Toshiro Tanimoto

Global upper mantle tomography of seismic velocities and anisotropies

A data set of 2600 paths for Rayleigh waves and 2170 paths for Love waves enabled us to retrieve three-dimensional distributions of different seismic parameters. Shallow layer corrections have been carefully performed on phase velocity data before regionalization and inversion at depth. The different seismic parameters include the five parameters of a radially anisotropic medium and the eight azimuthal anisotropic parameters as defined by Montagner and Nataf. It is found that the lateral heterogeneities of velocities and anisotropies in the upper mantle are dominated down to 250–30 km by plate tectonics with slow velocities below ridges, high velocities below continents and a velocity increasing with the age of the seafloor. Anisotropy is present in this whole depth range and the directions of maximum velocities are in good agreement with absolute plate velocities. Below 300 km, there is a sharp decreasing of the amplitude of lateral heterogeneities of seismic velocities and anisotropies. Below 450 km, lateral heterogeneities display a degree 2 and to a less extent a degree 6 pattern. Therefore, between 250 km and 450 km, there is a transition region where vertical circulation of matter is possible as shown by subducted slabs and “plumes” of slow velocities but which probably separates two types of convection. The first one is closely related to plate tectonics and to the distribution of continents. The second one dominates below 450 km and is characterized by two downgoing and two upgoing flows.

High‐resolution global upper mantle structure and plate tectonics

A global high-resolution S wave velocity model RG5.5 is obtained for the upper 500 km of Earths mantle using a 5° × 5° equal-area block parameterization. The data set consists of some 18,000 seismograms associated with 971 events with magnitudes larger than 5.5. Fundamental modes (Love and Rayleigh waves) are used with periods from 75 to 250 s. The horizontal resolution length is around 1000 km, and the vertical resolution varies with depth from 60 to 250 km. Model RG5.5 has many features consistent with previous three-dimensional global and local seismic studies, but many new features are found. The S waves under mid-ocean ridges have broad slow velocity and have very slow velocity in the upper 100 km below the surface. The minimum velocity is at depths near 50 km or shallower. The lateral extent of the slow velocity region across ridges increases with spreading rate. The S wave velocities under ridges are strongly correlated with spreading rates at shallow depth, but the correlation decreases with depth and almost disappears at 100 km. The slow velocities shift off the current spreading positions below 100 km depth under the Mid-Atlantic Ridge and may record past positions of the ridge and/or be related to hotspots near the ridge. Some major hotspots are associated with slow-velocity anomalies with magnitudes of about 1–2% slower than the global average and with lateral dimension larger than 1000 km at depths between 100 and 200 km. Differences in the upwelling structure between ridges and hotspots are indicated. The S wave velocity structures may suggest an active mechanism for the East African Rift Valley and a plate extension mechanism for the Baikal Rift Valley.

PLATE TECTONICS AND HOTSPOTS : THE THIRD DIMENSION

High-resolution seismic tomographic models of the upper mantle provide powerful new constraints on theories of plate tectonics and hotspots. Midocean ridges have extremely low seismic velocities to a depth of 100 kilometers. These low velocities imply partial melting. At greater depths, low-velocity and high-velocity anomalies record, respectively, previous positions of migrating ridges and trenches. Extensional, rifting, and hotspot regions have deep (> 200 kilometers) low-velocity anomalies. The upper mantle is characterized by vast domains of high temperature rather than small regions surrounding hotspots; the asthenosphere is not homogeneous or isothermal. Extensive magmatism requires a combination of hot upper mantle and suitable lithospheric conditions. High-velocity regions of the upper 200 kilometers of the mantle correlate with Archean cratons.

Earth's continuous oscillations observed on seismically quiet days

Analysis of IDA gravimeter data between 1983 and 1994 and GEOSCOPE data between 1988 and 1994 show that fundamental modes of the Earth, for frequencies between 2 and 7 mHz, are excited even on seismically quiet days. Amplitudes of acceleration are slightly less than one ngal(10−9gal). Examination of a sequence of shorter time interval records suggests that the Earth is oscillating continuously. Currently, both atmospheric excitation and tectonic motions are possible cause(s) of these oscillations.

