The use of direct shear waves in quantifying seismic anisotropy: Reference station technique applied on the Northeastern Tibet
Abstract
Using direct shear waves in addition to SKS waves for splitting measurement would be advantageous in splitting measurements since they sample possible anisotropic structures in the upper mantle with an increased range of incidence angles and backazimuths. However, this is rarely done because of the potential contamination of the direct S-waves by source-side anisotropy. To overcome the influence of the source we use a new approach that we call "reference station method". The method utilizes the observations of direct shear waves from two receiving stations at a station pair, one as a reference station with a well-known SKS splitting parameter and the second one as a target station with unknown splitting parameters. The method depends on maximizing the correlation between these seismic traces at reference and target stations after correcting the reference station for known receiver side anisotropy effect. This algorithm also provides a delay time between both stations which represents the isotropic heterogeneity. This delay time can be used to correct body wave S tomographic images for potential biases due to unmodelled splitting effects. The procedure effectively assumes the same source side anisotropy affecting the two stations for the same seismic event which is reasonable for teleseismic arrivals where two ray paths will be very close in the upper mantle near the source. Synthetic tests performed using various hypothetical anisotropic models show sufficient stability of direct S-based splitting parameters with those obtained from a SKS method even where variability in near surface properties (i.e. thickness and velocity of sediment layer) exists. We also applied the reference station technique to data from the INDEPTH IV and ASCENT seismic experiments at the northern margin of Tibet. Average splitting parameters, obtained from the analysis of direct shear waves recorded at possible station pairs within a range of interstation distance less than 300 km are mostly similar to the analysis previously carried out using the SKS method. Where differences exist, the resolved shear wave fast polarization estimates from direct S indicate a higher degree of internal consistency for closely spaced stations than those derived from SKS. This is probably due to the much larger number of S waves available for splitting measurements compared to SKS for the same observational period. This and other tests show that the reference station method is robust and has the potential to provide improved splitting measurements for temporary stations operational for too short a time or in locations unfavourably located with regard to SKS. Even where good measurements are available from SKS measurements, the S measurements sample the anisotropic layer with different angles of incidence and at different backazimuths, providing additional constraints on more complicated anisotropic structures such as multi-layer or dipping anisotropy.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFMDI21B2357E
- Keywords:
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- 7208 SEISMOLOGY / Mantle;
- 7218 SEISMOLOGY / Lithosphere;
- 8110 TECTONOPHYSICS / Continental tectonics: general