Geoscience Reference
In-Depth Information
2nd scenario. When a fault outcrops on the surface (of the bottom), or in the case
of horizontal movements of the bottom, turbulence generation is possible in
the near-bottom region owing to the shear instability. It may be assumed that
intense turbulent mixing will involve the near-bottom region of water of insignif-
icant thickness of the order of the height of the bottom inhomogeneity or of
the vertical displacement within the fault.
3rd scenario. Seismic movements lead to formation on the water surface of stand-
ing wave structures (parametric resonance). In a number of cases, such waves
are characterized as extremely violent storm waves. The excitation of turbulence
occurs when the waves collapse. Turbulent mixing only embraces the upper layer
(several tens of meters). In shallow water the entire water column will evidently
be involved in intense mixing. For standing waves to arise during an earthquake it
is necessary for the bottom to oscillate with infrasonic frequencies (
0.1-1 Hz).
An alternative, here, can be represented by elastic oscillations of a water column
several kilometers thick, which arise in the case of any (not necessarily oscilla-
tory) movements of the bottom and proceed with the same infrasonic frequencies.
4th scenario. The development of turbulence in the case of cavitation effects.
5th scenario. Intensive movements of the bottom form non-linear currents (in-
cluding acoustic wind [Ostrovsky, Papilova (1974)]). Vertical transfer is realized
directly by these currents and by the turbulence resulting from the instability
of these currents. This phenomenon takes place at any depths and may involve
the entire thickness of the water column.
For
scenarios 1 and 2
the ocean depth is a critical parameter in the sense that surface
effects of mixing will not be present where the depth is sufficiently great. Thus, for
example, an SST anomaly will be related, here, to small depths along the coastline,
and, consequently, the area of the anomaly will be small.
The
3rd scenario
is already capable of providing surface effects over significant
areas, comparable to the area of the pleistoseist zone of the earthquake. But, owing
to mixing only involving the near-surface layer, no significant change in the surface
temperature is to be expected to accompany the total stratification destruction.
Cavitation effects in the case of underwater earthquakes are little studied till now.
Therefore, we shall be prudent and only make the conclusion that the
4th scenario
may, probably, provide insignificant SST deviations over areas inferior to the area
of the pleistoseist zone.
The most widespread SST anomalies with significant temperature deviation from
the initial values can be provided for by the
5th scenario
. It is possible that the fifth
scenario includes the case, when on board a vessel 600 km from the coast the sea wa-
ter was found to be mixed with sand. Another striking event, which, most likely,
demonstrates the transfer of cold depth waters up to the ocean surface, is to be con-
sidered the case, when 'temperature of the air fell so low, that on the shore bathing
stopped and people looked for a shelter'.
Often, in the case of underwater earthquakes the water is seen to churn and foam,
or 'to boil'. The origin of such a phenomenon is due to non-linear currents, cavita-
tion effects or to convection flows (for example, in the case of underwater eruptions).
Of course, in the latter case, an enhancement will be observed of the temperature of
the water and the air.
∼
