Award Abstract # 1460205
Anisotropic Layering in the North American Upper Mantle Using a Combination of Seismological Approaches

NSF Org: EAR
Division Of Earth Sciences
Recipient: REGENTS OF THE UNIVERSITY OF CALIFORNIA, THE
Initial Amendment Date: June 8, 2015
Latest Amendment Date: June 8, 2015
Award Number: 1460205
Award Instrument: Standard Grant
Program Manager: Margaret Benoit
mbenoit@nsf.gov
 (703)292-7233
EAR
 Division Of Earth Sciences
GEO
 Directorate for Geosciences
Start Date: July 1, 2015
End Date: June 30, 2020 (Estimated)
Total Intended Award Amount: $354,291.00
Total Awarded Amount to Date: $354,291.00
Funds Obligated to Date: FY 2015 = $354,291.00
History of Investigator:
  • Barbara Romanowicz (Principal Investigator)
    barbara@seismo.berkeley.edu
Recipient Sponsored Research Office: University of California-Berkeley
1608 4TH ST STE 201
BERKELEY
CA  US  94710-1749
(510)643-3891
Sponsor Congressional District: 12
Primary Place of Performance: University of California-Berkeley
2150 Shattuck Avenue, Suite #300
Berkeley
CA  US  94704-5940
Primary Place of Performance
Congressional District:
12
Unique Entity Identifier (UEI): GS3YEVSS12N6
Parent UEI:
NSF Program(s): EARTHSCOPE-SCIENCE UTILIZATION
Primary Program Source: 01001516DB NSF RESEARCH & RELATED ACTIVIT
Program Reference Code(s): 017F
Program Element Code(s): 017F00
Award Agency Code: 4900
Fund Agency Code: 4900
Assistance Listing Number(s): 47.050

ABSTRACT

The formation and evolution of the oldest parts of continents is still not fully understood, and in particular why they have cold but light roots in excess of 200 km thickness that have survived destruction over billions of years. Recent seismological studies of their fine scale structure has revealed the presence of fine scale structure within these roots. There is an indication that the shallower half has a distinct structure from the deeper part which suggests that they may have formed successively, through somewhat different processes. Applying state-of-the-art tools of seismic imaging to a high quality dataset (from the USArray seismic network of Earthscope) that allows unprecedented illumination of the deep structure of the conterminous United States, down to and beyond the continental root, this project aims at detailed mapping of the major structural elements and their transitions across the north American continent. This information will serve as a starting point for other geophysicists and geochemists to further address the fundamental questions of continental formation and coupling of tectonic plates with deep mantle flow. The methodologies developed will also serve as basis for studies in other parts of the world. The proposed work will support the career development of a post-doc and a graduate student at Berkeley, and a senior female seismologist, whose influence as role model may be appreciated.


Evidence for widespread layering in the cratonic upper mantle in north America comes primarily from two types of seismic studies: 1) anisotropic tomography, and 2) receiver functions (RF). Both indicate the presence of a mid-lithospheric boundary (MLD) in cratonic areas. However, the nature of this MLD is not known nor is it clear whether the two types of data see the same boundary. The goal of this proposal is to combine high resolution continental scale anisotropic waveform tomography in the craton and proterozoic provinces of north America (NA), with 1D forward modeling of local layered anisotropic structure using a novel approach based on fully non-linear trans-dimensional Bayesian inversion, in order to map major structural and clarify the nature of lithosphere and asthenosphere layering east of the Rocky Mountain Front. It will shed light on the nature of the MLD and of the lithosphere-asthenosphere boundary, which is consistently detected as an anisotropic boundary, but is not consistently detected by RFs under the craton. The models and methodologies produced in this study will be made available to other investigators through the Earthscope/IRIS websites. As part of the proposed work, we will develop general methodologies for probabilistic integration of different data types, at different scales (frequencies). We thus expect the theoretical framework and algorithms developed here in the framework of trans-dimensional inference can be extended to include other types of data and will be useful to other researchers in seismology and Earth sciences.

