Scenario-based earthquake risk assessment for Bucharest, Romania

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Abstract

In this study, a seismic risk analysis for Bucharest (Romania) is performed using six intermediate-depth Vrancea earthquake scenarios, defined as a function of magnitude, focal depth and source-to-site distance. For each scenario, 2000 individual ground motion simulations for Bucharest are considered. The simulations are performed using a recently derived spatial correlation model for Vrancea intermediate-depth seismic events. Subsequently, for each individual earthquake scenario, an estimation of the damage, losses and casualties is obtained. Moreover, a community resilience metrics, defined in terms of recovery of the housing capacity is evaluated using a model available in the literature. Finally, based on the recent experience of Italian earthquakes, an estimation of the number of operators and of the number of tents necessary after an earthquake is provided.

Introduction

Bucharest, the capital city of Romania, is one of the cities with the highest seismic risk in Europe. Almost 40 years have passed since the last devastating Vrancea intermediate-depth earthquake in March 1977. Between one and six Vrancea seismic events with MW≥7.0 have occurred in each century [17]. The last three Vrancea earthquakes with moment magnitudes MW≥7.0 occurred in November 1940 (MW=7.7, h≈150 km), March 1977 (MW=7.4, h =94 km) and August 1986 (MW=7.1, h=131 km). A MW 6.9 earthquake took place in May 1990 and since then, a single MW 6.0 seismic event occurred in October 2004. The peak ground accelerations obtained for Bucharest from the seismic hazard models developed within the SHARE project [26] and BIGSEES project (Pavel et al. [14]) are in excess of 0.2g (or 0.3g depending on the soil conditions). The disaggregation of seismic hazard [13] shows the dominant influence of the Vrancea intermediate-depth seismic source on the seismic hazard of Bucharest.

The experience of the Vrancea 1977 earthquake and its effects on Bucharest are a vivid reminder of the possible consequences of such a devastating event. A World Bank report from 1978 reveals that the total losses caused by the earthquake are of 2.05 bill. USD (around 8% of GDP) or around 8 bill. USD if one uses the conversion rate for USD from 1977 and up to now. Over 50% of the losses were attributable to the buildings affected by the earthquake and another 25% were caused by the affected industry. Georgescu and Pomonis [8] show that 12% of the existing building stock at that time either was seriously damaged (or collapsed) or had to be strengthened or repaired. Nevertheless, the rapid and efficient response of the Romanian Government at that time is highlighted by the same World Bank Report [27]. The Report also states that those whose residences were destroyed by the earthquake have been housed (mainly by giving them priority for newly completed housing) and provided with essentials by the Government. Moreover, the data from the Annuary of Statistics for 1977 [19] shows that, despite of the earthquake, the GDP (Gross Domestic Product) of Romania increased with around 10% compared to that from the previous year.

In this context, in this study, we aim at evaluating the seismic losses for residential buildings in Bucharest in the event of Vrancea scenario earthquakes. The magnitudes of the earthquake scenarios are MW=7.0, 7.5 or 8.0 and their source-to-site distances correspond to events close or far from Bucharest. Thus, a minimum and maximum value for the seismic losses can be computed for each of the three magnitude levels. Previous studies with the same topic of Lang et al. [9] or Toma-Danila et al. [20] have shown that even the impact of MW=7.0 seismic events on the residential building stock of Bucharest is considerable. The much larger earthquakes can severely damage tens of thousands of buildings and cause losses exceeding 10 bill. Euro. In this study, unlike in the previously mentioned papers, the ground motion distribution throughout Bucharest is evaluated using a recently developed spatial correlation model. 2000 simulations of ground motion distributions are obtained for each individual earthquake scenario and for each simulation, the seismic damage, losses and resilience metric are evaluated. Another study authored by Pavel et al. [15] refers to the probabilistic evaluation of seismic risk for Bucharest and the impact of incorporating a spatial correlation model on the seismic damage and losses. The same building stock characteristics are used in this study, as well, but a different approach (scenario-based vs. probabilistic) is employed here. In addition, more risk-related parameters and a seismic resilience metric are evaluated in this research with respect to the previous study of Pavel et al. [15].

Section snippets

Residential building stock evaluation and earthquake scenarios

The characteristics of the residential building stock of Bucharest are based on the data from the most recent census performed in 2011. The results of Lang et al. [9] and Toma-Danila et al. [20] are obtained using building stock information collected in a census performed in 1999. The total number of inhabitants of Bucharest is 1846939 which inhabit 131875 residential buildings spread on an area of more than 240 km2. The average area of a dwelling in Bucharest is 46.6 m2, while the average

Seismic risk assessment

Seismic risk analyses were conducted for each of the six intermediate-depth Vrancea earthquake scenarios. However, in order to account for the ground motion variability, a recently developed spatial correlation model [16] is also employed. This simple spatial correlation model has an exponential functional form (widely used in literature) and has a correlation distance for peak ground acceleration of 32.3 km. The spatial correlation is applied for both intra- and inter-event components. The

Evaluation of seismic resilience metric

The topic of seismic resilience is becoming more and more important in Romania as there are almost 40 years since the last devastating Vrancea intermediate-depth earthquake. One of the main issues regarding the seismic resilience is the evaluation of the recovery of the housing capacity after an earthquake. For Romania, the seismic risk assessment and its metrics have been studies by several researchers named in the references. However, the seismic resilience part has not been studied up to now

Conclusions

In this study, a deterministic evaluation of seismic risk and resilience for the residential buildings in Bucharest is performed using data from the most recent census from 2011. Only intermediate-depth Vrancea earthquake scenarios are used in the computations since this seismic source is the most important contributor to the seismic hazard of Bucharest. 2000 simulations are performed for each of the proposed six scenario earthquakes in order to take into account the spatial variability of the

Acknowledgements

This work was supported by a grant of the Romanian National Authority for Scientific Research and Innovation, CNCS – UEFISCDI, project number PN-II-RU-TE-2014-4-0697. The authors gratefully acknowledge the financial support. The constructive feedback from two anonymous reviewers is greatly appreciated as it has helped us to considerably improve the quality of the manuscript.

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