The effect of the initial earthquake phase shift on the inversion of regional surface wave recordings for the estimation of crustal structure
Özcan Çakir, Murat Erduran and Salih Livaoglu
Department of Geophysics, Black Sea Technical University, 61080, Trabzon, Turkey
(Received 03 December 1999; accepted 24 February 2000)
Abstract: The single-station surface wave inversion is a powerful tool for crustal structure exploration. High quality earthquake recordings at far-field seismic stations can be used to perform such kind of inversion. In this process, the frequency dependent phase shift that seismic waves acquire at the focal area, should be removed from the observed dispersion curves. This is often a difficult task to achieve particularly for minor earthquakes. In the present work, we attempt a numerical simulation of this problem by employing synthetic seismograms and investigate the case where no correction is made for the source phase shift before the group velocity inversion. After performing the inversion, very satisfactory results were obtained particularly for shallow depths, i.e. upper and middle crustal levels. In some cases, inversion gave poor resolution for velocities at lower crustal levels, for which a forward synthetic seismogram modeling is suggested in order to recover the missing information at those levels. The results of the present study show that synthetic modeling is necessary for modeling surface wave recordings at local and regional distances. The case of anisotropic wave propagation in the crust has also been studied with an emphasis on the false identification of anisotropy provided that no corrections are applied for the source phase shift. This erroneous anisotropy identification is caused by the Rayleigh and Love wave incompatibility and can have serious practical consequences for surface wave modeling.
Key words: Source Phase Shift, Focal Mechanism, Source Time Function, Regional Earthquakes, Group Velocity Inversion, Anisotropy.