An Observation of PKJKP: Inferences on Inner Core Shear Properties
An Observation of PKJKP: Inferences on Inner Core Shear Properties
Credit:
Aimin Cao, Barbara Romanowicz • University of California, Berkeley; Nozomu Takeuchi • University of Tokyo/IRIS Consortium
Description
(A) Observed vespagram for PKIKP+PKiKP and their depth phases (the energy level is amplified 1.6 times). The center of the energy maximum is for a slowness of ~ 1.9 s/deg, which is the average of slownesses of PKIKP (1.85 s/deg) and PKiKP (2.04 s/deg) predicted from PREM. The following weaker energy maximum corresponds to pPKIKP+pPKiKP, and has the same slowness, as predicted from PREM. (B) Stacked waveforms for PKIKP+PKiKP and their depth phases for the energy maximum in (A). (C) Observed vespagram for the potential PKJKP (energy level is amplified 40 times). The slowness of the energy maximum is ~ -1.6 s/deg, close to the PREM prediction of -1.43 s/deg. The arrival time is also compatible with PREM (1695 sec for the maximum energy, compared to a prediction of 1690 sec for the high frequency onset of the pulse). (D) Stacked waveform corresponding to the energy maximum in (C). (E) Vespagram in the back-azimuth and travel time domain. This shows the direction of arrival of the detected energy, which we identify as PKJKP, in the negative slowness range of Figure 2C. The estimated back-azimuth is ~ 223°, which shows that the observed energy propagates along the major arc from the source (the expected back-azimuth of PKJKP is 218°). This indicates that the observed phase is not a near-array scattered phase, and provides additional evidence for its identification as PKJKP (Cao et al., Science, 2005).
The seismic phase PKJKP, which traverses the inner core as a shear wave, and would provide direct evidence for its solidity, has been difficult to detect. Using stacked broadband records from the Grafenberg array in Germany, we document a high signal-to-noise phase, whose arrival time and slowness agree with theoretical predictions for PKJKP. The back-azimuth of this arrival is also consistent with predictions for PKJKP, as is the comparison with a pseudo-liquid inner core model. Envelope modeling of the PKJKP waveform implies a slightly larger shear velocity gradient with depth in the inner core than that of the PREM model.
A. Cao, B. Romanowicz, N. Tacheuchi, An observation of PKJKP: Inferences on inner core shear properties, Science, (10.1126/science.1109134), 14 April, 2005.
NSF Grant No. EAR-0308750
Photographer / Contributor: Aimin Cao, Barbara Romanowicz • University of California, Berkeley; Nozomu Takeuchi • University of Tokyo
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