Seismic Hazard

Earthquakes, through their devastating effects - due to ground motion, earth faulting, tectonic deformation, soil liquefaction, landslides - are a serious and difficult problem facing modern society. Seismic hazard problem becomes more severe in recent years due to the fact that vulnerability is increasing by increasing urbanization and industrialization in exposed areas. The impact of the strong earthquakes occurred recently in different regions of the globe, draws attention to the necessity of taking urgent measures to reduce casualties and social and economic damages. An important tool to increase preparedness for earthquakes and to improve disaster prevention policies in densely populated areas is the seismic damage scenarios approach including loss estimation.The first step in building damage scenarios is to estimate seismic hazard.

Seismic hazard (H) is defined by the probability of occurrence of an event with the potential for destruction in a defined area and a given time interval. Vulnerability is defined by the expected degree of loss (0 < V<1) due to a destructive event, where: 0 means no loss and 1 - total loss. Seismic Risk (RS) is defined by the degree of loss to a particular event i and the likelihood of Hi, (RS)i = V x Hi. Seismic risk is a combination of hazard and vulnerability (Fig. 1).

High hazard is not always high-risk! This happens when the vulnerability is low. Low density population, correctly used land, secure and properly designed buildings will lead to low risk even in high-hazard areas.

Seismic hazard analysis requires expertise in fields other than seismology as well. Geology is required to determine the location, configuration and to define the potential seismic sources, in particular the known active faults. Geophysical techniques are needed to define the buried seismic sources. Analysis of historical data can play a decisive role in the assessment of earthquakes with no instrumental measurement.

There are currently two approaches in earthquake hazard assessment: deterministic approach and probabilistic approach.

Deterministic approach develops scenario for a particular earthquake – an earthquake with specified size, produced in a specified location - which is assessed based on ground motion at the site of interest. Standard deterministic methodology consists of the following steps:

 

Figure 2. Main steps for deterministic seismic hazard approach

Step 1 is to identify a source or several possible sources that may affect the site. Configuration of individual sources can by points, lines, areas or volume, depending on the source type and the possibility to define them geologically.

Step 2. At this stage select the so-called control earthquake. Seismic potential of each seismic source, as described in Step 1, depends on the maximum earthquake that can be generated in the source. This can be the expected earthquake, the maximum credible earthquake or any other type of earthquake. Selection of the specific criterion is one of the most important elements in determining the conservatism level. Responsibility for this control earthquake choice is immense and this constitutes the most vulnerable part in the deterministic analysis. Earthquake magnitude and epicentral intensity are commonly used to define the size (which here can be: duration, magnitude, maximum acceleration, etc..) of the earthquake. In addition to these sizes, there is an appropriate distance, which is the distance (usually the nearest) between the source and location. One of these hypothetical earthquakes will be the control earthquake, an earthquake that generates indices (intensity, peak acceleration, relative speed, etc. predominant period.) which will dominate the effects of other earthquakes. This earthquake will be considered to be the most important in defining the seismic hazard. In this stage can be used several control earthquakes because it is not always clear which event is associated with the largest movement of land at the site of interest. This can happen when a seismic source is important as other, or when using multiple parameters of the site terrain in defining the seismic hazard (eg, maximum acceleration, maximum relative velocity, maximum relative displacement, the fundamental period, maximum spectral acceleration, etc.).

Step 3 is to determine the earthquake effects, ground motion in the location of study. This is done by estimating the intensity, displacement, velocity or acceleration of land for different epicentral distances. They consist of a curve approximated by observable data when they exist (the relationship of attenuation).

Finally, the analysis provides deterministic scenario representing the most severe situation expected, but without giving information on its occurrence in time. Thus, the deterministic approach does not indicate the probability of control earthquake occurrence during the life of the building structures from the site.

Representative papers on deterministic method for assessing seismic hazard in Romania in the extensive cooperation with the Institute of Theoretical Physics from Trieste and Department of Earth Sciences, University of Trieste (Italy) (COPERNICUS project, NATO SFP Project, GO WEST grants, NATO Linkage Grants, bilateral projects):

• “Seismic Hazard of the Circum-Pannonian Region”, eds. G. F. Panza, M. Radulian, C.-I. Trifu, Pure and Applied Geophysics Special Volume 157, 221-247, 2000.

• “Impact of Vrancea Earthquakes on the Security of Bucharest and other Adjacent Urban Areas”, eds: G.F. Panza, M. Radulian, I. Kuznetov, Independent Film, Bucharest, Romania, 2007.

