SIT35-P01

Room:Convention Hall

Time:May 26 18:15-19:30

The boundary mode of axially symmetric MAC waves can exist in the stratified layer at
the top of the Earth’s outer core
NAKASHIMA, Ryosuke1∗ ; YOSHIDA, Shigeo2
1

Department of Earth and Planetary Sciences, Graduate School of Sciences, Kyushu University, 2 Department of Earth andPlanetary Sciences, Faculty of Sciences, Kyushu University

Seismological observations (e.g. Helffrich & Kaneshima(2010), Kaneshima & Helffrich(2013) ) and theoretical predictions
show the existence of a layer stratified by the accumulation of light fluid at the top of the Earth’s outer core. Helffrich(2014)
suggested that its most probable origin is the vestige of primitive Earth. MAC waves, which arise from the balance amongmagnetic, Archimedes and Coriolis forces, can exist in the stratified layer. To explain the cause of the 60-year variations of
the geomagnetic field, Braginsky(1993) examined axially symmetric approximate solutions of MAC waves theoretically. In his
model, a boundary exists, where the fluid density is discontinuous between the layer and the bulk of outer core, and the buoyancy
frequency is constant within the layer. (Recent seismological observations, however, indicate that the density jump is unlikely
at the boundary.) The latitudinal phase velocity of the solution is equal to the Alfvén wave velocity multiplied by the buoyancy
parameter (clat = VA ·Bu = VA ·N / f, where clat is the latitudinal phase velocity, VA is the Alfvén wave velocity, Bu is thebuoyancy parameter, N is the buoyancy frequency, and f is the Coriolis parameter), and the vertical structure is expressed asa superposition of sine waves. The decay rates of the wave are proportional to the magnetic diffusivity. Since the latitudinal
phase velocity is proportional to buoyancy frequency, the stratification can be estimated if the phase velocity is determined
observationally. If the 60-year variation of the geomagnetic field is identified as the fundamental mode with the latitudinalwavenumber l=2, the buoyancy frequency is estimated to be about twice the angular velocity of the Earth’s rotation.
We have found that Braginsky’s(1993) equations also have the solutions localized at the layer boundary, which we refer toas the boundary mode. This mode has a time scale smaller than the solution within the layer (Braginsky’s(1993) solution),and spreads through magnetic diffusion. The phase propagates away from the layer boundary. The frequency of the boundarymode does not depend on the buoyancy frequency within the layer. The frequency and the vertical wavenumber depend on
the magnitude of the density discontinuity, the latitudinal wavenumber, and several parameters. The wave amplitude decreases
exponentially with the distance from the layer boundary. As the density jump or the latitudinal wavenumber increases, temporaland spatial decay rates increase. Therefore, small density jumps and small layer thicknesses are required to find the boundarymode observationally, and waves with smaller latitudinal wavenumbers are expected to be observed more easily. If the 60-year
fluctuation of the geomagnetic field is identified as the boundary mode with the latitudinal wavenumber l = 2, the ratio of density
discontinuity is estimated to be about 10−4 . Furthermore, the boundary in contrast to the MAC wave within the layer, the spatial
and temporal decay rate of the boundary mode decreases as the magnetic diffusivity increases.

Keywords: MAC waves, the top of the Earth’s outer core, H layer

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SIT35-P02

Room:Convention Hall

Time:May 26 18:15-19:30

Effect of sulfur on the reaction between iron and water under high pressure and temperature
MITAI, Shingo1∗ ; KONDO, Tadashi1 ; TAKUBO, Yusaku1 ; KIKEGAWA, Takumi2
1

Graduate School of Science, Osaka University, 2 Institute of Material Structure Science

