Attenuation relation of Arias intensity for Zagros Mountains region (Iran)

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Abstract

Arias intensity is considered as a shaking parameter suitable for characterizing earthquake impact on ground stability. Within the framework of a study aimed at providing tools for the assessment of hazards related to earthquake-induced slope failures, Arias intensity attenuation relations were determined for the Zagros Mountains region, an active tectonic belt elongated NW–SE in the western and south-western part of Iran. The calculation of relation coefficients was based on strong-motion data of earthquakes located in the Zagros area and recorded by Iranian stations managed by the Building and Housing Research Center of Iran (BHRC). Five models of attenuation relation were considered and their coefficients were estimated through a least-square regression analysis. The relations obtained were then applied to a data sample different from that used for regression and the root mean square (RMS) of residuals was examined in order to compare effectiveness of different relations in probabilistic estimates. Furthermore a comparison made with attenuation relations obtained for Alborz and the central part of Iran showed significant differences possibly related to structural differences.

Introduction

Some of the most severe effects of earthquakes are related to the triggering of ground-failure phenomena. The most common of such phenomena are landslides, which, on occasions of strong earthquakes, can impact on property and infrastructures, leading to large economic losses and fatalities.

This problem is particularly serious in areas where active seismicity is combined with rough topographic relief as in some regions of Iran. This country is located in a tectonically active belt between two major plates: the Arabian plate, including Saudi Arabia, Persian Gulf and the Zagros Ranges of Iran, and the Eurasian plate, which incorporates Europe, central and East Asia, as well as interior Iran. The tectonic activity is forced by convergent movements between these two plates, with the Arabian plate subducting beneath Eurasia at a rate of 3 mm/yr [1]. The collision front is located in the region of the Zagros Mountains, a NW–SE elongated belt located in the western and south-western part of Iran, but intense seismicity affects also the most internal parts of the country such as the Central Iran plateau and Alborz Mountains to the north (Fig. 1).

In the recent years, several earthquakes caused many fatalities and damages to civil facilities, e.g. the Manjil (1990), Avaj (2002), Bam (2003) and Firuzabad-e-Kojur (2004) earthquakes. In all these events a noticeable contribution to damaging was derived from the mobilization of a large number of landslides. For instance, the Manjil Earthquake (Ms=7.7) in 1990 triggered catastrophic landslides that buried Fatalak village, killed more than 130 peoples and cut many important roads and other lifelines, resulting in major economic disruption [2].

In spite of the high earthquake-induced landslide hazard affecting Iran, very few attempts have been made to develop studies for hazard estimates and damage mitigation. Only some statistical studies have been reported about the earthquakes that hit Manjil [3], [4], [5], [6], Avaj [7], [8] and Firuzabad-e-Kojur [9].

Understanding where landslides are most likely to occur is crucial in reducing property damage and loss of life in future earthquakes. For this purpose approaches for earthquake-induced landslide hazard at regional scale have been developed both to predict scenarios of expected events (e.g. Ref. [10]), and to make probabilistic estimates of landslide triggering in a given time interval (e.g. Refs. [11], [12]). In these approaches, but also in other seismic hazard estimate applications, Arias intensity [13] has been increasingly used by many researchers as a parameter for describing ground shaking (e.g. Refs. [14], [15], [2]).

For the Iranian area, Mahdavifar et al. [16] derived an Arias intensity attenuation relationship from accelerometric recordings of seismic events located in the Alborz and Central Iran regions. This attenuation relationship was used for hazard estimates of seismically induced landsliding in such regions following the Jibson approach [10], [2]. To extend these studies to the Zagros region, a definition of attenuation relations applicable to this region is needed. The geological and seismotectonical differences of the Zagros region with respect to Alborz and Central Iran do not allow assumption of the same attenuation relationship. This motivated us to conduct a specify study and to define an Arias intensity attenuation relationship for the Zagros area.

