SEISMOTECTONICS AND TECTONIC
HISTORY OF THE ANDAMAN SEA
Eguchi, T., Uyeda, S. and Maki, T., 1979 .
Earthquake Research Institute, University of Tokyo(Japan)
Tectonophysics, 57 (1979) 35-51
INTRODUCTION
 The Andaman sea is an active spreading – arc basin.
In the eastern side of this island the mid-oceanic ridge migrate
towards northward the Ninety-East ridge collide with the
western end of the old Sunda trench in the middle or late
Miocene (10-20 m.y.B.P).
 This phenomenon creating a tremendous stress release in the
back arc area.
 The northward movement of the Indian plate and its
collision with Euration plate exert a drag in the back arc region
causing the opening of Andaman sea.
 This ocean basin formed by a oblique extensional rifting.
 In this paper Author examine the seismicity and present
tectonism in the Andaman sea region. And he try to correlate
the spreading of eastern Indian-ocean and the northward
movement of Great India.
Karig 1971 divided back arc basins in to two.
1. Active marginal basins.
2. Inactive marginal basins.
 Geomorphic and seismic evidences of this area
shows that active extension is taken place
along the central Andaman(Fitch 1972).
 Mid-oceanic ridge migrate towards northward
the Ninety-East ridge collide with the western
end of the old Sunda trench.
 Northward movement of the Indian plate and
its collision with Euration plate exert a drag
here.
Sunda trench
Seismicity
From Jan-1964 to Dec-1973 scientist examine the all earthquakes >4.5
between the latitudes 0 ̊ N and 20 ̊ N and longitudes 90 ̊ E and 100 ̊ E.
 All the epicenters are more in the
concave-side of the Andaman arc(fig-1).
This may due to the highly oblique
subduction of Indian plate in the
Andaman sea.
 All the epicenters are narrow down to
wards the back arc tectonic zone and
having a trend of NNE-SSW in the north
Andaman sea. This seismic zone playing
an important role for the spreading of
Andaman sea.
The rectangle showing the central
Andaman sea rift valley during the study
period a few earthquake occurred in this
area (mb4.5)
Deeper earthquake >150km occurred
twice in the south during the study period.
The northern Andaman experiencing comparatively shallower earthquake
than south Andaman.
Rectangle section from the previous fig, scientist make an E-W-vertical
cross section according to the latitudes and the depth of the earthquake.(11̊
N and 13̊ N)
Hear we can able to see the two seismic belts
TECTONIC HISTORY IN THE ANDAMAN SEA
Greater India's northward movement
from Australia
Other recent tectonic movement
mentioned in the previous part.
From the magnetic anomalies we getting
the information about the age of ocean
bottom
Eastern part of Ninety east ridge increases
southward while that the western part of
this ridge increases northward (Sclter and
Fisher 1974).
This ridge is the active right lateral
transform fault (between 53-32 my)
By (Sclter and Fisher 1974).
On the other hand the Stein and
Okla. 1978 suggest small left
lateral movement
Any way the models prepared
by Tapponnier and Molnar (1975)
supposing there was little relative
movement either side of the ridge.
 For example in the given figure
showing the 20 ̊clockwise rotation
of the Sumatra around the center
of Sunda strait (Point ‘C’).
 Spreading might have arrive at
the point ‘B’ and at point ‘A’
rotation of Sumatra started(15my).
This process completed at 10 my
at this time the spreading of the
Andaman sea started.
Conclusion:
The stage that the Andaman sea is appeared in between Burma
and west Sumatra as a combined result of ridge subduction and
collision with India and Asia
The former and the later collision probability start at about 20-10
and 30-10 m.y. B.P., respectively.
This all theories what ever proposed by the authors in this paper
similar to the theory proposed by Alfred Wagner in his classic book
“Continental drift”.