Passage:
In 1906, seismologist Richard Dixon Oldham established that seismic P-waves (compressional waves) and S-waves (shear waves) travel through the Earth's interior at variable velocities dependent on rock density and elasticity. Oldham also observed that direct P-waves abruptly ceased to appear on seismograms at recording stations situated more than 100 degrees of epicentral distance from an earthquake’s origin, reappearing only beyond roughly 140 degrees. This apparent void, termed the 'shadow zone,' led Beno Gutenberg in 1914 to calculate the depth of the core-mantle boundary at approximately 2,900 kilometers, proposing that the Earth possessed a single, homogeneous fluid core that sharply refracted P-waves downward, thereby creating an expansive ring on the surface where direct waves could not emerge.
For over two decades, Gutenberg’s homogeneous core hypothesis remained the orthodox model among geophysicists. However, the installation of more sensitive Galitzin and Wood-Anderson seismographs during the late 1920s began to complicate this paradigm. Seismological stations in Europe recorded subtle, unexpected P-wave arrivals—designated as P-prime or PKP waves—within the theoretical shadow zone following major seismic events in the South Pacific, notably the 1929 Murchison and 1931 Hawke’s Bay earthquakes in New Zealand. To preserve Gutenberg’s framework, prevailing theorists attributed these anomalous readings either to turbulent diffraction around the sharp curve of the core-mantle boundary or to instrumental noise.
In 1936, Danish seismologist Inge Lehmann published a landmark paper titled 'P′,' in which she systematically contested the diffraction thesis. By carefully recalculating the travel times and ray paths of waves recorded at distant observatories, Lehmann demonstrated that diffraction was mathematically insufficient to account for the substantial amplitudes and sharp onset times of the shadow-zone signals. Instead, Lehmann postulated that the Earth’s core is not uniform, but stratified into two concentric zones: a liquid outer core and a distinct, higher-velocity inner core. Under Lehmann’s mathematical model, seismic rays entering the outer core encounter an internal discontinuity where the sudden increase in density and velocity partially reflects and sharply refracts some compressional energy back into the shadow zone at angles between 104 and 140 degrees.
Crucially, Lehmann did not initially claim that the inner core was solid; she strictly confined her mathematical deduction to the velocity jump and geometric boundary required to redirect the observed rays. It was only in the late 1930s and 1940s, through the independent theoretical work of Harold Jeffreys and Keith Bullen on shear wave transmission, that the solid physical state of the inner core gained empirical confirmation. Nevertheless, Lehmann’s empirical reinterpretation of the PKP travel-time curves successfully dismantled the assumption of core homogeneity using only meticulously calibrated readings from existing mechanical seismograms.
According to the passage, which of the following accurately characterizes Inge Lehmann's 1936 hypothesis regarding the Earth's inner core?
- AIt established that the inner core was composed of a rigid solid based on the direct detection of shear wave transmissions.
- BIt relied upon modern satellite telemetry and digital seismometer networks to measure the precise depth of the core-mantle boundary.
- It deduced the presence of an internal boundary with increased velocity to account for redirected seismic energy without asserting that the inner core was solid.Cevap
- DIt attributed anomalous shadow-zone P-wave arrivals primarily to turbulent diffraction around the sharp curve of the core-mantle boundary.
- EIt demonstrated that compressional waves and shear waves propagate at identical velocities when traversing homogeneous fluid structures.