Seismic Anisotropy: GFZ

Seismic Anisotropy

Topography map of the Tibetan Plateau (center) and study area (upper right corner). Hypocenter distributions of the analyzed events are given in the upper left corner. 31 SKS splitting parameters (solid bars in magenta) represent the average of 619 station averaged SKS splitting parameters (Becker et al., 2012) combined in 2° x 2° cells. The location of average splitting parameters in this map corresponds to the average location of the group of stations in each cell. In the topographic maps, blue, gray, green, black, and white solid lines represent, suture zones, thrust, right lateral strike-slip, left-lateral strike-slip, and normal faults respectively. LT, Lhasa Terrane; QT, Qiangtang Terrane; SGT, Songpan-Ganzi Terrane; JS, Jinsha Suture; QB, Qaidam Basin; SKF, South Kunlun Fault; NKF, North Kunlun Fault; NKT, North Kunlun Thrust. Brown rectangle in the central map indicates the study region enlarged in the upper right corner.

Generally, shear wave splitting measurements are done on SKS waves, as the propagation through the liquid outer core removes any splitting acquired near the source, allowing interpretation of measured splitting fast directions and delays in terms of the structure in the immediate vicinity of the receiver. However, in many experiments the number and azimuthal range of SKS arrivals is severely limited such that it would be desirable to make use of direct S waves, too. This is not often done because splitting measured on S phases could have been acquired near the source. A new technique exploits the fact that for stations within a regional array, the ray paths are quite close to each other except near the stations and that therefore the source side splitting on S arrivals, if present, is identical for both. By comparing the horizontal waveforms at a station to those of a nearby reference station with assumed known splitting, the receiver side splitting at the target station can be deduced.

The use of body waves in quantifying seismic anisotropy

Summary
When mantle rocks are subject to large strain, the mineral grains of olivine and some other minerals align, causing the velocity of seismic waves passing though this material to vary as a function of direction of propagation and polarization (seismic anisotropy). By mapping seismic anisotropy we can thus put constraints on the past and present deformation in the upper mantle.
The main objective of the current project is to investigate seismic anisotropy within the upper mantle beneath various parts of the world by using an integrated approach where we combine different measures of anisotropy, which had been analyzed separately in most previous studies. We mainly analyze polarizations of body waves recorded during international passive seismic experiments. At the initial stage, we examine teleseismic observables recorded at the INDEPTH IV experiment at the northern margin of Tibet with significant participation of the GFZ Potsdam and their seismic pool. In particular, we use a comprehensive body wave analysis based on shear-waves splitting and P-polarization measurements. To be able to better sample the upper mantle, we develop the techniques which allow measuring receiver-side anisotropy from the polarization analysis of direct S-waves in addition to the core-mantle SKS phases in shear-splitting measurements by eliminating source-side anisotropy.
Body-wave anisotropic parameters later will be interpreted to retrieve the actual 3-D orientation of the anisotropic models of the upper mantle. The proposed project will improve our knowledge of the upper mantle structure and tectonic evolution of the Earth’s interior under different tectonic regimes.

Contact:
Prof. Dr. Frederik Tilmann (GFZ Potsdam)
Dr. Tuna Eken

overview