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Long-Duration Geodetic Data to Understand Mantle Structure


Researchers: Aizhi Guo, Sidao Ni, Jun Xie, Yong Wang, Baolong Zhang, Ziye Yu, Yixin Yao, and Wanjun Ma, Chinese Academy of Sciences; and Jeffrey T. Freymueller, Michigan State University.

Written by Linda Rowan
9 March 2020


Summary

High-rate GPS observations over long time periods after an earthquake were compiled to resolve weaker ground motions from large events. Analysis of long duration GPS signals for the 2011 Tohoku-Oki earthquake from the Plate Boundary Observatory (PBO) network in the contiguous western U.S. identifies multiple shear wave reflections and surface waves that traverse the globe multiple times. This analysis opens up a significant new way to understand the structure of the upper mantle using geodesy.


Observations

The magnitude 9, 2011 Tohoku-Oki earthquake generated massive ground motions, lots of seismic waves and many surface waves with a global effect. The main island of Japan was shifted east by about 2.4 meters. The Earth shifted on its axis by tens of centimeters and increased Earth’s rotation rate. Seismometers around the world recorded the seismic waves and researchers used seismic techniques on these recordings to gain a better understanding of the structure of Earth’s interior.

Japan’s dense geodetic network of GPS monuments, called GEONET, recorded the immediate ground motions caused by the earthquake. Additional work showed that geodetic data recorded before, during and right after a large earthquake provides information about the earthquake process.

Here, the authors consider GPS data collected at more than 600 sites in UNAVCO’s dense geodetic network (formerly PBO and now Network of the Americas) in the contiguous western United States for the Tohoku earthquake over a longer time period after the event (i.e., as much as 30,000 seconds after the event). The geodetic data can be stacked together via common methods used for seismic data to enhance and recover weak ground motion signals that are related to multiple phase shear waves and multiple global traverses of surface waves. These waves penetrate more of the upper mantle and provide information about the structure of the upper mantle.


Results

The compiled, long-duration geodetic data from more than 600 sites after the 2011 Tohoku earthquake are compared to seismic data from broadband seismometers in southern California. A comparison shows the geodetic data accurately captures multiple shear wave phases and many global traverses by surface waves. The results show that the observed group velocities of the surface waves are slower than a one-dimensional model of the mantle structure and this indicates there is more complexity in the actual three-dimensional structure of Earth’s interior. As more data is collected, especially with more geodetic sites and with more GNSS constellations, the geodetic and seismic data combined will be able to decipher more details of the structure of the Earth’s interior.

Related Links


References

Millimeter-level ultra-long period multiple Earth-circling surface waves retrieved from dense high-rate GPS network, Aizi Guo, Sidao Ni, Jun Xie, Jeffrey T. Freymueller, Yong Wang, Baolong Zhang, Ziye Yu, Yixin Yao, and Wanjun Ma, Earth and Planetary Science Letters, 2019, doi: 10.1016/j.epsl.2019.07.007 .

Keywords

surface waves, shear waves, high-rate GPS, long-duration GPS


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Last modified: 2020-03-11  10:18:08  America/Denver