It is reported that hydride phase such as FeH and FeOOH appears by a reaction between Fe and H2 O under the Earth’s deep
environment during the formation stage (e.g.,Okuchi,1997;Ohtani et al.,2005). In this study, we added sulfur and observed areaction of the Fe-S-O-H system under high pressure and high temperature to investigate the effect of sulfur on Fe-O-H system.
Because hydrogen in metal hydride, which is expected to produce in the reaction, is dissipated from the sample at low pressure,
we carried out the experiments at High Energy Accelerator Research Organization (KEK), Photon factory (PF-AR-NE1A), andidentified phase transitions of the sample and the reaction products by in situ X-ray diffraction method.
We used a laser-heated diamond anvil cell installed at AR-NE1 to generate high pressure and high temperature. Starting materials were pelleted FeS and pure water in rhenium gasket. The pressure was measured using the equation of state of ice Ⅶ. The
double-sided heating with Nd:YAG laser to heat, the reaction temperature was estimated from radiation of the high temperature
portion of the sample.
In this study, the pressure was 24 and 33 GPa and the temperature was between 300 and 1200 K. As a result, FeS2 (Pyrite),
dhcp-FeHX and ²-FeOOH appeared as the reaction product. It was revealed that stability field of ²-FeOOH is much higher
puressure than that of Fe-O-H system, high temperature decomposition of ²-FeOOH was also constrained, FeS2 which doesn’t
appear in the Fe-S-H system is observed. We will present further result of SEM-EDS analysis of the recovery sample.
Keywords: Earth’s core, light element, hydrogen, synchrotron

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SIT35-P03

Room:Convention Hall

Time:May 26 18:15-19:30

Measurement of thermal conductiviy of mantle minarals at pressures of the transition zone
to the lower mantle
OSAKO, Masahiro1∗ ; YONEDA, Akira2
1

Department of Science and Engineering, National Museum of Nature and Science, 2 Institute for Study of the Earth’s Interior,
Okayama University
Knowledge of thermal diffusivity or thermal conductivity of the mantle is vital for study of the dynamics of the Earth. So far
we have measured thermal diffusivity and thermal conductivity of upper mantle minerals, i.e. olivine and garnet and hydrous
phases, i.e. serpentine and talc. All those data were obtained by the experiments at pressures up to 10 GPa and temperatures
to 1100 K. The measurements were conducted by a pulse-heating method of one-dimensional heat flow using the Kawai-typeapparatus at the Institute for study of the Earth’s interior, Misasa. This current method is a predominant one for study in deep
Earth’s materials under pressure. It has some advantages as follows:(1) comparatively small amount of samples (2) applicable tomaterials with anisotropy in thermal conduction (3) simple cell assembly. Moreover, this method enables to obtain specific heat
capacity under pressure.
In order to expand pressure range the cell assembly is needed to advance by reducing its dimensions. A new cell-assembly
similar to our previous one is designed for a sample of 2.6 mm in diameter and 0.6 mm in thickness. This smaller cell isinstalled in a 14 mm edged octahedral pressure medium in 7 mm trancated anvils. This cell enables to make measurements ofthe thermal properties at pressures exceeding 15 GPa, which will covers the condition in the mantle transition zone. The cell willbe also applied to pyroxene samples of which sizes are necessarily limited. Test measurements were made using garnet samples.
The results agree well with those of the previous experiments using the larger (18-11 and 14-8) cell, and the extrapolations
to zero-pressure coincide to values of other methods. Thus, the pulse heating method will be applied for thermal property
measurements of wadsleyite, ringwoodite and majorite. Using large anvils ( >46 mm), the method is probable to measure thethermal conductivity of MgSiO3 perovskite (bridgmanite). However, measurements at high temperature still have somewhat
problems in precision. Materials of impulse heater and external furnace should be re-considered. The precision of measurements
should be improved by well-controlled machining of the cell assembly and by refining the data acquisition system.