PUBLICATIONS PRODUCED AS A RESULT OF THIS RESEARCH

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Clouzet, P., Y. Masson and B. Romanowicz "Box tomography: first application to the imaging of upper mantle shear velocity and radial anisotropy structure beneath the north American continent," Geophysical Journal International , v.213 , 2018 , p.1849 doi: 10.1093/gji/ggy078
M. Calo, T. Bodin and B. Romanowiz "Layered structure in the upper mantle across NOrth America from joint inversion of long and short period seismic data" Earth and Planetary Science Letters , v.449 , 2016 , p.164
Roy, C. and B. Romanowicz "On the implications of a-priori constraints in trans-dimensional Bayesian inversion for continental lithospheric layering" J. Geophys. Res. , v.122 , 2017 https://doi.org/10.1002/2017JB014968
T. Bodin, J. Leiva, B. Romanowicz, V. Maupin and H. Yuan "Imaging anisotropic layering with Bayesian Inversion of multiple data types" Geophysical Journal International , v.206 , 2016 , p.605

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.

Intellectual Merit: The goal of this project has been to investigate the structure of the upper mantle beneath the north American (NAM) continent using a variety of seismological data and tools, and more specifically to try and constrain the anisotropic structure, taking advantage of the many high quality seismic recordings provided by the USArray deployment of the NSF funded EARTHSCOPE program.

Seismic anisotropy is a property of seismic waves whereby they travel at different speeds within a region, depending on the direction of travel. It can provide constraints on the microscopic fabric of the materials encountered at different depths and locations within the earth's mantle, which itself informs on the past and/or present deformation experienced. It is a powerful tool to decipher the formation and evolution of tectonic plates, and in particular, of the thick keels of ancient continents, such as NAM.

In this project we have used some classical approaches (surface wave dispersion) as well as experimented with state-of-the art new techniques: full waveform inversion and trans-dimensional Monte Carlo Markov chain Bayesian inversion.

 

We have assembled a very large dataset of high quality surface wave dispersion measurements (group and phase velocity between 12 and 240 s period) on regional and teleseismic paths sampling NAM. The variation with period (or dispersion) reflects variation of structure with depth. Before mapping these dispersion data into depth variations, an intermediate step is to regionalize them, that is to combine all the measurements on individual paths to obtain local dispersion curves related to the structure beneath each point on a geographical grid. In this process, we also solve for the variation of the local dispersion with direction (i.e. azimuth) (figure 1). In a second step, the dispersion maps obtained at different periods are combined to obtain 3D shear velocity structure in the upper mantle (figure 2).

 

We have found that the 3D structure obtained depends significantly on the assumptions made on crustal structure, which cannot be fully resolved with this type of data alone. When computing the predicted waveforms from the 3D model obtained assuming an existing crustal model (Crust1.0), we have found that this model does not explain the observed waveforms as well as a model previously developed at the global scale and at longer periods, using full waveform inversion (a state of the art technique relying on accurate numerical seismic wavefield computations). We are continuing to investigate the reasons of this poorer fit, which could be due to the inaccurate crustal structure or to the fact that the standard dispersion inversion does not take into account the 3D wave propagation effects into account fully.

 

Meanwhile, we have developed and tested a new approach for the joint inversion of dispersion curves and waveforms of converted seismic waves (conversions from shear to compressional waves occur at discontinuities in structure such as the Moho interface between the crust and mantle), which can provide finer scale resolution of crust and upper mantle structure beneath seismic stations. The approach is based on a so-called "trans-dimensional" Monte-Carlo Markov chain methodology, where sampling of the model space includes the number of model parameters as unknown. We also included the ratio between compressional and shear velocity in the crust as unknown. We found that, in addition to constraining the depth of the Moho and a mid-crustal discontinuity well, this approach provides better constraints on the location of major discontinuities within the continental lithosphere, which is important for better understanding the layering of structure in the continental roots, and thus its formation through geologic times.

Broader impacts: this work provides a large database of dispersion measurements in north-America (over 60,000 dispersion curves covering the entire continent, including Alaska) as well as 3D upper mantle models for use by other researchers in regional seismological, geodynamics and mineral physics studies. It has contributed to the training of 3 post-docs and an early career researcher.


Last Modified: 12/11/2020
Modified by: Barbara A Romanowicz

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