Whittier Narrows LomaPrieta Northridge Date 17/01/1994 09/02/1971 01/10/1987 18/10/1989 17/01/1994

Recording Station (ST) 127 Lake

Hughes # 9 127 Lake Hughes

# 9

24399 Mt Wilson-CIT

Seista

47379 Gilroy Array #1

90017 LA - Wonderland

Ave

Magnitude M = 6.7 ML= 6.6 Ms= 6.7

M=6.6 Ms=7.6

M= 6.0 ML= 5.9 Ms= 5.7

M =6.9 Ms = 7.1

M = 6.7 ML= 6.6 Ms= 6.7

Peak Acceleration (amax)

0.165 g 0.157 g 0.186 g 0.209 g 0.172 g

Closet to fault rupture (km)

26.8 23.5 21.2 11.2 22.7

Closet to surface projection rupture (km)

28.9 20.2 - 10.5 -

M = Moment magnitude, ML = Local magnitude, Ms = Surface wave magnitude

5 Results and discussion

The results of the site response analysis using spectrum compatible time histories developed from five strong motion records are summarized in Table 5. Despite the wide range of peak accelerations (0.154g to 0.21g) of the spectrum compatible time histories of input motions, the maximum acceleration at the ground surface varies over a small range from 0.22 to 0.278 g for location Type-1. Similarly, the maximum acceleration at the

0.62 m

5.0 m

18.0 m

21.0 m

Brownish gray clayey silt with brick bats (Fill) Medium light brownish clay with traces of mica (CL) Soft to medium light to deep gray clayey

silt with decomposed wood and organic matter (CI-OI)

Light gray medium silty sand with traces of mica (SM)

30 m

18 m

30

25

20

15

10

5

010 2 0 30 40 5 0 60

SPT - N

Dep

th (

m)

Type 4

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ground surface for Type-2 location varies from 0.26 to 0.32g, for Type–3 location from 0.21 to 0.27g and for Type-4 location from 0.28 to 0.34g. It is to be noted that the time for maximum acceleration at the ground surface of locations Type-1 to Type-4 is not the same for all the five events. This is because the time of maximum acceleration at the surface depends on the time history of acceleration of input motion which is different from one event to the other. For practical purposes, based on all five events, an average value of peak ground acceleration (PGA) of 0.244g, 0282g, 0.237g and 0.298g can be adopted for Type-1, Type-2, Type-3 and Type-4 locations respectively. 5.1 Amplification of ground motion parameters

The maximum amplification of ground motion parameters considering five spectrum compatible events for Type-1 location is shown in figure 9 (a). As observed from this figure, the maximum amplification ratio (A) varies over a small range of 4.1 to 5.0 for the corresponding frequency (f) range of 0.75 to 0.87 Hz. Similarly, the maximum amplification ratio with frequency range due to five events for locations Type-2 to Type-4 can be obtained from the table 5. These results clearly demonstrate that the amplification factor at Kolkata is significant. 5.2 Response spectra

Figure 9 (b) shows the variation of spectral acceleration (Sa) with period corresponding to 5% damping for the location Type-1 based on five earthquakes. It can be noticed from this figure 9 (b) that the maximum spectral acceleration varies from 0.8 g to 1.1 g. Similarly, the maximum spectral accelerations in the range of 0.79 g to 1.16 g, 0.64g to 0.92g and 0.81g to 1.19g can be obtained (see Table 5) due to five events for remaining locations Type–2, Type–3 and Type–4 respectively. However, based on the average of all five earthquakes, the maximum spectral accelerations of 0.947g, 0.911g, 0.777g and 0.943g may be obtained for respective locations.

Figure 6. Time history of selected strong motion records

0 5 10 15 20 25 30 35 40-0.2

-0.1

0.0

0.1

0.2

amax

= 0.165 g

Acc

eler

atio

n (g

)

Time (s)

Northridge (ST:127 Lake Hughes # 9)

(a)

0 5 10 15 20 25 30 35-0.2

-0.1

0.0

0.1

0.2

amax

= 0.157 g

Acc

eler

atio

n (g

)

Time (s)

Sanfernando (ST:127 Lake Hughes # 9)

(b

0.3

0 10 20 30 40-0.2

-0.1

0.0

0.1

0.2

amax= 0.186 g

Acc

eler

atio

n (g

)

Time (s)

Whittier Narrows (ST:24399 Mt Wilson-CIT Seista)

(c)

0 5 10 15 20 25 30 35 40-0.3

-0.2

-0.1

0.0

0.1

0.2

0.3

amax

= 0.209 g

Acc

eler

atio

n (g

)

Time (s)

Loma Prieta (ST:47379 Gilroy Array # 1)

(d

0 5 10 15 20 25 30-0.2

-0.1

0.0

0.1

0.2

amax = 0.172 g

Acc

eler

atio

n (g

)

Time (s)

Northridge (ST:90017 LA-Wonderland Ave)

(e)

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Figure 7. Comparison of target and response spectrum of Northridge (ST: 127 Lake Hughes # 9) earthquake time history

Figure 8. Target spectra matched time history of acceleration

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.00.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8 Target spectra (IS:1893-2002, Rock or Hard soil)

Spectra of original motion (Northridge,ST:127 Lake Hughes # 9)

Spectra of modified motionS

pect

ral A

ccel

erat

ion

(g)

Period (s)

0 5 10 15 20 25 30 35 40-0.2

-0.1

0.0

0.1

0.2

amax

= 0.154 g

Northridge (ST:127 Lake hughes # 9)