Keywords: mantle minerals, thermal diffusivity, thermal conductivity, high-pressure, Kawai-type apparatus

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SIT35-P04

Room:Convention Hall

Time:May 26 18:15-19:30

Lattice diffusion in MgO crystal from first principles simulation
HARADA, Takafumi1∗ ; TSUCHIYA, Taku1
1

Geodynamics Research Center, Ehime University

Rheological property is critical to understanding the mantle convection. Diffusion creep might be the dominant deformation
mechanism in the Earth’s lower mantle and super-Earths’ mantle (e.g., Karato, 2011). Thus several experimental and theoreticalstudies have tried to measure lattice diffusion coefficients under pressure, which are both still technically difficult. There are
two theoretical approaches to calculate self-diffusion coefficient in solids. One is based on the static lattice energy calculationand the other is based on the molecular dynamics simulation. In the former case, it is difficult to evaluate attempt frequency
and in the latter case, atoms are hardly mobile in actual computation time at the Earth’s lower mantle and super-Earths’ mantle
temperatures. These two approaches were previously applied to MgO, one of major deep mantle constituents (Ita & Cohen, 1997;
Ito & Toriumi, 2007). However reported pressure dependences of the self-diffusion coefficients are contradictive with each otherparticularly at high pressure over 80 GPa.
In this study, we develop a new theoretical method to calculate self-diffusion coefficient in crystals with charged vacancies(Schottky pair) within the first principles framework. This method was then applied to NaCl-type MgO. We found that the calculated pressure dependences of the self-diffusion coefficients in MgO are consistent with those of Ita & Cohen (1997). Diffusion
creep viscosity of MgO was then estimated using calculated diffusion coefficients. Our activation volumes are consistent withexperimental values at low pressure (Van Orman et al., 2003) and decrease rapidly with increasing pressure. It suggests thatsuper-Earths’ mantle would not be quite viscous and the constant activation volume extrapolation leads to overestimation ofviscosity in the deep mantle.
This method is widely applicable to other materials including bridgmanite, post-perovskite and CsCl-type MgO, which areimportant to analyze more realistic planetary interior dynamics.

Keywords: MgO, lattice diffusion, Earth’s lower mantle, super-Earths’ mantle, first principles

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SIT35-P05

Room:Convention Hall

Time:May 26 18:15-19:30

Are LLSVPs formed in the Earth’s lowermost mantle by the subduction of oceanic crusts?
HARADA, Akari1∗ ; KAMEYAMA, Masanori2
1

Dept. Earth Sciences, Ehime University, 2 Geodynamics Research Center, Ehime University

We conducted a series of numerical experiments of thermo-chemical mantle convection where a subduction is preferentiallyinduced at a continental margin, in order to verify a hypothesis that the Large Low-Shear Velocity Provinces (LLSVPs) in the
Earth’s lowermost mantle are formed by subduction of oceanic crust. In this study, we adopted a model of two-dimensional
rectangular box of 2900km height and aspect ratio 6 with reflective boundary condition in the horizontal direction. We placed an
immobile lid as a model of surface supercontinent which covers a third of the top surface. We also put a thin layer of chemicallydense materials as a model of oceanic crust, which may sink into the deep mantle along with cold descending flows from the top
surface.
Our calculations showed that the subducted oceanic crusts are preferentially provided under the continent when the subduction
at the margin of continent is stable. However, stable subduction caused strong convection and significantly stirred the mantle
under the continent. Therefore, subducted oceanic crusts were distributed almost uniformly under the continent without accumulating on the CMB. On the other hand, the cases with unstable subduction at the margin of continent showed a long-wavelength
mantle convection structure which has an ascending plume along the side wall under the continent and a descending plume at
the opposite side wall. The large-scale flow gathered subducted oceanic crusts under the continent and formed large piles on the
CMB.
Our results suggest that the LLSVPs are hardly formed in the presence of stable plate tectonics like the current one where astable plate motion including subduction stirs the mantle very effectively. In other words, the formation of large thermochemical
piles which are equivalent to the LLSVPs should have been completed before the plate tectonics is well established, assumingthat subducted oceanic crusts are the origin of LLSVPs.
Keywords: mantle convection, numerical simulation, LLSVP, plate subduction