Plume heads, continental lithosphere, flood basalts and tomography

Abstract High-resolution uppermantle tomographic models are interpreted in terms of plate tectonics, hotspots and plume theories. Ridges correlate with very low velocity areas to a depth of 100 km, probably a result of passively induced upwelling and partial melting. Past positions of ridges also exhibit very low seismic velocities in the uppermantle. At depths greater than 100 km, some low velocity anomalies (LVA) may record past positions of migrating ridges. Buoyant upwellings induced by spreading do not track the migration of surface ridges; they lag behind. At depths greater than about 150 km many LVA (Atlantic and Indian oceans) are more closely related to hotspots, and past positions of ridges than to current ridge locations. In the upper 200 km of the mantle, back-arc and continental extension areas are generally slower than hotspot mantle, possibly reflecting partially molten and/or hydrous mantle. The Pacific ocean ridges tend to be LVA, and probably hot, to about 400 km depth. The surface locations of hotspots, ridges and continental basaltic magmatism seem to require a combination of hot uppermantle and suitable lithospheric conditions, presumably the existence of tensile stresses. The high-velocity regions of the upper 200 km of the mantle correlate with Archaean cratons. Below 300 km the regions of generally fast seismic velocity, and therefore cold mantle, correlate with regions probably underlain by ancient slabs, where the uppermantle may be cooled from below. A moving plate, overriding a hot region, and being put into tension, will behave as if it were being impacted from below by a giant plume head. At sublithospheric depths there are very large LVA (VLVA) in the Pacific and Indian oceans and in the North and South Atlantics. The large continental and oceanic flood basalt provinces seem to have formed over these large, presumably hot, regions. These VLVA do not appear to be plume heads nor is there any obvious damage to the lithosphere under the present locations of flood and plateau basalt provinces. The uppermantle does not appear to be isothermal; the LVA are not restricted to hotspot locations. We suggest that LVA are hotcells in the uppermantle which reflect, in part, the absence of subduction cooling. Plate tectonic induced rifting causes massive magmatism if the break occurs over hotcells, i.e. low-seismic velocity regions. Flood basalts (CFB) may result from the upwellings of already hot, even partially molten, mantle. In contrast to plume heads and plume tails, hotcells are robust features which are fixed relative to one another. They are most pronounced in parts of the mantle that have not been cooled by subduction. There is a close relationship between CFB initiation sites, LVA and ridges and, we believe, hotcells.

Global Love wave phase velocity variation and its significance to plate tectonics

Abstract Global Love wave phase velocity variation was constructed for periods between 80 and 200 s by using approximately 9000 paths from 971 earthquakes (with M ≥ 5.5). The data set was from GDSN and GEOSCOPE networks between 1980 and 1987. We examined both the spherical harmonic expansion method and the block parameterization method. With a simple, constant damping parameter approach, synthetic tests showed that more accurate results were obtained by the block parameterization method than by the spherical harmonic expansion method. We adopted the block inversion method with a (nearly) equal area block (5° × 5° near the equator) discretization. The general pattern of the resulting maps were consistent with previous global and local studies. With a discretization of 5° by 5°, the maps were detailed enough to test some plate tectonic models. For the Pacific, Atlantic and Indian Oceans, surface-wave velocities increased smoothly to plate ages older than 100 Ma. Simple forward modeling showed that seismic phase velocity variation with a continuous thickening of lithosphere up to about 150 Ma fits the present observation, disagreeing with the model deduced from the heat flow and ocean depth data, which change variations at about 60–80 Ma. The seismic phase velocity variations in different oceans showed systematic differences at younger ages, and convergence beyond 100 Ma. The difference at younger ages implies a failure of scaling derived from a simple thermal boundary layer model for oceanic plates. Age-seismic phase velocity relationships on each side of ridges were also examined and asymmetric velocity variations were found, which suggests differences in thermal states from one side of the ridge to the other. In order to further examine the thermal state of the lithosphere, average age-phase velocity relations were established for each ocean, and subtracted from phase velocity variation maps. The results indicated broad, low-velocity regions in the south Pacific (super-swell region), the south and west Indian Ocean, and high-velocity regions east of the East Pacific Rise and in the north to northeast Indian Ocean. The results reflect the asymmetry of phase velocity variation about ridges. There is some correlation between hot-spot locations and low-velocity anomalies, but additional, large-scale thermal anomalies exist under old oceanic plate.

