3-Dimensional upper mantle velocity structure for Iranian Plateau revealed by Pn and Sn tomography


Studying crustal and upper mantel structures has been limited in the Iranian Plateau and there has been little work done in this field. In this paper we have a primary motive to map Pn and Sn velocities beneath most of the Iranian Plateau in order to test 3-D mantle models and to develop and test a method to produce Pn and Sn travel time correction surfaces that are the 3-D analogue of travel time curves for a 1-D model. To the new data set that we have relocated using HDC method which we have published in another paper; we apply the tomographic method of Barmin et al., augmented to include station and event corrections and an epicentral distance correction. The Pn and Sn maps are estimated on a 2o x 2o grid thoughout the Iranian Plateau. We define the phases Pn and Sn as arriving between epicentral distances of 3o and 15o. After selection, the resulting data set consists of 42,368 Pn and 10,897 Sn travel times distributed in-homogeneously across the Iranian Plateau. We used CRUST 5.1 as the starting Model. The Pn and Sn maps compare favorably with recent 3-D models of P and S in the uppermost mantle. The RMS misfit to the entire Iranian data set from the Pn and Sn model increases nearly linearly with distance and averages about 1.5 s for Pn and 3.1 s for Sn. Comparing with the starting model we have a better detail map of the region. Getting the vertical velocity derivative right may be more important in predicting regional travel times than mapping lateral variations. Recent models are providing new information about the vertical velocity gradient in the uppermost mantle which controls the depth of penetration. Further research remains to determine if these results improve regional location capabilities.