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SIT35-P06

Room:Convention Hall

Time:May 26 18:15-19:30

Experimental presentation of plate subduction using paraffin wax
VAGLAROV, Bogdan S.1∗ ; UEKI, Kenta1 ; SAKUYAMA, Tetsuya2 ; IWAMORI, Hikaru1 ; HANYU, Takeshi1 ;NICHOLS, Alexander3
1

Department of Solid Earth Geochemistry, JAMSTEC, 2 Department of Geosciences, Graduate School of Science, Osaka City
University, 3 Research and Development Center for Ocean Drilling Science, JAMSTEC
Experimental approaches using analogue materials have been widely used to understand kinematic behaviors of tectonic plates.
Previously molten paraffin in a tank inside a hot water bath has been used. Although tectonic plate-like behaviors, such as inclined
subduction and trench migration, have been observed, the “plate” in this case was too thin to reproduce the lithospheric strengthand the heat balance through the thermal boundary layer of the Earth. In order to simulate the plate and its motion as a welldeveloped thermo-mechanical boundary layer on top of vigorously convecting mantle, we have developed a tank apparatus andperformed preliminary experiments using paraffin wax.
To control the complex heat and convection processes and for easy observation we constructed a glass tank with an innersize 120x23x4cm. The walls are constructed from double pane glass with panes separated by air gap to reduce heat loss, andreinforced with aluminum plates and bars. The paraffin was melted from bellow by a copper heat-sink containing 24 ceramicheating elements. To reduce heat loss to the back wall, the wall was isolated with 8cm thick foam. At the boundary layer
where the paraffin wax was sticking as it cooled down we applied NiCr wire heater to the inner walls. All heating sections were
controlled by variable controllers. We cooled the top layer of the wax with a cold air flow carefully controlled with thin foam
plates from a vat filed with liquid Nitrogen.
The biggest challenge was the “frosting” effect especially on the front uninsulated wall that prevented the “subduction” of theforming “crust” to deeper levels. External wall temperature was 65 ℃, 70 ℃ was measured at the boundary level by the wire
heater, while the wax inside the tank was at 80 ℃. Some external force was necessary to initiate a start of subduction. The
maintenance of balance between the various heaters, the wall temperature, the wax temperature and the cooling rate was critical
for the successful completion of the experiment.
We observed continuous subduction and clear “crust” forming with subsequent “subduction” . We can say that our experiment
properly reproduces the general features of plate motion of the earth. Artificially fracturing or weakening the boundary layer
and applying a vertical, downward external force were required to initiate subduction in addition to collision of the plates. The
thickness of the plate was the primary parameter controlling subduction behavior and plate motion. The plate showed elastic andplastic behavior depending on its thickness and temperature. A cold and thick “plate” did not subduct even after applying an
external force, and formed a stagnant lid. A hot and thinner “plate” did not show continuous subduction behavior, plate motion
stopped soon after subduction was initiated, possibly because the slab pull force from the thinner partially subducted slab was
too weak. Our experiment results suggest that the driving force of subduction and plate motion is slab pull, not the thermal
convection of the molten paraffin or ridge push. We will present photos and videos of the observed processes.
Improvements to the tank and heating elements design are necessary to provide better and easier control over the experiments.
Keywords: analogue experiment, plate subduction, paraffin wax, glass walls tank, slab pull

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SIT35-P07

Room:Convention Hall

Time:May 26 18:15-19:30

Numerical experiments on mantle convection of super-Earths with variable thermal conductivity and adiabatic compression
YAMAMOTO, Mayumi1∗ ; KAMEYAMA, Masanori1
1

