Heinloo, A., Heinloo, O. and Sildvee, H., 1996. Historical overview of instrumental-seismical observations in Estonia, Bull. of the Geological Survey of Estonia, 6/1,

This paper represents the history of instrumental-seismical observations in Estonia during 1894-1910, 1931-1944, 1987-1996. The activity of historic persons prof. G.V. Levitski, A.J. Orlov, prof. J. Wilip and H.  Masing is analysed in detail. The descriptions of seismological stations in Tartu and Tallinn are given and some problems of observation are discussed.

Keywords: seismology, horizontal pendulum, earthquake, Estonia.

In the 90-ies of the previous century the reactionary policy of the Russian Empire became obvious for the Baltic countries.

In 1893 Dorpat (at present Tartu) was called Yuryev and the University was named Yuryev University.

In 1894 G.V. Levitski (1852-1917) was appointed the director of Astronomy Observatory of Yuryev (Tartu) University. Ukrainian Professor Levitski transformed from Kharkov University to Tartu a new scientific branch - seismology. Levitski had started in Kharkov in 1893 the research of horizontal pendulum in order to learn how to use it in measuring and registering of earthquakes.

By Levitski's request Repsold improved Zöllner's horizontal pendulum that was primarily meant to measure the lunisolar attraction (Levitskij, 1902).

Further on Levitski adjusted Zöllner's pendulum by raising its weight to measure nearby earthquakes. This heavy pendulum with mechanical way of registration has been named Zöllner's-Repsold's-Levitski's pendulum or Levitski's horizontal pendulum (W. Struve - nimelise ... , 1969).

At the end of 1896 Prof. Levitski installed apparatus with two Rebeur-Paschwitz horizontal pendulums in the cellar of the Yuryev University Astronomy Observatory and started to register earthquakes. In November 1897 he installed another Zöllner's horizontal pendulum made by Repsold. In 1897 eighty earthquakes from which 25 were powerful and very powerful were recorded at the Yuryev Seismological Station. The station did not work for 43 days. Thus regular recording of earthquakes had started (Levitskij, 1898).

In 1900 the Permanent Seismology Commission (PSC) was founded at the Imperial Academy of Sciences, St. Petersburg. One of the founders was Prof. Levitski. The aim of the commission was to work out the network of seismological stations in Russia.

Control and delivery of the apparatus ordered from abroad for Russian seismological stations were entrusted to Prof. Levitski. Later on Prof. Levitski organised the manufacturing of horizontal pendulums at mechanic Schulze's workshop in Yuryev.

Since 1902 Levitski was editor of the Seismology Bulletin issued by the PSC. This bulletin contained information on earthquakes that were recorded at the 1st range seismological stations of Russia as well as at the Yuryev Seismological Station. The funds of Tartu University Library contain Seismology Bulletins issued in 1902-1907.

Registration of the arriving time of seismic waves into seismological station was very seldom possible in 19th century because of the absence of trustworthy contact clocks. It was not possibile to compare contact clocks with exact clocks or chronometers. In case of horizontal penduli working with mechanical way of registration the allowed mistake made by contact clock was 2-3 seconds in 24 hours.

In 1903 Levitski checked in Tartu the contact clocks brought for Russian seismological stations from Germany (Levitskij, 1904). The biggest mistake in 24 hours was 6 sec. Levitski concluded that more expensive clocks should be brought for seismological stations. For example, at the Yuryev Seismological Station with contact clocks of high quality the maximum mistake a day was 0. 6 sec.

Big problems arose to get new high quality registration apparatus for the Yuryev Seismological Station. A.J. Orlov who observed a registrator in Tartu constructed by Strasbourg mechanic Bosch, wrote: "In spite of the fact that in Russia many horizontal penduli have been installed to record earthquakes, none of them owns a satisfactory registration. All registration apparatuses move so irregularly that it's impossible to record the pendulum movement and therefore seismograms are not suitable for precise research." Orlov found out that the mistake of determining the beginning the seismic wave by irregular movement of registration apparatus may be up to 6 seconds (Orlov, 1907).

