Vrancea Earthquakes: Tectonics, Hazard and Risk Mitigation

Striking progress has occurred in recording strong shaking from damaging earthquakes since the 4 March 1977 Vrancea, Romania, earthquake (M_w = 7.4). Free-field accelerometers in seismic regions have significantly increased in number, geographical distribution, and dynamic range. Many digital instruments are now operational. In Europe and the Middle East alone, the number of individual triaxial recordings during the last 30 years exceeds 2500. Of particular importance, the 1989 Loma Prieta, 1992 Landers, 1994 Northridge (all in California), and the 1995 Hyogo-ken Nanbu, (Kobe) Japan, earthquakes have provided not only hundreds of key free-field accelerograms but many instrumental records of building response.In Romania there are now over 70 strong motion accelerometer stations and an important data base of seismic ground motion records is accumulating, including records from the intermediate depth Vrancea earthquakes of 1977, 1986, and 1990. The Bucharest accelerograms of the 1977 earthquake are of the greatest value in predicting future strong ground motions in Bucharest. The 1 to 2 sec velocity pulse in the horizontal components (repeated in the 1986 earthquake) is generated by directivity focussing of energy from the moving source rupture at intermediate depth (about 80 to 120 km) towards Bucharest. The thick alluvial layer under the city further amplifies the 1 to 2 sec S wave energy. The 1977 Bucharest record may, therefore, be taken as “characteristic” of the maximum input design motions in Bucharest.Attenuation laws for hazard analysis of intra- and interplate crustal earthquakes in many seismic regions have recently improved. In the near field, the dominant velocity pulse (or “fling”) from the radiation mechanism and directivity for strike-slip and thrust sources has been found to differ significantly between fault-normal and fault-parallel azimuths. Also, reflections from deep crustal structure can contribute to the seismic wave strong motions. Unfortunately for ground motion estimation in Romania, near-source recordings from intermediate depth sources and M_w > 7.0 earthquakes around the world remain sparse. To obtain a comprehensive distribution of recordings for hazard estimation in Romania, many more digital instruments are needed in different geological conditions and at various azimuths.Forensic earthquake engineering to understand structural vulnerability depends on correlation between ground motion records near to engineered structures (particularly from near-structure reference stations) and recorded motions of the structure. As well, digital recording systems can now provide near-real-time warnings and intensity maps, time histories, and spectral variations over the shaken region. All high seismic hazard regions need such digital recording systems, linked to absolute time through inexpensive GPS satellite telemetry.

The seismicity of the Romanian Vrancea area has peculiar features: (1) strong earthquakes occur at intermediate depths in a very narrow source volume; (2) the seismogenic zone is situated beneath continental crust, at the SE corner of the highly arcuate Carpathian arc; (3) no evidence for active ongoing subduction is found today. Several geophysical models were developed that tried to provide an explanation for the localization of seismicity at depth (Fuchs et al., 1979; Oncescu, 1984; Tavera, 1991). They contain ideas on interaction of a paleo-subduction zone with more recent subduction and include initial concepts of slab break-off. In recent years new facts and concepts came up that merit a re-evaluation of the tectonic scenarios related to Vrancea seismicity.

On March 4, 1977, at 19:21 UT, a destructive earthquake with M_w= 7.4 hit the Vrancea region in the Eastern Carpathians. The 100 km deep event was felt at distances greater than 2000 km and produced extensive damage in the epicentral area as well in Bucharest. Ambraseys (1977) and Hartzell (1979) were the firsts to note the particular shape of the (single) strong motion record available, recorded by a Japanese SMAC accelerograph: a low frequency pulse-like signal with a peak value close to 210 cm/s2 at the INC site in Bucharest (see Figures 1 and 2). The complex character of the rupture and the focal mechanisms of the individual events have been widely studied by Müller et al. (1978), Fuchs et al. (1979), Räkers and Müller (1982) and Iosif et al. (1983).

