| NSF Org: |
EAR Division Of Earth Sciences |
| Recipient: |
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| Initial Amendment Date: | March 30, 2015 |
| Latest Amendment Date: | April 10, 2017 |
| Award Number: | 1464033 |
| Award Instrument: | Continuing Grant |
| Program Manager: |
Robin Reichlin
EAR Division Of Earth Sciences GEO Directorate for Geosciences |
| Start Date: | April 1, 2015 |
| End Date: | March 31, 2018 (Estimated) |
| Total Intended Award Amount: | $144,194.00 |
| Total Awarded Amount to Date: | $144,194.00 |
| Funds Obligated to Date: |
FY 2016 = $48,052.00 FY 2017 = $49,341.00 |
| History of Investigator: |
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| Recipient Sponsored Research Office: |
1033 MASSACHUSETTS AVE STE 3 CAMBRIDGE MA US 02138-5366 (617)495-5501 |
| Sponsor Congressional District: |
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| Primary Place of Performance: |
1350 Massachusetts Avenue Cambridge MA US 02138-3846 |
| Primary Place of
Performance Congressional District: |
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| Unique Entity Identifier (UEI): |
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| Parent UEI: |
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| NSF Program(s): | STUDIES OF THE EARTHS DEEP INT |
| Primary Program Source: |
01001617DB NSF RESEARCH & RELATED ACTIVIT 01001718DB NSF RESEARCH & RELATED ACTIVIT |
| Program Reference Code(s): | |
| Program Element Code(s): |
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| Award Agency Code: | 4900 |
| Fund Agency Code: | 4900 |
| Assistance Listing Number(s): | 47.050 |
ABSTRACT
Earthquakes produce sound waves that travel through the interior of the Earth and thereby sample the conditions along their paths. Collections of large numbers of arriving sound waves are used to reconstruct conditions and structure of the interior of the Earth. While traveling through the interior, these waves produce microscopic strains in the rocks they pass through. The response of the rocks to these strains depends on conditions such as temperature and chemical environment. Very large Earthquakes in subduction zones, for example the Andaman-Sumatra earthquake from 2004 or the Alaskan earthquake from 1964, produce enough energy for the Earth to ?ring like a bell?. Gravitational interaction of the Earth with the Moon and the Sun (tides) causes periodic deformation of the whole Earth that is comparable to the effects of large Earthquakes. Microscopic strains similar to those caused by earthquakes and tides can be investigated in the laboratory, and thereby also the effects of temperature and chemical environment on the response. The challenge of the proposed work is to apply the laboratory results to global observations from seismology and geodesy. The promise of this approach is to improve our understanding of the conditions in the interior of the Earth and its physical description. The findings will have implications for modeling of the ongoing rebound of the surface of the Earth since the last ice age, affecting determination of the causes of changes in sea level, as well as tidal modeling of other planets and moons.
We propose to combine observations of small-strain deformation at a broad range of timescales, from seismic body waves (seconds) to normal modes (minutes - hour) and tides (hours to years), to determine the frequency dependence of energy dissipation in the interior of the Earth. These observations will be combined in a normal mode/tidal model of a non-spherical Earth that includes anelasticity. Results from this model will be compared to laboratory-derived models for anelastic behavior of crystalline grains and their defects, which are the basis for predictions outside of the experimentally accessible parameter space. Comparison of experimental predictions with global models will help to constrain the applicability of the microphysical models to small-strain deformation due to seismic waves and tides, and ultimately also post-glacial rebound and (large-strain) convection. At the same time the combination of information from normal modes and tides will yield new constraints on the conditions and structure in the mid to lower mantle, which are difficult to obtain by other means.
PUBLICATIONS PRODUCED AS A RESULT OF THIS RESEARCH
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PROJECT OUTCOMES REPORT
Disclaimer
This Project Outcomes Report for the General Public is displayed verbatim as submitted by the Principal Investigator (PI) for this award. Any opinions, findings, and conclusions or recommendations expressed in this Report are those of the PI and do not necessarily reflect the views of the National Science Foundation; NSF has not approved or endorsed its content.
One of the outstanding questions in Earth science involves the rate at which the solid Earth evolves, which is a crucial factor in understanding the origin and evolution of the Earth's atmosphere, oceans, geology, climate and life.
The Earth's rocky mantle is in a state of convection, with hot, buoyant material rising and cold, denser material falling, and this circulation drives the plate tectonic motions we observe at the surface. An analog for this state of circulation is a lava lamp - if we consider, for the moment, that the Earth's mantle is akin to a lava lamp, then a seminal question is whether the lava lamp is hot, with actively rising and sinking blobs, or whether it is sluggish, with dense material at the base slowly moving upwards, colling and sinking back down.
The type of lava lamp - energetic and buoyant, versus sluggish - depends crucially on two parameters that govern the energetics of the convective flow. The first is the density difference between large scale structures in the deep mantle (known as Large Low Shear Velocity Provinces, or LLSVPs) relative to their surrounding (that is, the density of the blobs in a lava lamp relative to their surroundings) and the viscosity of the material these large scale structures are moving within.
The greater the density deficit between the LLSVPs (blobs) and the surroundings and the lower the viscosity of the surrounding material, the more energetic will be the convective flow (i.e., the more energetic the lava lamp). Conversely, the greater the density excess of the LLSVPs (blobs) and the surroundings and the higher the viscosity of the surroundings, the more sluggish the mantle convective flow (i.e., the more sluggish the lava lamp).
The work funded by this proposal has contributed two extremely important constraints that can be brought to bear on these issues with the goal of answering the overarching, unanswered questions in long term Earth system evolution.
First, using an entirely new method of imaging the Earth's interior - tidal tomography - we have demonstrated that the LLSVPs (blobs) are, in an average sense, more dense than their surroundings. The new technique makes use of remarkably precise space-geodetic (Global Positioning System, GPS) measurements of the deformation of the Earth's crust due to the gravitational pull of the Sun and the Moon (so-called body tides) to detect small perturbations associated the presence of the LLSVPs in the Earth's interior.
Second, using state-of-the-art methods for modeling the Earth's response to the ice age, we have provided a rigorous constraint on the variation with depth in the mantle of rock viscosity. In this regard, we have also determined why previous studies of mantle viscosity have not reached a consensus and have been the subject of active, often acrimonious debate. Our updated inference demonstrates that mantle viscosity increases by a factor of ~100 from the base of the Earth's tectonic plates to the base of the mantle, just above the fluid iron outer core.
These two contributions both strongly favor arguments that the Earth's mantle convective flow is relatively sluggish, and our conclusion will have immediate and (we believe) enduring impact on a wide range of sub-disciplines in long-term Earth system evolution, including research in plate tectonics, geochemistry, geobiology/paleontology, geodynamics and paleoclimate.
Last Modified: 07/30/2018
Modified by: Jerry X Mitrovica
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