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Published April 2014 | Published + Supplemental Material
Journal Article Open

Lithospheric waveguide beneath the Midwestern United States; massive low-velocity zone in the lower crust


Variations in seismic velocities are essential in developing a better understanding of continental plate tectonics. Fortunately, the USArray has provided an excellent set of regional phases from the recent M5.6 Oklahoma earthquake (6 November 2011, Table 1) that can be used for such studies. Its strike-slip mechanism produced an extraordinary set of tangential recordings extending to the northern edge of the USArray. The crossover of the crustal slow S to the faster S_n phase is well observed. S_mS has a critical distance of around 2° and its first multiple, SmS^2, reaches critical angle near a distance of about 4°, and so on, until S_mS^n merges with the stronger crustal Love waves. These waveforms are modeled in the period band of 2–100 s by assuming a simple three-layer crust and a two-layer mantle, which allows a grid-search approach. Our results favor a 15 km thick low-velocity zone (LVZ) in the lower crust with an average shear velocity of less than 3.6 km/s. The short-period Lg waves (S waves, at periods of 0.5–2 s) travel with velocities near 3.5 km/s and decay with distance faster than high-frequency S_n (>5.0 Hz) which travels at a velocity of 4.6 km/s and persists to large distances. Although these short-period waveforms are not modeled, their amplitude and travel times can be explained by adding a small velocity jump just below the Moho with essentially no attenuation. P_n is equally strong but is complicated by the interference produced by the depth phase sP, but well modeled. The P velocities appear normal with no definitive LVZ. While these observations of S_n and P_n are common beneath most cratons, the lower crustal LVZ appears to be anomalous and maybe indicative of hydrous processes, possibly caused by the descending Farallon slab.

Additional Information

© 2014 American Geophysical Union. Received 24 June 2013. Accepted 24 Mar 2014. Accepted article online 27 Mar 2014. Article first published online: 25 Apr 2014. We would like to thank the editor, Mark Panning, and three anonymous reviewers. Their suggestions and comments made significant improvements to the manuscript. Seismic data in this study were obtained from IRIS data management center. This research was supported by the National Science Foundation through grant EAR-1053064 and partially supported by funding from National Natural Science Foundation of China through grant 41322027, the National Basic Research Program of China (973 Program) through grant 2013CB733203, Institute of Geodesy and Geophysics through grant Y309211201. Please note: Wiley Blackwell is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

Attached Files

Published - ggge20431.pdf

Supplemental Material - ggge20431-sup-0001-suppinfo01.txt

Supplemental Material - ggge20431-sup-0002-suppinfo01.doc

Supplemental Material - ggge20431-sup-0003-suppinfofs01.pdf

Supplemental Material - ggge20431-sup-0004-suppinfofs02.pdf

Supplemental Material - ggge20431-sup-0005-suppinfofs03.pdf

Supplemental Material - ggge20431-sup-0006-suppinfofs04.pdf

Supplemental Material - ggge20431-sup-0007-suppinfofs05.pdf

Supplemental Material - ggge20431-sup-0008-suppinfofs06.pdf

Supplemental Material - ggge20431-sup-0009-suppinfofs07.pdf


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