Geoscience Reference
In-Depth Information
peculiarities of the emission and damping of wave energy. It has been established
that in the case of a large number of registered events, as the distance from the source
increases, the weakening of waves follow a power law
r
−
α
, where the exponent
α
varies within the limits from 0.5 to 0.66. This corresponds to values of the source's
symmetry parameter between 1 and 2, i.e. the source is an elongated ellipse. Similar
estimates can be found in the topic by Pelinovsky [Pelinovsky (1982)]. Note that
precisely such a shape is attributed to the source of a tsunami in many publications
dealing with the resolution of inverse hydrodynamic problems. In the case of conti-
nental earthquakes, the region of maximum shakings reconstructed in problems of
macroseismics also exhibits a strongly elongated oval shape.
Statistical treatment of the material in the tsunami database has recently revealed
an interesting periodicity in the appearance of tsunami sources [Levin, Sasorova
(2002)]. The time sequence of events generated alternately in the Earth's northern
and southern hemispheres is characterized by a 6-year period. Strong perturbations
on Earth, caused by astronomical processes and the mutual arrangement of bodies in
the Earth-Moon-Sun system arise with approximately the same periodicity. Further
development of databases and of computational methods will permit future revela-
tion of new laws in the processes of tsunami and underwater earthquake generation.
1.6 Seaquakes: General Ideas
Every year approximately 10,000 earthquakes are registered on Earth, of which
about 4,000 events are perceptible earthquakes, for which the velocity of motion of
particles in the wave or the mass velocity amounts to over 0.1 m/s. Of this amount
the largest part are underwater earthquakes. Strong earthquakes result in the appear-
ance near the coasts of gigantic devastating tsunami waves, while in the region of
the earthquake epicentre unusual hydrodynamic phenomena are observed that are
known to seafarers by the term seaquakes. In certain cases, the terms tide race and
wave crowd are also used.
The transverse dimension of the perturbated region of the sea surface during
a seaquake usually exceeds 50-100 km, while the duration of a strong seaquake may
amount to 10 min. During a seaquake sets of very steep standing waves form on
the surface of the aquatorium, individual vertical columns of water and solitary wa-
ter formations arise and strong acoustic effects are noted. Spray sultans may be ob-
served, as well as cavitational layers of water separating from each other and flying
apart. A ship that happens to be in the zone of influence of a seaquake turns out to be
surrounded by giant standing waves filling up the entire visible space. Terrible thun-
derous rumbling and howling are enhanced by sharp blows to the bottom, the most
strong shaking of the vessel and the destruction of deck structures that had in the past
endured more than a few storms.
We shall present the average quantitative characteristics of the phenomenon:
•
Height of waves and of surges of water—over 10 m
•
Velocity of surface motion up to 10 m/s
The acceleration of water particles may amount to 10 m/s
2
•
