| Date |
Cause |
Impact |
| 31
Dec 1881 (Source: Prof Roger
Bilham) |
A 7.9 Richter scale earthquake beneath Car
Nicobar |
Entire east coast
of India and Andaman & Nicobar Islands;
1m tsunamis were recorded at Chennai. |
| August 1883 (Source:
Dr. Arun Bapat) |
Explosion of the Krakatoa Volcano in Indonesia |
East coast of India was affected; 2m tsunamis
were recorded at Chennai. |
| 26 June 1941 (Source:
Dr. Arun Bapat) |
A 8.1 Richter scale earthquake in the Andaman
archipelago. |
East coast of India was affected but no estimates
of height of the tsunami is available |
| 27 November 1945 (Source:
Dr. Arun Bapat) |
A 8.5 Richter scale earthquake at a distance
of about 100km south of Karachi |
West coast of India from north to Karwar
was affected; 12m tsunami was felt at Kandla. |
The Survey of India maintains
a tide gauge network along the coast of India. The gauges
are located in major ports shown in the figure
below. The day-to-day maintenance of the gauges
is carried with the assistance from authorities
of the ports.
Given below are data from
four of the Survey of India tide-gauges. These
data were collected from respective port authorities
(Vishakapatnam Port Trust, Chennai Port Trust, Tuticorin Port Trust,
Kochi Port Trust and Mormugao Port Trust) and
were
processed
at
the
National
Institute
of Oceanography, Goa, to prepare the figure
below.
As seen in the figure above non-tidal oscillations
continued at all the four locations well after
the main event took place on the 26th December,
2004. Red arrow indicates the approximate time
of occurrence of the earthquake off Sumatra
and the blue arrow indicates the time of arrival
of the disturbance at the tide gauge (Vishakhapatnam
at 09.05 a.m.; Chennai at 09:05 a.m.; Tuticorin at 09.57 a.m.; Kochi
at 11.10 a.m.; Mormugoa at 12.25 p.m.). The discontinuities
in the sea
level curves indicate data gaps.
The earthquake of 26 December
2004 (Dr.
K.S. Krishna and Dr. K.A. Kamesh Raju)
The outer rigid layer (about 70-100 km thick)
of the Earth is divided into number of lithospheric
plates. There are about 12 major plates such
as
North American, South American, African, Indian,
Australian
and so on covering the entire earth surface.
The plates are bounded by mid-oceanic ridges,
subduction
zones and transform faults. These boundaries
are usually narrow deforming zones and accompanied
by earthquake activity, but the plate’s
interiors are rigid.
The Indian plate is bounded by Sunda Trench (subduction
zone) in eastern side and Central Indian and Carlsberg
ridges on western side. It is known that the earthquakes
occur at deeper depths in subduction zones, while
they occur at relatively shallow depth at mid-ocean
ridge regions. The earthquake of 26th December
2004 occurred off northwest of Sumatra is not an
unusual earthquake from the Plate Tectonics point
of view. It has occurred in the vicinity of seismically
active zone, close to Sunda Trench in the water
depths of about 1200 m. The earthquake epicenter
is located relatively at shallow depth, about 10
km below the ocean floor. The high magnitude, 9.0
Richter scale, of the earthquake and its shallow
epicenter may have triggered tsunami (also called
seismic sea wave) in the northeast Indian Ocean.
These were traveled in open Ocean of the Bay of
Bengal and subsequently transformed into a train
of catastrophic oscillations on the sea surface
close to coastal zones of Sri Lanka and east coast
of India.
Aftershocks
There were aftershocks observed after
the main event. These are mostly clustered in
Andaman-Nicobar Islands. Locations of these secondary
events are
shown in the figure below. The data shown in the figure
are obtained from USGS National Earthquake Information
Centre.
The figure below shows topography at the site
of the main (m9.0) event. This has been generated
using world digital topo data. According to
this, the location of the event is about 1300m
deep from surface of the water.
