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INNER CORE 2006 IRIS 5-YEAR PROPOSAL Hemisphericity and Regional Seismic Anisotropy in the Top 80 km of the Earth’s Inner Core Wen-che Yu, Lianxing Wen • SUNY at Stony Brook The IRIS Consortium has been providing freely available, high-quality broadband seismic data to the scientific community. The IRIS high-quality broadband data have led to many discoveries of unique features of the Earthʼs inner core. The global coverage of the Global Seismographic Network (GSN) and dense coverage of many regional seismic networks have revealed the hemispherical variations in seismic velocity and attenuation and regional seismic anisotropy in velocity in the top 80 km of the Earthʼs inner core (Niu and Wen; 2001; Wen and Niu, 2002). Information from seismograms, such as the arrival time and the wave shape of the seismic waves transmitted through the inner core (PKIKP), allows seismologists to study velocity and attenuation structures of the inner core. The observed PKiKP-PKIKP differential travel times exhibit a distinct east-west hemispherical pattern with the PKIKP waves sampling the eastern hemisphere being about 0.7 s larger than the western hemisphere (Niu and Wen, 2001; Wen and Niu, 2002). The above observations indicate that the east-west hemispherical variations in velocity with the eastern hemisphere being about 0.8%-1.3% higher than that in the western hemisphere. The observed amplitude ratios of the PKIKP/PKiKP show a similar pattern, but with those sampling the high-velocity eastern hemisphere having smaller amplitude ratios. The observed amplitude ratios indicate that the eastern hemisphere has higher attenuation. Recently, we also examine the PKiKP-PKIKP phases sampling the inner core globally and along various directions recorded in the GSN and many regional seismic networks (Yu and Wen, 2005). The PKiKP-PKIKP differential travel times do not show polar-equatorial anisotropy in most regions in the top 80 km of the inner core. However, in a localized region beneath Africa, the PKiKP-PKIKP differential travel times exhibit anisotropy in velocity with the velocity along the polar direction being about 1.3%-1.8% higher than that along the equatorial direction. The hemispherical variations in core may shed light on the dynamics of the inner core and possible mechanisms on the inner core formation. Map view of the PKiKP-PKIKP differential travel time residuals relative to PREM displayed as PKIKP ray segments in the inner core and symbols at their turning points. Circles and triangles indicate negative and positive differential travel time residuals. The differential travel time residuals along the polar (equatorial) paths are indicated by color (black) symbols. The size of the symbols is proportional to the magnitude of the differential travel time residuals. It is evident that the differential travel time residuals show a distinct east-west hemispherical variation in velocity and a localized anisotropy in velocity beneath Africa. Ray paths of the PKiKP and PKIKP are shown in the inset. Niu, F., Wen, L. Hemispherical variations in seismic velocity at the top of the Earthʼs inner core, Nature, 410, 1081-1084, 2001. Wen, L., Niu, F. Seismic velocity and attenuation structures in the top of the Earthʼs inner core, J. Geophys. Res., 107, NO. B11 2273 doi:10.1029/ 2001JB00170, 2002. Yu, W., Wen, L. Regional variations of seismic anisotropy in the top 80 km of the Earthʼs inner core, Gordon Research Conference, 06/12-06/17, Mount Holyoke, MA., 2005. 190
2006 IRIS 5-YEAR PROPOSAL INNER CORE Uppermost Inner Core Attenuation From PKP Data Observed at Some South American Seismological Stations Marian Ivan • University of Bucharest, Romania Vasile Marza, Daniel de Farias Caixeta, Tassia de Melo Arraes • University of Brasilia, Brasil More than 200 digital recordings form the online archive maintained by the Incorporated Research Institutions for Seismology (IRIS) have been examined for clear PKP phases. Q P factor at the top of the inner core has been derived using the amplitude spectral ratio (PKPbc versus PKPdf) method (e.g. Cormier, 1981; Bhattacharyya et al., 1993; Souriau & Roudil, 1995; Tseng et al., 2001; Helffrich et al., 2002) 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 inner core is centered beneath the Pacific Ocean, but some Q P values correspond to volumes under South Africa and under the Northern Atlantic. The obtained average value of Q P is 323 ± 16 (at 95% confidence level), close to a normal (Gaussian) distribution. The maximum depth of penetration of the PKPdf phase into the inner core is roughly 333 km, suggesting a possible increase of Q P with depth, but no definite geographical variation. Location of permanent stations and epicenters. Dotted lines show surface projections of the PKPdf rays. Piercing points to the inner core boundary are indicated by rectangles. Individual Q values are displayed in the proximity of the bouncing point surface projection. Q P values versus radius of the bouncing point of the PKPdf wave. 191
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SCIENCE SUMMARIES 2006 IRIS 5-YEAR
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2006 IRIS 5-YEAR PROPOSAL EARTHQUAK
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2006 IRIS 5-YEAR PROPOSAL MONITORIN
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