HR: 14:10h
AN: S32C-03    [PDF]
TI: Development of Automated SKS Splitting Measurement - an Additional Parameter to be Provided by the ISC
AU: * Evans, M S
EM: matt@isc.ac.uk
AF: School of Earth Sciences, University of Leeds, Leeds, LS2 9JT United Kingdom
AU: Kendall, J
EM: kendall@earth.leeds.ac.uk
AF: School of Earth Sciences, University of Leeds, Leeds, LS2 9JT United Kingdom
AU: Willemann, R J
EM: ray@isc.ac.uk
AF: International Seismological Centre, Pipers Lane, Thatcham, RG19 4NS United Kingdom
AB: We have implemented an automated system to measure shear wave splitting in SKS and SKKS arrivals for stations and networks where waveform data are freely available and archived. The ISC plans to include these measurements in its On-line Bulletin as an aid to studies of anisotropy in the Earth. We make automated measurements only for strong, clear signals, and at distances where there is no chance of contamination from S, ScS, Sdiff or other phases, as predicted by the IASP91 earth model. Phase onset is more accurately identified using a simple STA/LTA picker. When a polarised shear wave passes through an anisotropic medium, it is split into two perpendicular S-waves that are separated in time. In measuring splitting, we seek a polarisation angle and time shift that removes the effect of the anisotropy, minimising either the energy on the transverse component or the smaller of the two eigenvalues calculated from the covariance matrix. We report results from both methods, as we find differences between the two to appear to be systematic at some stations. Error estimates are calculated using both the methods of Silver and Chan (1991) and Sandvol and Hearn (1994). As well as the above error estimates, we include two other quality control parameters. We calculate the ratio of eigenvalues (min/max) calculated from the covariance matrix of both the uncorrected and corrected waveforms. For the uncorrected waveforms, if this ratio (min/max) is smaller than a set threshold, we conclude that there is insufficiant initial energy on the transverse component for results to reliably measure splitting. Similarly, for the corrected waveforms, if this ratio is too high, then the splitting parameters that we computed are unreliable because they failed to transform the particle motion to expected linearity. We find good correlation between these automated results and manual measurements made on data from stations in the Canadian National Seismic Network, and plan to apply these methods to data from stations distributed globally in the near future.
DE: 7200 SEISMOLOGY
DE: 7218 Lithosphere and upper mantle
SC: Seismology [S]
MN: 2003 Fall Meeting