Vrancea Seismic Zone Research Articles

Past Earthquake Simulations Using Ambient Seismic Noise in Vrancea, Romania

Abstract The Vrancea seismic zone (VSZ), located in Romania, at the sharp bend of the southeast Carpathians, is an anomalous intraplate seismic nest releasing the largest strain in continental Europe. The last powerful earthquake in Vrancea occurred in 1977 causing significant damage to the densely populated cities in southeast Romania. The seismic infrastructure was underdeveloped at that time and the earthquake was not well recorded locally. Using ambient seismic noise recorded at modern broadband seismic stations around Vrancea in the year 2020, and the approximate moment tensor solutions of significant earthquakes Mw>6 since 1977, we reconstruct ground-motion waveforms recorded by modern seismometers decades after their occurrence. The virtual earthquake approach uses empirical Green’s functions from ambient noise cross correlations between pairs of seismic stations and adds the signatures of a model earthquake: double couple mechanism, buried source, and a realistic earth model in the epicentral area. Ambient noise data capture the signatures of complex subsurface structures and the 3D path effects, providing a more physically accurate representation of the seismic wavefield than purely synthetic data. Our new results demonstrate the viability of this innovative method and provide a unique opportunity for more accurate seismic hazard analysis in intraplate seismic zones with insufficient instrumental data.

Exploring the Relationship between Geomagnetic Variations and Seismic Energy Release in Proximity to the Vrancea Seismic Zone

Understanding the seismo–ionospheric coupling mechanism requires a quiet geomagnetic condition, as this represents an ideal situation to detect abnormal variations in the geomagnetic field. In reality, continuous interactions between solar wind and Earth’s magnetosphere create many fluctuations in the geomagnetic field that are more related to sun–magnetosphere interactions than to seismotectonic causes. A triaxial magnetometer was installed at the Muntele Rosu Observatory near the Vrancea seismic zone in 1996 to measure the local magnetic field. Since 2002, the data have become more consistent, allowing for the representation of long time series. Since then, variations have been observed on the eastern component (By) of the magnetic field, which sometimes overlaps with significant earthquakes. Previous studies have shown that high decreases in amplitude recorded on the By component of the magnetic field measured at Muntele Rosu have been accompanied by higher seismicity, while small decreases have been accompanied by lower seismic energy release. This research analyzes the geomagnetic data collected between September 2002 and May 2008 from two geomagnetic observatories, one located in the proximity of the Vrancea seismic zone and another one situated 120 km away. For each geomagnetic anomaly identified, the daily seismic energy released was plotted logarithmically, along with seismicity and Kp indices. Additionally, the daily seismic energy released was also plotted logarithmically for all earthquakes with Mw ≥3. To identify variations in the By component, datasets recorded at Muntele Rosu (MLR) were compared with those recorded at Surlari National Geomagnetic Observatory (SUA), to discriminate between global magnetic variations associated with solar activity and possible seismo–electromagnetic variations. The standard deviation (SDBy) was calculated for each anomaly recorded on the By component of the magnetic field and compared with the cumulative seismic energy release. To determine if this type of variation was present in other components of the magnetic field, the following ratios were calculated for all data recorded at Muntele Rosu: Bz/Bx, Bz/By, and Bz/BH. The size of the anomalies resulting from the standard deviation measured on the By component (SDBy) partially validates the relationship between the size of the anomalies and the seismic energy release during the anomaly. The relationship between the released seismic energy and the anomaly magnitude is vaguely respected, but these variations seem to follow two patterns. One pattern is described by smooth decreases, and the other pattern involves decreases where the By component varies significantly over short periods, generating decreases/increases in steps. It was noticed that seismic activity is greater for the second pattern. Additionally, using standard deviation measured on the magnetic field represents a great tool to discriminate external magnetic field variations from local, possibly seismo–magnetic variations.