Levitski pointed out (Levitskij, 1903) that when comparing the data of one and the same earthquake given by different seismological stations, only the arrival time of the first wave can be trusted because in different countries different phases of wave are marked with one and the same letter. The required exactness of distinguishing wave phases on seismograms was less than 6 sec. Levitski used the seismograms of the Yuryev Seismological Station to determine the rate of spreading of seismic waves and the epicentres coordinates of strong earthquakes (Levitskij, 1906; 1907; 1908).

Levitski compared seismological observations made between December 19, 1906 and January 20, 1907 in Pulkovo, Russia (began its work on December 9, 1906) with Golicyn's seismograph and in Yuryev with Zöllner's seismograph. He studied the arriving time of P-wave into the above-mentioned stations. In this period all earthquakes registered in Pulkovo were also registered in Yuryev, besides 3 more earthquakes were recorded in Yuryev. The conclusion was that the sensibility of seismographs at Yuryev and Pulkovo stations was practically equal, but in the Pulkovo Station the microseismic background was stronger than in Yuryev (Levitskij, 1908).

In August 1908 Professor Levitski left Yuryev and started to work as a curator in Vilnius study area. Levitski's student A.J. Orlov (1880-1954) who had taken part in in-service courses at Göttingen University, was appointed the head of the Yuryev Seismological Station. Orlov began to use two horizontal penduli to study earth deformation under influence of tidal motion (lunisolar attraction). The penduli were still situated in Tartu on Toome hill, in the room of gunpowder cellar. Thanks to his research Orlov became world famous.

On October 14, 1910 in St. Petersburg at the meeting of PSC the status of the Yuryev Seismological Station was under discussion. Taking into consideration the fact that no local earthquakes had been recorded in Yuryev and there was no need to detect far-away earthquakes due to the existence of the Pulkovo Seismological Station nearby, it was decided to stop the regular observations at the Yuryev Seismological Station. It was decided that Orlov's research should be supported in the future. At the end of 1912 Orlov left Tartu and took the post of the head of astronomy observatory in Odessa. Thus, the Yuryev Seismological Station had finished its work. The Library of the Tartu Observatory contains seismograms of the Yuryev Station from 1904-1908.

The second period in the life of the Tartu (former Yuryev) Seismological Station began on January 1, 1931 (Frisch, 1932). The head of this station was the first Estonian Professor of physics Johann Wilip (Johan Vilip) (1870-1942). Prof. J.Wilip installed in the station a full set of Golicyn-Wilip-type seismographs which had been manufactured by precision mechanic Hugo Masing (1873-1939) in his firm in Tartu (Masing, 1985). Twenty two full sets of seismographs (one vertical seismograps and two horisontal seismographs), manufactured in this firm during 1926-1939, were sent to the biggest seismological stations of the world in their order. The best set, the twenty-third one, was installed at the Tartu Seismological Station. J.Wilip's seismological station was in work until the beginning of the World War II in 1939. Seismographs were taken away by the Germans in 1944 (Prüller, 1975).

The seismograms of the J.Wilip's seismological station from the years 1931-1939 are preserved in the funds of Tartu University Library.

Physicist M.Elango (1968) has compiled the list of J.Wilip's scientific works. Unfortunately only one popular-scientific article can be found there which was written by J.Wilip (1931)himself after opening the Estonian Seismological Station. He noted that the task of the station was to gather observation materials about earthquakes. In this article J. Wilip denied the possibility of earthquakes in Estonia. Besides the above-mentioned article there is one written by Karl Frisch (1932) where the parameters of Wilip's apparatus are given. In this article the author analyses 3 earthquakes registered in Tartu in 1931: the quakes in Mexico and China on January 15 and in New Zealand on February 3. According to the seismograms the distance and direction of epicentrum and the angles were fixed under which the first wave was relieved from the Earth, as well as the reflected waves.

Unfortunately the bulletins of J. Wilip's seismological station have not been found. The data of the station situated in Tartu are not fixed in the journals of this period neither in Russia nor Western Europe. It was unexpected because J.Wilip had got corresponding experiences - the Pulkovo Seismological Station started to issue the bulletins after J. Wilip had been appointed the head of the station in 1911. J. Wilip was the head of the Pulkovo Seismological Station until 1920 when he returned to Estonia. J. Wilip's (1930) main affection in the field of seismology was working out seismic equipment.

