Abstract.  The   present  paper   is  a   written  version of  the
oral  report  presented   at    Paleotsunami   Section   of    the
Kamchatka Tsunami  Workshop held  in Petropavlovsk-Kamchatskiy  on
August 21-24,   1996.  It  describes  the  results  of  the  field
works done in the  summer seasons of 1995   and 1996 on   the east
coast of Kamchatka in  the  area near the mouth of   the Zhupanova
river    (approximately    120       km    north-east           of
Petropovlovsk-Kamchatskiy).       The       principal   scientific
objective  of  this   study  was  to  prove   the  availability of
pre-historical  paleotsunami   traces  on   the  east   coast   of
Kamchatka and their applicability  to the restoration of  the late
Holocene seismicity and tsunami  history in the Kamchatka  region.

The assessment of the long-term earthquake and tsunami risk is essentially based on historical data which are collected, stored and distributed in the form of historical seismic and tsunami catalogs and databases. The existing tsunami catalog for the Kamchatka region, compiled mainly on the basis of the available historical documents, covers the period of about 250 years (from 1737 to 1995). Its overall length is much shorter than, for instance, the length of the tsunami catalog for the neighboring Japanese region covering more than 1300 years (Iida, 1984) and hardly exceeds the duration of the basic seismic cycle for this region which is estimated to be within 140-190 years (Vikulin, 1992).

Altogether, the Kamchatka tsunami catalog contains 21 regional events with 5 of them resulted in the destructive effects on the eastern Kamchatka coast (Soloviev, 1978, Zayakin, 1996). They are tsunamis of 1937, 1841, 1923, 1952 and 1969. Taking into account the amount of the work already done for the catalog compilation, one can conclude that its further extension and addition by means of the search for new historical evidence and documents is practically impossible. However, the extension of the catalog is possible on the basis of application of geological methods for detecting traces of ancient tsunamis in sedimentary layers of accumulative terraces, alluvial deposit of river valleys, vatts, marshes and bottom sediments of coastal lagoon lakes.

As it was first shown by Minoura and Nakaya (1991) and later confirmed by many other coastal geologists, the dynamics of water flood induced by a tsunami wave and invasion of a large volume of salty water into the land can produce a certain disturbance of the normal sedimentary process and leave peculiar deposits which remain in the intertidal environment for a long time. These tsunamigenic deposits, being found and interpreted in the correct manner, can represent a 'geological chronicle' of a local tsunami history for at least the upper Holocene (2-3 thousand years B.P.). If in a certain area these deposits can be found, identified and dated, the local tsunami history can be considerably extended into the pre-historical period (of course, on some other level of accuracy and confidence as compared to the historical data).

The present paper is a written version of the oral report presented at Paleotsunami Section of the Kamchatka Tsunami Workshop held in Petropavlovsk-Kamchatskiy on August 21-24, 1996. It describes the results of the field works done in the summer seasons of 1995 and 1996 on the east coast of Kamchatka in the area near the mouth of the Zhupanova river (approximately 120 km north-east of Petropovlovsk-Kamchatskiy). The principal scientific objective of this study was to prove the availability of the pre-historical paleotsunami traces on the east coast of Kamchatka and their applicability to the restoration of the late Holocene seismicity and tsunami history in the Kamchatka region.

A sketch map of the area under study is shown in Figs. 1 and 2. This area was selected for the search and study of tsunami deposits by the following reasons:

1. it is an area with a high seismicity level, and several large historical tsunamis are known to be observed in this part of the Kamchatka coast with run-up heights from 5 to 10 meters;
2. this part of the coast is open for tsunami waves approaching from different directions (north-east, east and south-east);
3. the morphology of the river dale and nearby peat plain allows the study of tsunami traces with different conditions of sedimentation, depending on heights of the coast rim (that varies from 3 to 5 m);
4. numerous tephra layers (known and dated for this area up to 5000 radiocarbon years B.P.) incorporated into the peat thickness along with tsunamigenic layers can be used as markers for bracketing and preliminary dating of tsunami deposits directly in the field.

