Geoscience Reference
In-Depth Information
Fig. 6.10 Photograph of wall of bore with strata of tephra and tsunamigenic horizonts in the soil.
The bore is located at the 20th terrace 500 m from the shoreline of today (peninsula of the
Kamchatka cape, Kamchatka) (see also Plate 12 in the Colour Plate Section on page 320)
and peat, as well as tsunamigenic and other deposits are identified. Samples are
selected from these prolayers for mineralogical, radio-carbonic, diatomic, spore-
pollen, granulometric and other sorts of analysis—for determining the genesis and
age of the prolayers.
In the Far East, for instance, on the Kamchatka, the investigation of tsunami de-
posits is closely related to the possibility of applying the method of tephrochronol-
ogy. The method is based on investigating and correlating the marking horizons
of volcanic ash (tephra), each of which exhibits a characteristic outlook, chemical
and mineralogical composition and abundance over a large territory. In studies of
tsunami deposits on the Kamchatka the tephrochronological method is applied as
the base method for correlating and dating deposits (Fig. 6.10).
When the description of geological cuts, the identification of deposits and the cor-
relation of tsunamigenic prolayers are completed, a synoptical geological column
and a geochronological (time) cut are compiled, from which it is possible to calcu-
late the tsunami repetition frequency on the coast. Such work has resulted in signif-
icant success, for instance, in Russia. Thus, for many regions of the Kamchatka and
Kuril coasts traces were revealed of deposits due to tens of ancient tsunamis that
occurred in the past 2,000-7,000 thousand years.
The data on the tsunami repetition rates at individual sites of the coast are insuf-
ficient for estimation of the intensities of ancient tsunamis and of the earthquakes
that caused them. To resolve such problems it is necessary to know the run-up
or inundation height, the penetration distance and the length of the coast, sub-
jected to the influence of each of the tsunamis identified. This task is very com-
plicated and complex. For each of the regions investigated it is necessary to perform