On October 25, 1976 there was an earthquake in Estonia near Osmussaar Island with a magnitude (M) of 4.75. Information on this earthquake can be found in several papers (Ankudinov et al.,1994; Kondorskaya et al., 1988; Miidel, 1994; Nikonov, Sildvee 1988; Nikonov, Sildvee, 1992; Sildvee, Vaher, 1995). At that time there was no seismological station in Estonia. The seismograms of the Osmussaar earthquake from seismic tapes of six Finnish seismological stations are published by Kondorskaya (Kondorskaya et al, 1988) and the source mechanism inferred from surface - wave recordings is given by Slunga (1979).

The third period in the work of the Tartu Seismological Station started on October 15, 1987, when seismologists from Byelorussia installed seismic equipment produced in the Construction Bureau of Sciences of the USSR in the cellar of the Tartu Old Observatory (lat= 58° 22.8´ N, lon = 26° 43.3´ E). The station lies 67 m above sea level, the soil under the station is moraine. At the same time the seismological station in Tallinn started working. The Tallinn Seismological Station (lat = 59^o26.2' N, lon = 24^o44.5') was situated on Toompea, 9 Toomkooli Str. in a specially furnished room of medieval cellar. The station lies 42 m above sea level, the soil under the station is limestone.

At both Estonian stations three components of surface motions have been measured (N­S, Z and E­W). The speed of self-recording tape is 60 mm/min at Tallinn and 30 mm/min at Tartu Station, the magnification coefficients are 4,000 and 10,000 correspondingly, the time indices are obtained from quartz clocks AKV-214 and AKV-2M. The beginning of seismological event can be determined with the exactness of  0.1 sec on the tapes of the Tallinn Station (in case the background doesn't disturb), and with the exactness of  0.2 sec on the tapes of the Tartu Station. At both stations short-period transducers of CM-3 type are oriented to detect weak local quakes. In 1988 - 1991 Estonian seismological stations were financed by the Institute of Geochemistry and Geophhysics of Byelorussian Academy of Sciences and seismic tapes of the Tallinn and Tartu stations were sent to Byelorussia. The tapes have not been returned up to the present day. Since 1991 the seismological stations are financed by the Republic of Estonia. The equipment installed at the Tartu Seismological Station was purchased from the Byelarussian Academy of Sciences by the Institute of Geology of Estonian Academy of Sciences, while that of the Tallinn Seismological Station already belonged to the Institute of Geology. Since 1994 both stations are in the possession of the Geological Survey of Estonia and belong to the monitoring network of the Republic of Estonia financed by the Ministry of Environment.

There are two types of seismic events registered at the Tallinn and Tartu seismological stations: 1) distant earthquakes (distance of epicentre > 1000 km); 2) adjacent technological explosions. The data of seismic events recorded at the Tallinn and Tartu stations are presented in Table.

Table. Number of seismic events recorded at the Tallinn and Tartu stations.

Year	Type of event	Tallinn	Tartu
1991	earthquakes technological explosions total	46 216 320	107 253 546
1992	earthquakes technological explosions total	118 342 486	113 407 529
1993	earthquakes technological explosions total	141 381 522	187 450 637
1994	earthquakes technological explosions total	131 354 486	334 500 835
1995	earthquakes technological explosions total	178 245 423	439 501 943

A bit less than a thousand earthquakes with a magnitude greater than 5 take place on the globe every year, almost a half of which are recorded by Estonian seismological stations.

A catalogue of faraway earthquakes registered at the Tartu and Tallinn seismological stations between 1991 and 1994 has been compiled in which the differences between theoretical and practical arriving times of a longitudinal wave at the stations are less than 5 sec.

The data of man-made seismic events obtained during the last four years were used for evaluation of seismological background when expert opinions of the depository of radioactive substances of SILMET Ltd. (Sillamäe town) were made in November 1996.