Earthquake catalogues are the primary input data for seismic hazard computations. An earthquake catalogue should be up-to-date, complete, homogeneous and accessible. There are several earthquake catalogues for the Romanian earthquakes published until now (e.g. Purcaru, 1979; Radu, 1979; Constantinescu and Marza, 1980; Radu, 1991; Trifu and Radulian, 1991), but none of them fulfil all above-mentioned criteria.

Analyzing twenty years recording of Albanian Seismological Network (ASN), in this paper are given some characteristics of recent seismicity of the country. The construction of the catalogue of earthquakes of Albania with M_L≥3.0 for the period 1976–1995 is also described here. On the basis of this catalogue, calculating the seismic energy released for 220 cells covering all the Albanian territory, a panorama of distribution of seismic energy and cumulative magnitudes is provided which gives insight to a better understanding of seismicity of the country and helps on its hazard assessment.

The accelerograph network of Building Research Institute INCERC Bucharest consists of 71 SMA-1 accelerographs located in all seismic zones of Romania. A number of 51 accelerographs are mounted at ground level in low-rise buildings. The INCERC network furnished more then 110 useful records obtained during the Vrancea earthquakes of March 4, 1977 (Richter magnitude M = 7.2), August 31, 1986 (M = 7.0), May 30, 1990 (M = 6.7) and May 31, 1990 (M = 6.1), codified in the following as events 771, 861, 901 and 902.

The free-field accelerograms from Republic of Moldova come from four seismic stations in Kishinev, capital of Republic of Moldova, as well as from other two stations: Kahul, near the Romanian border (≈46° lat.N), and Krasnogorka, 60 km from Kishinev.

We have been developing in France a specific methodology which takes community vulnerability into account. Those microzonings are normally presented as a map at scale 1: 5,000 or 1: 10,000 of the particular effects that are due to site effects, which concern the modification of the seismic signal; taking them into account leads us to the identification of homogeneous zones from a ground shaking view point. In order to determine site effects we use different methodologies: a numerical one of course but above all an experimental one. We use two kinds of experimental methods: the one uses micro earthquakes to establish transfer functions (comparing record at studied site and substratum), the other uses microtremor (H/V process). We translate the result into French regulation by drawing up a map with homogeneous areas where transfer function is the same. It is then possible to attribute to each of these areas a coefficient t’(T) that should be applied to the So(T) regulation spectrum. We also can draw a map: giving the equal amplitude curves or the equal frequency curves. These maps render not only the seismic response of the site but also the shape aspect of alluvium basins and in particular the depth of alluvium-substratum contact.

The Vrancea seismoactive region, characterized by intermediate-depth earthquakes, is the quake source that has to be taken into account for microzonation purposes of Bucharest that could suffer serious damage also because of the severe local site effects. The strong seismic events originating in Vrancea have caused the most destructive damage experienced on the Romanian territory and may seriously affect vulnerable high risk constructions (such as nuclear power plants, chemical plants, large dams, pipelines etc.) located on a wide area, from Central Europe to Moscow.Realistic numerical simulation, describing the propagation of the seismic wavefield generated by a given quake in a complex geological structure, is a powerful tool, that may be efficiently used to estimate the ground motion for microzonation of the whole Bucharest area.The realistic modelling of ground motion is carried out by means of a sophisticated hybrid technique that combines modal summation (Panza, 1985; Vaccari et al, 1989; Florsch et al., 1991; Panza, 1993; Romanelli et al., 1996) and finite difference (Fäh 1991; Fäh and Panza, 1994; Fäh et al., 1994). The input data necessary for computations are the laterally variable anelastic structural model and the focal mechanism of the seismic source.The medium is modelled with a regional layered structure (bedrock structure), containing the seismic source and assumed to be representative of the path from the source to Bucharest, and a local structure, that is a NE20°SW oriented cross section, describing the local structure of Bucharest, along the studied path. The seismic source is described by a double-couple, buried in a layered medium, and corresponds to the focal mechanism of the May 30, 1990 Vrancea earthquake. The upper frequency limit considered in the computations is 1.0 Hz, and this allows us the modelling of seismic input appropriate for ten storeys and higher buildings.The simulated signals are satisfactorily compared with the available instrumental records from Magurele station (44.347°N, 26.030°E), and stability tests are performed with respect to the variation of focal mechanism, regional and local structure.

