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Localized Temporal Change of the Earth’s Inner Core Surface
Lianxing Wen (State University of New York at Stony Brook)
The accumulation of seismic data recorded in the Global Seismographic Network (GSN) makes it possible for many discoveries
to be made related to the Earth’s inner core. One of these discoveries in recent years is the localized temporal change of the
Earth’s inner core surface. The discovery is made by comparing the seismic signals of an earthquake doublet, defined as earthquakes
that occurred at different times but in almost exactly same location, recorded by the GSN stations.
Seismic phases used are the compressional waves reflected off the inner core surface (PKiKP) and propagating through the
inner core (PKIKP) for an earthquake doublet occurring in South Sandwich on 12/01/1993 and 09/06/2003. Temporal changes
of PKiKP travel time and waveform are observed at three GSN stations, ARU, AAK and OBN (Fig. 1), while no discernable
temporal change at other seismic stations. The PKiKP and PKIKP phases recorded at station ARU arrives 0.11 s earlier and the
PKIKP phase about 0.04 s earlier in event 2003 than in event 1993 (Fig. 1b). Moreover, the PKiKP-PKIKP differential travel
time is about 0.07 s smaller in event 2003 than in event 1993 (Fig. 1c). The later portion of the AAK waveforms, with the energy
primarily associated with the PKiKP phases, arrives about 0.07 s earlier in event 2003 than in event 1993, while the earlier portion
of energy appears arriving at about the same time (Fig. 1d). The PKiKP waveforms observed at station OBN exhibit two
characteristics: 1) the PKiKP main phase in event 2003 arrives about 0.07 s earlier than in event 1993; and 2) the PKiKP coda
waves show waveform dissimilarities between the two events (Fig. 1e). These observations indicate a localized enlarged inner
core radius by 0.98 to 1.75 km beneath middle Africa between the occurring times of the doublet.
The discovered localized temporal change of the inner core surface will have many implications to our understanding of the
growth of the inner core, thermodynamic processes near the inner core boundary, convection in the outer core and driving
forces for geodynamo. The GSN contributes uniquely to the discovery with its anchoring stations providing continuous openaccess
data, making it possible 1) to discover and locate many earthquake doublets, and 2) to confidently identify the subtle signals
related to the temporal change of Earth’s inner core surface between the doublets.
References
Wen, L., Localized temporal change of the Earth's inner core boundary, Science, 314. no. 5801, pp. 967 - 970, DOI: 10.1126/science.1131692,
2006.
Acknowledgements: I acknowledge the Global Seismographic Network for providing seismic data. This work is supported by the National
Science Foundation, under grant #EAR 0609717.
Fig. 1. a) Ray paths of PKIKP (black) and PKiKP (light blue) waves.
b-e) Superimposed PKiKP-PKIKP waveforms of the doublet recorded
at stations ARU (b,c) and AAK (d), and PKiKP waveforms at OBN (e).
Waveforms in b), d) and e) are superimposed with time shifts that
account for the differences in relative origin time and hypocenter
position of the doublet. Panel c) is the same as panel b) except that
the waveform for event 2003 is shifted 0.04 s more backward in
time. Waveforms are filtered with the WWSSN short-period instrument
response. [From Wen (2006)].
2010 IRIS Core Programs Proposal | Volume II | Outer and Inner Core Structure | II-251