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

Larger peak ground accelerations in extra-Carpathian area than in epicenter

<p>Devasting ― and, in some sense, unforeseen  ―  earthquakes in Nepal, Sumatra, Haiti, Japan  and elsewhere have triggered a heated debate about the legitimity and limitations of probabilistic seismic hazard  assessment(PSHA). The authors are coming with many recorded data which will open up a new challenge to seismologists studiing nonlinear site effects in 2-D and 3-D irregular geological structures, leading them to a realistic research subject in earth physics, in nonlinear seismology. Shortly, why are we recording PGA values much higher than epicenter value? There was a need to create, for Europe, a unified framework for seismic hazard assessment and to produce a common integrated European probabilistic seismic hazard assessment (PSHA) model and specific scenario based on modeling tools. The leading question is, if this is happening only in this area of Europe. Vrancea is the site of strong intermediate-depth seismicity, down to 160 – 200 km depth and large magnitudes (M<sub>W</sub> ≤ 7.9 - 8.0) and is one of the most active seismic zones in Europe. The latest strong and deep Vrancea earthquakes occurred on August 30, 1986 (Mw = 7.1; h = <strong>131.4 km</strong>, in epicenter a=162.60 cm.s<sup>2</sup> and at Chisinău:212 cm/s<sup>2</sup>;Focsani:310 cm/s<sup>2</sup>;Iaşi:181 cm/s<sup>2</sup>; Otopeni: 220cm/s<sup>2</sup> etc.); May 30, 1990 (Mw = 6.9; h = <strong>90.9</strong> km; in epicenter: 157 cm/s<sup>2</sup>; Chişinau:189 cm/s<sup>2</sup>; Oneşti:242 cm/s<sup>2</sup>;Periş:242 cm/s<sup>2</sup>; Bolintin din Vale:219 cm/s<sup>2</sup>; Campina;271 cm/s<sup>2</sup> etc. & May 31, 1990 (M<sub>W </sub>= 6.4; h = <strong>86.9</strong> km, in epicenter: a=102 cm/s<sup>2</sup>;Focşani:162 cm/s<sup>2</sup>.There are more than 200 values larger than epicenter ones. More, on October 28,2018 an earthquake (Mw=5.89 and h= <strong>147.8 km</strong> ) generate  acceleration of 8.65 cm/s<sup>2</sup> in epicenter Vrâncioaia and accelerations of   69 cm/s<sup>2</sup> in Ploieşti; 65  cm/s<sup>2</sup> in Leova - Republic of Moldova etc. Why in this part of Europe/World there are many peak ground accelerations recorded and are larger than epicenter values ?. Surface waves Rayleigh and Love waves ( A third type of surface wave, the Stonely wave propagates along an interface between two media and is more correctly an interface wave and are not dispersive, thus they decrease in amplitude with distance from the interface) are seismic waves which are guided along the surface of the earth and the layer near the surface and they do not penetrate into the deep interior.</p><p>On the other time, the Alpine Tethys was linked to the Euro-Asian back-arc basins located further east through the Moesia - Dobrogea Transform [G. G. Stampfli; http://sp.Lyellcollection.org/by guest on November 14, 2019]. It is observed along new times that in Dobrogea area the peak ground accelerations recorded in last time are smaller than epicenter ones and our Nuclear Power Plant is  safe  to strong Vrancea earthquakes. Peak ground accelerations recorded   in Muntenia, Moldova   and   Republic of   Moldova are maily larger than   Vrancioaia epicenter values (Gh.Mărmureanu, Certainties/uncertainties in hazard and seismic risk assessment of strong Vrancea earthquake. Romanian  Academy Press,2016,330 page,ISBN 978-973-27-2629-7).</p>