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,
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), 26th August, 1833 (Nepal), 23rd 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 MSK64 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.
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EARTHQUAKE INTENSITY SCALES
RF - 1883
MERCALLI - 1902
MM - 1931
MSK - 1964
EMS - 1998
Isoseist
OLDHAM 1899 Isoseist
Isoseismal
Isoseismal
Isoseismal
Isoseismal
I
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I
I
I
I
II
II
III
III
IV
IV
V
V
VI
VI
VII
VII
VIII
VIII
IX
IX
X
X
XI
XI
XI
XII
XII
XII
II
II
II
6 III
III
IV
IV
III IV
5
V
V V VI 4 VII
VI VI VII
VIII
3
VII VIII
IX
2
VIII
IX
X
IX
X
1 X
Fig. 1 Comparison of different Intensity Scales used for the macroseismic survey of Indian earthquakes
