Articles
2
by Harsh Gupta
Mega-Tsunami of 26th December, 2004: Indian
initiative for early warning system and mitigation
of oceanogenic hazards*
Department of Ocean Development, ‘Mahasagar’ Bhavan, CGO Complex, Lodi Road, New Delhi – 110 003, India.
(E-mail: [email protected])
The 26th December 2004 earthquake of Mw 9.3 is the
second largest earthquake ever to have been recorded.
This generated a tsunami which affected several Asian
countries. In India, the Andaman & Nicobar group of
islands, and coastal states of Tamil Nadu, Andhra
Pradesh and Kerala were severely affected. Here, we
briefly provide an outline of the approach taken by India
for an early warning system for mitigation of
oceanogenic disasters.
Introduction
The December 26, 2004 Sumatra earthquake was so far given a magnitude of Mw 9.0, and was thought to be the 5th largest recorded
earthquake. However, a very recent publication by Stein and Okal
(2005) has estimated, based on longest period normal modes of the
earth excited by the earthquake, that the earthquake had a moment of
1.3 x 1030 dyn-cm, and a magnitude of Mw 9.3, making it the 2nd
largest ever recorded earthquake. According to latest compilations,
the source was about 1200 km in length, and an estimated displacement of 13 m. The earthquake and the tsunami caused by it created a
havoc in several Asian countries. A number of photographs and
description of the damage have appeared in the media. A detailed,
technical evaluation is being carried out by a dedicated team of scientists and engineers for the Indian region, and results would be
reported later.
Source region and aftershocks of the
earthquake
Figure 1 (data from USGS) shows the distribution of main shock and
aftershocks which are still continuing. Following are the focal parameters of the main shock and its largest aftershock:
Date:
26th December, 2004
Origin Time
06 Hours 29 Minutes of IST**
Magnitude
Mw 9.3
Epicentre
Latitude: 3.7 degree North
Longitude: 95.0 degree East
Region
Off west coast of Sumatra Island
(Indonesia)
Figure 1 Location of the 26th Dec 2004 Java earthquake extension
of aftershocks from Java epicenter to Andaman and Nicobar group
of islands, and several major aftershocks. (Data from USGS).
Soon after the Sumatra earthquake, an earthquake of Mw 7.3
occurred off coast of Great Nicobar Island. In the morning of Dec 26,
2004, originally it was thought to be an independent earthquake.
However, it soon became clear that it was an aftershock of the Sumatra event, with the following parameters:
*This article is based on development of the project by Department of Ocean Development, Government of India, with Department of Science & Technology,
Department of Space, and Council of Scientific & Industrial Research being major partners in this initiative.
**All times are given in IST (Indian Standard Time) which is 5:30 hrs ahead the Universal Time.
March 2005
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Largest aftershock
Date:
Origin Time
Magnitude
Epicentre
Region
Early Warning System for tsunamis
26th December, 2004
09 Hours 52 Minutes of IST**
Mw 7.3
Latitude: 7.3 degree North
Longitude: 92.7 degree East
Off coast of Great Nicobar
The earthquake activity is still continuing, and so far more than
200 aftershocks of M > 5 have occurred. However, for such a large
earthquake, aftershocks are expected to continue for years. It is interesting to see in Figure 2, after Stein and Okal (2005), the aftershock
zone, and fast and slow slips inferred from body waves and normal
modes.
If one looks at the distribution of earthquakes globally, more than
75% of earthquake energy is released in the circum-Pacific belt,
about 20% in the Alpine-Himalayan belt, and remaining 5% through
the mid-oceanic ridges and other stable continental region earthquakes. Figure 4 shows the major plate boundaries and distribution
of earthquakes. Due to the frequent occurrence of tsunami on the
coasts of the Pacific rim countries, a Consortium came into existence
in 1964 for warning the populations around the Pacific coast countries, in advance, about the arrival of tsunami waves at specific locations. Known as the “Tsunami Warning System in the Pacific”
(TWPS), it is supported by 26 Pacific rim countries.
As far as the Indian coastline is concerned, the areas which
could possibly generate tsunamigenic earthquakes are the extension
of Java-Sumatra earthquake belt into Andaman & Nicobar, and some
areas in the Arabian Sea (possibly extension of the faults responsible
for the 1819 Kuchh and 2001 Bhuj earthquakes into the Arabian Sea
and the Makran coast — Figure 4). Tsunami is not a common phenomenon on the Indian coasts. Murty et al (1999) have listed six
tsunamis dating back to 326 BC (Table 1). Out of these, definite evidence exists for the August 27, 1883 tsunami as a consequence of the
Krakatoa volcanic eruption.
