Marine Ecosystems Research Department - jamstec japan agency ...

JAMSTEC 2002 Annual Report
Institute for Frontier Research on Earth Evolution (IFREE)
the convection modeling studies using both analog
experiments and computer simulation with the Earth
Simulator, may elucidate the factors controlling convection,
and their role in the evolution of the Earth.
2. Tomographic studies of structure of the Earth's
interior
2.1. All Seismic Phase Project
The All Seismic Phase Project involves analysis of
various seismic phases, including later phases, to obtain
their arrival times. The arrival times are measured
in suitable ways depending on phase type, and
combined into a data-base of accurate arrival time data,
which will be used for seismic tomography to obtain
new models of Earth structure. We analyzed waveform
data from global networks such as IRIS, and regional
networks such as OHP, SPANET, JISNET, SKIPPY,
and Hi-net. We have collected , arrival time data,
which comprise hand-picked data of P, pP, sP, and PcP
waves, differential travel times of PP-P and S-P,
and arrival time data by cross-correlating observed
and synthetic seismograms for P, PP, and Pdiff.
Crustal effects on the waveforms have been taken into
consideration for the cross-correlation method. Figure
shows PcP-P times plotted at the PcP reflection points
on the CMB. Variations can be mainly attributed
Fig. 1 PcP-P travel time residuals plotted on the reflection points
at the CMB. Blue points denote faster than iasp91 and Red
ones are slower than iasp91.
to heterogeneous structure near the CMB. We will
continue to expand the data-base and apply the data to
seismic tomography.
2.2. Upper mantle structure of the Philippine Sea
using semi-broadband data from long-term
ocean bottom observations
We analyzed semi-broadband seismic data from the
ocean bottom and broadband data from the Japanese
Islands, to determine the seismic structure beneath the
Philippine Sea, which has a complex evolution history.
The ocean bottom data were from the Trans-Philippine
Sea array equipped with the long-term ocean bottom
semi-broadband seismographs, which were developed
and deployed by the OHP project. The land-based data
are from F-net and IRIS stations. This is the first longterm
ocean bottom broadband seismic observation, and
enables us to achieve a higher lateral resolution than
ever before for upper mantle structures beneath the
Philippine Sea. After a comparison of the PMD sensors
used in the long-term OBS and the STS- sensor, the
available frequency range of the PMD sensor turns out
to be .-. Hz. For this range, we measured
Rayleigh wave phase velocities from event-station
pairs using the two-station method. Figure shows the
phase velocity deviations from the average at frequencies
of . and . Hz. The northern Philippine Sea
has lower velocities than the Pacific plate at both frequencies.
We selected event-station pairs to compute
pure-path phase velocities for the Mariana Trough, the
Parece Vera Basin, the Shikoku Basin, and the
Minami-Daito Basin, which have different sea floor
ages. Comparing the pure-path velocities to those of
the Pacific plate for respective sea floor ages, all the
pure-path velocities in the Philippine Sea are slower
than those of the Pacific plate. This result, along with
positive residual gravity anomalies, positive sea floor
depth anomalies, and the geochemical signatures of the
basalts of the Philippine Sea, suggest the presence
of Fe-rich peridotite in the upper mantle beneath
the Philippine Sea.
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