Hemispherical Transition of Seismic Attenuation at the Top of the Earth’s Inner Core
Hemispherical Transition of Seismic Attenuation at the Top of the Earth’s Inner Core
Credit:
Aimin Cao and Barbara Romanowicz • University of California, Berkeley/IRIS Consortium
Description
(top) Differential travel time residuals (referring to PREM). The measurement error is less than ~ 0.1 second. (bottom) Qα with respect to the epicentral distance and depth beneath the ICB. The average standard deviations are from ~24, ~43, to ~50. High-lighted green squares show the data sampling offshore northwest of Africa. The event epicentral distances were all calibrated with a reference focal depth of 100 km (Cao and Romanowicz, 2004)
In contrast to the liquid outer core, the earth’s inner core is mostly solid, and its composition is more purely iron. Based on dynamic arguments related to the freezing process of the inner core, and the observation of much lower P-wave quality factor in the inner core (Qα <450) than in the outer core (Qα > 10,000), it has been suggested that a mushy layer with liquid inclusions may exist at the top of the inner core. On the other hand, seismic measurements indicate that Qα increases towards the center of the inner core. We here present estimates of Qα in the depth range 32-110 km beneath the Inner Core Boundary (ICB), based on the measurement of PKIKP/PKiKP amplitude ratios after a narrow band-pass filtering (0.7-2.0 Hz). Our measurements indicate that there are pronounced hemispherical differences in the values of Qα (~ 335 and ~ 160 in the western (180° W to 40° E) and eastern (40° E to 180° E) hemispheres, respectively), and in the depth of transition from decreasing to increasing Qα (< 32 km beneath the ICB in the eastern hemisphere and ~ 85 km in the western hemisphere). Below 85 km, the hemispherical pattern disappears. We also confirm the existence of a correlated hemispherical pattern in P velocity down to 85 km. The P velocity and Qα variations are compatible with an interpretation in terms of small hemispherical variations of temperature at the top of the inner core and their influence on the morphology of porosity and connectivity of liquid inclusions in the mushy zone. The disappearance of the differences in Qα beneath 85 km provide constraints on the likely depth extent of the mushy zone.
A. Cao, B. Romanowicz, Hemispherical Transition of Seismic Attenuation at the Top of the Earth’s Inner Core, Earth Planet. Sci. Lett., 228, 243-253, 2004.
NSF Grant No. EAR-0308750
Photographer / Contributor: Aimin Cao and Barbara Romanowicz • University of California, Berkeley
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