Geoscience Reference
In-Depth Information
Fig. 6.6 Location of
JAMSTEC bottom stations:
'Hatsushima' (A), 'Muroto'
(B), 'Kushiro-tokachi' (C)
Since 1997, the Japanese agency JAMSTEC has also organized a network of
deep-water bottom stations, equipped with seismometers, hydrophones and pressure
sensors. The latter are intended for operative tsunami forecasting and are even
called 'tsunami sensors'. Unlike the American system DART, described above,
the Japanese stations do not make use of a satellite communication channel, but
are connected to the shore by cable lines. The resolution of the pressure sensors
is
0
.
3 mm of the water column. The frequency of data discretization is essentially
higher than in the American system; it amounts to 1 Hz. At present, 5 bottom stations
are functioning: Hatsushima (1 sensor), Muroto (2 sensors) and Kushirotokachi
(3 sensors). Their locations are shown in Fig. 6.6. Measurement data are avail-
able at the address http://www.jamstec.go.jp/scdc/top e.html. Detailed descriptions
of the technical features of these systems can be found, for example, in [Hirata et al.
(2002)] or at the aforementioned network site.
Since installation, the system has registered numerous seismic events and
tsunamis of small amplitudes (microtsunamis). The exceptionally strong 2003
earthquake (
M
w
= 8
.
3), that occurred in the vicinity of Hokkaido Island was also
successfully registered by the system. The earthquake turned out to be tsunami-
genic, it caused a wave 4 m high on the coast of Hokkaido. Two bottom pressure
sensors happened to be in the immediate vicinity of the tsunami source area. This
was the first direct measurement in history, made at the source at the moment of
the wave origination. It permitted not only to measure the residual vertical defor-
mations of the bottom at the tsunami source [Watanabe et al. (2004)], to estimate
the velocity and duration of these deformations [Nosov (2005)], but also to reveal
low-frequency elastic oscillations of the water column. These results were analysed
in detail in Sect. 3.1.6.
The catastrophe that took place in the Indian Ocean on December 26, 2004,
once more demonstrated the necessity of developing a global system for control-
ling the ocean level. In spite of the high costs (a DART station costs 250,000$ US),
bottom measurements of the ocean level represent an extremely promising and reli-
able means of operative tsunami forecasting.
∼
