Abstract
The purpose of this paper is to examine and assess, in thelight of recent evidence, the theory lliat the Earth's inner core hasa significant rigidity.The presenee of an inner core in the Earth is revealed fromobservations of the seismie pliase PKP in the « sliadow zone » forwhich the epicentral distance A lies in the range 105" < A < 143".Miss I. Lehmann (r ) in 1936, followed by Gutenberg and Richter (2)in 1938, atlrihuted these observations to tlie presence of an innercore; and Jeffreys (3) in 1939 applied Airy's theory of diffraetionnear a caustic to sliow that the alternative theory of diffraetionround the outer boundary of the centrai core was not capable ofexplaining tlie observations in the shadow zone. The existence of theinner core has been fairly generallv accepted sinee tliis ealculationof Jeffreys.
Highlights
The presenee of an inner core in the Earth is revealed from observations of the seismie pliase PKP in the « sliadow zone » for which the epicentral distance A lies in the range 105" < A < 143"
The work of Jeffreys [4] on llie P—-velocity, « say, in the centrai core favours tlie view that the inner core is composite, consisting of an outer region F [5] occupying the range 1250 km < r < 1390 km, wliere r denotes tlie distance from the eentre, and an inner region G of radius 1250 km;
The existence of the region F in which the velocity gradient is negative is not accepted by ali authors, for example Gutenberg
Summary
The presenee of an inner core in the Earth is revealed from observations of the seismie pliase PKP in the « sliadow zone » for which the epicentral distance A lies in the range 105" < A < 143". The existence of the region F in which the velocity gradient is negative is not accepted by ali authors, for example Gutenberg (°ì; (the existence of F rests on a hypothesis of reflection at the inner core boundary whicli Jeffreys considers may well he true in the liglit of various supporting evidence).
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