The Structure of the Crust and Upper Mantle in Western Tibet from the Analysis of Fundamental Mode Rayleigh Waves Recorded by a Local Network
Abstract
The Himalayan Mountains and the Tibean Plateau are the most important natural laboratory for understanding the mechanical behaviour and rheology of the continental lithosphere under compression. A great deal is now known abut the structure of Eastern Tibet but very few studies have focused on Western Tibet. Western Tibet is narrower (~400km compared to ~1000km) and higher (mean elevation ~5000m compared to ~4500m) than Eastern Tibet. The little work that has been done in Western Tibet suggests that the crust thickens from south to north across the whole of Western Tibet, unlike Eastern Tibet which is thick in the south and thins to the north. More work is needed to confirm this. To gain greater insight into the structure of Western Tibet we use recently recorded broadband seismic data from the region west of 84E on the plateau. The array used covers an area which includes the Karakorum fault and Indus and Bangong sutures. Because of the the difficult access, few prior seismic data exist for this part of Tibet. We make three types of surface wave observations to determine the shear wave structure beneath Western Tibet. First, we use seismograms from 543 regional earthquakes with magnitudes > 4.5 to compute fundamental-mode Rayleigh wave group velocity dispersion for the period range 5-70s. Second, we use continuous time series to compute Green's functions from the ambient noise, from which we calculate further fundamental mode Rayleigh wave group velocity dispersion curves. Using these two datasets we have dispersion curves for 3624 new paths from the Western Tibet data, significantly increasing our prior coverage of the region. We use the group velocity observations to determine the lateral variations in group velocity across Western Tibet and the surrounding region. At short periods (<25s) variations in group velocity reflect the heterogeneity in the upper crust. At longer periods results show that Western Tibet is faster than Eastern Tibet. Finally, we compute fundamental mode Rayleigh wave phase velocity dispersion in the range 20-140s using the two-plane wave technique of Forsyth et al. At long periods (<60s) the phase velocity in Western Tibet is very similar to the phase velocity recorded in South East Tibet using the two-plane wave technique. We use these dispersion measurements to map the structure of the crust and upper mantle beneath Western Tibet, and examine how the crust and upper mantle of Western Tibet differs from that of Eastern Tibet.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFM.T31A2488G
- Keywords:
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- 8108 TECTONOPHYSICS Continental tectonics: compressional;
- 7205 SEISMOLOGY Continental crust;
- 7270 SEISMOLOGY Tomography