Geodynamics Research Center, Ehime University

Recently, many extra-solar planets have been discovered by improved observation technologies. Some of these planets, calledsuper-Earths, have small masses (up to 17 times the Earth’s) and high mean density (>5000 kg/m3). Numerical modeling ofmantle convection of super-Earths plays an important role in studying the occurrence of plate tectonics and the surface environments on these planets. On the other hand, when considering mantle convection of super-Earths, it is also important to take
into account the difference in (hydrostatic) pressure in the mantles. Since super-Earths have high inner pressure, there must
exist a strong change in physical properties and the effect of adiabatic compression. While the effects of physical properties
have been intensively studied so far, those of adiabatic compression have not been well studied in the previous models of mantle
convection of super-Earths. Here we conduct numerical experiments of thermal convection of highly compressible fluid in a
two-dimensional rectangular box whose thermal expansivity and conductivity are dependent on depth, viscosity is dependent on
temperature, in order to elucidate the mantle convection on super-Earths.
Our numerical experiments showed the change in convecting flow patterns depending on the temperature-dependence in viscosity, regardless of the depth-dependence in thermal conductivity. When a viscosity is sufficiently dependent on temperature,
horizontal flow becomes dominant in the mantle, with a very weak activity of hot plumes from the base of the mantle. This flow
pattern is quite similar to the ”stratosphere” in the field of meteorology. In addition, we found that the occurrence of ”stratosphere” is enhanced for a strong depth-dependent thermal conductivity. One reason for this is that high conductivity at depth
significantly reduces the difference in temperature between the basal thermal boundary layer and isothermal core. Our study
therefore suggests that the depth-dependent thermal conductivity is one of the most important agents which control the mantle
dynamics of super-Earths.
Keywords: suoer-Earths, mantle convection, adiabatic compression, thermal expansivity, thermal conductivity, viscosity

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SIT35-P08

Room:Convention Hall

Time:May 26 18:15-19:30

Implicit solution of the material transport of the core formation simulation
FURUICHI, Mikito1∗ ; MAY, Dave2
1

Japan Agency for Marine-Earth Science and Technology, 2 Institute of Geophysics, ETH Zurich

In order to investigate the long time-scales of the global core formation process in a growing planet, we are developing the
Stokes flow simulation code using MIC based techniques for material transport with a free-surface treatment. We are interested
in the dynamical change of the internal structure after solidification of magma ponds/oceans during the core formation under aself-gravitating field, especially because it might lead to an initial heterogeneous structure in the deep mantle.
However current numerical solution method is difficult to solve the system coupled with the energy equation because the numerical system becomes stiff when the dynamical balancing time scale for the increasing/decreasing load by surface deformationis very short compared with the time scale associated with thermal convection. Any explicit time integration scheme will requirevery small time steps; otherwise, serious numerical oscillation (spurious solutions) will occur.
In this work, we propose to treat the advection as a coordinate nonlinearity, coupled to the momentum equation, thereby defining a fully implicit time integration scheme suitable for stiff problems [Furuichi and May, Compt. Phys. Commum 2015]. We
utilize a Jacobian free Newton Krylov (JFNK) based Newton framework to solve the resulting nonlinear equations. We also
investigate efficient solution strategies to reduce the computational cost to evaluate the nonlinearity on MIC advection.
These implicit methods are implemented within FD framework [Gerya and Yuen, 2003]. We examine the solution quality andefficiency of these methods by performing numerical experiments we have performed a series of numerical experiments whichclarify the accuracy of solutions and trade-off between the computational cost associated with the nonlinear solver and time stepsize.

Keywords: core formation, Stokes flow, free surface, implicit time integration, JFNK

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SIT35-P09

Room:Convention Hall

Time:May 26 18:15-19:30

Modeling of SKS splitting parameters measured in Japan with Hi-net
OGAWA, Naoto1∗ ; KAWAKATSU, Hitoshi1 ; TAKEUCHI, Nozomu1 ; SHIOMI, Katsuhiko2
1

Earthquake Research Institute, the University of Tokyo, 2 National Research Institute for Earth Science and Disaster Prevention

