Shear Velocity Structure Beneath the Central United States From the Inversion of Rayleigh Wave Phase Velocities. Issue 11 (30th October 2021)

• Record Type: Journal Article
• Authors: Geng, Y.; Basu, U.; Powell, C. A.
• Is Part Of: Journal of geophysical research. Volume 126:Issue 11(2021)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2021-10-30
• Subjects: Geomagnetism -- Periodicals; Geochemistry -- Periodicals; Geophysics -- Periodicals; Earth sciences -- Periodicals; 551.1
• DOI: 10.1029/2021JB022632
• Languages: English
• ISSNs: 2169-9313

Abstract

Abstract: A three‐dimensional shear velocity model for the crust and upper mantle beneath the central United States is presented by inverting Rayleigh wave phase velocities from 20 s to 100 s periods. These phase velocities were determined using regional and teleseismic earthquakes recorded by the Northern Embayment Lithospheric Experiment stations, the CERI New Madrid Seismic Network, the Earthscope Transportable Array, and the Ozark Illinois INdiana Kentucky Flexible Array. A low Vs anomaly is imaged in the mantle below the Reelfoot Rift, which is the uppermost portion of connected low‐velocity zones dipping toward the southwest below the rift and to the northwest below the Illinois Basin. According to the analysis in previous tomographic studies using both Vp and Vs anomalies, the elevation of temperature and the enrichment of iron, water, and orthopyroxene contents are required factors to explain the reduced seismic velocities. These low‐velocity zones are produced by silica‐rich fluids rising from the stalled Farallon slab. Two areas with low shear strength characterized by low Vs are imaged below the Ste. Genevieve and the Wabash Valley seismic zones. The low‐velocity, weak areas may be responsible for stress concentration and thus the generation of intraplate seismicity. Plain Language Summary: We present a three‐dimensional seismic velocity model for the northern Mississippi Embayment and southern Illinois Basin derived from seismic waves that travel along the surface of the earth. These surface waves consist of many wavelengths. Short wavelengths travel through shallow parts of the crust while long wavelengths penetrate deeper into the earth, allowing construction of a velocity model from the near surface to a depth of 150 km. Our study utilizes seismic tomography to map velocity anomalies, much like tomography is used to produce images of the human body. We find anomalously high velocities in the crust below the three active seismic zones in the area, the New Madrid, Wabash Valley, and Ste. Genevieve seismic zones. Anomalously low velocities underlie the seismic zones at greater depths below the crust. The slow velocities imply that the rocks are weak, and their presence may be concentrating stress in the seismic zones. We find anomalously low velocities below the crust in the northern Mississippi Embayment that extend to the bottom of the model. This low‐velocity region has been detected in previous studies and is attributed to the effects of fluids coming from a trapped slab fragment at depths below 400 km. Key Points: We present a 3‐D Vs model for the upper mantle in the central United States by inverting Rayleigh wave phase velocities A low Vs anomaly is imaged below the Reelfoot Rift, which is the uppermost portion of a prominent LVZ in previous tomographic studies Low Vs anomalies are found in the mantle below the Ste. Genevieve and Wabash Valley seismic zones