Discontinuities and mantle deformation in the subduction zones of South America and Indonesia

• Applicant: Dr. Ingo Wölbern
• Subject Area: Geophysics
• Term: from 2010 to 2017
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 188582760

Subduction is an integral part of plate movement and of the dynamic processes in the Earth's interior. The deformation associated with subduction processes are not well understood, so far. Seismic anisotropy is a measurable physical expression of these dynamic processes allowing to infer mantle flow in the Earth's interior. Previous studies have often shown orientation of the fast polarization axis perpendicular to the movement of the subducting plate instead of a parallel orientation as it is expected from the alignment of mantle olivine. This led to the assumption of trench-parallel flow or mineralogical influences. Lateral changes of the anisotropy further show that simple two-dimensional models are often insufficient to explain the observations. And so, measurements at single station do not reliably image the processes in the Earth's interior. The GEOFON data archive in Potsdam may provide extensive seismological data sets recorded in the subduction zones of Indonesia and South America. The station number as well as the density of these monitoring networks significantly exceed those of previous studies and allow for high-resolution determination of the lateral shear-wave splitting and thus the anisotropy in these regions. The small station spacing allows for detection of small-scale lateral changes and may thus provide a detailed image of the complex dynamic processes. In order to model and correctly interpret the resulting observations a precise knowledge of the seismic structures, layer thicknesses and discontinuities is mandatory.During the first funding period new techniques have been developed to improve the methods of S-receiver functions and SKS-splitting analysis. The data required for the investigation have been requested and pre-processed. For the area of the central Andean subduction shear wave splitting has been investigated from SKS and additional local S-phases utilizing data from the ReFuCA project. Previous observations neither of trench-parallel orientations of the fast axes nor of the large delay times of up to 3 seconds could not be confirmed in this study. During the second application period the study will be extended and completed by other data sets (IPOC, ISSA) recorded in the central and southern Andes in order to obtain a detailed image op the complexity of the seismic anisotropy. The results will be compared to a study conducted in the area of central Java, Indonesia. The aim is to map anisotropy patterns from SKS- and local S-phases with high-resolution to image deformation processes in these subduction zones. FD waveform modeling will be used to test for the plausibility of different anisotropy models that may explain the observations. Finally, it has to be examined, why the new results do not match to the previous observations.

• DFG Programme: Research Grants