| NSF Org: |
EAR Division Of Earth Sciences |
| Recipient: |
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| Initial Amendment Date: | June 15, 2018 |
| Latest Amendment Date: | June 4, 2019 |
| Award Number: | 1830644 |
| Award Instrument: | Continuing Grant |
| Program Manager: |
Robin Reichlin
EAR Division Of Earth Sciences GEO Directorate For Geosciences |
| Start Date: | August 1, 2018 |
| End Date: | July 31, 2021 (Estimated) |
| Total Intended Award Amount: | $211,946.00 |
| Total Awarded Amount to Date: | $228,446.00 |
| Funds Obligated to Date: |
FY 2019 = $124,757.00 |
| History of Investigator: |
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| Recipient Sponsored Research Office: |
300 W. 12TH STREET ROLLA MO US 65409-1330 (573)341-4134 |
| Sponsor Congressional District: |
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| Primary Place of Performance: |
MO US 65409-6506 |
| 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): | Geophysics |
| Primary Program Source: |
01001920DB NSF RESEARCH & RELATED ACTIVIT |
| Program Reference Code(s): |
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| Program Element Code(s): |
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| Award Agency Code: | 4900 |
| Fund Agency Code: | 4900 |
| Assistance Listing Number(s): | 47.050 |
ABSTRACT
Numerous geoscientific investigations over the past 50 years have convincingly demonstrated that the Earth's surface is divided into seven large and many small blocks known as tectonic plates. The relative movement of the plates against each other produces earthquakes, volcanoes, majestic mountains, and deep ocean basins. The physical and chemical processes that drive plate motion, however, are still poorly understood. Improving our understanding of such processes is essential not only for understanding how the Earth works, but also for achieving the ultimate goal of reliably predicting and mitigating natural hazard such as earthquakes and volcanic eruptions. One of the effective tools to investigate plate motion is shear wave splitting analysis, which is based on the observation that when a shear wave originated from an earthquake travels through an anisotropic area formed by plate motion, it will split into two waves. Previous shear wave splitting studies were mostly conducted under the assumption that there is only one layer of anisotropy. The recent dramatic increase in the number of seismic stations and recorded earthquakes suggests that the actual situation is more complicated than this single layer assumption. This project will develop and test a set of sophisticated tools to systematically investigate complex anisotropy on a global scale, for the purpose of providing constraints on a number of hypotheses related to plate dynamics and plate motion. This project will also support training of a graduate student, two undergraduate students, and a summer intern.
Shear wave splitting (SWS) has been increasingly used to quantify seismic azimuthal anisotropy and to understand the geodynamic processes responsible for its formation. Mostly due to the limited amount of data available at most of the stations on Earth and the resulting poor azimuthal coverage of the incoming XKS (including SKS, SKKS, and PKS) rays, the vast majority of existing SWS studies, including most of the studies conducted by the PIs, assumed the simplest form of anisotropy, i.e., a single layer with a horizontal axis of symmetry. On the other hand, some SWS studies have identified systematic variations of the observed splitting parameters as a function of the azimuth of the incoming XKS rays. This dependence is the most important diagnostic of complex anisotropy (e.g., anisotropy with a dipping axis of symmetry and/or with two or more layers with a horizontal axis). By taking advantage of the recent dramatic increase in the quantity and quality of broadband seismic data, this project will address the important question of the pervasiveness of complex anisotropy using long-running seismic stations on the continents. The project also intends to characterize the complex anisotropy by determining the splitting parameters associated with the layers. The resulting spatial distribution of complex anisotropy will be a valuable constraint and input parameter for various geoscientific studies in the areas of geodynamic modeling, seismic tomography, and mineral physics.
This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.
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.
Intellectual Merit:
Wave birefringence (aka. shear wave splitting) measurements are utilized to study the material property of the crust and mantle to improve understanding of many geological features such as mountain ranges and slab subductions. The purpose is to achieve the goal of reliably predicting and mitigating natural hazard such as earthquakes and volcanic eruptions.
The shear wave splitting parameters, fast polarization direction and splitting time between the fast and slow split shear waves are measures of the orientation and strength of the material anisotropy, respectively. This project investigated seismic azimuthal anisotropy, that is, the directional dependence of the velocity of seismic waves in a medium, at different depths in various regions such as Alaska, Arabian plate, Australia, China, Myanmar, Southern California, and Tibet. Below are the major outcomes.
