Uppermost inner core attenuation from PKP data observed at some South American seismological stations

Abstract

S U M M A R YQ P factor at the top of the inner core (IC) is derived using the amplitude spectral ratio (PKPbc versus PKPdf) method applied to the waveforms of suitable, strong (magnitude >6.0) intermediate depth and deep (h > 150 km) earthquakes recorded at some selected South American stations. In most cases, the sampled volume of the IC is centred beneath the Pacific Ocean, but some Q P values correspond to volumes under South Africa and under Northern Atlantic. The obtained value of Q P is 323 ± 16 (at 95 per cent confidence level), close to a normal (Gaussian) distribution. The maximum depth of penetration of the PKPdf phase into the IC is roughly 333 km, suggesting a possible increase of Q P with depth, although the large scatter in data prevents a definite conclusion. As regards the geographical patterns no variation was observed.

Deep Earth Structure: Q of the Earth from Crust to Core

“…From early on, measurements of amplitude ratios of PKP(DF) (sampling the inner core) and PKP(BC) (sampling the outer core) constrained Q a in the top 500 km of the inner core to be in the range 200-600 (e.g., Bhattacharyya et al, 1993;Bolt, 1977;Choy and Cormier, 1983;Cormier, 1981;Doornbos, 1974Doornbos, , 1983Ivan et al, 2005;Niazi and Johnson, 1992;Sacks, 1969;Song and Helmberger, 1993;Tseng et al, 2001). Most studies have suggested an increase of Q a with depth, although some find that Q a is constant with depth (e.g., Bhattacharyya et al, 1993;Niazi and Johnson, 1992).…”

Attenuation in the uppermost inner core from PKP recordings at African seismological stations

“…This suggests an important geographical variation of the attenuation in the IC area beneath the Southern Indian Ocean, a region having some other particular properties outlined by Stroujkova and Cormier (2004). As compared to recent studies performed using exactly the same method (Ivan and Popa, 2004;Ivan et al, 2006), this study supports the basic observation in the time domain that the western hemisphere of the IC displays larger values of Q with respect to the eastern hemisphere (see also Cao and Romanowicz, 2004). However, the statistical support of this result is diminished if the possible variation of Q with depth is considered, because of the large variance values of the data sets and relative important errors associated with Eqs.…”

“…We performed a statistical analysis (Student's t-test) to determine wheather this average of Q is significantly different from other results derived by exactly the same methodology. We have used the average Q = 323 ± 16 (95% confidence level) obtained mainly beneath the Pacific Ocean, in the IC western hemisphere (180°W to 40°E) by Ivan et al (2006) …”

“…The applicability of the spectral ratio method is discussed in detail in several papers, both from a theoretical point of view or in relation to real cases (e.g., Cormier, 1981;Bowers et al, 2000;Ivan et al, 2006). (2) and (3) are the time windows of 128 points immediately following the arrivals of PKPdf and PKPbc, respectively.…”

“…A reliable method of evaluating the attenuation in the IC is represented by the amplitude spectral ratio of the core phases PKPdf and PKPbc (e.g., Dornbos, 1974;Cormier, 1981;Tseng et al, 2001;Helffrich and Kaneshima, 2002). Various authors have suggested radial and/or geographical variations of the lateral IC quality factor Q P (see the synopsis in Ivan et al, 2006), associated with the possible presence of a mushy zone at the top of the IC (Loper and Roberts, 1981). The anisotropy of the region of the IC below Africa has also been considered in the time domain by Souriau and Romanowicz (1996), with the lowest Q-value axis parallel to the Earth's rotation axis (see also Souriau and Romanowicz, 1997;Yu and Wen, 2006).…”