Shallow seismic structure around the Vrancea Seismic Zone from joint inversion of ambient noise H/V ratios and surface wave dispersion

The South-East Carpathians in Romania host the Vrancea Seismic Zone (VSZ), an intraplate seismic nest of frequent high magnitude earthquakes that cause unusually high damage over extended areas. Estimating the amplification of seismic signals strongly depends on the depth to the geophysical bedrock and the complex near surface geological structures. To improve seismic hazard estimates and ultimately mitigate seismic risk, we determine the frequency of resonance at 81 broadband seismic stations in southeast Romania using the horizontal-to-vertical spectral ratio (HVSR) of ambient seismic noise. We jointly invert HVSR functions with Rayleigh wave phase velocity dispersion curves using a Markov-chain Monte Carlo algorithm under the assumption of a diffuse field. Our results show excellent correlation between the fundamental frequency of resonance and local surface geology: stations located on foreland sedimentary basins show HVSR peaks in the range 0–1 Hz, while those located in orogenic nappes reach values as high as 10 Hz. Seismic bedrock topography mirrors the crystalline bedrock depth inferred from gravity modeling, but is consistently ∼2000 m shallower, corresponding to the interface between Paleozoic-Triassic to Jurassic-Cretaceous sedimentary levels on the Moesian Platform constrained by borehole data. The near surface seismic model shows anomalously low velocities in the Focsani forearc basin, adjacent to the VSZ, consistent with extremely low resonance frequencies and deep sedimentary layer successions (>5 km). In the back-arc of the Carpathian Orogen, at the contact with the Transylvanian Basin, average resonance frequencies are <2 Hz and inferred seismic bedrock depths vary laterally, likely affected by buried volcanic structures and diapiric salt domes. Our study presents the largest endeavor to study anomalous intensity patterns and local site effects in and around the VSZ and has fundamental implications for seismic risk evaluation and future civil engineering design planning.

Vrancea intermediate-depth focal mechanism catalog: a useful instrument for local and regional stress field estimation

The Vrancea seismic zone, located in the bend region of the South-Eastern Carpathians, is a unique area with both crustal and intermediate-depth seismic activity and is known as one of the most active seismic area in Europe. Moderate crustal seismicity is recorded all over the Carpathian region, but the far more intense activity occurs in a small subcrustal seismogenic volume beneath the SE‐bend of the Carpathian arc with about 20 × 50 km lateral and 110 km vertical extent (70–180 km depth). A unique slab geometry, likely preserved until the present, causes stress localization due to the slab bending and subsequent stress release resulting in large mantle earthquakes in the region. The main focus of this study is to determine the focal mechanisms for events with a magnitude larger than 2.7, between 2005 and 2020 and evaluate the current stress field along the Vrancea subcrustal region, from the derived fault plane solutions. The main style of faulting for Vrancea subcrustal events presents a predominant reverse one, with two main earthquakes categories: the first one with the nodal planes, oriented NE–SW parallel with the Carpathian Arc and the second one with the nodal planes, oriented NW–SE perpendicular on the Carpathian Arc. The results of stress inversion indicate a dominant thrust faulting style, with an average stress regime index of 2.87. The stress pattern shows similar partitioning with vertical extension in the slab and no preferred orientation in the overlying crust, showing a transition regime from the extensional regime in the Moesian Platform to the compressional regime in the Vrancea subcrustal zone.

Time-Dependent Framework for Analyzing Emergency Intervention Travel Times and Risk Implications due to Earthquakes. Bucharest Case Study

Earthquakes can generate a significant number of casualties within seconds, as well as high economic losses. The lack of rapid and coordinated emergency intervention can contribute to much greater losses. In this paper we develop a framework taking advantage of the ArcGis Network Analyst extension, able to account for post-earthquake conditions and reflect travel times. By combining 1) network characteristics with 2) direct seismic damage information, 3) models to determine road obstruction potential, 4) traffic information and time-dependent post-earthquake modeling but also 5) emergency intervention facilities (hospitals or fire stations) and considerations regarding their functional limitations, this framework can provide important support for the management of emergency intervention but also for risk reduction planning. Main results consist of maps showing travel times for various scenarios and moments after an earthquake, inaccessible areas, vital roads for access or an identification of important facilities. As case study we chose Bucharest, one of Europe’s most endangered capitals considering the seismic risk level. The city was and could be considerably affected by earthquakes in the Vrancea Seismic Zone, being characterized by a high number of vulnerable buildings and by one of the greatest typical traffic congestion levels in the world. Compared to previous network studies for Bucharest, the new approach is more complex and customable, providing means for real-time integration and time-dependent analysis. Results, for a worst-case scenario, prove that the risks could be even greater than expected, but also what should be done to mitigate them, such as the construction of a new hospital in the western part of the city, ensuring safe delimited routes for emergency vehicles or expanding the treatment capacity of actual hospitals—some of which also need seismic retrofitting. Results of this study will be integrated in the revised version of the National Conception for Post-Earthquake Response—an operational framework which will lead to risk mitigation through the improvement of post-disaster reaction.

