Geoscience Reference
In-Depth Information
If the area investigated is inhabited, inquiries must be made among the population
to clarify the beginning time of the tsunami, its period, the number of waves and so
on, and, also, to analyse thoroughly the destruction of buildings.
It is necessary to make use of the results of in situ studies, aerial surveys and
cosmic photography to compile the map showing the distribution of run-up heights
and distances over the entire territory, subjected to the influence of a tsunami.
Although tsunamis occur every year, strong and catastrophic tsunamis are quite
rare phenomena, and for most tsunamidangerous coasts the catalogue of these
events is insufficiently representative for statistical estimations. Therefore, studies
of
tsunami deposits
, both historical and prehistorical (
paleotsunami deposits
), are
carried out throughout the entire world since the end of the 1980s.
The first studies of tsunami deposits were performed in Japan in the middle of
the 1980s [Atwater (1987); Minoura et al. (1996)]. Subsequently, similar inves-
tigations were carried out on the West coast of the USA and Canada, in Chile,
Australia, New Zealand and in a number of other countries—in most of the regions
subject to tsunami. In Russia detailed work in this direction started in the middle of
the 1990s—on Sakhalin, Kamchatka, the Kuril islands—in the most tsunamidanger-
ous areas of the Russian Far East [Pinegina et al. (1997), (2000); Pinegina/Bourgeois
(2001)].
The following can be considered the main peculiarities of tsunami deposits:
1. They are related to the coastal belt outside the reach of storms and to various
hypsometric levels (approximately up to 30 m above the sea level).
2. The presence in deposits of sea sand and of smoothed out pebbles.
3. Insignificant thickness of deposits (from several millimeters up to several tens of
centimeters, rarely down to a few meters).
4. Periodicity of deposit formation (few tens-few hundreds of years).
Preliminary sites of searches for, and investigation of, tsunami deposits are chosen
after a thorough analysis of aerial-survey and cosmic photographs and topographic
materials. They are used for identifying key areas on the coasts, where tsunami-
genic deposits may have remained intact for a long time. These areas must not be
within zones subject to the influence of alluvial and slope processes (e.g. freshets,
rock slides, etc.). It is desirable for the coastal relief to have various height levels,
and for the coastal configuration not to hinder free tsunami penetration. Moreover,
the descriptions of historical tsunamis in the proposed search area are collected.
Deposits of historical tsunamis serve as benchmarks for revealing the intensities of
more ancient events.
During in situ work at the chosen coastal area levelling photography of the topo-
graphical profiles is carried out, as shown in Fig. 6.9, from the shoreline across
the beach and coastal swells up to the distance of maximum tsunami run-ups.
The profiles are usually determined for several kilometers along the shore. Along
the profiles geological bores are made, in terms of which geological cuts are de-
scribed. Usually from 5 to 20 bores are made along each profile. The average depths
of the bores are between 1 and 4 m—depending on the age of the surface and on
the growth rate of the soil. During geological description of the bores, strata of soils
