Modeling Crustal Structure through the Use of Converted Phases in Teleseismic Body-wave Forms by L. J. Burdick and Charles

By comparing records of the radial component of motion of teleseismic P waves to records of the vertical component, it is possible to identify S phases within the P wave form. These phases are generated by the mechanism of P to S conversion at discontinuities in velocity under the receiving station. Similar phases of the S to P converted type appear as precursors to the direct SV arrival. Models for the crustal structure can be tested by generating synthetic seismograms for both components of motion of both the P and SV waves and comparing with the data. The technique has been used to model the crustal structure at WWSSN stations CAR and COR. It has also been used to check a recently proposed model for the crustal structure in eastern Canada which contains a large low-shear-velocity zone at the base of the crust. This study indicates that the crustal structure in eastern Canada is highly non-uniform with perhaps few features common to the whole region. Finally, the technique is used to identify several stations in the WWSSN which appear to be located on highly anomalous structure. I N T R O D U C T I O N The displacement at the Earth's surface due to the arrival of a teleseismic body wave is affected by a number of unknown quantities. These include the source time function, the Earth's velocity structure near the source, the velocity structure near the turning point of the ray) and the crust and upper mantle structure under the observation point. A number of techniques have been developed to separate the effects of the structure under the observation point from the others and to translate this information into models for the crust and upper mantle. One method involves the use of synthetic seismograms to model S to P converted phases from the MohoroviSi5 discontinuity (Bath and Stefansson 1966; Jordan and Frazer, 1975). An attractive feature of this technique is that it is simple and unambiguous. A specific arrival in the wave train is associated with a specific discontinuity in the Earth. Until now, the method of using synthetic seismograms has been limited to studies of S P phases from the moho. However, the moho or any other boundary which produces an S to P converted phase strong enough to be used in a study of this type will produce strong reflections and conversions of other kinds as well. Some of these can be identified quite readily within the P wave form. The purpose of this paper will be to demonstrate that all types of converted phases can be used together to obtain a reliable model of the crustal structure. A second method of determining the receiver structure involves modeling the ratio of the Fourier spectral amplitude of the vertical displacement to the spectral amplitude of the horizontal displacement of incoming P waves. The theoretical basis for this type of analysis has been discussed by Phinney (1964), Hannon (1964), Kurita (1973a), Leblanc (1967), and Fernandez (1967). The technique has been used to model receiver structures by Fernandez and Careaga (1968), Bonjer et al. (1970), Kurita (1973b), and Leong (1975) among others. The main advantage of this technique is that it does not require a knowledge of the incident