Present-day stress field pattern in the Vrancea seismic zone (Romania) deduced from earthquake focal mechanism inversion

Vrancea seismogenic zone in the South-Eastern Carpathians is characterized by localized intermediate-depth seismicity. Due to its complex geodynamics and large strain release, Vrancea represents a key element in the Carpatho-Pannonian system. Data from a recently compiled catalogue of fault plane solutions (REFMC) are inverted to evaluate stress regime in Vrancea on depth. A single predominant downdip extensive regime is obtained in all considered clusters, including the crustal layers located above the Vrancea slab. The prevalent stress regime confirms previous investigations and requires some mantle-crust coupling. The S3 principal stress is close to vertical, while S1 and S2 are horizontal, oriented perpendicularly and respectively tangentially to the Carpathians Arc bend. This configuration is present at any depth level. According to seismicity patterns, there are two main active segments in the Vrancea intermediate-depth domain, at 55 – 105 km and 105 – 180 km, both able to generate major events. The configuration of the tectonic stresses as resulted from inversion is similar in both segments. Also, high fault instability (I &gt; 0.95) is characterizing the segments. The only notable difference is given by the friction and stress ratio parameters which drop down in the bottom segment from μ = 0.95 to μ = 0.55 and from R = 0.51 to R = 0.29. This variation is attributed to possible weakening processes activated below 100 km depth and can explain the intensification of seismicity production as earthquake rate and average energy release in the lower segment versus the upper segment.

Evaluating landslide response in a seismic and rainfall regime: a case study from the SE Carpathians, Romania

Abstract. There have been many studies exploring rainfall-induced slope failures in earthquake-affected terrain. However, studies evaluating the potential effects of both landslide-triggering factors – rainfall and earthquakes – have been infrequent despite rising global landslide mortality risk. The SE Carpathians, which have been subjected to many large historical earthquakes and changing climate thus resulting in frequent landslides, comprise one such region that has been little explored in this context. Therefore, a massive (∼9.1 Mm2) landslide, situated along the river Bâsca Rozilei, in the Vrancea seismic zone, SE Carpathians, is chosen as a case study area to achieve the aforesaid objective (evaluating the effects of both rainfall and earthquakes on landslides) using slope stability evaluation and runout simulation. The present state of the slope reveals a factor of safety in a range of 1.17–1.32 with a static condition displacement of 0.4–4 m that reaches up to 8–60 m under dynamic (earthquake) conditions. The groundwater (GW) effect further decreases the factor of safety and increases the displacement. Ground motion amplification enhances the possibility of slope surface deformation and displacements. The debris flow prediction, implying the excessive rainfall effect, reveals a flow having a 9.0–26.0 m height and 2.1–3.0 m s−1 velocity along the river channel. The predicted extent of potential debris flow is found to follow the trails possibly created by previous debris flow and/or slide events.

Liquefaction potential assessment of saturated loess

Usually, soils with mainly fine grain-sized content, as loess, are considered to have low liquefaction potential. Regardless of this, many researchers have analyzed and presented much field evidence that silty soil (in particular loess) liquefaction occurred under certain conditions. In Bulgaria, the first loess river terrace (T1) within the Danube River lowland areas is covered by low plasticity silty loess with a thickness of 10–12 m. Тhe groundwater level is often located between 5 m and 8 m in depth so that substantial part of loess deposits are saturated and immersed. Meanwhile, that region of North Bulgaria is under the influence of the Vrancea seismic zone in Romania, which is able to generate strong earthquakes with magnitudes M≥7.0. The present paper aims to assess the liquefaction potential of loess in a ground profile representative of the T1 loess river terraces by the so-called simplified procedure based on SPT, which is incorporated in the software code NovoLiq. The safety factor against liquefaction FSL is estimated at the respective depths in one-dimensional model of the ground profile for free-field conditions at varying peak ground accelerations amax. The critical amax, at which liquefaction of loess is possible according to the assumptions of the applied simplified procedure and the requirements of the National Annex of Bulgaria to Eurocode 8, has been established.

