Seismic Hazard Analysis for Surat City and Its Surrounding

Region, Gujarat

Thaker, T.P. Rao, K.S. Gupta, K.K.Research Scholar Professor Associate Professor

e-mail: tej_p_thaker@yahoo.co.in e-mail: raoks@civil.iitd.ac.in e-mail: kkg@civil.iitd.ac.in

Department of Civil Engineering, IIT Delhi, Hauz Khas, New Delhi

ABSTRACT

In the present paper, an attempt has been made to estimate seismic hazard at bedrock level in terms of PGA using

state of art probabilistic seismic hazard analysis. A detailed catalogue of historical and recent seismicity within

350 km radius around the city has been compiled and new seismotectonic map has been generated for the region.

The completeness of the data should be check before carryout hazard analysis so subsequently completeness has

been checked using cumulative visual interpretation technique. Finally earthquake data analyzed statistically

and the seismicity of the region around Surat city has been evaluated by defining ‘a’ and ‘b’ parameters of

Gutenberg- Richter recurrence relationship. Finally, probabilistic hazard maps corresponding to 10% and 2%

probability of exceedance in 50 years have been developed for the region.

Indian Geotechnical Conference – 2010, GEOtrendz

December 16–18, 2010

IGS Mumbai Chapter & IIT Bombay

1. INTRODUCTION

In recent years, the interest of the scientific community

regarding seismology and seismotectonic has greatly

increased in Peninsular India (PI), especially in the field

related to seismic risk assessment of urban seismic areas

and its possible reduction. This is due to the fact that PI

has experienced many strong to moderate earthquakes since

18th century. Surat is the port city located in PI on the bank

of river Tapi. As of 2007, Surat and its metropolitan area

has a population of more than five million. In recent time

city has emmerged as a hub for chemicals, minerals, textiles,

engineering, oil and post based industries. Surat has

infrastructure and conducive environment for industrial

growth, presence and contribution of these industries make

the city, one of the most industrialised cities of India. The

city is located nearly 350km away from the epicentre of

2001 Bhuj earthquake (Mw 7.7), and unexpectedly received

considerable damages. The seismic zoning map of India

(BIS: 1893-2002) marks the region in zone III, specifying

thereby basic peak ground acceleration as 0.16. Even for

the moderate earthquake, it is vulnerable because of the

presence of varied kind of structures founded on soft soils

of varied thicknesses and high water table as it is in the

high flood Zone of Tapi River. In addition of late, the river

valley is being filled up due to the large number of

constructions with loose/unconsolidated soils, which may

result in high ground motions.

Considering above, Seismic hazard assessment for

Surat region is very essential in order to mitigate the effects

of future earthquakes. Present study focus on the

probabilistic seismic hazard analysis in terms of Peak

Ground Acceleration (PGA) considering known seismic

sources selected by preliminary deterministic hazard

analysis. Finally, probabilistic hazard maps for Surat region

(latitude 21002’00" to 21016’00" and longitude 72036’00"

to 72055’00") corresponding to 10% and 2% probability of

exceedances in 50 years have been developed for the region.

2. SEISMICITY OF THE REGION

Gujarat is one of the most seismic prone intercontinental

regions of the world. It has experienced two large

earthquakes of magnitude Mw 7.8 and 7.7 in 1819 and

2001 respectively and seven earthquakes of magnitude

seven earthquakes of Mw 6.0, during the past two centuries.

The intense aftershock activity of 2001 Bhuj earthquake is

still continuing. Through March 2008, 14 aftershocks with

Magnitude 5.0-5.8, about 200 aftershocks with magnitude

4.0-–4.9, about 1600 aftershocks of with magnitude 3.0-

3.9, and several thousand aftershocks with magnitude < 3

have been recorded. Regional seismicity has also increased

with Mw 5.0 earthquakes and associated foreshocks and

aftershock sequences (Thaker et al.2010).

