Strong Vrancea Research Articles

Seismic Observations in Bucharest Area with a Raspberry Shake Citizen Science Network

Technological advancements and the appearance of low-cost Raspberry Shake seismographs have enabled the development of citizen science seismic networks in many areas worldwide. These networks can help reduce seismic risk and increase citizens’ understanding of seismology and earthquakes. Such a network exists in Bucharest, one of the cities in Europe that are struck and affected by strong Vrancea earthquakes. The paper aims to show that data from such networks can be used in both outreach programs and research studies. There are presented, for the first time, seismic observations collected over two years beginning in the summer of 2020 in the Bucharest area based on the low-cost seismometers from the citizen science Raspberry Shake network. A significant number of earthquakes from the Vrancea region were recorded by the Bucharest Raspberry Shake Seismic Network (BRSSN). Some of them were felt by Bucharest inhabitants. The National Institute for Earth Physics in Magurele (Romania) organizes educational events that promote geosciences among the population and presents the tools at its disposal for a better understanding of earthquakes and their effects, contributing this way to the development of the concept of citizen science. Citizens are the first witnesses to seismic events and the citizen science seismic network provides them with the first direct information about the event via web apps available for any internet-connected device. Their involvement as non-professional participants helps in providing data for scientists via questionnaire forms to improve scientific research for earthquake assessment. Since citizen seismometers are installed in urban areas, an analysis of the ambient seismic noise (ASN) was performed in addition to the analysis of recorded seismic events. The analysis indicates that the level of seismic noise is mainly controlled by human activities. At the same time, for one citizen seismometer installed in a school in Bucharest, the results show patterns of noise variations due to students’ activity.

Liquefaction Potential Analysis in Bucharest City as a Result of the Ground Shaking during Strong Vrancea Earthquakes

Bucharest, the capital of Romania with about 2.5 million inhabitants, is frequently struck by intense, damaging earthquakes (2–3 events per century). The Collaborative Research Center 461 (CRC-461) entitled: “Strong Earthquakes - a Challenge of Geosciences and Civil Engineering” was established in July 1996 and ended in December 2007, but some projects continued until 2010. It was funded by the German Research Foundation and involved the University of Karlsruhe which today belongs to Karslruhe Institute of Technology. The CRC aimed strategic research in the field of strong earthquakes with regional focus on the Vrancea seismic events in Romania. Between 1995–2007 several research works were done in Romania, with the support of several Romanian research institutes and the University of Bucharest. One of the research questions was to study the occurring of liquefaction during strong earthquakes within the shallow sandy layers in Bucharest. In suitable conditions, strong earthquakes can cause, under certain geologic conditions, liquefaction and therewith ground failure as sand boils, lateral spreading, or differentiated subsidence. In the present paper we analyze the liquefaction risk for Bucharest. For this purpose, at 10 representative sites in Bucharest, Seismic Cone Penetration Tests (SCPTu) were executed. An area-wide evaluation of the liquefaction probability in Bucharest was established. The factor of safety (FS) against liquefaction and the probability of liquefaction (PL) were computed from the obtained test-data. For the first time, maps of the liquefaction potential index (Li) for Bucharest were outlined. This map shows how severe the liquefaction phenomena might be during strong Vrancea earthquakes in Bucharest, amplifying the site effects. Keywords: hydrogeologic conditions, liquefaction probability, liquefaction hazard, Bucharest city, strong Vrancea earthquakes

GROUND MOTION PREDICTION AT THE DAM SITES IN SW ROMANIA FOR MODERATE VRANCEA SUBCRUSTAL EARTHQUAKES

The seismic hazard studies of the last 30 years have been largely carried out taking into account the needs of the construction engineers, by linking the specific quantities of soil movement with the physical parameters determined instrumentally, namely, with the maximum values of soil acceleration. At present, interest in the results of hazard studies has increased and has extended to other areas such as insurance or design companies, environmental protection, etc. A fundamental element in the estimation of seismic hazard is the variation of the amplitude of the movement of the soil according to the distance, magnitude and local conditions. To understand and prevent the effects of the strong Vrancea earthquakes in the dam sites located in the South West of Romania, we study the seismic waves attenuation relations using the accelerations recorded by the national network of K2 accelerometers, following the moderate Vrancea intermediate earthquakes. The study area includes the largest agglomeration of dams in Romania, with almost 100 dams out of the 250 large dams. One of the most important specific requirements towards dams' safety is the seismic risk and hazard assessment and the computation of attenuation relationships is one of the most important steps of the work. The main objective of the present work is the evaluation of the specific attenuation relationships of the seismic wave propagating from Vrancea subcrustal focus toward south-west, with direct application for the dams situated in the area.