This paper briefly reviews previous models published for finding attenuation relations of Arias intensity, describes the set of strong-motion data analyzed to generate new regression models and analyzes some Arias intensity models in order to optimize the choice of an attenuation relation for the Zagros area for purposes of seismically induced landslide hazard assessment.

The attenuation relations established in this study are then compared with those obtained for other Iranian regions and can be used to estimate Arias intensity in other areas that geologically and structurally are similar to the Zagros region.

Section snippets

Area study

From the seismotectonic and geological points of view, Iran can be divided into several tectonic provinces. The Zagros thrust fault zone is the main geological frontier, separating the Arabian Plate, including the Zagros region, from the Central Iran micro-plate (Fig. 1).

Another remarkable aspect of the Zagros Mountains is the presence of an extensive sedimentary record encompassing strata ranging in age from the latest Precambrian to the recent. The Zagros Fold and Thrust Belt (ZFTB) consist

Review of Arias intensity attenuation relationships

Arias intensity [13] is one of the shaking parameters commonly used in seismic hazard analysis to characterize the damaging potential of earthquakes. It is defined as a measure of seismic shaking in terms of the sum of energies stored by an infinite series of unit weight oscillators characterized by a single degree of freedom and having frequencies from zero to infinity. This parameter is directly proportional to the integral of square modulus of ground acceleration over the entire time history

Methodology and data

The main purpose of this study was to derive a “regional” attenuation relation of Arias intensity designed for estimation of seismically induced landslide hazard in the region of Zagros Mountains. In such a context the attenuation model requirements have features that allow some simplifications in comparison with relations of more general use. First of all the “regional” nature of the relation and its application to an area having a limited variability of structural tectonic properties make

Results and discussion

As result of regression analysis we obtained five expressions, with the relative standard deviation of estimator, σ, as follows:

Model 1:logIa=-3.790+M-2logRwith σ=0.555.

Model 2: logIa=-2.659+0.601M-0.011R-logRwith σ=0.430.

Model 3:logIa=0.512M-0.017R-0.279logR-2.848S1-2.961S2-3.123S3-2.714S4with (S1, S2, S3, S4) equal to (1, 0, 0, 0) for soil category 1, (0, 1, 0, 0) for soil category 2, (0, 0, 1, 0) for soil category 3, (0, 0, 0, 1) for soil category 4 and σ=0.394.

Model 4:logIa=0.188(M-6)-0.397(M-6)2-0.480logR

Conclusions

Attenuation relations of Arias intensity for the Zagros region, one of the main tectonic provinces of Iran, have been developed through a regression analysis on the basis of 37 accelerometric recordings purposely selected from a rich dataset.

Different attenuation models were adopted in regression analysis and their performances as Arias intensity predictors were compared on a different data set. On the basis of data analysis an expression based on the formulation proposed by Ambraseys and

Acknowledgments

The authors would like to thank Dr. F. Sinaiean for his kind permission to use his corrected data. They would also like to acknowledge Dr. Noorian for his effective guidance in statistical analysis. They also thank the three reviewers for insightful comments and useful suggestions.

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      Table 4 summarises the results obtained, compared to the performances of the preferred equation resulting from this study (i.e. Eq. (10)) and of equations, among those analysed in this study, whose formulation are more similar to those adopted by previous studies. Thus the formulae by Wilson and Keefer [72] and Jibson [21] are comparable to the equation including only the basic parameters a, b and c (code d0h0f0e0), that by Sabetta and Pugliese [75] is assimilable to the equation characterised by a two variable site term and lacking in anelastic attenuation and focal mechanism terms (code d0h1f0e2), the Mahdavifar et al. [36] formula is similar to the equation lacking in site and focal mechanism terms (code d1h1f0e0) and that by Rajabi et al. [50] excludes also the saturation parameter (code d1h0f0e0). It is apparent that, not only the Eq. (10) provides much more accurate Ia estimates than previously published formulae, but also equations of this study having a formulation similar to those proposed by other studies performs better, at least in terms of estimate errors (even though equations excluding parameter d do not appear better than their preceding homologues in terms of logarithmic errors and efficiency coefficient).

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