Evaluation of the man-made seismological background has been carried out in order to assess the state of the environment in quarries and neighbouring areas (Harku quarry near Tallinn, Karinu quarry near Järva-Jaani, Anelema quarry near Pärnu-Jaagupi), as well as for the expert survey of the main building of Püssi PPK Ltd. (influence of the blastings in Aidu quarry). In the vicinity of quarries the influence of blasting is notable but short-time observations do not guarantee a reliable overview of the seismological background.

At present in the seismological observations carried out in Estonia two trends can be distinguished: 1) global investigations and 2) local observations. Firstly, the Tartu Seismological Station participates in global seismological investigations. It has become possible due to the use of modern seismological equipment (Quanterra Q380 Very-Broad-Band seismograph together with STS-2 seismometer) installed by Potsdam Geoinvestigation Centre (GeoForschungsZentrum, Potsdam, GFZ) in June 1996. According to the agreement between GFZ and the Geological Survey of Estonia the apparatuses were installed at the expense of GFZ, while observations will be financed by the Estonian side. The Tartu Seismological Station is a part of GEOFON-project (GeoForschungsNetz) lead by GFZ. The GEOFON, consisting of about 20 seismological stations located all over the world, has been called into being in order to investigate the interior and seismic activity of the Earth by studying seismic waves reflected from deep-lying strata. Seismological studies and computer modelling should enable to predict powerful earthquakes in the future.

Processing the seismograms of all seismological stations requires remarkable computer resources. It is carried out mainly in USA at IRIS (Incorporated Research Institutions for Seismology), USGS (U.S.Geological Survey) and NEIC (National Earthquake Information Centre) where the data of GEOFON and other seismological networks are concentrated. Seismological data are used by different research centres all over the world (including GFZ). The data of earthquakes and registered seismograms can be obtained through SPYDER (System to Provide You Data from Earthquakes Rapidly) from the Internet address drm.gfz-potsdam.de (use Telnet protocol, login:drm, password:geofon). International code of theTartu Seismological Station is TRTE.

At present there is no computer technology enabling to carry out fundamental research at Estonian seismological stations. Therefore cooperation with neighbouring countries is important, especially with Sweden and Finland that have necessary equipment and rich experience in this sphere. Nordic countries are interested seismological data recorded in Estonia and in cooperation with GEOFON they are planning to switch the Tartu Seismological Station into the Nordic seismological network.

The second trend in seismological observations in Estonia is studying local weak earthquakes. These investigations are important as well, because at different times several local earthquakes have taken place here. During 1988 - 1991 Finnish seismological stations registered 50 weak earthquakes occurred in Estonia. The cooperation with Finnish colleagues in this sphere would enable to carry out more detailed studies. In addition to local seismological events all man-made events or blastings in quarries, as well as distant earthquakes are recorded at the Tartu and Tallinn stations.

Every year Estonian stations record several seismological events which are not reflected in charts of earthquakes or in the blasting reports of Northeast Estonian quarries. Unfortunately the present equipment of stations does not allow full identification of these events, because for proper location of a seismological event the data of three seismological stations are required. At present there are only two stations in Estonia, both writing on paper, which means that measurements are made manually they are not sufficiently exact. In Tallinn there is intensive microseismical background which disturbs investigations of locally important high-frequency district.

Lately the conditions of carrying out seismological observations have improved. The Tallinn Seismological Station has been transferred to Suurupi (about 15 km from Tallinn) where there is no high-frequency background conditioned by town noise. The recording system of the seismograph is planned to be reconstructed, in the result of which the seismograms will be recorded digitally on magnetic tape. The latter would enable to study the seismograms by computer.

The Tartu Seismological Station can use data of digital Quanterra seismograph. After purchasing a suitable computer the digital seismograms will be processed by computer. The vacant analogseismograph could be re-installed at another seismological station. Thus, there would be three seismological stations on Estonian territory that would enable to determine the location of seismological events recorded.

Finally it should be mentioned that in December 1996 was celebrated the 1st centenary of the first seismological station in Estonia, the head of which was Professor of Physics G.V. Levitski.

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