Cause of continuous oscillations of the Earth

Spheroidal fundamental mode oscillations of the Earth for frequencies between 2 and 7 mHz (millihertz) are observed even on seismically quiet days. Two hypotheses of the cause of these oscillations are investigated: the cumulative effect of small earthquakes and atmospheric pressure variations. The cumulative effect of earthquakes, assuming that earthquakes follow the Gutenberg-Richter law, is shown to be 1–2 orders of magnitude too small. The observed amplitudes of modes require an equivalent earthquake of magnitude 6.0 everyday, which cannot be achieved by summing up contributions from small earthquakes. The hypothesis of atmospheric excitation is favored because of the discovery of seasonal variations in stacked modal amplitudes for spheroidal modes between 0S20 and 0S40. It is also evaluated by comparing observed modal amplitudes with theoretical amplitudes, derived from a stochastic normal mode theory. The source of excitation is atmospheric pressure variations, which indicate turbulent motion of the atmosphere for the frequency range of interest and are estimated by barometer data. The observed modal amplitudes can be matched by the stochastic normal mode theory, indicating that atmospheric pressure variation is large enough to excite solid Earth normal modes up to the observed amplitudes. Therefore two lines of evidence, detection of seasonal variations and approximate match of overall modal amplitudes, support the hypothesis that the continuous background oscillations are excited by atmospheric pressure variations.

Waveform inversion for three-dimensional density and S wave structure

Long-period seismic data (longer than about 100 s) are sensitive to density as well as P wave and S wave structure in the Earth. Seismograms containing overtone and fundamental mode signals are inverted directly for density and S wave velocity structure. A simple relation between P wave and S wave velocity perturbations is assumed in order to reduce the number of unknowns. A model is parameterized by spherical harmonics up to degree and order 8 and by five knots in depth (at 30, 220, 450, 670 and 882 km) with linear interpolation between adjacent knots. Density resolution is good only at shallow depths; in fact, only the average in the upper 200 km is marginally resolved. S wave resolution is generally good throughout the depth range. S wave results show good correlation to surface geological features and are similar to previous studies. Density results show quite different patterns from S wave maps; high-density regions appear along the subduction zones surrounding the Pacific Ocean and low-density regions appear in most major continents. Most continents have low density and high S wave velocity, which suggests that continental mantle consists of compositionally different materials since thermal effects can never produce such perturbations. High S wave velocity also suggests a relatively cold geotherm for continents, and thus continents should consist of significantly low-density material such as depleted mantle rocks, as proposed by many researchers before. The density map also correlates well with a geoid map for spherical harmonic components l=4–6 but not for l=2,3. It suggests that sources for these geoid undulations l=4–6 are in the upper mantle, probably in the upper 300–400 km, but those for l=2,3 are in deeper regions.

Three‐dimensional S‐wave velocity structure in Southern California

[1] The three-dimensional S-wave velocity structure was constructed from Rayleigh wave phase velocities, measured using teleseismic TriNet data. The S-wave velocity maps show some features that are much more distinct than previous tomographic results in Southern California. There is a clear seismic velocity contrast across the San Andreas fault. The North American plate side is systematically slower than the Pacific plate side. Under the Eastern California Shear Zone, there are distinct slow velocity anomalies in the crust, suggesting a close connection to lower viscosity crust, previously concluded from observation of the post-seismic deformation after the Landers earthquake. Major upper mantle features include a fast velocity root under the Transverse Ranges, which is the dominant feature in the upper mantle, and a slow velocity anomaly under the Salton Trough. Our results support that the velocity contrast across the San Andreas fault extends below the Moho.

Estimate of Rayleigh‐to‐Love wave ratio in the secondary microseism by colocated ring laser and seismograph

©2015. American Geophysical Union. All Rights Reserved. Using a colocated ring laser and an STS-2 seismograph, we estimate the ratio of Rayleigh-to-Love waves in the secondary microseism at Wettzell, Germany, for frequencies between 0.13 and 0.30 Hz. Rayleigh wave surface acceleration was derived from the vertical component of STS-2, and Love wave surface acceleration was derived from the ring laser. Surface wave amplitudes are comparable; near the spectral peak about 0.22 Hz, Rayleigh wave amplitudes are about 20% higher than Love wave amplitudes, but outside this range, Love wave amplitudes become higher. In terms of the kinetic energy, Rayleigh wave energy is about 20-35% smaller on average than Love wave energy. The observed secondary microseism at Wettzell thus consists of comparable Rayleigh and Love waves but contributions from Love waves are larger. This is surprising as the only known excitation mechanism for the secondary microseism, described by Longuet-Higgins (1950), is equivalent to a vertical force and should mostly excite Rayleigh waves.