Geoscience Reference
In-Depth Information
Tidal oscillations of the ocean level usually provide for 85-90% of the total en-
ergy of long-wave 'noise', the spectral level, corresponding to 24-h and 12-h tides, is
of the order of 10
3
cm
2
. The tidal component, unlike pressure variations, related to
the random influence of atmospheric and other processes, is strictly deterministic—
it is determined by changeless astronomical constants.
An unavoidable task, related to singling out and analysing the tsunami signal in
a record obtained by a sensor of bottom pressure, consists in removal of the tidal
component. Tsunami wave frequencies usually exceed tidal frequencies by more
than an order of magnitude, and, moreover, tides are not stochastic processes. There-
fore, there exist no essential difficulties in singling out a tsunami signal. Certain
technical difficulties may arise during operational tsunami identification, since tun-
ing the filter, that cuts off low frequencies, requires significant time. But, since tidal-
level oscillations can be calculated aforehand from known constants, they can just be
subtracted from the record. The main drawback in this approach is that it cannot be
applied immediately after the sensor has been established on the ocean bottom. For
calculation of the constants with the necessary precision the level must be recorded
during at least 1 month.
Within the National Tsunami Hazard Mitigation Program (NTHMP, USA) there
exists a project aimed at revealing tsunami waves in the open ocean and estimating
their danger (Deep-ocean Assessment and Reporting of Tsunami, DART) [Bernard
et al. (2001); Eble et al. (2001); Kong (2002); Meinig et al. (2005); Green (2006)].
DART buoy stations are established near the regions, within which the formation of
destructive tsunamis is possible (Fig. 6.4). Information from the stations is sent to
tsunami warning centres in a real-time mode via the satellite communication chan-
nel. The choice of sites for the stations guarantees, that a tsunami arising in these
regions will be revealed in 30 min after having originated.
The system is based on deep-water pressure sensors, developed in one of the
NOAA divisions (Pacific Marine Environmental Laboratory, PMEL). A DART
Atlantic
Pacific
Equator
Indian
Fig. 6.4 Location of buoy stations, for registering tsunamis in the open ocean (as of November
2007). The figure is taken from http://www.ndbc.noaa.gov/dart.shtml (see also Plate 8 in the Colour
Plate Section on page 316)
