Attenuation relationship for the horizontal component of peak ground acceleration and acceleration response spectra in NW Iran

The region of northwestern Iran is exceptional within the Arabian-Eurasian continental collision zone. The tectonics is dominated by the NW-SE striking right-lateral North Tabriz Fault (NTF) and regional seismicity (historical and modern one) concentrates. The NTF is a major seismogenic fault in NW Iran. The last damaging earthquakes on this fault occurred in 1721، rupturing the southeastern fault segment، and in 1780، rupturing the northwestern one. The understanding of the seismic behavior of this fault is critical for assessing the hazard in Tabriz، one of the major cities of Iran; the city suffered major damage in both the 1721 and 1780 events. The north of the NTF seismicity is rare، and almost nothing has been revealed about activity of the structures until now. On 11th of August 2012، the region was surprisingly struck by a shallow Mw 6. 5 earthquake with a pure right-lateral strike-slip character only about 50 km to the north of the NTF. An east-west striking surface rupture of about 20 km length was observed in the field by Geological Survey of Iran. Only 11 minutes later and about 6 km further to NW، a second shallow event with Mw 6. 2 occurred. It showed an NE-SW oriented oblique thrust mechanism. This earthquake sequence provided an opportunity to understand better the processes of active deformations and their causes in NW Iran. Ground-motion relations describing the expected amplitudes of this motion as functions of the magnitude and distance are key components of seismic hazard analyses. Ground-motion (attenuation) relations are used to estimate strong ground motion for many engineering and seismological applications. Where strong motion recordings are abundant، these relations are developed empirically from strong-motion recordings. Where recordings are limited، they are often developed from seismological models using stochastic and theoretical methods. The stochastic model is a widely-used tool to simulate acceleration time series and develop ground motion relations (Hanks and McGuire، 1981; Boore، 1983; Boore and Atkinson، 1987; Atkinson and Boore، 1995 and 1997; Atkinson and Silva، 1997 and 2000). The stochastic method begins with the specification of the Fourier spectrum of the ground motion as a function of magnitude and distance. The acceleration spectrum is modeled by a spectrum with ω2 shape، where ω is the angular frequency (Aki، 1967; Brune، 1970; Boore 1983). Finite fault modeling has been an important tool for the prediction of ground motion near the epicenters of large earthquakes (Hartzel، 1978; Irikura، 1983; Joyner and Boore، 1986; Heaton and Hartzel، 1986; Somerville et al.، 1991; Tumarkin and Archuleta، 1994; Zeng et al.، 1994; Beresnev and Atkinson، 1998a). One of the most useful methods to simulate ground motion for a large earthquake is based on the simulation of some small earthquakes as subfaults that comprise a big fault. A large fault is divided into N subfaults، and each subfault is considered as a small point source (introduced by Hartzel، 1978). In this study، the first region-specific ground motion relations were developed for seismic hazard analysis of NW Iran. The attenuation relation for the horizontal acceleration response spectrum in a period of 0-4 s، with a magnitude range of Mw=5 to 7. 7 and distances up to 150 km were established. We used 51 waveforms recorded on a rock site in the NW Iran. Due to the paucity of the data at small distances and large magnitudes، we applied the stochastic method to simulate waveforms for different magnitudes and distances. To overcome the incompleteness of the data set، we simulated 1240 acceleration time series for magnitudes from M5. 0 to M7. 7 and magnitude steps of 0. 2 units for the North Tabriz fault and M5. 0 to 6. 7 and magnitude steps of 0. 5 units for the Ahar fault. The relations were derived by a maximum likelihood regression algorithm from Joyner and Boore (1993) on a set of 1240 simulated strong-motion records and 51 observed ground motions recorded on the rock site in this region. The theoretical-empirical ground motion relation for NW Iran was compared to the ground motion relations for the other regions and had a good agreement with them especially with Akkar and Bommer relations for Europe، the Mediterranean Region and the Middle East. The present results will be useful in estimating strong ground motion parameters and in the earthquake resistant designs in this region.