Seismic hazard analysis by using a non-poissonian probabilistic approach

Tahernia N.

Department of Physics, Parand Branch, Islamic Azad University, Parand, Iran

[e-mail]

Iran is one of the most seismic countries of the world. It is situated over the Himalayan-Alpied seismic belt and is one of those countries which have lost many human lives and lot of money due to the occurrence of earthquakes. In this country, a destructive earthquake occurs every several years due to the fact that it is situated over a seismic zone. Tehran city, the capital of Iran, has its special features including highly dense population (more than 10 million people), as well as political and economic centralization, that make it prone to more severe earthquake damage here will affect the whole country, and therefore, the evaluation of the severity of earthquake occurrence is indeed very necessary. The existence of the active North Tehran thrust, the active faults like Mosha and North and South Rey, the alluvium deposits of Tehran plain and Rey city, and the occurrence severe past earthquakes, all indicate the high seismicity of this region and they have caused the probability of occurrence of severe earthquakes with magnitudes over 7 to be very high [5].

The seismic hazard assessment of this region is of great importance to minimize the seismic risk and to predict earthquakes accurately. Seismic hazard may be analyzed using an empirical-statistical approach, which is based on historical data, or a deterministic approach, when a particular scenario is assumed, or a probabilistic approach, in which uncertainties in earthquake size, location, and time of occurrence are explicitly considered. A hybrid method in which the advantages of both deterministic and probabilistic methods are involved can also be used.

The widespread application of probabilistic seismic hazard analysis has, in virtually every case, been based on the memoryless Poisson model of earthquake occurrences. The Poisson model has been commonly used for several reasons. But perhaps most importantly, it is the simplest model that captures the basic elements of the problem. The standard Poisson seismic hazard model requires only an average arrival rate, to provide a complete statistical description of seismic occurrences [3, 4]. Should an alternative model be considered, questions arise, first, as to which alternative model should be considered and, second, as to how in practice to estimate both the additional model parameters and the initial conditions upon which Non-Poisson predictions may depend. Various past studies have considered specific Non-Poisson models and applications [1, 2, 6, 7].

Based on the modified methodology of probabilistic seismic hazard analysis and Non-Poisson recurrence time model, the present study makes available more reliable seismic hazard maps for the Tehran region. Our purpose here is to apply the suitable distribu­tions for interoccurrence times of earthquakes in two major seismotectonic provinces of Iran, Alborz- Azarbayejan and Central-East Iran, which include our study area, for probabilistic seismic hazard analysis with the help of renewal process. Two seismic hazard zoning maps based on PGA over bedrock for 10% and 63% probability of exceedance in 50 years are provided.

Application of renewal process, which is a counting process that time intervals between two events are independent with the same arbitrary distribution, can makes it possible to use a non-Poisson temporal distribution function in the seismic hazard analysis. In this study, the seismic hazard analysis of Tehran region and surrounding areas is conducted using the probabilistic approach and renewal process and also applying the selected distributions function of the seismotectonic provinces (Tahernia et al., 2011) where the study region is located.

By using a computer program that has been written for this purpose the seismic hazard assessment is carried out for a grid of points, in the area where encompassed by the 49.5°–53.5°E longitudes and 34°–37°N latitudes including Tehran region and surrounding areas. The seismic hazard zoning maps were provided accordingly. These maps show the non-Poisson probabilistic hazard analysis for the return periods of 50 and 475 years. According to the maps which have been presented in this study, PGA values are estimated to be 0.30g-0.32g for 10% probability of exceedance in 50 years, 0.14g-0.115g for 63% probability of exceedance in 50 years for non- Poisson approach for Tehran region.

Fig. Seismic zoning map of the Tehran region for (a) 10% probability of exceedance in 50 years, (b) 63% probability of exceedance in 50 years.

References:

1. Aki, K. (1965) Maximum likelihood estimate of b in the formula logN=a-bM and its confidence limits, Bull. Earthq. Res. Inst., Tokyo Univ., 43, 237-239.

2. Anagnos, T., and Kiremidjian, A.S. (1984) Stochastic time-predictable model for earthquake occurrences, Bull. Seismol. Soc. Am. 74, 6, 2593-2611.

3. Cornell, C.A. (1968) Engineering seismic risk analysis, Bull. Seism. Soc. Am. 58, 1583-1606.

4. Der Kiureghian, A., and Ang, A.H-S. (1977) A fault rupture model for seismic risk analysis, Bull. Seismol. Soc. Am. 67, 4, 1173-1194.

5. Ghodrati-Amiri, G., Μotammed, R. and Rabet-Eshaghi, H. (2003) Seismic hazard assessment of metropolitan Tehran, Iran, J. Earthq. Eng., 7, 347-372.

6. Kiremidjian, A.S., and Anagnos, T. (1984) Stochastic slip-predictable model for earthquake occurrences, Bull. Seismol. Soc. Am. 74, 2, 739-755.

7. Vere-Jones, D. (1970) Stochastic models for earthquake occurrence, J. Roy. Stat. Soc. B (Methodological) 32, 1, 1-62.

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