Insight into the lithospheric structure and deformation in Eastern Tibet from splitting and traveltime variations of core phases S. Sol, A. Meltzer, B. Zurek, X. Zhang, J. Zhang 1 Lehigh University, Bethlehem, PA 2 Institute of Geology & Min. Res., Chengdu, China Geodynamics of Indentor Corners

Map of the Eastern part of the Tibet plateau Study area

LOCATION OF THE SEISMIC ARRAY

Shear-wave splitting or birefringence  t=L*  Approximate expression for small anisotropy Fast polarization (  Delay time { As a shear (S) wave propagates through an anisotropic medium, it will split into two roughly perpendicularly polarized shear waves and its components will propagate at different speeds, resulting in a differential arrival time (split time,  t) between the components (shear-wave splitting or birefringence). Objectives: Assess the presence of seismic anisotropy and illuminate important questions about the processes of deformation in the Earth

Where do we have evidence of anisotropy within the Earth? Causes of anisotropy? - Thin isotropic layers with different velocities - Aligned, fluid fractures or cracks - LPO of lower crustal phyllosilicates minerals - LPO of mantle minerals (Olivine) CRUST MANTLE

Lattice-preferred orientations of olivine The splitting technique can provide important information about mantle strain field. Pole figures of crystallographic orientation of deformed olivine aggregates - A-axis parallel to ductile shear - S-wave polarization parallel to a-axis

Paths of seismic phases used for the splitting analysis

Seismograms showing the arrival of different phases Local event SKS The bulk of the evidence for mantle seismic anisotropy comes from observations of SKS and SKKS phases.

Seismograms filtered appropriately for SKS phases

Example of SKS splitting analysis

From Savage, 1999 How do you better constrain the depth of anisotropy of the observable splitting? We can also use other geophysical methods!

Physical processes for producing mantle anisotropy From Silver, 1996.

Pacific events Bolivia event Topographic map displaying splitting results from two distinct azimuths

Topographic map showing the preliminary splitting parameters at all the stations of the Eastern Syntaxis Experiment

Comparing preliminary GPS results and splitting direction GPS measurements are courtesy of Liu Yuping and Peter Koons

Complex anisotropic structure at the LHASA station (LSA)      

Preliminary SKS relative traveltime analysis