Overview

I use long period seismology as a tool to study the Earth's deep interior which holds clues to the state of the early Earth and its subsequent tectonics. I seek a better understanding of how the Earth formed and evolved to its present state. Present and past surface environment conditions are rooted in forces deep within the cooling a planet. I investigate planet history using seismic waves from earthquakes which are sensitive to characteristics of deep mantle rocks. Combining seismic obserations with studies from mineral physics (lab and computer experiments on rocks at high temperatures and pressures), I perform 3D mapping of rock properties from the deepest, oldest, most distant regions within the Earth.

Research

Tomography

Shear wave model HMSL-S06: Click to download
Compressional wave velocity model HMSL-P06: Click to download
Houser et al. GJI, 2008
These Vp and Vs models are based on surface wave data for the upper mantle and body wave travel time data for the lower mantle. The bulk of the S, SS, P, and PP travel times were measured using cluster analysis. Phases such as ScS-S, SS-S, and PP-P from previous studies were also included. While tomographic models are nice to look at, they are also useful tools for extracting information about Earth dynamics and state as described below.
Shear wave velocity model HMSL-S06. The tomographic inversion is based on the ray theory approximation using the LSQR algorithm. The file columns are longitude, latitude, depth, and dV/V starting with the bottom layer with 4 degree spacing. When plotting, it is customary to remove the mean value of each layer.
These models and others can be viewed using IRIS tools or SubMachine. Please contact me if you have any questions. Enjoy!
Shear velocity model

Earth evolution:

The BEAMS model (bridgmanite enriched ancient mantle structures) proposes that the Earth's lower mantle could be more silica rich than the upper mantle and shows how the subsequent evolution is consistent with current seismic structure. The high viscosity of the lower mantle for Earth and other rocky planets could help sustain global mantle convection for billions of years.
BEAMS cartoon: The stronger Si-rich regions (gray) of the lower mantle persist and preserve the channels of oceanic plate subduction (blue) and return flow of heat from the core-mantle boundary (red).
BEAMS cartoon

Plate tectonics across the mantle:

We see seismically fast (usually plotted blue) regions that extend from current and ancient subduction zones which appear to be subducted oceanic crust and upper mantle plunging across the mantle and collecting above the core-mantle boundary. Seismically slow (usually plotted red) areas occupy continent-sized regions at the core-mantle boundary with slow velocities extending to the upper mantle which appear to be linked to ocean island volcanos and hot spots.
The iron spin crossover, when electrons transition from a high-spin to a low spin state, has been expected to cause changes in P-wave velocity in the mid-mantle (see velocity sensitivity figure at right) which has eluded seismic detection for over a decade. The question was: are the predictions of the iron spin crossover applicable to our understanding of Earth's composition?
Predicted velocity and density anomalies for changes in temperature for a uniform composition of harzburgite (orange), pyrolite (blue), and perovskitite (magenta). The ab initio calculations were carried out for each composition with a 500 K temperature decrease and a 500 K temperature increase at the top of the lower mantle compared to the ambient values which have an anchor temperature of 1873 K. The high-to-low spin crossover in ferropericlase is temperature dependent, so the anomalies for temperature increase (dashed lines) are not simply a mirror image of those for a temperature decrease (solid lines). The grey line is the difference between the predicted velocities and densities for ambient perovskitite versus ambient harzburgite while the black line is for ambient perovskitite versus ambient pyrolite. Note that changing the temperature by only 500 K produces the same magnitude of a velocity or density anomaly as the entire range of compositions explored here.
Velocity sensitivity
Combining observations from seismic tomography models using the vote map method, mineral physics calculations, and insights from geodynamics, we identified the unique seismic signal of iron spin crossover in these fast and slow velocity regions. The observation is possible because these regions are concentrated in the mineral ferropericlase (Mg,Fe)O. Oceanic basaltic crust forms from partial melting of the upper oceanic mantle leading to a crust that is rich in Si and mantle rich in Mg. Identifying oceanic plates in the lower mantle and their regional concentration rather than even distribution across the mantle indicates that the mantle is a mosaic of rock with differing compositions and temperatures which hold the key to our planet's history.
Side view of Earth's mantle highlighting where seismic tomography models identify fast (blue) and slow (red) rock in the lower mantle. The S-wave models show greater continuity and agreement than P-wave models. The iron spin crossover does not affect S-wave velocity, but has a temperature dependent effect on P-wave velocity localized to the mid-mantle. The greater coherence of S-wave velocity with a disruption in P-wave velocity matches the unique seismic signal predicted for the iron spin crossover.
Iron spin crossover signal in slabs.

Transition Zone Topography

I also make maps of topography of the 410 km and 660 km discontinuities and likewise transition zone thickness Houser et al., GJI, 2008. I followed up with analysis to interpret these maps along with seismic tomography to understand the thermal and composition contributions to these seismic observations.
Transition zone thickness, blue areas are thicker and red areas thinner than average. The thick transition zone in the circum Pacific results from the chilling effect of subducting ocean plates passing through and sometimes stalling on their way to the lower mantle.
Transition zone thickness

Water in the Mantle

While some minerals in the mantle transition zone are capable of holding water, is it actually there? Using results from the studies mentioned above, transition zone topography and seismic tomography, the mantle transition zone appears to be quite dry. If the transition zone minerals are dry, then it is likely that the mantle as a whole is also dry. Thus, it may not take much water to explain the amount of water we observe: 1 ocean at the surface, maybe 1-2 oceans worth of hydrogen in the upper mantle, and 1-2 oceans worth of hydrogen in the transition zone. We studied the flux of water between the surface and interior by using the hydrogen isotopes, that is the ratio of deuterium to hydrogen, D/H. There are different scenarios that can explain the current observations, so more measurements in older rocks are necessary. Of course when discussing water in a planetary context, questions of "habitability" often arise; see this comment on why that may not be the best approach when considering rocky planets.
Water effects the transition zone mineral phase changes in ways that differ from temperature. Cold subducting ocean plates lead to fast velocities, but velocity decreases in the presence of water (actually hydrogen at these high pressures/temperatures). In comparison, dry areas would be faster than wetter areas. Investigating global patterns in velocity and topography, it is reare to find the combination consistent with water, thick transition zone and low shear velocity. Thus, we do not observe the signal of water trapped in the mantle transition zone as ocean plates and deep mantle plumes circulate across the planet.
Water in the mantle transition zone

Collaborations

Communication

Contact

📄 CV (PDF) 📄 Publication list 🔬 Google Scholar 📘 ResearchGate

🏛️ Affiliation

Earth-Life Science Institute (ELSI)
Institute of Future Science
Institute of Science Tokyo

📮 Mailing address

2-12-1-I7E-311 Ookayama, Meguro-ku,
Tokyo 152-8550, JAPAN

📍 Physical address

Ishikawadai building No.7, 3rd floor room 316,
1 Chome-31 Ishikawacho, Ota-ku,
Tokyo 145-0061