Acc

eler

atio

n (g

)

Time (s)

0 5 10 15 20 25 30 35-0.2

-0.1

0.0

0.1

0.2Sanfernando

amax= 0.172 g

Acc

eler

atio

n (g

)

Time (s)0.2

0 5 10 15 20 25 30 35 40-0.2

-0.1

0.0

0.1

0.2

amax

= 0.167 g

Acc

eler

atio

n (g

)

Time (s)

Whittier Narrows

0.2

0 5 10 15 20 25 30 35 40-0.2

-0.1

0.0

0.1

0.2

amax

= 0.158 g

Loma Prieta

Acc

eler

atio

n (g

)

Time (s)

Time (s)

0 5 10 15 20 25 30

-0.2

-0.1

0.0

0.1

0.2 Northridge (ST:90017 LA-Wonderland Ave)

amax = 0.21 g

Acc

eler

atio

n (g

)

Time (s)

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Figure 9 . Variation of amplification ratio and spectral acceleration at location Type - 1 (damping 5%)

6 Concluding remarks

Site–specific seismic response analysis of soil deposits for the city of Kolkata is carried out. Physiographic and geologic settings, seismotectonics and seismicity of the study area were outlined. The peak ground acceleration for rock sites in the study area was obtained from Global Seismic Hazard Assessment Programme (GSHAP) map. Spectrum compatible time histories of acceleration as input motions were developed from wavelet based target spectrum matching technique. Site response analysis was performed using equivalent linear method. From the results of the study, it has been shown that the alluvial deposits in Kolkata have tendency to increase the amplification ratio of acceleration from 1.34 to 1.73 and amplification of ground motion parameters in the range of 4.46 to 4.82. The maximum spectral acceleration at four locations in Kolkata varies in the range of 0.777g to 0.947 g. This aspect must be taken in to account in the study area while designing the infrastructures for earthquake forces.

Table 5. Summary of the results of site response analysis

7 Acknowledgement

The authors are thankful to Prof. Vinay K.Gupta, Department of Civil Engineering, Indian Institute of Technology, Kanpur (India) for his valuable suggestions and help in development of synthetic earthquake motions for our studies.

0 3 6 9 12 150

1

2

3

4

5

6 Type-1

Northridge (ST:127) SanFernando Whittier Narrows Loma Prieta Northridge (ST:90017)

A = 4.1 - 5.0f = 0.75 - 0.87 Hz

Am

plifi

catio

n R

atio

Frequency (Hz)

(a)

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.00.0

0.2

0.4

0.6

0.8

1.0

1.2 Type - 1

Northridge (ST:127) Sanfernando Whittier Narrows Loma Prieta Northridge (ST:90017) Average

Sa = 0.8 - 1.1g

Spe

ctra

l Acc

eler

atio

n (g

)

Period (s)

(b)

Parameter Type - 1 Type - 2 Type - 3 Type - 4

Range 0.22 - 0.278 0.26 – 0.32 0.21 – 0.27 0.28 – 0.34 Peak ground surface acceleration (g) Average 0.244 0.282 0.237 0.298

Range 4.1- 5.0 4.4 – 4.9 4.1 – 4.7 4.4 – 5.1 Maximum amplification of ground motion parameters

Average 4.5 4.64 4.46 4.82

Range 0.75 – 0.875 1.13 – 1.25 1.0 – 1.13 1.13 – 1.25 Frequency for maximum amplification (Hz) Average 0.85 1.23 1.10 1.23

Amplification factor of acceleration based on site response analysis

1.42 1.64 1.34 1.73

Range 0.80 – 1.10 0.79 – 1.16 0.70 – 0.92 0.81 – 1.19 Maximum spectral acceleration (g) Average 0.947 0.911 0.777 0.943

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8 References

Chatterjee, R.S., Benedicte Fruneau., Rudant, J.P., Roy, P.S., Pierre-Louis Frison., Lakhera, R.C., Dadhwal, V.K. and Ranajit Saha. 2006. Subsidence of Kolkata (Calcutta) city, India during the 1990s as observed from space by Differential Synthetic Aperture Radar Interferometry (D-InSAR) technique , Remote Sensing of Environment, 102, Issues 1-2, 176-185.

IS 1893 (Part 1) : 2002. Criteria for earthquake resistant design of structures , Bureau of Indian Standards Manual on SHAKE2000.“A computer program for the 1-D analysis of geotechnical earthquake engineering problems”,

University of California, Berkeley.

Murty,C.V.R. 2006. http://www.telegraphindia.com/1060113/asp/propertt/ story_5714487.asp

Nandy, D. R. 1994. Earthquake hazard potential of central and south Bengal Basin, Indian Journal of Earth Sciences , 21(2), 59-68.

Nandy, D.R. 2007. Need for seismic microzonation of Kolkata megacity, Proceedings of workshop on microzonation, Indian Institute of science, Bangalore, India, June 26-27.

Sushovan Mukherjee and Vinay K. Gupta. 2002. Wavelet-based generation of spectrum-compatible time-histories, Soil Dynamics and Earthquake Engineering, No.22, 799-804.

Shannon Hsien-Heng Lee.1992. Analysis of the multicollinearity of regression equations of shear wave velocities, Soils and Foundations , No. 32, 205–214.

Vucetic, M. and Dobry, R. 1991. Effect of soil plasticity on cyclic response, Journal of Geotechnical Engineering, ASCE, 117(1), 89-107.

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