In general terms, the seismic hazard defines the expected seismic ground motion at a site, phenomenon which may result in destructions and losses.

Тwo major approaches – **deterministic** and **probabilistic** – are worldwide used at present for seismic hazard assessment.

The **deterministic approach** takes into account a single, particular earthquake, the event that is expected to produce the strongest level of shaking at the site.

The outputs – macroseismic intensity, peak ground acceleration, peak ground velocity, peak ground displacement, response spectra – may be used directly in engineering applications.

In the **probabilistic approach**, initiated with the pioneering work of Cornell [1], the seismic hazard is estimated in terms of a ground motion parameter – macroseismic intensity, peak ground acceleration – and its annual probability of exceedance (or return period) at a site.

The method yields regional seismic probability maps, displaying contours of maximum ground motion (macroseismic intensity, PGA) of equal – specified – return period.

*[1] Cornell C. A., 1968. Engineering seismic risk analysis, Bull. Seism. Soc. Am., 58, 1583–1606.*

## Deterministic versus probabilistic approaches

Both – **probabilistic** and **deterministic** – methodologies have a role in seismic hazard assessment, the two methods can complement one another to provide additional insights to the seismic hazard problems.

One method will have priority over the other depending on how quantitative are the decisions to be made, depending on the seismic environment, and depending on the scope of the project (single site or a region).

The **probabilistic hazard assessment** (which accounts for event recurrence) is important for developing viable insurance policies, and to increase earthquake preparedness, by planning adequate mitigation actions.

When dealing with cultural heritage – for which the consequences of failure or loss are intolerable – or critical structures (*e.g.*, nuclear power plants, deposits of pollution materials and radioactive waste, etc.) – whose failure after a strong earthquake would represent a serious risk for the natural and human environment – the **deterministic approach**, based on maximum possible seismic input, is relevant.

The lessons from the destructive earthquakes which occurred worldwide during the last decade pointed out that a single hazard map cannot meet the requirements from different end-users.

In many situations a recursive analysis, where deterministic interpretations are triggered by probabilistic results and vice versa, may result in the most realistic evaluation and allow the most informed decisions to be made.

The results of a seismic hazard analysis can be convolved with a seismic fragility function – which quantifies the probabilities of various levels of damage to a facility as a function of ground motion – to obtain a **seismic risk** analysis, which indicates the probabilities per unit time of different levels of failure or loss.

Therefore the seismic hazard analysis is a primary input in **risk assessment**.

## Seismic hazard assessment for Romania

The level of the seismic hazard in Romania is determined by the presence of several seismogenic zones with damage potential over a larger or smaller area.

The most dangerous seismic source is located in the subcrustal lithosphere (in the depth range 60-200 km) at the bending of the Eastern Carpathians – Vrancea region.

Beside this intermediate-depth zone, several shallow-depth seismic areas of local importance for the seismic hazard are present as well: the Făgăraş – Câmpulung zone in the eastern part of the Southern Carpathians; the Danubian, Banat and Crişana – Maramureş zones in southwestern, western and northwestern Romania, respectively; the crustal-depth Vrancea zone; the Bârlad Depression and the Predobrogean Depression in eastern Romania; the Intramoesian Fault in southeastern Romania; the Transylvanian Depression in central Romania (Radulian et al., 2000). The south-easternmost part of the country is also exposed to the effects of Shabla seismogenic zone, located in northeastern Bulgaria.

A description of the seismic sources – boundaries, seismicity characteristics, tectonics – is given here Seismicity of Romania.

Several papers dealing with the seismic hazard assessment on the territory of Romania have been published in reputable international journals.

The studies are carried out following both neo-deterministic and probabilistic approaches.

The probabilistic analyses (which are more numerous) apply methodologies which differ to some extent from the standard procedure (although they incorporate the basic principles of the classical approach [1]). Moreover, the considered input data – earthquake catalogues, seismic zonation – as well as the selected ground motion parameter – macroseismic intensity, PGA – differ from one author (or group of authors) to another. Nevertheless, the results of the analyses are all consistent, showing a notably good agreement.

#### Seismic hazard analyses for Romania

Radulian M., Vaccari F., Mandrescu N., Panza G. F., Moldoveanu C., 2000. Seismic hazard of Romania: A deterministic approach, *Pure Appl. Geophys.*, 157, 221-247.

Musson R. M. W., 2000. Generalised seismic hazard maps for the Pannonian Basin using probabilistic methods, *Pure Appl. Geophys.*, 157, 147–169.

Mäntyniemi P., Mârza V. I., Kijko A., Retief P., 2003. A new probabilistic seismic hazard analysis for the Vrancea (Romania) seismogenic zone, *Nat. Haz.*, 29, 371–385.

Enescu D., Mărmureanu A., Enescu B.D., 2004. A procedure for assessing seismic hazard generated by Vrancea earthquakes and its application. II. Attenuation curves, *Romanian Reports in Physics*, 56 (1), 147-159.

Ardeleanu L., Leydecker G., Bonjer K. P., Busche H., Kaiser D., Schmitt T., 2005. Probabilistic seismic hazard map for Romania as a basis for a new building code, *Nat. Haz. Earth Syst. Sci.*, 5, 679–684.

Enescu D., Enescu B.D., 2007. A procedure for assessing seismic hazard generated by Vrancea earthquakes and its application. iii. A method for developing isoseismal and isoacceleration maps, *Romanian Reports in Physics*, Vol. 59 (1), 121–145.

Moldovan I.A., Popescu E., Constantin A., 2008. Probabilistic seismic hazard assessment in Romania: application for crustal seismic active zones, *Rom. Journ. Phys.*, 53 (3–4), 575–591.

Leydecker G., Busche H., Bonjer K. P., Schmitt T., Kaiser D, Simeonova S., Solakov D., Ardeleanu L., 2008. Seismic hazard maps for Bulgaria and Romania, *Nat. Haz. Earth Syst. Sci.*, 8, 1431–1439.

Sokolov V. Yu., Wenzel F., Mohindra R., 2009. Probabilistic seismic hazard assessment for Romania and sensitivity analysis: A case of joint consideration of intermediate-depth (Vrancea) and shallow (crustal) seismicity,* Soil Dyn. Earthquake Eng.*, 29, 364–381.

Mărmureanu G., Cioflan C.O., Mărmureanu A., 2011. Intensity seismic hazard map of Romania by probabilistic and (neo)deterministic approaches, linear and nonlinear analyses, *Romanian Reports in Physics*, 63 (1), 226–239.

More published papers (with authors from NIEP) treating various aspects related to the seismic hazard assessment on the territory of Romania may be found at the publications page (http://publicatii.infp.ro/)