The first reconnaissance trip to this area was undertaken by T.Pinegina and A.Storcheus in August of 1995. At that time about 40 excavations on the both banks of the Zhupanova river were made. The left river bank is very flat (its slope is less than 0.0001). It is covered with peat marshes up to 20 km from the sea coast. Several hills ( 'sopka', as they are called on Kamchatka) are located on the right river-bank at distances up to 1 km from the coast. Peat-bogs are located in front of the hills and separated from the sea by a gently sloping rim of 3-5 m height. Excavations in this area were made on low sea accumulative plains, and surface terraces, i.e. in the places, where the tephra and tsunami deposits could be well preserved without being reworked. They revealed a well-developed system of sedimentary layers where paleotsunami traces have been well preserved.

Typically, tsunamigenic layers consist of coarse marine sand (black or gray ) mixed with small to medium gravel and pebbles sometimes also with organic remnants. Usually, tsunami layers are well distinguished from peat-layers, and also from tephra, eolian -and alluvial sand, and other sediments of the non-sea origin. Excavations on the right bank in the estuary of the Zhupanova river, made on high sopka terraces (up to 15-30 m), discovered traces of only strong tsunami-events with the run-up heights exceeding the terrace height. These strong tsunamis correspond to the sediments found at large distances from the coast line ( up to 10 km ) along the left bank of the river. The highest concentration of tsunami sediments was found in excavations located at 300-700 m from the sea-coast. In excavations located closer than 300 m to the coastline we could find only the trace of the 1952 tsunami, which was the last destructive event in this area.

One of the problems in this type of research in Kamchatka is the dating of sedimentary layers. Due to the high cost of the radiocarbon analysis we had a limited possibility to use this method of dating. However, application of the tephrachronological scale, well-developed for many areas in Kamchatka, gave us a possibility for a rough time bracketing of tsunamigenic layers and, in some cases, for a more precise estimation of their age. The method of tephrachronology, developed by the Kamchatka volcanologists, gives a possibility to estimate the rate of peat sedimentation and to date tsunamigenic layers. Since 1970, scientists of the Institute of Volcanology have been studying distribution of volcanic tephra from eruptions of Kamchatka volcanoes in Holocene (Braitseva et al, 1992, 1994). These studies resulted in well-established and reliable date marking tephra layers, which can be easily identified in the field by a lot of attributes. Application of the tephrachronological scale allows us to estimate the age of paleotsunami deposits and helps to correlate tsunami traces in different sections. Age of tsunami traces, occupying the intermediate location in the section between marking horizons of tephra, can be estimated by measuring their exact position between tephra markers (on the

Fig.1. A location map of the Kamchatka Peninsula . The shaded box represents the area under study shown in Fig.2

Fig.2. A location map of the area under study. The land is shown in the gray color. The solid circles show some of excavation sites

Fig.3 A combined section of tsunami traces found near the mouth of the Zhupanova river. The vertical axis represents years before present.

Fig.4. A histogram of tsunami occurrence within unequal time intervals defined by the identified tephra markers. The numbers at the vertical axis mean the number of tsunami traces per 100-year interval. Note the increased number of tsunami traces within 1800-2000 years B.P.

Fig.5. The number of tsunami traces (per 1000 year interval) depending on the distance of sites from the coastline.

basis of the hypothesis of the constant rate of peat accumulation). Errors of such estimates vary from several years for the recent historical tsunamis, to the first decades of years for the more ancient tsunami events.

Some preliminary results of analysis of the data obtained in two field trips are shown in Figs.3,4,5. Fig.3 represent a combined stratigraphic section for the whole area near the mouth of the Zhupanova River. Altogether, we could have identified 56 tsunamigenic layers for the last 5000 radiocarbon years. It gives, on average, one tsunami event (with run-up height greater than several meters) in 100 years, that roughly coincides with the estimates obtained from the available historical catalog..