To systematically investigate the spatial variation of seismic anisotropy around Japanese islands, we measured splitting parameters (fast polarization direction φ, delay time δt) of teleseismic SKS phases observed by Hi-net (Ogawa et al., 2014, SSJ). The
results indicated regional scale variations of splitting parameters that are apparently related to subduction systems. In order toinvestigate detailed anisotropic structures (fabric in mantle wedge, subducting slab, and asthenosphere), we conducted forward
modeling using synthetic seismograms. We modeled the SKS phases by the ray theory. We assumed that the SKS ray is straightand that each region has homogenous anisotropy. We rigorously calculated the phase velocity in each region by solving theChristoffel matrix. The preliminary analysis indicates that the measured splitting parameters appear to be primarily affected by
the A-type fabric in subducting slab (oceanic lithosphere) whose a-axis aligns in the direction of the fast axis observed at thesurface by using our OBS data.
Keywords: seismic anisotropy, s-wave splitting, modeling

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SIT35-P10

Room:Convention Hall

Time:May 26 18:15-19:30

Constraining radial anisotropy in the upper mantle with multi-mode surface waves
YOSHIZAWA, Kazunori1∗
1

Faculty of Science, Hokkaido University

The radial anisotropy of shear waves represents the differences in the propagation speeds between vertically polarized shear
waves (SV) and the horizontally polarized ones (SH), and can be a key to the understanding of the dynamic processes in theupper mantle. Seismic surface waves are the most powerful tool to determine the spatial distribution of the radial anisotropy.
Some recent studies have revealed the existence of a layer with strong radial anisotropy (with SH >SV) beneath the lithosphere;e.g., under the Pacific plate (Nettles & Dziewonski, 2008, JGR) and the Australian continent (Yoshizawa, 2014, PEPI). Thisis, however, not always the case and there are also some studies on radial anisotropy that do not show such a clear layer with
SH>SV beneath the lithosphere. These differences may be related to the differences in model parameterization.
For the inversions of multi-mode phase speeds of Rayleigh and Love waves for radial anisotropy of shear waves, we can use
either set of model parameters for the representation of the anisotropic S velocity; i.e., (A) SV velocity (Vsv) and SH velocity(Vsh), or (B) SV velocity (Vsv) and radial anisotropic parameter ξ =(Vsh/Vsv)ˆ2. The choice of model parameters for inversion
is arbitrary, but, through synthetic experiments, we have confirmed that this difference causes non-negligible effects on thereconstruction of radial anisotropic properties of shear waves. This is mainly caused by the differences in the sensitivity kernelsof Love-wave phase speeds to Vsv, Vsh and ξ.
For the set of parameters (B) [Vsv, ξ], Love waves always have the largest sensitivity to Vsv with suppressed sensitivity to ξ,
and the kernel shapes for both Vsv and ξ are nearly identical. On the other hand, for the parameterization with (A) [Vsv, Vsh],
Love wave phase speeds are controlled primarily by the kernels for Vsh, which have the largest sensitivity to Love wave phasespeeds with little influence from Vsv, which can be better (and independently) constrained by Rayleigh waves.
Such intrinsic differences in the sensitivities of surface waves can lead to the different results in the estimation of radialanisotropy. Our synthetic experiments suggest that the parameterization with [Vsv, Vsh] would be preferable particularly when
the radial anisotropy with SH>SV is caused by anomalously slow SV velocity, which is consistent with the recent anisotropy
models reported in the fast moving Pacific and Australian plates. We have also found that the strong dependence of the retrieved
anisotropy on the initial model, when we use [Vsv, ξ] as model parameters.
Keywords: radial anisotropy, surface waves, upper mantle, lithosphere, asthenosphere

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SIT35-P11

Room:Convention Hall

Time:May 26 18:15-19:30

Teleseismic P- and S-wave tomography beneath Japan Islands
FUJISAWA, Moeto1∗ ; ZHAO, Dapeng1 ; TOYOKUNI, Genti1 ; KITAGAWA, Hiroki2
1