Table 1 Tsunamis in the North Indian Ocean (after Murty et al,
1999).
Date
Figure 2 The diagram shows a comparison between the
aftershock zone with the minimum area of slow slip inferred
from normal modes (after Stein and Okal, 2005).
Tsunami
The tsunami generated by the Sumatra earthquake is probably the
worst in the total recorded history of tsunamis globally. The tsunami
devastated several countries. The toll was very heavy in Indonesia,
Sri Lanka, India and Thailand. The estimated loss of human lives has
exceeded 2,00,000, and the figures of economic and financial losses
are still being estimated. As far as Indian coasts are concerned, an
amplitude of 4.5 m was estimated at Port Blair and 3.2 m at Chennai.
The Mt for the tsunami is estimated to be around 9.5. Figure 3 gives
the Tide-Gauge recordings of this tsunami at Port Blair, Chennai and
Cochin (please see Figure 5 for the location of these cities). At Port
Blair it arrived at 07:14 hrs IST, at 09:06 hrs IST at Chennai, and at
11:12 hrs IST at Cochin. It may be noted that at Port Blair, there was
a shift of about 90 cm in the median line. This is an evidence of the
sinking of the base where the tide gauge was located, by 90 cm.
Detailed geodetic surveys now being carried out are confirming the
same.
Episodes, Vol. 28, no. 1
Remarks
326 BC
Army of Alexander the Great suffered
Between 1st April
and 9th May 1008
Tsunami on the Iranian coast from a
local earthquake
27th August 1883
Krakatoa: 1.5 m tsunami at Madras, 0.6 m
at Nagapattinam, 0.2 m at Arden
1884
Earthquake in the western part of the Bay
of Bengal, Tsunamis at Port Blair, Dublet
(mouth of Hooghly River)
26th June 1941
8.1 quake in the Andaman Sea at 12.9ºN,
92.5ºE. Tsunamis on the east coast of India
with amplitudes from 0.75 to 1.25 m
27th November 1945
8.25 quake 70 km south of Karachi at
24.5ºN, 63.0ºE. Tsunami amplitude
at Kutch was 11.0 to 11.5 m.
An often asked question after the tragedy of the 26th December
tsunami is that why it was not observed by the data buoys deployed
in the Bay of Bengal. It is important to note that the ocean surface is
not quiet at any given time, and in open oceans, waves of several
meters are a common happening. However, these waves have no
signature when you go to depths of 200 m or so. At the bottom of the
ocean, it is very quiet. However, when a tsunami is generated, the
whole column of water gets affected and as this disturbance moves,
a transducer set at the ocean bottom can detect a tsunami which
could not be done by ocean surface data buoys.
What is being done for India
Tsunami is one among the most destructive coastal hazards in the
world. 85 % of the Tsunamis occur in the Pacific Ocean; the
Mediterranean and Caribbean Seas have histories of some locally
destructive Tsunamis. The Indian Ocean region has been scarcely
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Bay of Bengal and 10 Cyclones in the Arabian Sea,
resulting in Storm Surges of even 7–8 metres of
height.
The recent Indian Ocean Tsunami (December 26,
2004), considered to be one of the strongest in the
world for the past 40 years, resulted in devastations
amounting to national calamities in several parts of the
Indian Ocean. As compared to the most severe
Tsunamis occurred over the last 30 years in the Pacific,
the loss of lives in the Indian Ocean Tsunami has been
higher by several orders of magnitude thereby calling
for development of Tsunami Warning System on a
war-footing.
The coastal population being the victims of
Storm Surges and Tsunami, it is obvious that the systems for their mitigation have several commonalities
(in terms of observational network, data base on
bathymetry and coastal topography, data communication, dissemination of warnings, training and education, operational practices) and hence it is prudent and
cost-effective to address them together. India's plan is
for development of an integrated mitigation system for
the oceanogenic disasters viz. Tsunami and Storm
Surges in the Northern part of Indian Ocean region
with an ultimate goal to save lives and property.
