عنوان مقاله [English]
نویسندگان [English]چکیده [English]
Crustal velocity structure of a seismically active region such as the Alborz located in North of Iran has a great influence on the precise location of earthquakes, attributing the seismicity to the active faults, and improving the reliability of the seismic hazard assessment. In spite of several researches carried out on the Central Alborz crustal structure, very little is known about the structure and thickness of the crust beneath the Western Alborz which is significant due to occurrence of the 1990 Manjil-Tarom earthquake with Ms = 7.3. In this study, we intend to examine the thickness and structure of the crust beneath three stations of Zanjan, Roudbar and Ghazvin located in Western Alborz by joint inversion of the receiver functions and Rayleigh wave group velocity dispersion measurements. A combined inversion of Rayleigh wave group velocities and body wave receiver functions increases the uniqueness of the solution over separate inversions and also facilitates explicit parameterization of layer thickness in the model space. The time-domain iterative deconvolution procedure, which has higher stability with noisy data compared to frequency-domain methods, is employed to deconvolve the vertical component of the teleseismic P waveforms from the corresponding horizontal components and obtain radial and transverse receiver functions for two broadband stations of ZNJK and RUD, and one short-period station of GZV. The waveforms were corrected from the instrument response before proceeding with the receiver function deconvolution. High frequencies were filtered using a Gaussian filter, at 2.5, 1.6, and 1.0, giving an effective high-frequency limit of about 1.2, 0.8 and 0.5 Hz, respectively. As the structure may vary with azimuth and with epicentral distance, all the observations were grouped by azimuth (< 10°) and distance (∆ < 10°). To increase the signal-to-noise ratio of the deconvolved traces, the individual receiver functions were aligned according to the P-wave arrival and point-to-point stacked waveforms. The stacked receiver function was then allocated an average slowness and the back-azimuth of every event was included in the stack. Rayleigh wave group velocity dispersion came from tomographic images in a time period between 10 s and 70 s produced by a study of regional fundamental modes of Rayleigh waves propagating across Iran and surrounding regions. Fundamental-mode Rayleigh wave group velocities to each stations were taken from the corresponding tomographic cell containing the station.
The results show that the crust beneath the Roudbar station has a thickness of 36 ±3 km. A shallow low velocity sedimentary layer, of about 4 km thickness, and a velocity discontinuity at a depth of ~12 km is observed in the crustal model of this station. Beneath the Zanjan Station, the Moho depth varies from 38 to 42 km. The same sedimentary layer as beneath the Roudbar station and an interface at about 15 km depth are observed. Toward east, beneath the Ghazvin station, the thickness of the crust increases up to 52 km which is close to that proposed for the crust of the Central Alborz. Our seismological results show that the Western Alborz has a moderate crustal root but of insufficient thickness to compensate the elevation of the range.