Figure 3. Deterministic seismic hazard obtained by considering the maximum possible earthquake with Mw 7.7 (10 November 1940) (Deterministic seismic hazard analysis in Romania, Cioflan OC, ICTP, Trieste, Italy, November 17-18, 2009).

Probabilistic approach, proposed by the pioneering work of Cornell (1968), has become a standard method widely accepted and used worldwide. It consists of four basic steps, some of which partially overlap with those of the deterministic approach (Reiter, 1990).

Figure 4. Main stages of the probabilistic seismic hazard analysis

Step 1. At this stage we define seismic sources. It is generally similar to the deterministic analysis of phase 1 except that the sources are explicitly defined as having a constant seismic potential, ie, the probability of occurrence of earthquakes or an earthquake by a certain size is the same in the source. Sources may vary from line sources to seismotectonics regions (eg Vrancea seismogenic zone).

Step 2. At this stage we define recurrence seismic characteristics for each source. This step is fundamentally different from stage 2 of the deterministic analysis. Instead of checking the control earthquake or maximum earthquake within each source, here each source is characterized by a recurrence relation or a probability distribution of occurrence of earthquakes. A recurrence relationship indicates the probability of a given size earthquake, with the epicenter anywhere in the source, within a timeframe, usually one year, to take place. It is selected a maximum earthquake for each source. In contrast with the deterministic procedure, this earthquake is not only the maximum considered earthquake, but the upper limit of the size of earthquakes that will enter the analysis for each source considered.

Step 3. Now estimate the earthquake effects in site location like deterministic analysis, but in probabilistic analysis is a family of attenuation curves for each magnitude. Each attenuation curve has its degree of uncertainty with the known data set (curves M1, M2, M3 ... in Fig.3).

Step 4. consists in integrating the entire range of magnitudes and distances for each seismic source to obtain - in particular location - probabilistic hazard values in the form of cumulative distributions for parameters that describe the movement of land. The effects of all earthquakes of various sizes, produced in different locations and different seismic probabilities are integrated into a single curve, which expresses the probability of exceedance in a specified time period, certain values of parameters describing the seismic motion on site.

Figure 5. Probabilistic seismic hazard in terms of macroseismic intensity (MSK scale), obtained by considering all seismic zones, for a return period of 475 years (after Ardeleanu et al., 2005)  

 

Comparing the two methodologies for seismic hazard assessment, we note that the deterministic approach is more intuitive, it requires well-defined production of an earthquake (in terms of magnitude and source distance - the site), subsequently used in all successive steps of the analysis. However, the choice of control earthquake is largely the result of reasoning, and not a quantitative one.

Conversely, probabilistic analysis allows the use of various events and mitigation models. The estimated hazard incorporate the estimated effects of all earthquakes considered capable to affect a particular location and can be used in mitigation account several models, each with its uncertainty, also the probability of occurrence of different sizes are included in the analysis.

A detailed and quantitative overview of the steps followed in a probabilistic seismic hazard estimates can be found in the book "Methods and statistical models with applications in seismology in the complex study of earthquakes in some areas of Romania”, Moldovan, I.A. (2007).

Hazard analysis is a fundamental input in the process of risk assessment and establishing policies to limit the destructive effects of the earthquakes. Both methodologies - deterministic and probabilistic - have a role in seismic hazard and risk analysis, whose purpose is the decision-making and selection criteria and levels of building design and rehabilitation, financial planning for earthquake losses (insurance levels or self-insurance or reinsurance), investment in industrial systems, emergency response planning and intervention for post-quake recovery plan for the long term.

In Romania, the highest level of seismic hazard is related to the presence in the Eastern Carpathians bend zone of a source of destructive earthquakes of intermediate depth, which may affect a wide area from Central Europe to Moscow. In the last hundred years, in the Vrancea area occurred four major subcrustal earthquakes - November 10, 1940, magnitude 7.7 Mw, March 4, 1977, magnitude 7.4 Mw, 30 August 1986, magnitude 7.1 Mw, May 30 1990, magnitude 6.9 Mw - the first two with disastrous impact. In the event of 4 March 1977 died 1570 people, 11300 were injured and 32500 houses and 763 industrial units were destroyed or badly damaged (Sandi, 2001). Most casualties and material damage were registered in Bucharest.

