IGCP- 490 ‘Holocene Catastrophes’

Field workshop

17-19 August 2005, Chennai

REEVALUATION OF EARTHQUAKE TRIGGERED DAMAGES

IN AND AROUND KOLKATA

Sujit Dasgupta

Geological Survey of India

27 J.N. Road

Kolkata- 700 016

Catalogue of pre-instrumental, historic and pre-historic earthquakes for any area is one of

the vital inputs for seismic hazard assessment for the region. Recorded documents

indicate that though Kolkata has not suffered major damages from earthquakes during the

last 300 years, there are many records of felt earthquakes, and some have also triggered

moderate level of damages. As per the Earthquake Catalogue of India (T. Oldham, 1883)

at least 30 earthquakes were felt at Kolkata during the period from 1803 to 1869. During

the following years of the nineteenth century there are further records of felt earthquakes

from Calcutta and earthquakes for the subsequent instrumental era are relatively better

located for Calcutta. While many of these earthquakes are of ‘far source’, some of them

are definitely of ‘near source’ origin. Based primarily on the seismic history, Seismic

Zoning Map of India prepared by the Bureau of Indian Standards (2002) indicate that

Kolkata is located at the border of seismic Zone III and IV; hence for all practical

purpose Kolkata comes under seismic zone IV.

To start this reevaluation and reconciliation we would like to look back the two recorded

events of the 18th

Century, of 11th

October, 1737 and 2nd

April, 1762. For the former

event Bilham (1994) and Nandy (1994) have already argued that evidence available do

not favor a major earthquake but the catastrophe though briefly described cannot be

overlooked and underestimated, particularly after witnessing the series of events

following the 26th

December 2004 Sumatra earthquake. Even today Press reports

overstate damage and casualty figures in the aftermath of a major disaster. The original

report says ‘300,000 souls are said to have perished!’ Though population of Kolkata in

the year 1752 was of the order of 117,742 and in 1737 it must have been far less, this

mismatch cannot be the sole criteria to underestimate the disaster. Let us consider the

other descriptions (Oldham, 1883). There happened a furious hurricane at the mouth of

Ganges which reached 60 leagues (285 km) up the river; the water rose 40 feet (12m)

higher than usual in the Ganges; barques (vessels) of 60 tons were blown 2 leagues (9.5

Km) up the river; two hundred houses were thrown down along the river Ganges; 20,000

ships including boats, canoes etc have been cast (throw off, lose) away; eight of nine

ships lost etc. These descriptions do not indicate a simple hurricane; rather suggest a

tsunami due to a strong distant earthquake that may have originated in the Bay of Bengal,

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could be from the region between coastal Burma and the Andaman Island. Further

research is warranted on this event as this is related to the seismic and tsunami hazard of

Kolkata.

Let us look into the next recorded major earthquake of 2nd

April 1762. A very destructive

and violent earthquake felt all over Bengal and Arakan (Burma); Chittagong suffered

very severely; great explosions heard and opening in the earth formed with water spouted

like fountain; earth continued to sink day by day; 60 sq miles permanently submerged;

two volcanoes (mud volcano) said to have opened on the Sita Kunda hills; in Calcutta

water tank rose by 6 feet (1.8 m; seiches); at Ghirotty (Gorhatty), 18 miles (28 km) above

Calcutta river rose more than 6 feet (1.8m); at Dacca water rose so suddenly as to carry

up hundreds of boats, and many lives were lost. Captain Halsted visited coastal Arakan

for survey work in 1841. He documented several uplifted coastal area (13 feet at

Terribles; 22 feet at northwest of Cheduba; 9 feet at Foul island etc.) that have been

related to the earthquake. This earthquake and the sequence of events described also calls

for further research.

Comparing the effects of the 26 December 2004 earthquake, I suggest both these events

of 1737 and 1762 were major earthquakes originating at the Indian plate margin from the

Coastal Burma region that generated tsunami and had impact in the up-streams of the

Ganges at least up to Kolkata and Dacca.

Among the 30 odd felt earthquakes of the nineteenth century between 1803 and 1869,

some of the strong far source earthquakes that caused damage to Kolkata are those from

1st September, 1803 (Mathura/ Nepal), 26

th August, 1833 (Nepal), 23

rd March, 1839

(Burma), 11th

November, 1842 (Bengal- Assam) and 10th

January, 1869 (Cachar, Assam).

The 31st December 1881 Nicobar earthquake that generated tsunami in the Andaman

Islands was also strongly felt at Kolkata. Another major far source 19th

century

earthquake that was widely felt at Kolkata is the great earthquake of 12th

June 1897. The

epicenter of this earthquake was 470 km towards N35E from Calcutta. Considerable

damage including partial collapse of a number of buildings was reported from Calcutta.

Calcutta Town hall, High Court, St Thomas Church, Loreto Convent etc was damaged.

Oldham (1899) assigned intensity 3 (Oldham Scale) to Calcutta which is equivalent

intensity VII in EMS/MSK scale.