The seismic design of low frequency systems differs considerably from the normal practical situation, in the sense that is based on spectral velocities and displacements rather than on spectral accelerations. This fact imposes some limitations on the standard spectral analysis that must be considered. First, the amount of processing spurious noise present in the strong motion records must be known and kept bounded at all times. The above is particularly true for analog accelerograms, for which the instrument characteristics and the procedures used to retrieve the original data affect significantly the reliability of the long period range of response spectra. Other sources of enor, however, are not man-induced and therefore escape human control. This is the case of uncertainties that emanate from geologic or geophysical phenomena, like anomalous attenuation laws (e.g., Vrancea earthquakes in Romania), fault directivity, site effects, and so on. Actually, all this factors decrease the reliability of the spectral ordinates at low frequencies and need to be carefully evaluated. Besides they are important in assessing the seismic performance of base-isolated structures at high risk conditions -such as epicentral areas, soft soil sites, etc.— for which special caution and restrictive design measures are specified in the corresponding building codes

Bucharest suffered on repeated occasions strong damages as a result of the Vrancea intermediate-depth earthquakes, located in a confined isolated focal volume beneath the Eastern Carpathians Arc bend. The particular seismic energy radiation and the seismic cycle in Vrancea focus, characterized by 2–3 major shocks (M _w > 7.0) per century, lead to a high level of the seismic risk in Bucharest.

The estimation of seismic site effects is becoming an important part of the scenario-like modelling approaches used to predict the seismic strong motion in vulnerable environments. We show how detailed numerical modeling can lead to estimates of the seismic amplification due to site effects that are consistent with observations.Such modeling allows for the full utilisation of both the seismological and the large amount of geological, geophysical and geotechnical data, already available. Therefore, detailed modelling of the ground motion for heterogeneous media (up to several Hz) can be used for immediate microzonation purposes, without the need to wait for the occurrence of the next big event in the not always densely instrumented area of interest.

The importance of site effects due to soil condition in Bucharest was firstly observed during the 1977 Vrancea earthquake. A substantial understanding of the effect of near-surface geology on the characteristics of the ground motion came later, from the analysis of the frequency content of the 1986 Bucharest accelerograms and of corresponding soil profiles at recording sites. The paper focuses on the correlation of the frequency content of the recorded accelerograms in Bucharest with the properties of the near-surface geology (stratigraphy, dynamic characteristics of the soil layers, etc.).To a great extent, the near-surface geology study for the city of Bucharest is based on laboratory tests made by Metroul S.A., during Bucharest subway construction works. Soil layers in thousands of meters of boreholes were analysed.

To quantify the variations of the site responses in downtown Bucharest the horizontal- to vertical-component spectral ratio method (H/V) on different kinds of seismic data was applied. Starting September through November 1997 we have recorded ambient seismic noise at 16 sites in Bucharest. In addition, we used ambient noise and small earthquake records from the instruments which had been deployed during an aftershock campaign in June/July 1990. Furthermore, we have examined S-waves of strong Vrancea earthquakes.The ambient noise ratios, determined on two profiles which cross in the center of Bucharest, show a remarkable constancy of the main resonance period. They range in a narrow band from 1.2 to about 1.6 seconds with an average value of T₁=1.36±0.14 s. A second but small amplitude resonance at even greater periods is recognizable with T₂=5.22±0.9 s.First recordings of Vrancea earthquakes by the new K2 network corroborate that the high frequency peak ground accelerations (PGA) may differ up to 2–3 times in downtown Bucharest. However, lowpass filtering with a corner frequency of 1Hz reduces these differences from several 100% to about ±15%, thus confirming the almost constant level of the main soil resonance at an average period of about 1.4 seconds of the noise data in downtown Bucharest.

