Long wavelength characteristics of Earth structure

Considerable efforts have been paid to analyse digital seismic network data and ISC (International Seismological Center) data during the last decade. Although there are still uncertainties in seismic maps, some consistent results for long wavelength characteristics of Earth's heterogeneity have emerged. We briefly summarize those features in this paper

Ridges and hotspots: perspectives from global tomography

Resolution in global tomography has improved to a level of about 1000 km due to a rapid increase of digital data during the last decade. We have started to see various important tectonic features in some detail. We will attempt to summarize our current observations for ridges and hotspots

Oceanic Lithosphere: perspectives from Love wave phase velocity

Global Love wave phase velocity variations are constructed for periods between 80 and 200 seconds by using approximately 9,000 paths from 971 events (with M ≥ 5.5). The block parameterization approach (5° X 5° near equator) is used in this study. With improved resolution of about 1000 km, we attempt to study the oceanic lithosphere structure by using these results, because they are very sensitive to structure variation in the upper 200 km near the surface

Mapping convection in the mantle

Long-period (100-250 sec.) Love and Rayleigh waves are used to map heterogeneity and azimuthal anisotropy in the upper mantle. Spherical harmonic descriptions of anisotropy up to ℓ = m = 3 and 2θ are derived. Azimuthal anisotropy obtains values as high as 1½%. There is good correlation of fast Rayleigh wave directions with upper mantle return flow directions derived from kinematic considerations. This is consistent with the a-axis of olivine being aligned in the flow direction. The main differences between the flow models and the Rayleigh wave azimuthal variation maps occur in the vicinity of hotspots. Hawaii, for example, appears to perturb the return flow. There is strong correlation of the geoid and surface wave velocity at ℓ = 4 and 5. Slow regions at this scale are associated with geoid highs and high heat flow, consistent with upwelling convective flow or with isostatically compensated regions of low density. The correlation of azimuthal anisotropy with upper mantle return flow directions, rather than with plate directions, suggests that part of the return flow is in the upper mantle and this, in turn, implies a low viscosity channel

Lateral Heterogeneity and Azimuthal Anisotropy of the Upper Mantle: Love and Rayleigh Waves 100-250 s

The lateral heterogeneity and apparent anisotropy of the upper mantle are studied by measuring Rayleigh and Love wave phase velocities in the period range 100-250 s. Spherical harmonic descriptions of the lateral heterogeneity are obtained for order and degree up to 1 = m = 10. Slow regions are evident at the East Pacific Rise, northeast Africa, Tibet, Tasman Sea, southwestern North America, and triple junctions in the northern Atlantic and Indian oceans. Fast regions occur in Australia, western Pacific, and the southern Atlantic. These features are also found by a completely different analysis based on the Backus-Gilbert method. The Backus-Gilbert method also shows that the obtained phase velocities are averaged values within an area of about 2000-km radius and the errors are about 1% of the phase velocity in the zeroth-order spherically symmetric earth. Inversion for azimuthal dependence shows that for low angular order the fast phase velocity directions seem to correlate well with the plate motion vectors. However, resolution analysis by the Backus-Gilbert method shows that the current data do not have enough resolution for everywhere on the globe. Only a few regions currently have adequate azimuthal coverage. Thus confirmation of the above correlation requires a more complete data set

Seismic constraints on a model of partial melts under ridge axes

In a recent global scale seismic study, the correlation between S wave velocity under ridge axes and spreading rate was pointed out. The correlation is strong for depths to about 70 km, but it diminishes below this depth. We present the correlation plots at four depths, 38, 66, 90, and 110 km, for which correlation is strong at 38 and 66 km but is weak at 90 km and is almost nonexistent at 110 km. We present a model to explain this behavior, which includes a thermal conduction model for the development of lithosphere and a simple melt percolation. Thermal effects on S wave velocity are assumed to be accounted for entirely by the plate cooling (thermal conduction) model. We point out that the thermal model under this assumption predicts asymptotically no correlation between S wave velocity and spreading rate, specifically for spreading rate larger than about 3 cm yr^(−1). This contradicts the correlation observed in the data at shallow depths. The existence of partial melt is thus required to explain the observed behavior at 38 and 66 km depths. We start from four basic equations that govern the distribution of partial melt and derive the relation between the amount of partial melt and the spreading rate. We adopt a simple power law relation between permeability (k) and porosity (ƒ) by k(ƒ) = k_0ƒ^n, where k_0 and n are constants and assume that pores are filled with melt. We then set up an integral relation between S wave velocity and spreading rate. The final formula indicates that the gradient in the correlation plots is the inverse of the power (1/n) in the permeability-porosity relation, thus enabling us to constrain n as well as k_0 from seismic data. The data also have some sensitivity to the depth to solidus. We show that (1) the depth to solidus is probably within the range 60–100 km and (2) if the power n is n = 2–3, then k_0 = 10^(−8) - 10^(−10) m^2. These parameters predict that porosity and fluid velocity are 1–2% and about 0.5 m yr^(−1), respectively. The depth to solidus is consistent with previous estimates by petrological data but is perhaps the first and direct seismological evidence of partial melt from surface wave data. Analytical forms for the dependence on depth and spreading rate of porosity, fluid velocity within permeable rocks, and ascent times of magma are also obtained

The relationship between upper mantle anisotropic structures beneath California, transpression, and absolute plate motions

We calculated SKS splitting parameters for the California Integrated Seismic Network. In southern California, we also estimated splitting in the upper 100 km using azimuthal anisotropy determined from surface waves. The inferred splitting from surface waves in the mantle lithosphere is small (on average < 0.2 s) compared with SKS splitting (1.5 s) and obtains a maximum value (0.5 s) in the transpressive region of the Big Bend, south of, and aligned with, the San Andreas Fault (SAF). In contrast, the SKS splitting is approximately E-W and is relatively uniform spatially either side of the Big Bend of the SAF. These differences suggest that most of the SKS splitting is generated much deeper (down to 300–400 km) than previously thought, probably in the asthenosphere. Fast directions align with absolute plate motions (APM) in northern and southeastern California but not in southwestern California. We interpret the parallelism with APM as indicating the SKS anisotropy is caused by cumulative drag of the asthenosphere by the overlying plates. The discrepancy in southwestern California arises from the diffuse boundary there compared to the north, where relative plate motion has concentrated near the SAF system. In southern California the relative motion originated offshore in the Borderlands and gradually transitioned onshore to the SAF system. This has given rise to smaller displacement across the SAF (160–180 km) compared with central and northern California (400–500 km). Thus, in southwestern California the inherited anisotropy, from prior North American APM, has not yet been overprinted by Pacific APM

Breakout Sessions <Special Contents: 2010 Hiroshima University Symposium on Special Needs Education "The Nature of Education after the Ratification of the Convention on the Rights of Persons with Disabilities : The Linkage between Special Needs Education and Conventional Education">

日時: 2010年6月20日(日) 10:00-16:50 ; 場所: 広島大学東広島キャンパスおよび広島大学サタケメモリアルホー