Accounting for site effect in probabilistic assessment of seismic hazard for Romania and Bucharest: a case of deep seismicity in Vrancea zone

The paper presents recent achievements in evaluations of site-dependent seismic hazard in Romania and the capital city of Bucharest caused by the Vrancea focal zone (SE-Carpathians). The zone is characterized by a high rate of occurrence of large earthquakes in a narrow focal volume at depths 60–170 km. The database that was used for the hazard evaluation includes parameters of seismicity, ground-motion source scaling and attenuation models (Fourier amplitude spectra), and site-dependent spectral amplification functions. Ground-motion characteristics were evaluated on the basis of several hundred records from more than 120 small magnitude (M 3.5–5) earthquakes occurred in 1996–2001 and a few tens of acceleration records obtained during four large (M 7.4, 7.2, 6.9 and 6.3) earthquakes. The data provide a basis for probabilistic seismic hazard assessment in terms of peak ground acceleration, peak spectral acceleration and MSK intensity using Fourier amplitude spectra for various exceedance probabilities or average return periods. It has been shown that the influence of geological factors plays very important role in distribution of earthquake ground-motion parameters along the territory of Romania.     

Average structure of the crust and upper mantle in East Africa

An average crust-mantle model has been derived for the East African Rift system based on a number of presently available seismic data. The inversion of experimentally determined spectral transfer ratios of long-period body waves recorded at stations AAE (Addis Ababa, Ethiopia), NAI (Nairobi, Kenya) and LWI (Lwiro, Zaïre) requires at least a two-layer crust. Except for station AAE. the observed P-wave delays can be accounted for by differences in the deduced crustal structure. Phase- and group-velocity measurements of Rayleigh waves along the path AAE-NAI provide additional information on the gross structure of the crust and upper mantle. Only a well-developed asthenosphere channel can explain the observed surface-wave dispersion. It is shown that the average model MS-71 permits a satisfactory interpretation of all the data presented in this paper.     

Ground-motion prediction equations for the intermediate depth Vrancea (Romania) earthquakes

We present the regional ground-motion prediction equations for peak ground acceleration (PGA), peak ground velocity (PGV), pseudo-spectral acceleration (PSA), and seismic intensity (MSK scale) for the Vrancea intermediate depth earthquakes (SE-Carpathians) and territory of Romania. The prediction equations were constructed using the stochastic technique on the basis of the regional Fourier amplitude spectrum (FAS) source scaling and attenuation models and the generalised site amplification functions. Values of considered ground motion parameters are given as the functions of earthquake magnitude, depth and epicentral distance. The developed ground-motion models were tested and calibrated using the available data from the large Vrancea earthquakes. We suggest to use the presented equations for the rapid estimation of seismic effect after strong earthquakes (Shakemap generation) and seismic hazard assessment, both deterministic and probabilistic approaches.     

A uniform approach to seismic site effect analysis in Bucharest, Romania

As a uniform approach to the assessment of ground motion variation within the Romanian capital Bucharest we analyze and compare strong motion records from analog recorders, weak motion data from a modern digital accelerometer network, and intensity observations of previous strong earthquakes. These different data sets allow to clearly characterize geographical trends in the distribution of ground shaking in the city for future earthquakes. Below 2 Hz the variability is small. Between 2 and 5 Hz, however, variations by a factor of 3–4 have to be expected. As the key source for the seismic hazard—the intermediate depth Vrancea earthquakes—remain at hypocentral distances in excess of 150 km from the city the ground motion variation must be predominantly attributed to site effects. This geometry of Vrancea sources to the site of Bucharest is ideal for the application of source-site separation techniques. However, despite this fact site effect amplification functions display a very large amount of aleatory uncertainty. In other words the standard source-site parameterization is too simple and we do not yet fully understand the cause and size of site effects.     

Slab break‐off – abrupt cut or gradual detachment? New insights from the Vrancea Region (SE Carpathians,Romania)

Slab break-off is a plate-tectonic process which does not only return lithospheric material into the deeper mantle, but also has severe effects on surface movements and seismic hazard: slab-pull induced seismicity is reduced when the subducted slab decouples from the overlying crust. In the Vrancea region (SE Carpathians), strong earthquakes frequently occur at intermediate depths (70–180 km) in a laterally small volume, while the crust shows low but distributed seismicity. The stress pattern shows similar partitioning with vertical extension in the slab and no preferred orientation in the overlying crust. Both features indicate a decoupling between slab and overlying crust, either by slab break-off or delamination. However, the strong vertical elongation of the slab requires that the upper end of the slab is fixed vertically. Thus, a 'soft' coupling is assumed that is strong enough to enable elongation of the slab, but weak enough to inhibit 'quasi-static' stress transfer to the overlying crust.     

The 2005 Heidelberg and Speyer earthquakes and their relationship to active tectonics in the central Upper Rhine Graben

We determine the source parameters of three minor earthquakes in the Upper Rhine Graben (URG), a Cenozoic rift, using waveforms from permanent and temporary seismological stations. Two shallow thrust-faulting events (M _L = 2.4 and 1.5) occurred on the rift shoulder just south of Heidelberg in March 2005. They indicate a possible movement along the sediment–crystalline interface due to tectonic loading from the near-by Odenwald. In February 2005, an earthquake with a normal-faulting mechanism occurred north of Speyer. This event (M _L = 2.8) had an unusual depth of about 22 km and a similar deep normal-faulting event occurred there in 1972 (M _L = 3.2). Other lower crustal events without fault plane solutions are known from 1981 and 1983. At such a depth, inside the lower crust, ductile behaviour instead of brittle faulting is commonly assumed and used for geodynamic modelling. Based on the newly available fault plane solutions we can confirm the brittle, extensional regime in the upper and lower crust in the central to northern URG indicated in earlier studies.