The design of the System is based on end-to-end
principle, encompassing:
1. upgrading wherever necessary and connecting several existing Seismic Stations of the Department of
Science & Technology, for near-real time determination of Earthquake parameters in Tsunamigenic
zones,
2. establishing observational network of 8–10 Bottom
Pressure Recorders (typically DART System of
NOAA, USA) around the Tsunamigenic areas of
Northern part of Indian Ocean,
3. a chain of 45–50 Real-time Sea level monitoring
stations (Tide Gauges) at strategic locations in the
mainland, islands and offshore platforms,
4. establishment of 10 Radar-based monitoring stations for real time measurement of Surface Current
and Wave,
5. establishment of a network of 8–10 deep Sea Current Meter moorings around the Indian subcontinent
6. numerical modelling for Tsunami, Storm Surges
with all associated data inputs,
7. generation of Coastal inundation and Vulnerability
maps,
8. development of Tsunami Warning Centre in the
country and its operation on 24x7 basis for generation of timely advisories for implementation, and
9. capacity building, education, and training for all
Figure 3 Tide data from the Acoustic Tide Gauges installed at (a) Port Blair, (b)
stakeholders.
Chennai, and (c) Cochin.
Project will be implemented by the Department
of Ocean Development through its Institutions, with
active participation from (a) Department of Science and Technolaffected by Tsunamis (with only two events reported over the last 65
ogy, (b) Department of Space, (c) Council of Scientific and Indusyears in the North Indian Ocean region).
trial Research, and (d) University departments.
What would be important for India is to try to locate tsunamiThe Project will be completed within 30 months at a cost of US $
genic earthquakes in near real time, deploy Ocean Bottom Sensors of
30 million. With the commencement of the project in February 2005,
the kind deployed in the Pacific region to detect generation of
operational warning of Tsunamis and Storm Surges along with their
tsunamis, calculate the travel time from possible sources to Indian
likely landfall and inundation could be provided from September 2007.
coasts based on bathymetry and other relevant information, prepare
It may also be noted that the tsunamigenic zones which can proinundation maps and the affect on coastal areas, and develop a
duce tsunamis for the Indian coasts are also the principal source of
proper mechanism of communication as early as possible to the
tsunamis for the rest of the Indian Ocean rim countries. We shall be
stakeholders. While worrying for tsunamis, it is also important to
providing products of our work to all the countries and centre of
improve India's surveillance and forecast of storm surges resulting
research, through the mechanism of Indian Ocean GOOS or other
from tropical storms. It is well-known that 13% of world's Cyclones
suitable mechanism. India shall also be interacting and coordinating
occur in the North Indian Ocean and that 75% of this occurs in Bay
with other Tsunami Warning and Research Centres globally.
of Bengal. Over the last 35 years, nearly 60 Cyclones have hit the
March 2005
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Figure 4 Diagram showing the distribution of earthquakes and major plate boundaries. It may be noted that globally, more than 75% of
earthquake energy is released in the circum-Pacific belt, about 20% in the Alpine-Himalayan belt, and remaining 5% through the midoceanic ridges and other Stable Continental Region earthquakes. For a tsunami to hit Indian coast, it is necessary that a tsunamigenic
earthquake occurs and its magnitude should be larger than M 7, and the possible locations of such events are enclosed in blue circle and
ellipse.
References
T.S. Murty et al., 1999, Tsunamis on the coastlines of India: Science of
Tsunami Hazards, Vol 17, No. 3, p. 167.
Seth Stein and Emile Okal, 2005, Ultra-long period seismic moment of the
great December 26, 2004 Sumatra earthquake and implications for the
slip process: E-Journal Veda.
Figure 5 Epicentres of earthquakes of magnitude 6.0 and above,
from 1800 to Feb 2003, with major tectonic fault features;
tentative location of Seismic Stations to be interconnected for near
real time determination of earthquake parameters; and proposed
locations of DART kind of Ocean Bottom Pressure sensing Data
Buoys.
Episodes, Vol. 28, no. 1
Harsh Gupta is Secretary to the
Government of India, Department of
Ocean Development. He obtained
his PhD in Seismology from the University of Roorkee in 1970. He has a
unique combination of rich scientific
research and science-administration
experience, with specialization in
Earth Sciences and their application
to address problems of continents
and oceans, administration of educational and scientific institutions.
He has published 3 pioneering
books, edited over 15 volumes, and
published about 130 papers. He is
Bureau Member of the IUGG, Fellow of Third World Academy of Sciences, Indian National Science
Academy, etc. and has been Councilor of IUGS during 2000–2004.