During the 20th

Century with the advent of instrumental seismology earthquake source

regions are relatively better located. The Srimangal earthquake of 8th

July 1918 was

located some 350 km N55E from Calcutta. It was felt by nearly everyone indoors in

Calcutta. Ominous cracks appeared in many new and old buildings of Calcutta due to this

earthquake. Stuart (1926) following Oldham scale assigned isoseist 5 (later changed to 6)

at Calcutta, which is equivalent to intensity IV-V as per EMS/MSK scale. Similar grade

damage (isoseist 5 of Oldham Scale ≈ intensity IV-V of EMS/MSK Scale) was reported

(Gee, 1934) from Calcutta due to Dubhri earthquake 3rd

July, 1930 which was located

some 360 km towards N20E from Calcutta. The Bihar-Nepal earthquake of 15th

January,

1934 which was located 480 km N20W of Kolkata, also caused substantial damage to

buildings. At 2:40 PM Calcutta time the earthquake was felt for about 5 minutes.

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Considerable movement in lake water, damage to St Paul’s Cathedral reported. Dunn et

al (1939) assigned intensity of VI in Mercalli Scale (≈ VI-VII of EMS/MSK) for

Calcutta.

Near source earthquakes located within 100 km of Calcutta being mostly of moderate to

low size do not give well constrained locations particularly for events prior to 1964.

Considering documented records of many felt earthquakes of the nineteenth century that

indicate high frequency response at Calcutta accompanied by sounds, point towards its

near source. Nevertheless except two 20th

century earthquakes, documented records of

earthquake damage for Calcutta are not available. In the earthquake of 29th

September,

1906, a number of buildings at Calcutta suffered damage in the form of development of

serious cracks. The isoseismal map based on Rossi-Forel scale (Middlemiss, 1908)

indicates maximum intensity of VI-VII in and around Calcutta which is equivalent to

intensity V-VI of EMS- 98/MSK- 64 Scale. The earthquake of 15th

April, 1964 whose

epicenter was 100 km south of Calcutta, was more damaging with the development of

serious gapping cracks and fall of plaster in many old and new buildings. Jhingran et al

(1969) assigned intensity VI in Mercalli scale at Kolkata which is equivalent to intensity

VI-VII in EMS/MSK scale.

From these accounts it is noticed that different intensity scales were used to draw

isoseismal maps for different earthquakes and there is an urgent need to re-evaluate

original damage data of each events and prepare fresh maps based on EMS-98 or MSK-

64 scale for comparison and using it for inputs for seismic hazard assessment of Calcutta.

Nevertheless, till it is attempted, from visual comparison of different intensity scales (Fig

1), it is observed that maximum intensity documented at Calcutta is VII on EMS/MSK

scale which was recorded from both the near source earthquake of 1964 and distant

earthquakes of 1897 and 1934. From this evaluation of intensity an average peak velocity

of 8-12 cm/sec and acceleration of 0.10g- 0.15g (100-150 cm/sec2) can be expected at

Calcutta due to distant and near source earthquakes. A recent estimate (GSHAP, 1999),

also indicate that Calcutta could expect earthquake inflicted PGA values in the range of

0.08g to 0.13g, in next 50 years with 10% probability. Another issue that need be

considered for seismic design of structures is the predominant frequency of seismic

waves that will excite Calcutta due different earthquakes. Though it is subject of research

including considering scenario earthquake with different source- path- site situation, in

general high frequency content of seismic waves will be predominant for near source

events while long period waves will travel through Calcutta from distant earthquakes.

Except for the Bihar- Nepal earthquake of 1934 all other far source events of the 20th

century (also the Shillong earthquake of 1897) that affected Calcutta are of magnitude

less than 8.0. These source regions in the Himalaya and northeastern India are capable of

generating earthquakes of magnitude above 8.5. In such scenario damage intensity at

Calcutta would likely be VIII + in EMS/MSK scale. And an event of the size of the 26

December, 2004 within 500 km from Calcutta either from the north/ northeast (Himalaya/

Shillong Plateau)) or from the southeast (coastal Bangladesh/Burma) could be something

for which no one in Calcutta is perhaps prepared.

EARTHQUAKE INTENSITY SCALES

RF - 1883 OLDHAM -

1899

MERCALLI - 1902 MM - 1931 MSK - 1964 EMS - 1998

Isoseist

I

II

III

IV

V

VI

VII

VIII

IX

X

Isoseist

7

6

5

4

3

2

1

Isoseismal

I

II

III

IV

V

VI

VII

VIII

IX

X

Isoseismal

I

II

III

IV

V

VI

VII

VIII

IX

X

XI

XII

Isoseismal

I

II

III

IV

V

VI

VII

VIII

IX

X

XI

XII

Isoseismal

I

II

III

IV

V

VI

VII

VIII

IX

X

XI

XII

Fig. 1 Comparison of different Intensity Scales used for the macroseismic survey of Indian earthquakes