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Tsunami Occurrence in the Makran Region
Mokhtari, M., Farahbod, A.M.
Tsunami Seminar, Tehran, 26th
February 2005
Background Studies
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The first
geological/geophysical survey and mapping in the Gulf of Oman was done by
the John Murray-Mabahiss Expedition in 1933, using then new echo sounder.
The essential features noted then have since been confirmed by extensive
academic research.
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In summer 1997 a
scientific cruise (SONNE-123) conducted by GEOMAR, BGR, Hannover, Germany,
the University of Cambridge and the National Institute of Oceanography,
Pakistan, set out for a vast survey that included swath mapping,
high-resolution and single-channel reflection seismics, ocean-bottom
seismology, micro-seismicity monitoring, magnetics and gravity.
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In year 2000 offshore of
Makran was covered by 2D Seismic data (PC2000).
Characteristics of the Makran Accretionary Complex
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The east-west oriented
accretionary complex is more than 900 km long, bounded to the east and west
by large transform faults defining plate boundaries.
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The distance from the
accretionary front to island arc volcanics (the Bazman, Taftan and Sultan
calc-alkaline andesitic volcanic chains) is 500-600 km.
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Convergence rate between
the Arabian and Eurasian Plates has been estimated to 30-50 mmy-1 (Platt et
al. 1988).
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The thrust faults are
oriented nearly perpendicular to the direction of convergence.
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There is no obvious
topographic trench associated with the present accretionary front.
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There is a thick
sedimentary cover on oceanic crust, 6-7 km of terrigenous sediments in the
abyssal plane.
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There are no obvious
magnetic anomalies related to ocean floor spreading in the Oman Sea. The
oceanic crust formed during the Cretaceous quiet zone (108-79 Ma)
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There are no indications
of active volcanism or intrusions in the accretionary complex.
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The subducting plate has a
northward dip of >20 till 270N, then bending down to an angle of ~300
Seismicity related to the Makran Accretionary Complex
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In a plate tectonic
setting like that of the Makran Accretionary Complex a fairly high
earthquake activity would be expected, as in many of the other major
accretionary complexes/subduction zones around the world
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But the Makran zone is
remarkably low in seismicity.
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Between two major
transform faults most earthquakes (magnitude >3) are related to
wrench-faults, or boundaries of basement blocks at the segmented margin of
the Eurasian Plate.
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Teleseismic activity in
the Makran region prior to the great earthquake of 1945 can be characterized
in terms of two stages (Quittmeyer 1979).
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First, during the period
30 (or more) to 10 years prior to the main event, the frequency of
occurrence of moderate to large earthquakes was relatively high. These
events probably occurred near the down-dip limit of seismic activity within
the subducted slab.
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Second, activity was
concentrated along the coast during the ten years immediately preceding the
great earthquake and most of this activity was confined to the vicinity of
the epicenter of the 1945 earthquake.
An Example of 2D seismic data from PC2000
The profile is in North-South
direction offshore Oman Sea
North-south
seismic line through the offshore part of the Makran Accretionary Complex
Tsunami in Makran
Off the Makran Coast
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Date 28th November 1945
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Epicentre: 87.1 kms SSW of
Churi (Baluchistan), Pakistan Latitude: 24.500° N
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Longitude: 63.000°E
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Origin Time: 21:56 UTC
(03:26 IST)
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Magnitude: Mw 8.0, Ms 7.8
Moment: 10.2*10*20 Nm
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Underwater and
above-water explosions.
Tsunami impact
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This was the last major
tsunami-generating earthquake in the Arabian Sea. More than 4000 people were
killed on the Makran Coast by both the earthquake and the tsunami.
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The most significant
aspect of this earthquake was the tsunamis that it triggered. The tsunami
reached a height of 40 feet in some Makran ports and caused great damage to
the entire coastal region.
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There was similar loss of
life and crafts along the coasts of Makran, Iran and Oman. The towns of
Pasni and Ormara were badly affected. Both were reportedly "underwater"
after the tsunami.
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The tsunami was also
recorded at Muscat and Gwadar.
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The waves were 6.5 feet
high in Karachi. The first wave was recorded at 5:30am, then at 7:00am,
7:15am and finally at 8:15am. The last wave at 8:15 was the biggest.
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The tsunami had a height
of 11.0 - 11.5 m in Kutchh, Gujarat. At 8:15am, it was observed on Salsette
Island i.e Mumbai. It was recorded in Bombay Harbour, Versova (Andheri),
Haji Ali (Mahalaxmi), Juhu (Ville Parle) and Danda (Khar).
Major earthquakes and
Tsunamis in Oman Sea, Pakistan, India and surrounding regions
Socio-Economical Factors
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Rapid growth & development
of coastal areas
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Living people on or quite
near the coast
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Foreign trade necessitates
to maintain large fleets of ships & major port facilities
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Fishing industries
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Aqua cultural industries
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Great loss of life
Conclusions
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The east-west oriented
accretionary complex is more than 900 km long, bounded to the east and west
by large transform faults defining plate boundaries.
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There is no obvious
topographic trench associated with the present accretionary front.
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Makran Accretionary
Complex is characterized by low seismicity.
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Quittmeyer (1978) claims
that the recent seismicity in the region west of the area affected prior to
1945 suggests that the western region may be the site of the next large
earthquake.
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Therefore this region
should be closely monitored in the future.
References
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History of Tsunamis in
Pakistan/Arabian Sea. PAKISTAN METEOROLOGICAL DEPARTMENT, January 2005.
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Reports of National
Iranian Oil Company.
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Quittmeyer, R.C., and
Jacob, K.H., "Historical and Modern Seismicity of Pakistan, Afghanistan,
N.W. India and S.E. Iran," Bulletin of the Seismological Society of America,
69/3, pp. 773-823, 1979.
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