Tectonic regimes and stress patterns in the Vrancea Seismic Zone: Insights into intermediate-depth earthquake nests in locked collisional settings

Earthquake nests are anomalous clusters of seismicity located far from active collisional systems in intraplate, locked suture zones, or the deep part of relic subducted slabs, challenging classic earthquake generation mechanism theories. The Vrancea Seismic Zone in Romania is such an upper-mantle seismic nest located in the SE Carpathians, releasing the largest strain in continental Europe. To better understand earthquake generation and the relationship with lithospheric deformation, we estimate earthquake source parameters in Vrancea and surrounding regions between 2014 and 2020, and determine the stress field via focal mechanism inversion and unsupervised machine learning. In the crustal domain, maximum horizontal stress is in agreement with surface fault kinematics and GPS-derived S-SE trending horizontal plate velocities relative to Eurasia, implying that tectonic stress is vertically coherent on a crustal scale. The stress regime changes from transpression beneath the orogen to transtension towards the foreland where movement is accommodated along major crustal faults, and tension further away from the epicentre, in the Moesian Platform and the North Dobrogea Orogen. Inside the seismogenic body vertical tension and an overall compressive regime dominates, implying that vertical elongation may be the driving mechanism for brittle failure and that stress is transmitted along the sinking slab to the surface. However, the retrieved stress ratios are low: ~0.2 for mantle earthquakes Mw>4 and ~0.4 for Mw<4, challenging the brittle failure assumption. Increased pore fluid pressure has been shown to lower stress ratios, implying that dehydration embrittlement may contribute to generating intermediate-depth seismicity in the Vrancea slab. Comparisons with seismic tomography and anisotropy studies show excellent correlations between maximum horizontal stress directions, possible slab strike orientation, and seismic anisotropy, especially below ~130km depth, suggesting ambient mantle flow may also promote in-slab stress build-up and seismic potential.

PROCESSING AND ANALYSIS OF THE RESULTS FROM INSTRUMENTAL OBSERVATIONS AT THE PERMANENT SEISMIC MONITORING NETWORK, RIVNE NPP

The article presents information about the organization and arrangement of the seismic monitoring network in the Rivne NPP area, as well as the results of the analysis of interfering conditions and the influence of microarrays of various orderтon the ability to record useful signals on the network elements. The average values of the microseismic background at the checkpoints were calculated based on the records obtained using seismic sensors installed in instrument wells and on the day surface. In the course of instrumental observations, local, regional, and teleseismic events of different nature and energy levels were recorded. The vast majority of recorded earthquakes are located in Romania in the Vrancea seismic zone, in Poland, and in Southern Belarus. According to the results of seismic record processing, the intensity of shaking in the region of Rivne NPP was determined and conclusions were made regarding the necessity of changes of the initial data for the assessment of the seismic stability of the RNPP site.

Instrumental observations at the Rivne NPP permanent seismic monitoring network

In accordance with the contract № 1881 dated 30.11.2018 between the State Enterprise «National Atomic Energy Generating Company "Energoatom"» and the National Center for Management and Testing of Space Means, specialists of the Main Center of Special Control carried out work on the analysis of the results of instrumental seismic observations on a permanent seismic network Rivne NPP monitoring. The article presents information about the organization and arrangement of the seismic monitoring network in the Rivne NPP area, as well as the results of the analysis of interfering conditions and the influence of microseims of different order on the ability of recording useful signals on the network elements. According to the records obtained using seismic sensors installed in instrument wells and on the day surface, the average values of the microseismic background at the checkpoints were calculated. In the course of instrumental observations, local, regional and tele-seismic events of different nature and energy levels were recorded. The vast majority of recorded earthquakes are located in Romania in the Vrancea seismic zone, Poland and southern Belarus.According to the results of seismic record processing, the intensity of shaking in the region of Rivne NPP was determined and conclusions were made regarding the necessity of changes of the initial data for the assessment of the seismic stability of the RNPP site.

Upper mantle deformation signatures of craton–orogen interaction in the Carpathian–Pannonian region from SKS anisotropy analysis