Fig.4 shows some results of processing of the data obtained over the whole area under study. It gives the histogram of tsunami occurrence within unequal time intervals lying between the identified (in most of sections) tephra markers. However, the recurrence of strong tsunamis is non-uniform during the last 5000 years. The clear maximum of tsunamigenic layers falls within the interval between 1800 and 2000 years B.P. This period coincides with the time of the increased tectonic activity in Kamchatka which was accompanied also by great volcanic-explosions ( Melekestsev et al, 1994).

Fig.5 shows the numbers of tsunamigenic layers (averaged in the 1000-year interval) found in a single borehole depending on the distance from the coastline. As one can expect, it exponentially decreases with distance, with only 1 event per 1000 years penetrating deeper 10 km in-land. For such a deep in-land invasion, the initial wave height at the coastline should be about 20-30- meters, that would correspond to the maximum possible tsunami of intensity of 4 and 5, known from historical evidence (Soloviev, 1972)..

The present paper gives only the first preliminary results for one area on the east coast of Kamchatka where paleotsunami traces have been specially sought and studied .The first findings turned out to be very promising. In the future, we plan to make a similar study of other parts of Kamchatka east coast, first of all, to investigate several bay areas south of Petropavlovsk-Kamchatskiy where. many historical tsunamis were observed. Analysis of the combined section of tsunami traces obtained for an extended part of the coast (hundreds of kilometers) will give an opportunity to make a spatial correlation of tsunami traces and, on this basis, to estimate magnitudes of the pre-historical events as well as to distinguish the regional events from the distant ones. It gives a potential possibility to extend the historical tsunami catalog for the region by at least 10 times, and on this basis to obtain more reliable estimates of the long-term tsunami risk for the Kamchatka east coast..

The study of paleotsunami traces on the east coast of Kamchatka was partly supported by RFBR Grants No. 96-05-64764 and 96-05-65938.

REFERENCES

Braitseva O.A., Melekestsev I.V., Ponomareva V.V., Kirianov V.Yu., Litasova S.N., Sulerzhitsky L.D. (1992) Tephra of the largest prehistoric Holocene volcanic eruptions in Kamchatka, Quaternary International, v.13/14, 177-180.

Braitseva O.A., Melekestsev I.V., Ponomareva V.V., Sulerzhitsky L.D., Litasova S.N. (1994) Ages of active volcanoes in the Kurile-Kamchatka region, Volcanology and seismology, No.4-5, 5-33 (in Russian).

Iida K. (1984) Catalog of tsunamis in Japan and its neighbouring countries, Aichi Inst. of Tech., Special Report, 52 pp.

Melekestsev I.V., Kurbatov A.V., Pevzner M.M., Sulerzhitsky L.D. (1994) 'Prehistoric' tsunami and large earthquakes in the Kamchatka peninsula (Kamchatka) as shown by tephrochronological data // Volcanologiya i seismologiya, No.4-5, 106-116 (in Russian).

Minoura K., Nakaya S. (1991). Traces of tsunami preserved in inter-tidal lakustrine and marsh deposits: some examples from northeast Japan, J. of Geology, 99, 2, 265-287.

Soloviev S.L. (1972) Earthquake and tsunami recurrence in the Pacific ocean, Volny Tsunami, Trudy SAKHII, vyp.29, Yuzhno-Sakhalinsk, 7-47 (in Russian).

Soloviev S.L. (1978) Basic data on the tsunamis on the Pacific coast of the USSR, 1737-1976 , Izuchenie Tsunami v Ontkrytom Okeane, Moscow, Nauka, 61-136 (in Russian).

Vikulin A.A. (1992). Migration of the largest earthquake foci in Kamchatka and north Kuriles, reiteration of earthquakes, Vulcanologiya i Seismologiya, 1, 46-69 (in Russian).

Zayakin Yu.A. (1996) Tsunamis on the Far East of Russia ,Petropavlovsk-Kamchatskiy, 85 pp. (in Russian).