Romania is a country of moderate seismicity, with 2–3 major earthquakes (M _w > 7) per century. These events occur at subcrustal depths (60 < h < 200 km), in a confined epicentral area (approximately 3,000 km2), located at the Eastern Carpathian Arc bend, in the Vrancea Mountains. There are only few seismic regions in the world with comparable focus isolation and persistence (e.g., Hindu Kush in Afganistan and Bucaramanga in Columbia). The large Vrancea earthquakes severely affect not only the Romanian territory, but also extended areas in the neighboring countries.

The nonlinear effects in the ground motion during strong earthquakes are a controversial issue between seismologists and geotechnical engineers. The soil effect, is one of filtering the seismic motion, increasing the amplitude in some frequency ranges and decreasing it in others. The transverse body S waves induced by strong earthquakes, with possible shear strains γ≅10−3 (as it is the case of March 4, 1997, Vrancea event, M_s=7.2) have the most distructive effect. The nonlinear viscoelastic behavior of the superficial soil deposits for such large deformations has an important influence on the propagation of the seismic waves in the hazard and microzonation studies.

The present paper is the attempt to generalize the results of extensive study of local soil conditions influencing earthquake motion in the territory of the Republic of Moldova. It has been shown that for specific regional conditions such factors as thickness of soft soil, shear waves velocity, high impedance contrast between the clay-sandy layers and limestone basement play a decisive role in forming the amplitude level and spectral composition of motions occurring on surface. It has also been established that the influence of waterlogging, lithology is less than traditionally adopted.The unusually high level of damage to modern structures has been observed, resulting from resonance phenomena pertinent to soil-structure system.Regional methodology of seismic microzonation based on a complex study of soil dynamic properties is being advanced.The results obtained may be of use in solving problems of seismic microzonation and soil profile types classification.

Romania is a country with severe seismic conditions: more than half of its territory is subjected, periodically, to earthquakes having intensities I = VII … IX MM. After the strong earthquake of November, 1940 (Richter M = 7.4), another severe seismic event occurred on March 4, 1977; it produced heavy damage: 32 high-rise buildings collapsed in Bucharest, two small towns - Zimnicea and Alexandria- were, practically, destroyed. About 3000 people were killed and 12,000 injured.

The conventional methods for earthquake-resistant structural design use high strength or high ductility concepts to mitigate damage from seismic effects. In the first case, corresponding to shear wall structures, generally the design is problematic in that their fundamental frequency of vibration is in the range of frequencies where earthquake energy is the strongest, resulting in a very high floor acceleration, which may cause damage to equipment or machinery. The second, the capacity design method, incorporates that a part of the energy transmitted into the structure by. an earthquake is dissipated by plastic deformations. The capacity method mostly used for flexible structures as frames, provided that plastic deformations occurred in structural elements, which are designed to undergo such large deformations. Therefore, the design of such yielding zones has to be planned carefully. However, this concept may lead to a very high interstory drift, causing P-Δ effects and damage to non-structural elements. Thus, the costs for retrofitting or strengthening after a strong earthquake can be very high. An alternative approach consists in isolating the structure base from the ground by using rubber bearings.

The article outlines the mobility, over the last 60 years, of design requirements for earthquake resistance of structures and seismic zonation in Romania.

Eurocode 8, published in 1994 as a European prestandard, is considered to represent the latest word in codified earthquake design. On the other hand, in Romania there is a large practical experience in seismic design and the Romanian seismic design codes were recently revised (1992 and 1996), including the latest experience and concepts in seismic design.

The objective of this paper is to compare the structural design and detailing provisions included in Eurocode 8 (the values in brackets) to the corresponding provisions in the recent Romanian Codes of Practice. The comparison refers to regular multistory frame structures. Both Romanian and European Codes are based on the capacity design procedure and for both of them, for multistory flexible buildings, the interstory drift control governs the design. So, some concluding remarks, referring to the major differences between the two Codes of Practice can be pointed out.

