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Wide-Scale Detection of Earthquake Doublets and Further
Evidence for Inner Core Super-Rotation
Paul G. Richards (Lamont-Doherty Earth Observatory of Columbia University), Jian Zhang (Scripps Institution of
Oceanography), David P. Schaff (Lamont-Doherty Earth Observatory of Columbia University)
The best evidence for a super-rotation of the Earth's
inner core is provided by comparing the signals of earthquakes
occurring years apart but in almost exactly the
same location (doublets). The method was developed in
the context of high-quality doublets by Li and Richards
and has been used by many subsequent authors. Zhang et
al. [2008] report on more than 100 earthquake waveform
doublets in five subduction zones, including an earthquake
nest in Bucaramanga, Colombia. Each doublet is
presumed to be a pair of earthquakes that repeat at essentially
the same location. These doublets are important for
studying earthquake physics, as well as temporal changes
of the inner core. In particular we have data for six different
paths. They partition into three paths for which the
travel time of PKIKP changes significantly and measurably
(on the order of 0.01 s per year), and three paths that
show little or no change (no more than 0.005 s per year, if
any) of inner core travel-times. In the former set of three
paths, there is a high angle between the ray path within
the inner core, and the equatorial plane. In the latter set
of ray paths, the path in the inner core is nearly parallel
to the equatorial plane. Such a pattern of observations
showing both presence and apparent absence of innercore
travel-time change can be explained by the geometry
and relative directions of ray path, lateral velocity
gradient and inner-core particle motion due to an eastward
super-rotation of a few tenths of a degree per year.
References
Paul G. Richards and Anyi Li, Inner core rotation, in
Encyclopedia of Geomagnetism and Paleomagnetism, ed.
David Gubbins and Emilio Herrero-Bervera, Encyclopedia
of Earth Sciences Series, general editors Rhodes Fairbridge
and Michael Rampino, pages 423-425, Springer: Dordrecht,
The Netherlands, 2007.
Jian Zhang, Paul G. Richards, and David Schaff, Wide-scale
detection of earthquake waveform doublets and further evidence
for inner core super-rotation, Geophys. J. Int., 174,
993-1006, September 2008.
Acknowledgements: Our research was largely funded by two
grants from NSF/EAR.
An apparent temporal change (about 0.1 s) of inner core travel-times observed from
an Aleutian Islands doublet (about 7 yr apart in time) recorded at station BOSA. Top:
Map view of the ray path projected on the Earth’s surface. Star represents the doublet
location. Triangle represents station BOSA. Blue curve represents the ray path
projected on the Earth’s surface. The green part of the curve represents the projected
part of the ray path within the inner core. Bottom: Comparison of the highly similar
waveforms of an Aleutian Islands doublet recorded at BOSA. PKP signals within the
box in the upper panel are superimposed and enlarged in the lower panel, showing
an apparent change of both inner core travel-times and PKP(DF) coda.
2010 IRIS Core Programs Proposal | Volume II | Outer and Inner Core Structure | II-261