Geoscience Reference
In-Depth Information
•
The amplitude of pressure in the underwater wave of compression—up to 2 MP
(20 atm)
Such effects may result in the destruction of structures on the ship, demoralization
of the crew, the rise of critical and emergency situations on the vessel and the death
of sea animals and fish. We shall point out the main parameters of a seaquake that
are important for developing a physical model of the phenomenon:
1. The ratio of the water depth
H
and the characteristic horizontal dimension of
the source
L
lies within the interval between 0.001 and 0.1.
2. The range of frequencies of the sea-floor oscillations is 0.001-10 Hz.
3. The acceleration amplitude of motions of the sea-floor amounts to 10 m/s
2
.
Modern catalogues, articles and scientific publications contain over 250 descriptions
of seaquakes in various regions of the World Ocean. Among the recent events one
must note the strong damage of eight fishing boats in the region of the South Kuril
islands after the Shikotan earthquake of 1994 and the destruction of the huge tanker
'Exxon' in the Gulf of Alaska resulting from a seaquake in 1988. Below follows
a scenario of the development of a 'generalized' seaquake, based on materials of
individual descriptions due to witnesses [Levin (1996)].
Synthesized description of a seaquake
On a totally breezeless day the smooth mirror-like sea surface became all of
a sudden covered with bumps. These aquatic bumps, that looked like waves, did not
move away in any direction, but they did not remain motionless, either. They grew
rapidly up to a height of about 8 m and then shrank down forming deep craters in
the place of the recent bumps.
The oscillations were rapid, we were dazzled by these unusual boiling waves fill-
ing up all the visible sea space. The aquatic surface seethed and jumped up and
down, as if it were in a red-hot kettle with salt brine. The boat was thrown up
and down, and it rocked ominously on these jumping waves. They were as steep
as the most ferocious storm waves, but no longer than 20 m. The keel rocking was
so strong, that the propeller was several times seen to be completely dry in air, and
the wheel of the ship compass fell off its pivot.
All the passengers and crew poured out on deck. The bright sunshine and total
calm only enhanced the tension of this terrifying spectacle of a sea gone crazy.
Less than a minute passed, but no willpower was left to resist this monstrous gal-
loping, which once in a while weakened, then strengthened again. Hands clinging
to the ship's sides felt how unreliable this plaintively creaking vessel was in front of
the mysterious and incomprehensible sea catastrophe.
The aquatic bumps started to become smaller, while the frequency of their blink-
ing increased. At the same time, from somewhere out of the depth a low thunderous
rumble arose, that supressed all willpower and reason. People started thrashing
around the ship full of panicky fear. Many passengers, and even sailors, could not
bear such torture and, having evidently lost their minds, started to jump overboard.
Against the background of these blinking waves there started to appear very high
jets of water, that collapsing created a strange rustling sound.
All of a sudden the ship was shaken by a most strong blow. Several persons were
thrown overboard. The blows to the bottom of the ship came one after another. The