Surat, which is at around 350 km from the seismically

active sources of Kutch receives unexpectedly a considerable

164 T.P. Thaker, K.S. Rao and K.K. Gupta

damages from 2001 Bhuj earthquake of Mw 7.7 and lies

in seismic zone III according to IS 1893:2002, and may

experiences high seismic activity from the far field sources.

Surat is considered to in an earthquake prone area and has

been shaken by several time since historical time. Cambay,

Rann of Kutch and Narmada- Tapti regions are recognised

as one of the most active regions of the Peninsular India.

Jaiswal and Sinha (2008) reported that most of the historical

earthquakes in Peninsular India have been concentrated

near the weak rifting zones (e.g., Rann of Kutch, Narmada

lineament)

3. SEISMOTECTONIC MODEL

Seismotectonic Atlas (SEISAT, 2000) which is generated

by Geological Survey of India (GSI) has been used for

identification of seismic sources in the study area. Total of

17 sources have been identified in an area of 350km radius

around the city. Earthquake data collected from various

agencies and available literature have been superimposed

on the base map along with all tectonic sources. Figure 1

shows the seismotectonic model for Surat region.

Fig. 1: Seismotectonic Model

The earthquake catalogue for this area was prepared

by combining and consolidating the available information

from different sources and covers the time period 1820-

2008. The earthquake data were collected from different

sources, i.e., Geological Survey of India (GSI), Indian

Meteorological Department (IMD), International

Seismological Centre (ISC), National Geophysical Research

Institute (NGRI), Gujarat Engineering Research Institute

(GERI), Institute of Seismological Research (ISR), and

United States Geological Survey (USGS). In addition to

that, a few more data were collected from the catalogues

published by different researchers like Chandra (1977);

Oldham (1883); Malik (1999) and Jaiswal and Sinha

(2005).

The earthquake events collected are about 303 with

minimum moment magnitude of 3.0 and maximum of 7.7.

After removing the dependent events, the data set contains

150 events between magnitude 3 to 3.9, 111 events from 4

to 4.9, 36 events from 5 to 5.9 and 4 and 2 events of between

magnitudes 6 to 6.9 and more than 7.0 respectively.

4. SEISMIC HAZARD ANALYSIS

The Seismic hazard analysis refers to the estimation of some

measure of the strong earthquake ground motion expected

to occur at a selected site. This is necessary for the purpose

of evolving earthquake resistant design of a new structure

of importance. One important application of hazard

analysis is the preparation of seismic zoning maps for

generalized applications. Present study deals with the

estimation of peak ground acceleration based on the two

state of art methods of seismic hazard analysis, viz.,

deterministic and probabilistic.

5. DETERMINISTIC ESTIMATION OF PGA

MODEL

The deterministic approach for seismic hazard analysis is

not well documented in literature, and it is practiced

differently in different part of the world and even in different

application areas (Gupta, 2002). In the present analysis

Deterministic Seismic Hazard Analysis (DHSA) has been

carried out for entire Surat region considering the seismic

events and Seismotectonic sources from the newly

developed seismotectonic model for the region around

350km around the city. The maximum possible earthquake

magnitude for each of the seismic sources within the area

has been estimated. Shortest distance to each source and

site of interest has been evaluated and taken as major input

for conducting DHSA. The number of earthquake sources

on which earthquake magnitude greater than 4.0 moment

magnitude have occurred are 16 faults and lineaments

which are listed in Table 1. The entire region is divided in

to the grid of 1’ each. Attenuation relationship developed

by Iyenger and Raghukanth (2004) considered for the

analysis and PGA has been calculated at each grid cross

section. Maximum value of PGA has been taken amongst

the PGA calculated by various source at each point. finally

deterministic PGA model has been generated for the entire

region, which is given in Fig. 2. Focal depth of 10 km is

arrived by considering the past events of earthquake in the

region.