RCPEVE, Tohoku Univ., 2 Shujitsu High school

The Japan Islands are characterized by complex structure and tectonics caused by four plates which are interacting with eachother. Many seismic tomography studies using local seismic data have been made to investigate the 3-D velocity structure
beneath Japan. However, the study areas of the previous researches are limited to the shallow part, in the crust and shallow
mantle, whereas the deep structure beneath Japan (a depth range of 200-700 km) is not well known. Investigation of the deep
structure is very important for improving our understanding of the subducting slabs and mantle upwelling, as well as subduction
dynamics.
In this work, we apply teleseismic tomography to relative travel-time residuals of teleseismic events to study the 3-D P- andS-wave velocity structure of the Japan subduction zone. Although there have been several studies using P-wave data so far (e.g.,
Zhao et al., 1994, 2012; Abdelwahed and Zhao, 2007), few studies using S-wave data have been conducted. Using both Pand S-wave data, we can determine not only the P- and S-wave velocity structures but also other physical parameters such as
Poisson’s ratio, which are useful for better understanding the physical property of the mantle.
Part of the P-wave relative travel-time residuals used in this work were chosen from the data collected by the previous studies(Zhao et al., 1994, 2012; Abdelwahed and Zhao, 2007). We selected 130 teleseismic events from the previous data sets based on
the following criteria: (1) The epicentral distances range from 30 to 100 degrees from central Japan; (2) Each event was recordedby over 100 seismic stations on the entire Japan Islands. In addition, we newly collected data from 38 teleseismic events so thatthe event distribution becomes more homogeneous. Thus, our data set contains 168 teleseismic events which generated ˜60,000
P-wave arrivals. Our S-wave data set contains ˜40,000 arrivals from 56 teleseismic events.
We also selected ˜3,000 local shallow and deep earthquakes from the JMA Unified Catalog, each of which was recorded byover 100 seismic stations. We applied the tomographic method of Zhao et al. (1994, 2012) to invert the local and teleseismic
data simultaneously.
Both of our P- and S-wave tomographic images show that low-velocity anomalies exist in the mantle wedge beneath thevolcanic front and back-arc area, which reflect hot upwelling flow in the mantle wedge associated with slab dehydration. Highvelocity anomalies are revealed, which reflect the subducting Pacific and Philippine Sea slabs where intermediate-depth and deep
earthquakes are located. Beneath western Kyushu, dipping high-velocity anomalies are visible down to ˜400 km depth in both Pand S-wave tomography, suggesting that the Philippine Sea slab has subducted aseismically down to the mantle transition zone
depth. In addition to the P- and S-wave tomography, we will also present images of Poisson’s ratio and the R value (dlnVs/dlnVp),
which provide additional information on physical properties of the Japan subduction zone.

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SIT35-P12

Room:Convention Hall

Time:May 26 18:15-19:30

Seismic evidence for a mantle plume beneath the Cape Verde hotspot
LIU, Xin1 ; ZHAO, Dapeng1∗
1

Tohoku University, Department of Geophysics

The Cape Verde hotspot is located in the African plate, about 2000 km east of the nearest plate boundary. It is composedof a group of late Cenozoic oceanic islands resting on a broad bathymetric swell on mature (>110 Ma) oceanic lithosphere.
This hotspot has a positive surface heat flow, high geoid anomaly, and long-term volcanism. The last known volcanic eruption
occurred at Fogo volcano in 1995.
We determined P- and S-wave tomography of the upper mantle beneath the Cape Verde hotspot using arrival-time data measured precisely from three-component seismograms of 106 distant earthquakes recorded by a local seismic network. Our results
show a prominent low-velocity anomaly imaged as a continuous column <100 km wide from the uppermost mantle down toabout 500 km beneath Cape Verde, especially below the Fogo active volcano, which erupted in 1995. The low-velocity anomaly
may reflect a hot mantle plume feeding the Cape Verde hotspot.
Keywords: Cape Verde, hotspot, mantle plume, seismic tomography, mantle transition zone

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