SUMMARYSince the Mesozoic, central and eastern European tectonics have been dominated by the closure of the Tethyan Ocean as the African and European plates collided. In the Miocene, the edge of the East European Craton and Moesian Platform were reworked in collision during the Carpathian orogeny and lithospheric extension formed the Pannonian Basin. To investigate the mantle deformation signatures associated with this complex collisional-extensional system, we carry out SKS splitting analysis at 123 broad-band seismic stations in the region. We compare our measurements with estimates of lithospheric thickness and recent seismic tomography models to test for correlation with mantle heterogeneities. Reviewing splitting delay times in light of xenolith measurements of anisotropy yields estimates of anisotropic layer thickness. Fast polarization directions are mostly NW–SE oriented across the seismically slow West Carpathians and Pannonian Basin and are independent of geological boundaries, absolute plate motion direction or an expected palaeo-slab roll-back path. Instead, they are systematically orthogonal to maximum stress directions, implying that the indenting Adria Plate, the leading deformational force in Central Europe, reset the upper-mantle mineral fabric in the past 5 Ma beneath the Pannonian Basin, overprinting the anisotropic signature of earlier tectonic events. Towards the east, fast polarization directions are perpendicular to steep gradients of lithospheric thickness and align along the edges of fast seismic anomalies beneath the Precambrian-aged Moesian Platform in the South Carpathians and the East European Craton, supporting the idea that craton roots exert a strong influence on the surrounding mantle flow. Within the Moesian Platform, SKS measurements become more variable with Fresnel zone arguments indicating a shallow fossil lithospheric source of anisotropy likely caused by older tectonic deformation frozen in the Precambrian. In the Southeast Carpathian corner, in the Vrancea Seismic Zone, a lithospheric fragment that sinks into the mantle is sandwiched between two slow anomalies, but smaller SKS delay times reveal weaker anisotropy occurs mainly to the NW side, consistent with asymmetric upwelling adjacent to a slab, slower mantle velocities and recent volcanism.

Macroseismic Intensity Estimation from Instrumental Ground Motion Recordings in the Case of Small and Moderate Vrancea Subcrustal Earthquakes

Romania is one of the most seismic-prone countries in Europe due to the periodically occurrence of strong intermediate-depth earthquakes in Vrancea seismogenic zone. The Vrancea area is located beneath the South-Eastern Carpathian Arc bend, at the contact between the East - European plate and the Intra-Alpine and Moesian sub-plates. An intense seismic activity is recorded in the mantle, within a narrow, almost vertical descending volume between 60 and 180 km depth. Earthquakes with magnitudes larger than 5.0 (Mw) and macroseismic effects exceeding V MSK degrees on extended populated area occur in Vrancea seismic zone, with a return period of around 2 years. Besides the extended scientific studies, the near real time estimation of the macroseismic intensity recently became mandatory for the insurance companies to cover some of the losses and damages that earthquakes might cause to houses, belongings, and other buildings. The first approach to obtain intensity values was to develop an online environment for collecting people feedback regarding the effects of earthquakes and for the automatic approximation of the intensity. The automatic intensity estimation code from the online feedback proposed by [1], [2] and adopted by [3] for the Romanian earthquakes was improved and used for this study. Additionally, there were used prediction equations to obtain intensity values from the epicentral intensity, and epicentral and hypocentral distances. Besides the estimation of macroseismic intensities from online people feedback and from attenuation relations, equations for conversion of peak ground acceleration in macroseismic intensity were also required for a rapid evaluation of ground motion effects. In order to avoid the increase of the prediction uncertainties, the data must be selected in such a way that they represent the parameters’ rank for which the prediction will be accomplished and the selected data must be representative for the investigated regions and seismic source. In this paper were developed conversion methods from PGA to macroseismic intensity for moderate intermediate depths earthquakes with 4.5<Mw<6.0. All the studies were realised in the case of Vrancea recent (2014-2019) moderate intermediate depth earthquakes, felt on the extra-Carpathian region.

Experiential Learning for Seismic Protection Using Living Lab Approach in Romania

The purpose of the research: The paper is going to present the role of seismic areas as living labs/natural laboratories for studying the behavior of buildings and of people and establishing a relationship between a research, a training-education and a knowledge dissemination. The results materialized by experiential actions to reduce negative impacts of disasters are highlighted. Methodology: Engineering learning from earthquakes, based on a long-term seismic culture, became a continuous activity based on field observation, since more than a century. On the other hand, creating earthquake disaster prevention awareness and disaster preparedness can be achieved nowadays by education and active learning addressed to all levels of population, from elder generations to the younger ones, and from usual people to the representatives of authorities. The link between these two approaches is becoming more and more necessary in the context of high local seismicity: investigating the behavior of people, while using emerging technologies (computer-generated imagery, immersive virtual reality etc.), to obtain an active individual attitude. The general concept of awareness and preparedness trends evolved. To deal with earthquake-related situations, people are taught to take measures to prevent severe earthquakes effects, but we must take a step forward to get closer to the system that includes: research knowledge, innovation, GIS and specialized software, and more importantly, the access to all of these, through public and private involvement. Conclusions: The Vrancea seismic zone of Romania, as a natural laboratory, involves us with knowledge and individual perception and attitudes and we have time for improved engineering codes, exploring and experiencing some new learning modalities, refining new policies and regulations from this field, new scenarios for implementation of disaster measures.

