| Summary: | 碩士 === 國立中央大學 === 地球物理研究所 === 86 === Inner core attenuation properties from two spots were examined
in detail scale using PKIKP and PKP(BC) phases with distance
range between 147 and 155 degrees recorded by both of the short-
period regional seismic array in Taiwan and the broadband
stations of the German Regional Seismic Network. Six earthquakes
from the Northern South America and one from Fiji Islands
regionwere investigated in this study. The ray path of South
America to Taiwan samples the inner core region beneath the east
of northern Pacific Ocean and the ray path of Fiji to Europe
reflects the inner core boundary under the west of northern
Pacific Ocean. The Qp values of inner core were estimated using
Fourier spectrum ratios of core phases PKIKP and PKP(BC) on the
short-periodfrequency band around 1 Hz. The multiple taper
method was adopted to reduce the leakage effect due to the short
duration of these two phases. Within the depth range of 160~340
km under the inner core boundary (ICB), the weighted means of Qp
estimated from east Pacific Ocean are 246±19 (frequency band
appropriate) and 305±25.4 (frequency band fixed). The average
Qp is 192±28(appropriate) and 241±21.8 (fixed) in depth
shallower than 210 km from ICB. In contrast, the average Qp
obviously higher at depths more than 270km as a range from
31±15.7 (appropriate) to 337±46.5 (fixed). Our study
demonstrated that Qp is increasing with depths and the gradient
might probably be 1/km in this region. We observed a few of
anomaly spectral characteristics during analysis, but there are
not enough evidences to understand the primary causes. The
average Qp calculated from west Pacific Ocean is 160±33.5 (
frequency band at 0.5~1.5Hz) within depth range from 178 to 265
km, and the value is much lower than that of east Pacific Ocean.
The discrepancy of attenuation property between these two
regions doesn't result from anisotropyof inner core, and might
be related to the regional variation in the uppermost inner
core.
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