6. PROBABILISTIC ESTIMATION OF PGA

MODEL

Probabilistic seismic hazard analysis (PSHA) was initially

developed by Cornell (1968), which incorporates the

influence of all potential sources of earthquakes and the

activity rate assigned to them. There after many researchers

have adopted this methodology for evaluating hazard and

recently this method has been adopted by Iyengar and

Seismic Hazard Analysis for Surat City and Its Surrounding Region, Gujarat 165

Ghosh (2004), Raghu Kanth and Iyengar (2006),

Ambazhagan et al. (2008), and Vipin et al. (2009) for the

probabilistic seismic hazard analysis of Delhi, Mumbai,

Bangalore and Peninsular India respectively. The procedure

for carrying out PSHA has been demonstrated by Kramer

(1996) is adopted for the present analysis. Based on the

deterministic analysis, total of five sources which produce

PGA more than 0.01g has been selected of probabilistic

analysis which is maked bold as shown in Table 1. Seismic

activity of the region is characterized by recurrence

relationships. In the present investigation truncated

exponential recurrence model developed by Mcguire and

Arabasz (1990) and is given by following expression;

(1)

Where β=2.303b and Ni(m

o) is the weightage factor for a

particular source based on recurrence. Threshold value of

magnitude 4.0 as adopted in the study.

Table 1: Estimation of PGA: Deterministic Approach

Fig. 2: Deterministic PGA Model

Mulargia and Tinti (1985) proposed a method for

calculating completeness interval based on the

Cummulative Visual Interpretation (CUVI) technique has

been used to estimate seismicity parameters ‘a’ and ‘b’.

Figure 3 shows the seismicity parameters along with

seismicity deaggregated to each zone. Finally, probabilistic

hazard maps for Surat region corresponding to 10% and

2% probability of exceedances in 50 years have been

developed for the region which is shown in Fig. 4 and 5.

Fig. 3: Regional Recurrence Along with Deaggregation

As the local soil plays a significant role on the ground

motion parameters. Hence Uniform hazard response spectra

considering all classes (BSSC, 2001) have been developed

for the region for above mentioned probabilities. UHRS of

10% and 2% probabilities in 50 years have been shown in

Fig. 6.

Fig. 4: PGA Corresponding to 10% Probability in 50 Years

Fig. 5: PGA Corresponding to 2% Probability in 50 Years

166 T.P. Thaker, K.S. Rao and K.K. Gupta

7. RESULTS AND DISCUSSION

In the present study, the seismic hazard analysis was carried

out for the establishment of PGA at bed rock level for Surat

city based on deterministic approach also attempt has been

made to evaluate the seismic hazard in terms of PGA and

SA at rock level and PGA at ground level, based on different

site classes for Surat city based on probabilistic hazard

analysis. There are no previous engineering studies in open

literature on probabilistic seismic hazard analysis covering

Surat region. Hence, results of the present study are

compared with the studies reported in the literature on the

Peninsular India and also with IS code.

Fig. 6: UHRS for 10% and 2% Probability of

Exceedances in 50 Years

Seeber et al. (1999); Raghukanth and Iyenger (2004);

Ambazhagan et al. (2008); Jaiswal and Sinha (2008) and

Vipin et al. (2009) carried out a study for different region

of Peninsular India and reported b value in the range of

0.80 to 0.89. These values are nearly same as that obtained

here for the control region around Surat city.From the

Deterministic analysis it has been found that PGA value at

rock level varying between 0.09 to 0.29g, Hazira port region

at south western side receiving max. PGA up 0.18 to 0.19

g which much higher than the value reported in the IS

code for this region. From the probabilistic analysis surface

level PGA reported for the region. The PGA value for 10%

probability of exceedance in 50 years for Surat city,

considering different sites varies from 0.100 g to 0.210 g,

and 0.138 g to 0.335 g for 2% probability in 50 years which

is again much higher than reported in the IS 1893:2002.

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