Depth-dependent attenuation structure of the
inner core inferred from short-period Hi-net data
Kazama, T., Kawakatsu, H., and N. Takeuchi,
Phys. Earth Planet. Int.,
, inpress, 2008 (preprint PDF).
Depth-dependent attenuation structure in the inner core is studied utilizing the
short-period Hi-net data. The commonly used method of estimating the inner core
attenuation (differential spectral slope analysis) is not suitable because it is too
sensitive to the presence of small amplitude phases. Instead, we found that the
method of relative and differential amplitudes is more robust estimator of the inner
core attenuation. We use the relative amplitude of PKIKP (DF) to PKPs (BC or
AB) around 1Hz to estimate t* as a function of the epicentral distance. Geometrical
ray theory was employed to correct the source and propagation effects except that
of the inner core attenuation. The resulting t* for South American events shows a
clear peak around an epicentral distance of 152., whose corresponding bottoming
depth is about 300km below the inner core boundary (ICB). This pattern can be
explained by a low QP (< 200), high attenuation region in a depth range of 200-
300km. We also found that, in this depth range, there exists positive correlation
between attenuation and velocity, indicating seismic scattering due to anisotropic
structure of iron crystals at the intermediate depth of the inner core.