The seismic observations on the territory of the Republic of Moldova on regular basis started in 1949 - on December 20 the first seismogram was received on the seismic station "Kishinev". The year of 1963 could be considered the starting point of the scientific investigations into earthquake engineering, when the first volume of scientific publications was issued dedicated to problems of tectonics and seismology of Moldova, prepared by the group of young scientists of Institute of Geology and Mineral Resources of the Academy of Sciences of Moldavian Soviet Socialist Republic (MSSR). After reorganization of two above-mentioned institutions into Institute of Geophysics and Geology in 1967, seismological investigations in Moldova have experienced qualitative changes. Yet, in 1968 in fundamental work "Seismic zonation of USSR" the results of the investigations were published about seismicity of the territory of Moldova and adjacent territories of Ukraine, on the basis of which the new map for seismic zonation for this territory was worked out as a part of the general map of seismic zonation of the territory of the USSR (till 1991 Republic of Moldova was the part of the USSR). The basis of that map, which was introduced into the code for the following 10 years, formed the data from instrumental seismic records, macroseismic observations and the results of detailed geological investigations, particularly the tectonics of the region. Generally, that map had the descriptive character.
Starting from 1968, the team of seismologists from Moldova, working together with their colleagues from Moscow (Institute of Physics of Earth, Academy of Sciences of USSR), launched research into quantitative methods of the seismic assessment. In late 60's and in 70's the complex of works for the evaluation of seismic intensity was performed; this method was proposed for the first time by Reznichenko J.V, correspondent member of the Academy of Sciences of the USSR.
The calculations of seismic intensity were performed not only for the territory of the Moldova, but for the whole Carpathian and Balkan territory, as for shallow foci, as for intermediate-depth earthquakes. Partially this work was carried out in frame of UNESCO contract (1975). In 1978 the work for upgraded map of the seismic zonation of the territory of the Moldova was finalized. The map compiled in terms of seismic intensity, based on macroseismic and tectonics data, as well as on quantitative estimates of seismic hazard (in frame of the method of seismic intensity evaluation), contains also probabilistic terms of the assessment of the given intensity recurrence in form of indexes attached to intensity values. This map is still present in building code. Three strong earthquakes (1977 with ¦=7.2, 1986 with ¦=7.0, 1990 with ¦=6.7), during two of which intensity 8 was observed on the southern territory of the Moldova, "tested" the reliability of this map. The group of the seismologists from the Institute was awarded the Moldavian State Award in field of science and technology for the work on compilation of the map.
From 1971 the investigations of seismic impact on buildings and structures were launched, the small network of the seismic stations was created, the methods of quantitative assessment of seismic intensity were developed as according to macroseismic, as to instrumental data. The seismic network from Kishinev includes stations in "free-field" as well as on the basement and different levels of buildings.
In 1979 the theory statistical interpretation of instrumental records in seismology and seismological survey was applied, based on the polarized analysis of seismic waves.
The years of 80's were the period of intensive development of the methods of mathematical modeling, development of databanks and interpretational methods of geological-geophysical data based on computer simulation. The logical scheme of the method of seismic hazard assessment was created, including elements of non-parametrical statistics, the seismic events are described with the help of Markov's processes, which reflects seismic field as a function, determining probability of occurrence of seismic event with certain intensity in given interval of time.
On the basis of data from the network of seismic stations, placed in buildings of different type, constructed in regions with different soil conditions, the engineering investigations of seismic resistant capacity of buildings were held, particularly changes of the dynamical properties of buildings after successive strong seismic motions. These investigations resulted in construction of regional spectral response function, which was introduced into the new building code. All strong and weak earthquakes occurred during the activity of the Institute of Geophysics and Geology, were recorded by the network of seismic stations, while the majority of the obtained seismograms was processed and subjected to spectral analysis. After each significant seismic event the comprehensive macroseismic investigations of large constructed areas were carried out. The archives of the Institute contain data on the recorded damage for thousands of buildings of different type.