The experience of recent strong earthquakes having occurred in Romania or abroad, some of them followed by a disastrous impact, besides the general evolution of the level of information and knowledge, led in Romania to an increasing concern for earthquake protection, within the community of specialists, among decision makers, as well as for the population as a whole. This evolution is not singular. It could be witnessed at an earlier stage in countries that are at the same time more developed and affected by more severe seismic conditions, first of all in Japan and US (California).

The Vrancea region is the source of subcrustal (60–170km) seismic activity which affects more than 2/3 of the territory of Romania, important parts of the territory of Republic of Moldova and small areas in Bulgaria and Ukraine. The Vrancea source induces a high seismic risk in the densely built zones of the South-east of Romania. On March 4, 1977, during the most severe Vrancea earthquake of this century, in the city of Bucharest more than 1570 casualties were registered, 11300 persons were injured, 35000 families lost their houses and 32 reinforced concrete multi-storey buildings completely collapsed.

Using the available information about regional structural models, past seismicity, and the seismotectonic regime in Italy, we have generated a set of synthetic seismograms covering the whole Italian territory on a 0.2° × 0.2° grid. Peak values of ground motion (displacement, DMAX, and velocity, VMAX) and Design Ground Acceleration (DGA) based on Eurocode 8 (EC8, 1993), extracted from the synthetic signals, have been compared with the macroseismic intensities felt in Italy. The correlation relations that we have obtained are in a good agreement with empirical relationships given by other authors and compare quite well with the few observations available in the Italian territory.

In the upcoming years, efforts in the field of earthquake disaster risk assessment and management should focus on three main issues: (1) urban risk, (2) a holistic, multidisciplinary approach, and (3) the implementation and dissemination of current knowledge. This paper introduces a series of three complementary projects—in risk assessment, risk management, and risk forecasting, and describes how, individually and collectively, they embody a new philosophy built on these three main issues. The first study attempts to assess the relative overall earthquake disaster risk of cities worldwide, and the relative contributions of various factors to that risk. The second seeks to compare the cost-effectiveness and feasibility of different risk mitigation strategies for a city. The third aims to forecast how a city’s risk, and therefore the most appropriate mitigation strategies, will change over time.

The awareness on the threat to society represented by the seismic risk is increasing due to several reasons. The impact of several destructive earthquakes having occurred during the last decades has made people more sensitive to this issue. The activities conducted worldwide under the auspices of IDNDR (International Decade for Natural Disaster Reduction) and WSSI (World Seismic Safety Initiative) contributed considerably to this too. Direct experience showed how high the toll of human lives can be, especially in less developed countries (Tangshan, 1976), but also that the most advanced and best protected countries of our time still have much to do in order to avoid heavy losses (Northridge, 1994, Kobe, 1995).

Experience with em ergency response following major earthquakes shows, that approximately the first three days after an earthquake event are essential for the performance of the relief efforts. Usually the time until help from outside the disaster area will be available takes too long and therefore one of the main efforts in disaster relief is to strengthen the local emergency response. One of the problems a local emergency planner is faced with in this context is to find good locations for relief resources like search and rescue equipment, first aid kits or emergency shelter. For best results the specific conditions arising from the threat of an earthquake have to be taken into consideration.

Some aspects about basic data obtaining, are presented in this paper, for 3-D model construction using photogrammetric methods. The data was completed with the data obtained by digitization of large scale cadastral plans. Analytic photogrammetric methods have been used with a good efficiency and accuracy for 3-D model urban areas reconstruction. 3-D model construction was made with the AutoCAD software. The disaster evaluation effects are also described.

Management procedures in general, and disaster management in particular, depend on information. The central questions are however, which information is necessary in order to draw for a particular decision, when is this information needed and at which resolution it is required. “Resolution” in this context is defined as the “level of detail”. Quality of management is thus directly related to the quality of information.

Early Warning Systems (EWS) have the potential to warn the population, government officials, police, fire department, and operators of vital and/or hazardous facilities. The options available for response within the warning time depends very much on its length. For instance, the EWS of Mexico City provides 60 to 75 seconds of advance warning. This represents the time delay between strong motion recorded at the subduction zone where the Cocos plate slides beneath North America and felt in the city 300 to 350 km away.