Inversion of Rayleigh waves group velocity to shear wave velocity structure in the NW Iran



We determined inter station shear wave structure using data from a temporary network of 23 broadband stations in the north west of Iran. Waveforms were used from 230 tele-seismic and regional earthquakes to obtain inter station dispersion curves of the group velocity of the Rayleigh waves. Events in the epicentral distance range of 250 to 3000 km with magnitudes 3.0 ≤ Mw ≤ 7 were used. The individual dispersion curves of the group velocity of Rayleigh waves for each source-station path have been calculated; Then via double-station method we calculated 20 dispersion curves for inter station paths. The group velocities are available in the range of 6-48 sec; in general it is only possible to resolve the parameters of upper mantle and crust. We divided study area to 5 regions, and then we calculated the average dispersion curve in each region. These curves have been used to determine shear wave structure in each region via non-linear Hedgehog inversion method. We need to initial velocity model to start non-linear inversion process, therefore initial model are calculated via linear inversion method.
In additional, the obtained velocity models show that crustal thickness in these 5 regions varies between 40 and 56 km. Also the boundary between Upper and Lower crust changes between 12 and 28 km. The results from the non-linear Hedgehog inversion as applied to derived dispersion curves show a crustal thickness of approximately 40 km in the west part of studied area, 56 km in the middle of studied area and 43 km in the western coast of Caspian Sea.
Based on obtained results the Moho depth varies from 56 km to 40 km when you move from the middle of study area to western coast of the Caspian Sea. We propose that under thrusting of Caspian Sea basement beneath the Talesh Mountains impresses Moho depth in Talesh zone. But no geological observation prove the under thrusting of Caspian Sea basement beneath the Talesh Mountains, thereforewe cannot be certain about this propose. In other hand, Talesh zone is located in passive continental margin of Caspian Sea; these kinds of margins have complicated structure. We can assume that observed results in Talesh zone have been created by passive continental margin of Caspian Sea. Also we observed a low velocity and warm (probably) anomaly in range of depth 12-22 km beneath the Sahand volcano.
We derived attenuation effects of south Caspian basin when periods bigger than 32 seconds of fundamental mode Rayleigh waves propagate across the south Caspian Basin. We used 20 events in along the Apsheron Sill and calculated dispersion curves of these events at our stations. We collected 172 waveforms from used events; we found only 31 fundamental mode waveforms of Rayleigh waves. In other waveforms energy of fundamental mode was diffused and we cannot specify any trend for dispersion. The South Caspian Basin contains one of the thickest sedimentary deposits in the world. In the South Caspian Basin, based on Priestley (2001), attenuation of surface waves is largely controlled by sediments in the basin. Therefore we guess that our observations about attenuation of the Rayleigh waves are related to sediments in this basin.


Main Subjects

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