Re-evaluation of the macroseismic effects produced by the March 4, 1977, strong Vrancea earthquake in Romanian territory

<p>In this paper, the macroseismic effects of the subcrustal earthquake in Vrancea (Romania) that occurred on March 4, 1977, have been re-evaluated. This was the second strongest seismic event that occurred in this area during the twentieth century, following the event that happened on November 10, 1940. It is thus of importance for our understanding of the seismicity of the Vrancea zone. The earthquake was felt over a large area, which included the territories of the neighboring states, and it produced major damage. Due to its effects, macroseismic studies were developed by Romanian researchers soon after its occurrence, with foreign scientists also involved, such as Medvedev, the founder of the Medvedev-Sponheuer-Karnik (MSK) seismic intensity scale. The original macroseismic questionnaires were re-examined, to take into account the recommendations for intensity assessments according to the MSK-64 macroseismic scale used in Romania. After the re-evaluation of the macroseismic field of this earthquake, the intensity dataset was obtained for 1,620 sites in Romanian territory. The re-evaluation was necessary as it has confirmed that the previous macroseismic map was underestimated. On this new map, only the intensity data points are plotted, without tracing the isoseismals.</p>

Estimates of some ground motion parameters, related to the damage potential of strong intermediate-depth Vrancea earthquakes

This study is related to the particular strong intermediate-depth Vrancea earthquakes. Its purpose is to confirm once again the significant influence of the seismic source mechanism to the seismic input at a given site through estimates of different engineering parameters, used to describe the damage potential of the ground motion due to an earthquake. Real and synthetic time-series have been used in the computations. The synthetic seismic wavelets have been computed applying the neo-deterministic seismic hazard assessment procedure and the relevant software and hardware facilities at the Department of Earth Sciences, University of Trieste, Italy. To derive the ground motion parameters of interest, all seismic signals have been uniformly processed by the Seismosignal software.

A Summary of Instrumental Data on the Recent Strong Vrancea Earthquakes, and Implications for Seismic Hazard

The paper is intended to summarize the most important instrumental data of direct relevance for engineering activities, obtained in connection with the strong Vrancea earthquakes of 4 March 1977, 30 August 1986, 30 May 1990, and 31 May 1990, and to point out some significant consequences and conclusions derived on this basis. Two main objectives of this analysis may be emphasized: (a) in-depth analysis of the radiation pattern; and (b) analysis of the spectral contents of ground motion in connection with the features of local conditions, and with the intention of assessing the relative importance of two main factors: source mechanism and long-distance wave propagation, versus features of local geological conditions. Some specific methodological developments used in this context may be mentioned: (a) use of a new approach to the quantification of ground motion intensity on the basis of instrumental (accelerographic) information; (b) analysis of radiation pattern in spectral and directivity terms; (c) parametric analysis of site-specific transfer functions for the local sequences of geological layers; and (d) a critical view on the outcome of post-earthquake survey techniques, keeping in view the implications of the spectral features of ground motion. The main results obtained are related to: (a) ground motion radiation features that have to be taken into account in connection with the data on the source mechanisms of the successive events dealt with; (b) expected spectral features of future strong ground motion at different sites; (c) methodological developments proposed for the assessment of local transfer functions; and (d) implications for microzonation activities.

Near surface shear wave velocity in Bucharest, Romania

Abstract. Bucharest, the capital of Romania with nearly 2 1/2 million inhabitants, is endangered by the strong earthquakes in the Vrancea seismic zone. To obtain information on the near surface shear-wave velocity Vs structure and to improve the available microzonations we conducted seismic refraction measurements in two parks of the city. There the shallow Vs structure is determined along five profiles, and the compressional-wave velocity (Vp) structure is obtained along one profile. Although the amount of data collected is limited, they offer a reasonable idea about the seismic velocity distribution in these two locations. This knowledge is useful for a city like Bucharest where seismic velocity information so far is sparse and poorly documented. Using sledge-hammer blows on a steel plate and a 24-channel recording unit, we observe clear shear-wave arrivals in a very noisy environment up to a distance of 300 m from the source. The Vp model along profile 1 can be correlated with the known near surface sedimentary layers. Vp increases from 320 m/s near the surface to 1280 m/s above 55–65 m depth. The Vs models along all five profiles are characterized by low Vs (<350 m/s) in the upper 60 m depth and a maximum Vs of about 1000 m/s below this depth. In the upper 30 m the average Vs30 varies from 210 m/s to 290 m/s. The Vp-Vs relations lead to a high Poisson's ratio of 0.45–0.49 in the upper ~60 m depth, which is an indication for water-saturated clayey sediments. Such ground conditions may severely influence the ground motion during strong Vrancea earthquakes.

