Marine Ecosystems Research Department - jamstec japan agency ...

JAMSTEC 2002 Annual Report
Institute for Frontier Research on Earth Evolution (IFREE)
1944 Tonankai Eq. [Tanioka and Satake, 2001]
KR0108-5
0
decollement
Depth [km]
5
10
Accretionay prism
splay fault
plate boundary
Philippine sea plate
oceanic
crust
90
80
70
60
50 40
Distance [km]
30
20
10
0
Fig.19 Micro-seismicity around an up-dip limit of seismogenic zone observed at the Kumano-nada.
around the plate interface (Fig.). The seaward limit
of this seismicity is characterized by clusters of earthquakes
with very similar waveforms. These earthquakes
are considered to occur at small asperities in
the aseismic-seismogenic transition zone along the
plate interface. Off Kii peninsula, the rupture area of
the Tonankai earthquake, seismicity is locally
active around the toe of the accretionary prism. These
earthquakes were located at the seaward limit of the
coseismic slip area of the Tonankai earthquake.
By contrast, micro-seismicity in the rupture area of the
Tonankai earthquake is very low. Recent GPS
surveys show a fully coupled plate interface along the
Nankai trough seismogenic zone. The Tonankai
Earthquake was explained by a rupture on a single
asperity without small scale segments on the basis of
seismic and tsunami waveforms. These observations
imply the existence of a large asperity with a uniform
interplate coupling.
(iii) Integrated onshore-offshore seismic survey from
the southwestern Japan to Japan Sea
From August to September , we carried out an
integrated onshore-offshore seismic survey from
southwestern Japan to the Japan Sea in cooperation
with the Earthquake Research Institute (ERI),
University of Tokyo. One aim of this study is to image
the subducted Philippine Sea plate beneath the southwestern
Japanese islands and the Japan Sea, and
to obtain the structure of the island arc - Japan Sea
transition zone, which is believed to be closely related
to the origin of Japan sea and the Japanese island
arc. Along the offshore part of the profile (km
long), from off Tottori to the southwestern Yamato
Basin, we acquired wide-angle seismic data using the
seismic system of JAMSTEC's R/V Kaiyo ( OBSs,
, cu. inch air-gun array) as well as the Japan
Meteorological Agency's R/V Seifu-maru.
ERI, Chiba University, and other universities have
deployed more than land seismic stations along
the onshore part of the profile. These stations recorded
seismic signals from explosion sources (three of
kg TNT and six of kg TNT). We are planning
to investigate the subduction structure of the
Philippine Sea plate from all of the onshore-offshore
wide-angle seismic data.
() Seismogenic zone study in the Japan Trench region
We conducted seismic velocity structure analyses
using wide-angle reflection-refraction seismic survey
data to clarify the seismogenic mechanisms in the
Japan Trench forearc region. In the rupture zone of
both the Tokachi-oki and Sanriku-oki
earthquakes, we obtained P-wave velocity structure
using seismic survey data, and made a seismic reflection
image using our newly developed method, which
100