ML Velocity Tomography in Iranian Plateau

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Abstract

We useWe use ML shear wave velocity to derive a high resolution 2D ML shear wave velocity map for the Iranian Plateau. The ML amplitudes and arrival times are routinely measured for the calculation of local magnitude. ML shear wave velocity is very sensitive to the lateral change of crustal thickness and switches between the velocity of Lg and Sn waves. An Lg wave will die out as soon as encounter a sudden crustal change in favor of formation of mantle Sn wave. The collected data base is consisted of 56152 ML velocity belong to 2943 precisely relocated events happened during 1996 to 2012. The arrival time of ML amplitudes were read from waveforms of permanent and temporary networks in Iran. Using the arrival time of an ML amplitude and its ray length, we calculate average shear wave velocity for each ray. The selected events are consisted of 63 clusters with epicentral location uncertainty of 5 km or less. The cluster approach adopted in this work allows us to easily calculate empirical velocity error for each summary ray connecting a given observing station to the corresponding cluster. This also reduces drastically the number of initial 56152 rays to just 3107 summary rays and thus significantly reduces the required computation time for the seismic tomography. Except for the Makran region, the summary rays provide a good coverage for most of Iran. Using a constrained direct damped weighted least square inversion scheme, we inverted the ML velocity for a 2D ML shear wave velocity map of Iran along with its cluster and station correction terms. In our tomography, we constrained the velocity of each cell based on the azimuthal coverage of the hitting rays. The input average velocity for each ML ray was also weighted based on its empirical reading spread. The computed ML shear velocity varies mostly between 2.9 and 3.6 km/s, so suggesting that the majority of the rays are indeed Lg rays. The map shows a general similarity with previous maps of Pn velocity indicating that ML shear wave velocity is strongly affected by lateral changes of crustal thickness and upper mantle velocity. Our results show that Caspian Basin, and Zagros regions are Lg blocking regions. We speculate that the blockage of Lg wave in Zagros is related to strong lateral crustal thickness changes caused by the orogenic processes. We also noted that the shear wave velocity border between the Zagros and Central Iran is considerably deviating from Zagros suture line indicating a partial underthrusting of the cold Arabian plate beneath the Central Iran. The Lg blockage in South Caspian basin is either related to its postulated oceanic type crust and/or strong lateral change in its crustal thickness. East of the Caspian Sea shows high velocities likewise its interior, implying the low plain is underlane by either an oceanic type crust or a transitional crust with large lateral variations of crustal thickness. The ML velocity map also shows a velocity in the range of Lg velocity for the Lut block and thus implying a continental nature for the unknown Lut block. Alborz, most of the Central Iran and especially the northwestern Iran show rather low Lg velocities suggesting a warm continental crust.

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References

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Journal of the Earth and Space Physics
Volume 42, Issue 2 - Serial Number 2
September 2016
Pages 263-279

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