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A pkp study of the earth's core using the warramunga seismic array Bertrand, Aimee Elizabeth Surrendra

Abstract

PKP core phases recorded at the U.K.A.E.A. - type seismic array, WRA, in Northern Territory, Australia over the distance range 113° to 176° are used to determine a velocity-depth model for the earth's core. Paper recordings played out at high speed (40 mm/sec) from analog magnetic tape with amplitude gain control and narrow bandpass filters provided the data available for analysis. Such data enabled precision measurement of relative times between seismic traces. Four distinct travel time branches were observed. Two of these are the well-known PKP[formula omitted] and PKP[formula omitted] branches which are trans- mitted through the outer and inner cores, respectively. The two additional branches are forerunners to the DF branch at distances less than 143°. In recent years, the existence of one of these branches designated PKP[formula omitted], has been accepted to account for ob-served precursors. However, the existence of a distinct second branch, designated PKP[formula omitted], has been a subject for debate. Since the precursor branches provide information about the "transition region" between the fluid outer core and the solid inner core, the identification in this study of two distinct sets of forerunners is of some importance. Traveltime and traveltime gradient (dT/d∆ ) measurements of each phase observed on the seismograms were made. The dT/d∆ values were measured by a least-squares technique. These measurements were strongly perturbed by structure beneath the array and it was necessary to correct for the effect by an empirical approach. The corrected dT/d∆ values were smoothed by the method of summary values and, for comparison, by a polynomial regression technique. The smoothed dT/d∆ values for all four branches were inverted by the Herglotz-Weichert method in order to obtain a velocity-depth model for the earth's core. The final velocity model, UBC1, determined in this manner was the one which gave the best fit to all observations in this study. Near a depth of 4000 km., UBC1 required a -slight reduction in the Jeffreys-Bullen (1940) velocities in order to obtain travel times that agreed with the observations for the outer core. The model exhibits three velocity discontinuities at depths 4393 km, 4810 km, and 5120 km. The magnitude of the velocity increases at the discontinuities are 0.10, 0.24 and 0.92 km/sec, respectively. These discontinuities define two shells surrounding the inner core. In the outermost shell, 417 km thick, the velocity gradient is near zero in magnitude. In the second shell, 300 km thick, the velocity gradient is slightly negative.

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