AMPLITUDES OF CORE WAVES NEAR THE PKP CAUSTIC, FROM NUCLEAR-EXPLOSIONS IN THE SOUTH-PACIFIC RECORDED AT THE LABORATOIRE DE DETECTION ET GEOPHYSIQUE NETWORK, IN FRANCE

The purpose of this paper is 2-fold. It is first a continuation of the study of Massot and Rocard (1982) on the amplitude variations of the core waves near the PKP caustic, for French nuclear explosions in the South Pacific recorded on the LDG network in France. Taking advantage of a larger available data set, and using current methods to compute s ynthetic seismograms, such as the WKBJ method, we reinvestigate the co re waves amplitude versus distance variation pattern derived by Massot and Rocard (1982). The second purpose is to give precise constraints on the PKP caustic phenomenon, using waveform and amplitude comparison s between data and synthetics. WKBJ synthetics are computed for severa l global spherical Earth models. Recent models with a D'' discontinuit y, or a low velocity gradient at the top and the bottom of the liquid core are also tested. Our computations do not support the highly varia ble amplitude versus distance profile of Massot and Rocard, which disp lays a major peak at 145.6 degrees and many secondary interference fri nges. Between 142 degrees and 147 degrees, the theoretical amplitude v ariations we have calculated are dominated by the large PKP caustic pe ak, and only modulated by the simultaneous arrival of the PKIKP wave n ear 145 degrees. Using a data set of 17 nuclear explosions, we have me asured relative amplitudes as a function of distance, in the 142 degre es to 147 degrees distance range, and analyzed the waveform variations . Our observations (amplitudes and waveforms) are well modeled by the 1066B model of Gilbert and Dziewonski (1975). The agreement on amplitu de variations is still good for models PREM (Dziewonski and Anderson, 1981) and IASP91 (Kennett and Engdahl, 1991). Accurate quantitative co nstraints are derived on the PKP caustic, among which the position of the B caustic point (144.5 degrees +/- 0.5 degrees) for the mean 296 d egrees Mururoa-LDG network azimuth. This location depends, as a whole, on the depth of the core-mantle boundary, and on the velocity gradien ts in the mantle and in outermost core. Although the depth of the core -mantle boundary, and the existence of a discontinuity at the top of D '' can slightly affect the position of the B caustic point, the focusi ng effect at the PKP caustic is rather a global derivation of the mode ls.