NIEP have developed, over time, a series of studies to estimate the seismic hazard in Romania. Among the most recent include:

 

Intensity seismic hazard map of Romania by probabilistic and (neo) deterministic aproaches, linear and nonlinear analyses, Mărmureanu Gh., Cioflan C.O., Mărmureanu A., Romanian Reports in Physics, Vol. 63, No. 1, P. 226–239, 2011

 

New seismic hazard map of Romania by probabilistic and deterministic approaches, linear and nonlinear analyses. Cioflan C.O., Mărmureanu Gh., Mărmureanu A., 14ECEE, 2010

 

Estimări ale hazardului seismic probabilist pentru teritoriul României, Ardeleanu L.A., ISBN 978-973-702-808-2, Tehnopress Iaşi, 2010

Probabilistic seismic hazard in terms of intensities for Bulgaria and Romania – updated hazard maps, G. Leydecker, H. Busche, K.-P. Bonjer, T. Schmitt, D. Kaiser, S. Simeonova, D. Solakov, and L. Ardeleanu, Nat. Hazards Earth Syst. Sci., 8, 1431–1439, 2008

 

Probabilistic seismic assessment in Romania: application for crstal seismic active zones, I. A. MOLDOVAN, E. POPESCU, A. CONSTANTIN, Rom. Journ. Phys., Vol. 53, Nos. 3–4, P. 575–591, 2008

Ground Motion Patterns of Intermediate-Depth Vrancea Earthquakes: The October 27, 2004 Event, in “Harmonization of Seismic Hazard in Vrancea Zone”. Bonjer K.P., Ionescu C., Sokolov V., Radulian M., Grecu B., Popa M., Popescu E., (Eds. A. Zaicenco, I. Craifaleanu, I. Paskaleva), NATO Science for Peace and Security Series – C, Springer, 47-62, 2008.

Recent achievements of the neo-deterministic seismic hazard assessment in the CEI region.

 

Panza G.F., Kouteva M., Vaccari F., Peresan A., Cioflan C.O., Romanelli F., Paskaleva I., Radulian M., Gribovszki K., Herak M., Zaichenco A., Marmureanu G., Varga P., Zivcic M., Proc. 2008 Seismic Engineering Conference Commemoreting the 1908 Messina and Reggio Calabria Earthquake (eds. Santini A. and Moraci N.), Amer. Inst. Physics, vol. 1020, 402-409, Melville, USA, 2008.

 

Cercetari privind hazardul seismic la nivel national si local. Harti de hazard seismic general si local (microzonare), Editura Tehnopress, (Editura acreditata de CNCSIS); ISBN:978-973-702-701-6; Coordonator Gh Marmureanu, Iasi 2009.

 

Cercetari privind managementul dezastrelor generate de cutremurele romanesti, Editura Tehnopress, (Editura acreditata de CNCSIS); ISBN:978-973-702-701-6; Coordonator Gh Marmureanu, Iasi 2009.

 

Probabilistic seismic hazard map for Romania as a basis for a new building code, Ardeleanu, L., Leydecker, G., Bonjer, K.-P., Busche, H., Kaiser, D., and Schmitt, T., Nat. Hazards Earth Syst. Sci., 5, 679-684, doi:10.5194/nhess-5-679-2005, 2005.

Figure 6. The Mediterranean harzard map showing PGA on rigid soil in units (g) for a 10% probability of exceedance in 50 years (Jiménez et al., 2001)

References:

Engineering seismic analysis, Cornell C.A. Bull. Seismol. Soc. Am., 58, 1583 – 1606, 1968.

Earthquake hazard analysis: issues and insight, Reiter L., Columbia University Press, New York, 1958.

Estimări ale hazardului seismic probabilist pentru teritoriul României, Ardeleanu L.A., ISBN 978-973-702-808-2, Tehnopress Iaşi, 2010.

Unified seismic hazard modelling throughout the Mediterranean region, M.J. JIMÉNEZ, D. GIARDINI, G. GRÜNTHAL, and SESAME WORKING GROUP, BOLLETTINO DI GEOFISICA TEORICA ED APPLICATA VOL. 42, N. 1-2, PP. 3-18, 2001.

Metode si modele statistice in seismologie cu aplicatii in studiul complex al cutremurelor din unele zone ale Romaniei, I.A. MOLDOVAN, Editura MOROSAN. Bucuresti, 2007, ISBN 978-973-8986-01-5 (230 pag), 2007.