Romanian Seismic Network development

The Romanian Seismic Network consisting of 8 short-period stations (S13), 60 Altus — K2 and Q330 seismic stations, is primarily designed to survey the Vrancea seismic region (strong and moderate Vrancea earthquakes). Since July 2002, a new seismic monitoring system, Bucovina Seismic Array (BURAR) has been operating. BURAR consists of 12 seismic stations distributed on a 5 km × 5 km area. 9 stations are equipped with short-period (SP) vertical sensors (GS-21 res) and one station is equipped with broad-band (BB) three component sensor (KS 54000). Broad-band sensors GMG40T (1000V/cm/s) were installed at 5 of the short-period stations, at the beginning of 2008. Recently, NIEP has developed its real-time digital seismic network. This network consists of 21 broad band stations and one seismic array. In the next year NIEP will install additional broad band stations in the central part of Romanian territory and other 40 strong motions stations in Bucharest. At the National Data Centre (NDC) runs BRTT’s Antelope™ 4.9 data acquisition and processing software on two HP workstations for real-time and post processing. The Antelope Real-Time System is also providing automatic event detection, arrival picking, event location and magnitude calculation. The Romanian Seismic Network is already linked with IRIS and ORFEUS organizations and other European countries via Internet and is contributing in near real time with waveform data from 5 broadband stations. The seismotectonic characteristics of the Vrancea region offer the opportunity to create and develop a rapid seismic warning system. This system allows warning in an approximately 25 seconds time window for Bucharest.

A new ground motion model - Methodological approach

The investigations presented in this paper were aimed at empirical definition of ground motion under Vrancea earthquakes. They were based on the recorded accelerogrammes from the occurred strong Vrancea earthquakes of 1977, 1986 and 1990 obtained from the accelerographs installed in the territory of former Yugoslavia, Romania and Bulgaria. A methodological approach to empirical prediction of ground motion parameters under strong earthquake effects was developed and empirical attenuation laws of horizontal peak ground acceleration (PGA) were defined.

Deterministic Approach for the Seismic Microzonation of Bucharest

The mapping of the seismic ground motion in Bucharest, due to the strong Vrancea earthquakes is carried out using a complex hybrid waveform modeling method which combines the modal summation technique, valid for laterally homogeneous anelastic media, with finite-differences technique, and optimizes the advantages of both methods. For recent earthquakes, it is possible to validate the modeling by comparing the synthetic seismograms with the records. We consider for our computations the frequency range from 0.05 to 1.0 Hz and control the synthetic signals against the accelerograms of the Magurele station, low-pass filtered with a cut-off frequency of 1.0 Hz of the 3 last major strong (M w > 6) Vrancea earthquakes. Using the hybrid method with a double-couple seismic source approximation, scaled for the source dimensions and relatively simple regional (bedrock) and local structure models, we succeeded in reproducing the recorded ground motion in Bucharest at a satisfactory level for seismic engineering. Extending the modeling to the entire territory of the Bucharest area, we construct a new seismic microzonation map, where five different zones are identified by their characteristic response spectra.

Vrancea Source Influence on Local Seismic Response in Bucharest

The mapping of the seismic ground motion in Bucharest, due to the strong Vrancea earthquakes, is carried out using a complex hybrid waveform modeling method that allows easy parametric tests. Starting from the actually available strong motion database, we can make realistic predictions for the possible ground motion. The basic information necessary for the modeling consists of: (a) The representative mechanisms for the strong subcrustal events, (b) the average regional structural model, and (c) the local structure for Bucharest. Two scenario earthquakes are considered and the source influence on the local response is analyzed in order to define generally valid ground motion parameters, to be used in the seismic hazard estimations. The source has its own (detectable) contribution on the ground motion and its effects on the local response in Bucharest are quite stable on the transversal component (T), while the radial (R) and vertical (V) components are sensitive to the scenario earthquake. Although the strongest local effects affect the T component, both observed and synthetic, a complete determination of the seismic input for the built environment requires the knowledge of all three components of motion (R, V,T). The damage observed in Bucharest for the March 4, 1977 Vrancea event, the strongest earthquake to strike the city in modern times, is in agreement with the synthetic signals and local response.

Possible Cause-Effect Relationships Between Vrancea (Romania) Earthquakes and Some Global Geophysical Phenomena

The possibility that the Earth's tides are a triggering factor of Vrancea subcrustal earthquakes is investigated in the first part of this paper. A possible correlation between Vrancea subcrustal earthquakes and geomagnetic jerks is demonstrated in the second part. The last part of the paper presents a number of results concerning a possible relationship between the regularities of strong Vrancea subcrustal seismicity and the Chandler nutation parameters. An attempt is made to integrate all of these phenomena in a more general framework that takes into account physical processes in the Earth mantle and core. A long-term prediction of the next strong Vrancea earthquake is finally attempted.