Fast Moment Magnitude Determination from P-wave Trains for Bucharest Rapid Early Warning System (BREWS)

Bucharest, with a population of approximately 2 million people, has suffered damage from earthquakes in the Vrancea seismic zone, which is located about 170 km from Bucharest, at a depth of 80–200 km. Consequently, an earthquake early warning system (Bucharest Rapid earthquake Early Warning System or BREWS) was constructed to provide some warning about impending shaking from large earthquakes in the Vrancea zone. In order to provide quick estimates of magnitude, seismic moment was first determined from P-waves and then a moment magnitude was determined from the moment. However, this magnitude may not be consistent with previous estimates of magnitude from the Romanian Seismic Network. This paper introduces the algorithm using P-wave spectral levels and compares them with catalog estimates. The testing procedure used waveforms from about 90 events with catalog magnitudes from 3.5 to 5.4. Corrections to the P-wave determined magnitudes according to dominant intermediate depth events mechanism were tested for November 22, 2014, M5.6 and October 17, M6 events. The corrections worked well, but unveiled overestimation of the average magnitude result of about 0.2 magnitude unit in the case of shallow depth event (H < 60 km). The P-wave spectral approach allows for the relatively fast estimates of magnitude for use in BREWS. The average correction taking into account the most common focal mechanism for radiation pattern coefficient may lead to overestimation of the magnitude for shallow events of about 0.2 magnitude unit. However, in case of events of intermediate depth of M6 the resulting Mw is underestimated at about 0.1–0.2. We conclude that our P-wave spectral approach is sufficiently robust for the needs of BREWS for both shallow and intermediate depth events.

Scaling of ground motions from Vrancea (Romania) earthquakes

This paper evaluates the scaling of ground motions recorded from nine intermediate-depth earthquakes produced in the Vrancea seismic zone in Romania. The considered ground motion database consists of 363 horizontal recordings obtained on soil classes B and C (according to Eurocode 8). An analysis of the inter- and intra-event spectral accelerations is performed in order to gain information regarding the magnitude and distance scaling of the Vrancea ground motions. The analyses reveal a significant influence of the earthquake magnitude and focal depth on the distance scaling and different magnitude and distance scaling for the two soil classes. A linear magnitude and distance scaling is inferred from the results for the range of magnitudes <TEX>$5.2{\leq}M_W{\leq}7.1$</TEX>. The results obtained are checked through stochastic simulations and the influence of the stress drop and kappa values on the ground motion levels is assessed. In addition, five ground motion models which were tested in other studies using recordings from Vrancea earthquakes are analyzed in order to evaluate their corresponding host stress drop and kappa. The results show generally a direct connection between the host kappa values and the host stress drop values. Moreover, all the ground motion models depict magnitude dependent host kappa and stress drop levels.

Investigation on directional effects of Vrancea subcrustal earthquakes

This paper evaluates possible directional effects from strong ground motions recorded during the last four significant subcrustal earthquakes produced in the Vrancea seismic zone (Romania) in August 1986 (MW = 7.1), May 1990 (MW = 6.9 and 6.4) and October 2004 (MW = 6.0). Several measures of the horizontal component of the ground motion given in the literature (Boore et al., 2006; Boore, 2010) are computed and are related to the geometric mean of the as-recorded horizontal components. ANOVA method is applied in order to quantify the influence of the earthquake magnitude and of the soil class on some strong ground motion parameters (e.g. Arias intensity, PGV/PGA, or mean period TM). The study of the directional effects is performed using the distribution of IJMA and of the rotation angle corresponding to a measure of the horizontal component of the ground motion (RotD100) and the results reveal a visible directional pattern for IJMA, while in the case of RotD100 the directional patterns are less visible.