In 70-80's the works for seismic microzonation of the territories of the most important cities of the Republic - Kishinev, Bendery, Beltsy, Cahul, Komrat, Leovo - were performed. Also a big amount of works for seismic microzonation of the construction sites of large industrial objects was done, such as nuclear power plant stations, hydrological structures, plants, etc. These applicative works, as well as special investigations in seismic testing sites, along with data on detailed investigations on soils settlement due to strong earthquakes from 1977 and 1986 allowed to evidence on total seismic intensity the influence of such geological factors as content of water and settlement, lithological content of soils and their thickness. Simultaneously, during these investigations there were tested different methodological approaches of seismic microzonation, such as method of acoustic stiffness, blasts and microseismic records.
Along with experience in regional seismological data investigations, the team of Moldavian seismologists gained experience in the analysis of the global seismicity. In 1981 the Laboratory of Seismology was suggested to participate (in frame of contract) in the international project under the auspices of International Oceanographic UNESCO Committee on edition of geology-geophysics atlas of the Atlantic and Pacific oceans. A small team of researchers of 6 persons, with the participation of professor Shebalin (Institute of Earth Physics, Moscow), headed by professor Drumea, had to prepare for the above mentioned two atlases chapters "Seismicity". Practically, the work has started in 1982 and lasted 6 years. Taking into considerations the fact that almost 90% of the earthquakes of the Globe occurs in Pacific zone, the volume of the initial seismological data was great. The content of work was the following: to collect and classify global and regional catalogues of the earthquakes; unification of the parameters of seismic events for the creation of basic catalogues; development of the legend for the maps; construction of general and detailed regional maps of epicenters; construction of deep cross-sections and recurrence laws of the most significant seismic zones; to introduce into the catalogues of the available focal mechanisms data; construction of time-frequency, energy and depth distribution of the earthquakes. In 1988 this work was completed and after its positive evaluation by international commission, which included leading geophysists from USSR, Japan, France, Great Britain, USA and Argentina, all the materials were passed to publishing house. In 1990 the Atlas of the Atlantic Ocean was published. Two years later the Atlas of the Pacific Ocean was planned to be published, yet, due to collapse of Soviet Union and financial difficulties resulted from that, its publishing didn't happen.
Simultaneously with the work on that contract the laboratory of seismology carried out a set of investigations on the seismicity of the oceans, resulted in series of articles and scientific reports.
Utilization of personal computers in scientific research occurred in our country with certain delay, in comparison with the rest of the world. The first personal computers appeared in Institute in late 80's. That allowed intensifying significantly the research and broadens the spectrum of the problems to be solved. The possibility to process vast amounts of data became real, graphical presentations of the results eased construction of maps. These years, the database of the seismological information was created in the Institute, including as the catalogue of the earthquakes and focal mechanisms of the studied region, as macroseismic information (data for maps "points-seismic grades"). The statistical algorithms for interpretation of field models of seismic intensity and seismic impact, alternative models of its assessment are considered in frame of probabilistic representation of seismic hazard.
In late 80's and early 90's the group of seismologists from the Institute has performed the series of works for determination of the velocities of seismotectonic deformations. In the basis of that method, which provided quantitative description of the contribution of seismicity into tectonic deformation, is the model of the seismic flow of rock masses suggested in 1977 by Reznichenko. For calculation of the velocity of deformations during seismic flow of rock masses the complex of parameters of seismic stage is investigated for certain region as well as components of tensors of seismic moments, calculated from the solutions for focal mechanisms and magnitudes of seismic events. The velocity of the seismotechtonic deformation was calculated and graphically represented in form of maps (with isolines) for Vrancea zone and Carpathian-Balkan region, as well as for Caribbean region and Southern Sandwichev islands (in North-Eastern part of this structure there exists a local zone of high concentration of intermediate-depth seismic sources with small stable recurrence interval of magnitudes, similar to Bukaramang in Columbia and Gindukush in Afghanistan). These local zones were considered having in view the similarities with Vrancea zone for better understanding of the seismotectonic processes in intermediate-depth seismic zones.