1. Established a shear wave splitting parameter database and studied seismic azimuthal anisotropy beneath the Arabian Plate
The database contains about 4000 pairs of well-defined shear wave splitting parameters from data recorded by 182 seismic stations. Most of the stations beneath the Arabian Plate show dominantly north-south fast orientations with large splitting times in the western region of the Arabian Peninsula and decrease eastward.
2. Anisotropy layering beneath Southern California
Similarities between the resulting shear wave splitting parameters at different depths suggest that the lower mantle in the study area is azimuthally isotropic. Significant azimuthal anisotropy is not present in the mantle transition zone bounded by 410 and 660 km. Anisotropy measurements using shear waves with different depths of origin suggest that the Earth?s upper mantle is the major anisotropic layer beneath the area.
3. Mantle flow systems associated with slab subduction and absolute plate motion beneath Alaska
Alaska is home to some of the largest earthquakes and violent volcanic eruptions on Earth. These natural hazards are mostly caused by the subduction of the Pacific Plate beneath the North American Plate along the Aleutian Trench. Such subduction has not only led to the rising of the magnificent mountain belts seen on the surface, but also caused the mantle beneath the lithosphere to flow. The model established in this study area suggests that materials beneath the subducting Pacific-Yakutat slab are driven northeastward by the southward retreat of the Aleutian Trench. At the eastern edge of the subducting slab, they split into two branches, with one continuing eastward and another going around the slab edge and entering the area above the slab. The observations and the new flow model can be used to better understand forces and processes inside the Earth, which are the ultimate reasons for the earthquakes and volcanoes in tectonically active areas such as south-central Alaska.
4. Mantle flow system by shear save splitting in central Myanmar
Myanmar is located at the boundary between the Indian Plate and the Eurasian Plate. Here, the Indian Plate moves northward at a rate that is faster than most other tectonic plates on Earth and subducts obliquely beneath the Eurasian Plate. This subduction not only causes a strong deformation of the Earth's surface, forming the approximately 1,250 km long, N?S trending Indo-Burma Ranges, but also results in pervasive crustal deformation and possibly modulates the mantle flow field in the area. In this study, data from 71 seismic stations deployed in central Myanmar were used to analyze seismic azimuthal anisotropy at different depths. Based on the established relationship between seismic anisotropy and mantle flow, a model was proposed to explain the observed trench-parallel mantle flow beneath the subducted Indian Plate. Above the plate, there are two flow systems with trench-parallel and trench-orthogonal orientations, respectively, with spatially varying strengths.
5. Spatial dependent seismic azimuthal anisotropy beneath Sichuan Basin
When a recording station is located near the boundary of two or more regions with different anisotropy materials, the observed splitting parameters are dependent on the location of the ray. Such a dependence is clearly observed at three stations situated near the northeastern edge of the Sichuan Basin in central China. The observed splitting parameters from the stations are spatially most consistent when they are projected at a depth of ∼250 km, and can be explained by shear strain associated with the plate motion and mantle flow deflected by the root of the basin.
Broader Impacts:
Five graduate students and six undergraduate students from Missouri S&T were actively involved in projects in addition to two summer interns (one from Brown University and one from University of Houston) recruited through the Incorporated Research Institutions for Seismology. The students are trained for data processing and analysis skills and presented their results at the American Geophysical Union Fall meetings and the Geological Society of America meetings. Three doctoral dissertations were produced from the project. Results from this project have been integrated in teaching several courses at the university and used by a wide range of geoscientists beyond the seismological community.
Last Modified: 11/22/2021
Modified by: Kelly H Liu
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A three-dimensional schematic model showing the mantle flow fields in Alaska. Solid red arrows represent the flowlines outside of the slab, and dashed arrows indicate those beneath the slab. The two red columns represent the two flow branches. The sawteeth mark the trench of the Alaska subduction.Yang et al., 2021Public DomainKelly H LiuSeismic anisotropy and mantle flow beneath Alaska -
Schematic model showing the mantle flow field beneath the Sichuan Basin and adjacent areas. The white and red bars are shear wave splitting measurements. The black dashed lines with arrows indicate the direction of mantle flow deflected by the keel of the thick lithosphere beneath the basin.Jia et al., 2021Public DomainKelly H LiuSeismic anisotropy and mantle flow beneath Sichuan Basin, China
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