In the history of the development of human society, earthquakes and their destructive consequences hold a significant place in terms of material losses and human sacrifices. Generally, earthquakes have a volcanic or tectonic origin, but there can also be artificial causes such as dynamiting explosions in quarries or underground atomic explosions.
Tectonic earthquakes are the most frequent and the energy they release extends over larger areas of the earth’s surface, having the most emphasized destructive effect. Suffice it to mention the California earthquake from 1906 that has caused the longest fault line, of 270 miles, known as the San Andreas Fault line.
The seismic hazard in Romania is due to the subcrustal seismic source Vrancea and several surface seismic sources (Banat, Shobla, Fagaras, Dobrogea). Vrancea source is responsible for the seismic hazard in about two-thirds of the Romanian territory, while the surface sources contribute more to the local seismic hazard. Generally, the earthquakes foci are concentrated inside the earth’s crust, at depths of 5-70km.
Historical sources mention numerous earthquakes produced on the territory of Romania, accompanied by significant loss of human lifes and material damages.
Among the strongest earthquakes, which have had a high destructive effects, the histography of earthquakes mentions the following: the earthquake from August 22, 1473 when the Neamț monastery was damaged, the one from June 11, 1738 which destroyed the tower of the Royal Court from Bucharest, the biggest earthquake from 26 November 1802 that has devastated Bucharest causing the complete collapse of the Colței Tower. But the biggest earthquakes recorded in the last century were: the one from November 1940 (with a depth of 160 km and a magnitude of 7.4 degrees on the Richter scale) and the earthquake from March 4, 1977 (with a depth of 90-100 km , and a magnitude of 7.2 degrees on the Richter scale), as a result of which many victims, numerous damages and destruction of buildings and significant material losses were recorded.
In March 1977, the first recordings of the seismic movement were obtained, by using seismographs and accelerometers installed in Bucharest at the INCERC headquarters and in the E5 block of flats from Balta Alba. Recordings were also obtained in Galati and Vrancioaia, which processing allowed a deeper knowledge of the characteristics of the seismic movement. The recordings highlighted the type of the movement from March 4, 1977 consisting in:
- An aftershock with a magnitude of 5 degrees on the Richter scale, followed by 2 separate main shocks with a magnitude of 6.5 degrees on the Richter scale.
- The final shock with a magnitude of 7.2 degrees on the Richter scale occurred 19 seconds after the first rupture.
The extent of the destruction and damages produced in 1977 required a broad analysis of the behavior and damage of various categories of buildings at the national level in order to better substantiate the seismic zoning of Romania’s territory, the level of insurance required as well as the design norms for the future.
The analysis carried out by researchers from the National Institute of Earth Physics, INCERC Bucharest and Iasi, Universities and the relevant design institutes in the “Monograph of the earthquake from March 4, 1977” showed the following observations and conclusions:
- The old constructions (built between the two wars) behaved inappropriately, recording collapses and serious damages, because at the time of their design and construction there was no obligation to calculate the seismic actions.
- The new constructions built after 1963 based on the norms developed in 1963 and 1970 had, with few exceptions, a satisfactory behavior, although some water castles and the OD16 block of flats from Balta Alba (Bucharest) have collapsed. The structure of the blocks of flats with a flexible ground floor also behaved inappropriately, even registering a collapse in Lizeanu area.
- The earthquake of March 1977 highlighted the need to increase the level of insurance against the seismic action compared to the provisions of the regulations from 1963 and 1970, as well as the need to endow the territory with the necessary equipment for tracking and recording the seismic movements.
- The national legislation must include, in addition to the seismic design codes of the various categories of works, the necessary measures to ensure the protection of the population and property, taking into account 3 categories of measures:
o preparation measures for a strong earthquake consisting in material and logistical insurance for the necessary emergency interventions;
o Emergency intervention measures to avoid fires, temporary support, rescuing the population from under the ruins, etc.
o Rehabilitation and consolidation measures for buildings affected by strong earthquakes.
The design codes must take into account the main factors that ensure a good behavior and for the removal of the risk of collapse, among which are mentioned:
- Structural type and spatial distribution of stiffnesses;
- The most significant geometric sizes such as the shape in plan and the total height;
- Materials used and their physical-mechanical properties;
- Distribution of partition walls and the percentage of holes;
- Type of foundation adopted and foundation conditions;
- Construction destination and its category of significance;
- Execution quality;
- The effect of earthquakes over the buildings is complex and cannot be defined exclusively by its degree without taking into account the factors mentioned above that must be taken into account when designing and building a structure.
The causes that can lead to the destruction of a building must always be taken into account, namely:
- Execution errors
- Design errors related to the wrong conception of the resistance structure, the resistances distribution, the materials quality, the spatial configuration of the structure
- Calculation errors due to false schematizations of the resistance structure, the use of wrong analysis methods, the erroneous assessment of input data, the undersizing of some elements and parts that are heavily demanded
- Errors in the geotechnical data regarding the compressibility of the foundation soil, its sensitivity to moisture, susceptibility to subsidence, the dominant period of the soil
- Errors in the design rules: a project, even if it is rigorously drawn up based on the design rules, can be compromised if the rules provisions do not comply with the actual facts occurred during a strong earthquake.