Time Series in the Study of Seismic Regime of Vrancea (Romania) Seismic Zone

In the series of monthly number of earthquakes is present long-term systematic component. The assumption about stationary of the average value and the variance is rejected. Statistically significant autocorrelation coefficients mean that the time series is not random, and there is some connection between successive levels. In order to predict a series of exponential smoothing method was used. Best model is simple seasonal for the logarithm of the levels, and Winters additive for the square root level of the original series. The research of time series confirms that the ARIMA (0, 1, 2) x (1, 0, 1) s model can be used to the analysis and prediction of uniform non-stationary time series with a nonlinear trend, such as a polynomial of low degree. Prediction for 2012 is computed using simple Simple seasonal, Winters and ARIMA models.

Possible Triggered Seismicity Signatures Associated with the Vrancea Intermediate-Depth Strong Earthquakes (Southeast Carpathians, Romania)

Online Material: Vrancea earthquake catalogs. To date, no generally applicable procedure is available for distinguishing triggered earthquakes from other seismic events that occur, apparently randomly, within a certain region (see discussion in van der Elst and Brodsky, 2010). In particular, it is not habitual to utilize diagrams illustrating the time evolution of the seismic release (e.g., Mantovani et al. , 1987; Husen et al. , 2004) as a diagnostic tool. However, that investigation method has been shown in the present study to be outstandingly useful for highlighting similarities existing between the patterns of seismic activities recorded in two distinct and possibly interacting lithospheric domains associated with the Carpathians orogenic belt in Romania. The main potentially causative earthquakes occurred in the Vrancea seismic zone (hereafter VSZ), located at intermediate depths (60–180 km) beneath the southeast Carpathian Arc bend (Fig. 1), whereas the resulting seismicity enhancements (recorded several months to tens of months afterward) developed at less than 60 km depth in the adjoining Carpathians‐foredeep region. The concerned Carpathians Mountains foreland basement is built up of two distinct lithospheric plates: the east European plate to the north‐northeast and the Moesian plate to the south‐southwest (Fig. 1), which are separated by a regional system of northwest–southeast‐oriented fractures (the so‐called Trotus and Peceneaga‐Camena faults). In coincidence with that contact, a shallow seismicity domain, designated (Raileanu et al. , 2007) as the Marasesti–Galaţi–Braila lineament (hereafter MGBL), has also been delineated. Figure 1. General seismotectonic setting of the considered area (geology after Sandulescu, 1984). The Carpathian Mountains accretionary wedge has overthrust (saw‐tooth boundary) the corresponding foreland, which extended on two major plates (east European and Moesian). The roughly oval, solid contour bounds the epicentral domain of the intermediate‐depth (≥60 km) Vrancea earthquakes. Large black symbols …

Nature of the Vrancea seismic zone (Eastern Carpathians) – New constraints from dispersion of first-arriving P-waves

The Vrancea region of the southeastern Carpathians is one of the most active seismic zones in Europe and it is well-known for its strong intermediate depth earthquakes. Seismic tomography had revealed a high-velocity body beneath Vrancea and the Moesian platform that extends to a depth of at least 350 km and can be interpreted as descending lithosphere. The strong earthquakes occur within the northeastern part of this high-velocity body, in a very limited seismogenic volume at intermediate depth (70–180 km). Several geodynamic models have been proposed for this area. They can be split into two main categories, in terms of the nature of the high-velocity anomaly, which may (a) be associated with descending relic oceanic lithosphere beneath the bending zone of the SE-Carpathians, either attached or already detached from the continental crust; or (b) it may represent continental lithosphere that has been delaminated, after continental collision and orogenic thickening. Based on currently available information, it appears difficult to distinguish between these two types of models. In this paper we attempt to shed more light on the nature of the seismic anomaly, as well as that of the origin of the intermediate depth seismicity in the Vrancea zone, by investigating the waveform character of P-waves excited by local earthquakes beneath this area, and in particular the dependence of group arrival times on frequency. We present observations of such a dispersion from stations situated at the bending zone of the SE-Carpathians. On the other hand, signals from the same earthquakes, but observed at reference stations outside of the anomalous zone do not show that frequency dependence. A natural explanation for these observations is that it is caused by the presence of a low-velocity channel at the top of the seismic anomaly, which is too thin to be resolved by classical seismic tomographic techniques. Similar observations of dispersed first-arriving P-waves have been made above subduction zones around the world, in which low-velocity layers with a thickness of several kilometers are known to exist. This suggests that a tabular slab of subducted oceanic crust is present within the seismic anomaly under the Vrancea region, and that the anomaly consists of subducted oceanic lithosphere rather than continental lithosphere, at least at depths shallower than the seismically active zone.