In 1990-95 the Laboratory of the Seismology performed the investigation of the horizontal discontinuities of the upper mantle for the territory of Moldova and neighboring Romania on the basis of the analysis of teleseismic P-wave propagation and Stuart method, based on differences in (¦¦-¦) waves. For calculation purposes the computer program was compiled, which used methods of spheroid geodesy for the determination of reflection points' coordinates and analysis of equations of seismic raze.
During the years of the activity of the Laboratory for Seismology the work for compilation and upgrade of catalogues of earthquakes of the studied region has been carried out as on the basis of own seismic network, as on the basis of data from more modern Romanian network, as well as the seismic network of former Soviet Union, and later Russia and Ukraine. According to the demands and tasks the data from the international seismological agencies, such as ISC, NEIS, are used. The information on the historic seismicity was reviewed and reassessed on the basis of different archive data and new map of seismic isoseists for the period of 1738-1995 was compiled. The permanent analysis of variation of the degree of seismic activity and focal mechanisms of Vrancea earthquakes is performed. Among the significant results of last years in this regard one can mention establishment of correlation between the position of the focus on one of the edges of epicentral zone, type of focal mechanism and the direction of rupture and macroseismic effect on the territory of Moldova (for close values of magnitude). It was established that the most the most severe are those foci, which have the orientation of rupture surface in azimuth 30-600.
The Laboratory of Survey of seismic effects has launched recent years project aimed on utilization of GIS technology for storing and processing of the available information. This project allowed constructing of seismic macrozonation map of the territory of the Republic in digital format using recent studies [Publications in English, 33] on attenuation functions for Vancea seismic zone, for PGA and EPA for different return periods. Besides, the study of the central part of Kishinev using GIS technology [Publications in English, 37,42] allowed making certain advances in seismic risk and seismic microzonation studies.
Seismic network of the Republic of Moldova
The seismic network of Moldova consists of 5 seismic stations, situated in Kishinev, Cahul, Leovo, Soroky and Djurjuleshti. The names of the stations correspond to the name of the city or village. Such, seismic observations in the Republic are provided currently by stations "Kishinev", "Cahul", "Leovo", "Soroki" and "Djurjuleshti", while station "Kishinev" is the basic one, while the rest four of the stations are regional.
Such, currently the above-mentioned stations form the network of Center for Experimental Seismology of the Institute of Geophysics and Geology Moldavian Academy of Sciences.
Seismic station "Kishinev" (KIS)
It is located on the ground floor and basement of 2-storey building of geophysical observatory built specially for this purpose in Kishinev. Coordinates of the station: Latitude 47.00 N, Longitude 28.82 E, height 185 m. Subsoil: silty clay, clay. Thickness of the sedimentary soft soil deposit - 150-170 m. Seismographs are installed on the basement on the special concrete blocks. The recording instruments are situated on the ground floor in separate rooms for each type of the equipment.
All the stations are equipped with instruments with galvanometer recording, while the recording is performed directly on photo paper. The recording instruments of the station "Kishinev" is working in different amplitude and frequency ranges. In the rest of the stations, the recording instruments working in short frequencies are installed.
As far as the Republic of Moldova is situated in seismic active zone, stations are also equipped with strong motion instruments, working in "waiting mode" (triggered at certain threshold shaking level).
The seismic network of Moldova performs permanent seismic monitoring and interpretation of the obtained results. The work is based on the Instruction for Performing and Processing of Records on Seismic Stations for the Unique Network of Seismic Survey (UNSS).
That reduces recording of both near and body waves from distant seismic events. At average, per year the station records and processes round 1500 earthquakes. The information is sent by phone and fax to Central Experimental Methodological Group of Geophysical Service of Russian Academy of Sciences (CEMG of GS of RAS). The work of the station is monitored by group of geophysical service of Russian Academy of Sciences. According to the results of the processed records the concluding reports are performed, seismological bulletins, urgent notes about the global severe earthquakes, all the information is transmitted to CEMG of GS of RAS, Obninsk by phone and fax.
In view of better integration of the seismic network from the Republic of Moldova into international cooperation, the upgrade of the available recording instruments would be desirable in nearest future.
