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CHIKYU MAIL NEWS Vol. 74, November 16th 2007

This mail news service provides you with information
about the Deep Sea Drilling Vessel "Chikyu" and
science and technology for the IODP Earth exploration.

CHIKYU HAKKEN WEB SITE
http://www.jamstec.go.jp/chikyu/
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[JAMSTEC Press Release]
November 14, 2007

3D Geological Structure Analysis below the sea-floor
at Sea of Kumano off Kii Peninsula
Unraveling Great Tsunami Genesis
http://www.jamstec.go.jp/e/about/press_release/20071114/
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1. Abstract
Gregory Moore, Asahiko Taira and Shin'ichi Kuramoto at the Japan
Agency for Marine-Earth Science and Technology (JAMSTEC;
Mr. Yasuhiro Kato, President) have clarified 3D geological structure
under the sea-floor by analyzing data obtained from 3D seismic
reflection survey conducted at Sea of Kumano off Kii Peninsula
in April through May 2006 (refer to our press release on April 6, 2006).
This analysis showed Megasplay fault derived from plate boundary
is the source of the great tsunamis.

This result will be published on webpage of the USA science magazine,
"Science" on November 16.

2. Background
The fundamental cause of Tsunami is sudden vertical change of
bathymetric feature. It is considered to be induced by fault activities
from plate boundary to near the sea-floor. At Nankai Trough
accretionary prism, Megasplay fault derived from boundary fault
between subducting plate (Philippine Sea Plate) and upper plate
(Eurasia Plate) had been discovered. And the idea that the Megasplay
fault induces the change of bathymetric feature and resulted in
generation of tsunami has been controversial.

3. Details
(1) Method
At the Sea of Kumano, 3D seismic reflection survey was implemented
to create 3D visualization of Megasplay fault, as a preliminary survey
for Nankai Trough scientific drilling by the Deep-sea Drilling Vessel
"Chikyu".

(2) Analysis result
Based on the analysis of 3D geological structure acquired from (1),
the followings became clear.
a) Megasplay fault is derived from the plate boundary fault.
b) Megasplay fault cut through the old fault which is forming
accretionary prism.
c) Structure of Megasplay fault and tracks of large-scale landslide
on the sea-floor suggest that this splay fault is still active and new
fault evolves landward (north) direction.
d) The Megasplay fault gradually evolves in higher angle from seaward
(south) to landward and it changes bathymetric feature more vertically.

4. Discussion
These analyses clarified that the activities of Megasplay fault
in Sea of Kumano is the key for bathymetric feature changes.
The Megasplay fault has been derived from the plate boundary.
It is very active with evidence of recent activities. Therefore, it is
considered that the Megasplay fault is still active and this is the one
that caused historical tsunamis such as the tsunami of Tonankai
earthquake in 1944 (magnitude 8.1)

(*1) Nankai Trough Accretionary Prism
Nankai Trough derived from Suruga Bay runs to off the coast of
Tokai - Kii Peninsula - Shikoku - Kyushu, where the Philippine Sea
Plate subducting under the west-south Japanese archipelago,
(Usually, 'trough' means long thin ditch on the sea-floor, but here
it means 'trench' which length is approximately 770km).
Along with this plate subduction, geological phenomenon that
sediment on the plate is taken off and added to the plate on land
side is occurs at this area in the ocean, such newly formed
geological sediments are called "accretionary prism".
It is recognized that its progress is very important as a basic
process of orogenesis, and also the front line research is conducted
as the seismogenesis zone.

Contacts:
(For the Research)
Mr. Shin'ichi Kuramoto, e-mail: cdex@jamstec.go.jp	
Group Leader, Program Operation Group, IODP Department
The Center for Deep Earth Exploration (CDEX)
Japan Agency for Marine-Earth Science and Technology

(For Publication)
Mr. Shinji Oshima, e-mail: press@jamstec.go.jp 
Manager, Planning Department Press Office
Japan Agency for Marine-Earth Science and Technology

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CHIKYU HAKKEN web site introduces you to every
aspect of the Deep Sea Drilling Vessel "Chikyu" and
science and technology for the IODP Earth exploration.
http://www.jamstec.go.jp/chikyu/

The Integrated Ocean Drilling Program (IODP) is an
international marine research drilling program dedicated
to advancing scientific understanding of the Earth by
monitoring and sampling sub seafloor environments.
IODP drilling operations aboard the Chikyu are managed by
JAMSTEC through CDEX, Japan Implementing Organization.
http://www.iodp.org

"CHIKYU HAKKEN" means Discover the Earth in Japanese.
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