Seismic Anisotropy of Upper Mantle Beneath the Ordos Block and Adjacent Regions

• Published in: Chinese journal of geophysics 2011-05, Vol.54 (3), p.315-326
• Main Authors: HU, Ya‐Xuan, CUI, Du‐Xin, QIN, Shan‐Lan, JI, Ling‐Yun, HAO, Ming
• Format: Article
• Language: English
• Subjects: Dynamic causes; Horizontal velocity field; Ordos block and adjacent regions; Shear wave splitting
• ISSN: 0898-9591; 2326-0440
• DOI: 10.1002/cjg2.1614

Based on the polarization analysis of teleseismic SKS waveform data recorded at 90 broadband stations in the Ordos block and adjacent regions, the splitting parameters of fast polarization direction and delay time between the fast and slow shear waves are determined for each station using the minimum transverse energy method, yielding the images of upper mantle anisotropy. Combined with the GPS horizontal velocity field with respect to the Ordos block during 1999~2007, we analyze the seismic anisotropy as well as the crust‐mantle coupling beneath the Ordos block and its adjacent regions. At last, a new understanding of the block movement and the dynamic causes of the adjacent fault zones is achieved. The results reveal that there are nulls in the block which show an obvious isotropic characteristic. The delay time of the adjacent stations is obviously greater than those in the central block, which indicates an obvious anisotropy. The fast polarization directions are compatible with the tectonic strike direction and the horizontal velocity direction, which shows the crust‐mantle coupling. The differences between the directions at some stations may be associated with the early block activities. According to the SKS shear wave splitting results and the current GPS velocity field, we suggest that the Ordos block has no rotation. Meanwhile, the horizontal and ascending intrusion and the transverse extension of the plastic material in the upper mantle lead to the bifurcated flow around the Liupan Shan Mountains in the southwestern margin of Ordos. The generation and development of the Fenwei graben are the result from upward intrusion and lateral spreading of upper mantle material along deep faults.