Geoscience Reference
In-Depth Information
1. The discharge into water of a large volume of matter (from slow lava flows to
explosive eruptions)
2. The collapse of a caldera (explosion of a volcanic island)
3. Pyroclastic flows, landslides etc.
4. Volcanic earthquakes
In the case of underwater volcanoes the first two mechanisms are prevalent. The
third (landslide) mechanism may be more peculiar to volcanoes on coasts, although
the possibility cannot be excluded of underwater landslides and mudslides initiated
by an underwater eruption.
In certain cases, volcanic eruptions can provocate enormous collapses. Thus, for
example, in [Ward, Day (2001)] the possibility is indicated for part of the La Palma
island (Canary islands) to collapse during the next eruption of the volcano, located
there. Geological estimates reveal that the volume of such a collapse may amount
to 500 km
3
. A tsunami wave caused by such a colossal collapse would be capable
of crossing the Atlantic Ocean and reaching the coasts of America with a height
exceeding 10 m.
In this section we shall only deal with those original mechanisms of tsunami
formation that are peculiar precisely to volcanic eruptions. Thus, here we shall not
consider wave generation by volcanic earthquakes, as well as by volcanogenic land-
slides.
We shall first dwell upon certain peculiarities of tsunami formation in the case
of a caldera collapsing and being subsequently filled up with water. If one speaks
of an underwater volcano, then the description of the waves generated fully reduces
to the problem of tsunami generation by deformation of the ocean bottom, which
has been investigated in detail in Chapters 2 and 3. Truly, in the case of a collapsing
caldera, the amplitude of the 'bottom deformation',
10
2
-10
3
m, and the hori-
η
0
∼
10
3
-10
4
m, may turn out to be com-
parable to the ocean depth. Note that underwater volcanoes may be located both at
small (shelf) and at large (abyssal) depths. If the eruption of a volcanic island takes
place, then the water filling up the caldera, like the waters surrounding the island,
are evidently characterized by shelf depths (
zontal dimension of the deformation area,
D
∼
10
2
m).
A suddenly generated caldera (in total absence of obstacles to water entering
it) will be filled with water in a time
T
∼
D
/
√
g
H
, where
D
is the diameter of
the caldera and
H
is its characteristic depth. Taking advantage of the aforementioned
ranges of these parameters, we obtain that the quantity
T
varies within the limits of
30-300 s. In the case of the most probable development of events, when obstacles
to the arrival of water do exist, the time the caldera will take to fill up may increase
significantly.
Thus, the caldera collapsing results in a source of waves (a flow of mass) with
a characteristic action time of 10
2
-10
3
s. The volume of water, taking part in the pro-
cess, can be estimated as
V
∼
0
.
3-30 km
3
. The obtained characteristics
of the source are quite in agreement with the values for a seismotectonic tsunami
source. Caldera collapses are capable of generating powerful long-period tsunami
waves. Simulation of tsunami wave propagation, due to activity of the Krakatau
volcano in 1883, performed within the framework of longwave theory [Choi et
D
2
H
/
4
∼
π
∼
