Geoscience Reference
In-Depth Information
seismological classification, since this wave is registered as the third phase (tertiae)
after the appearance of phases P (primae) and S (secondae). The range of frequen-
cies, usual for the T-phase, is 1-100 Hz. The lower boundary of this range, most
likely, depends on the conditions for propagation of the signal along the SOFAR
(low velocity waveguide).
Seismic deformations of the bottom are also capable of causing low-frequency
(
0
.
1 Hz) elastic oscillations of the water layer. These oscillations exist near
the tsunami source. Their nature is related to the multiple reflection of the elas-
tic wave from the water-air and the water-bottom surfaces. These effects are dealt
with in detail in Chap. 3.
Registration of the T-phase is possible not only with the aid of seismographs,
but also with hydrophones. The latter method, for instance, is actively used in the
American system SOSUS (SOund SUrveillance System) [Fox, Hammond (1994)],
operating from the middle of the 1950s and initially intended for searching for
submarines. The system represents a set of hydrophones, connected with the coastal
services by a cable line. Registration of a T-phase signal by the SOSUS system per-
mits to successfully determine the coordinates of epicentres of underwater earth-
quakes, which serves as a successful alternative to traditional seismological methods
(http://www.pmel.noaa.gov/vents). Similar hydroacoustic systems were also created
some time ago in the USSR [From the history (1998)].
In Russia, several recent years saw the revival of research aimed at making use of
hydroacoustic signals from underwater earthquakes for tsunami warning [Sasorova
et al. (2002)]. If a tsunami is excited by a nearby earthquake, the modern tsunami
warning system has very little chance of providing a timely alert signal, since the
time provided by nature for reacting (the time interval between the arrival of the seis-
mic signal and the first tsunami wave) amounts to less than 5 min. At present,
the only promising way of withstanding local tsunamis consists in making use in
good time of available information on the preparatory stages of a developing under-
water earthquake.
Analysis of the records of oceanic hydroacoustic noises, obtained by the Rus-
sian multi-purpose antenna AGAM within the framework of the international
programme ATOC (Acoustic Thermometry of the Ocean's Climate) between 1998
and 1999 revealed promising results. The set of hydrophones established on the Pa-
cific shelf of Kamchatka registered hydroacoustic signals of seismic origin in
the 3-70 Hz frequency range, which appeared much earlier than the first blow from
the earthquake (from hours down to several minutes) [Lappo et al. (2003)]. The
signals were generated by microearthquakes in the preparation area of a strong
earthquake and were evidence of the development of the event's critical stage.
It must be noted that signals of a similar type, caused by microdestructions of
rock (acoustic emission and so on) and propagating in ground and rock dampen
very rapidly and are practically imperceptible by land stations already at a distance
of several kilometers from the source. The amplitude of an acoustic signal drops
exponentially with the distance, and the exponent is proportional to the signal's
frequency. The damping factor in water for a signal of frequency 100 Hz amounts
to 0.0006 dB/km, in magmatic rock it is approximately 0.01 dB/km, in sedimentary
rock and sand of the order of 0.1 and 0.5 dB/km, respectively. A signal of frequency
∼
