Upper mantle P velocity structure beneath the Midwestern United States derived from triplicated waveforms
Upper mantle seismic velocity structures in both vertical and horizontal directions are key to understanding the structure and mechanics of tectonic plates. Recent deployment of the USArray Transportable Array (TA) in the Midwestern United States provides an extraordinary regional earthquake data set to investigate such velocity structure beneath the stable North American craton. In this paper, we choose an M_w5.1 Canadian earthquake in the Quebec area, which is recorded by about 400 TA stations, to examine the P wave structures between the depths of 150 km to 800 km. Three smaller Midwestern earthquakes at closer distance to the TA are used to investigate vertical and horizontal variations in P velocity between depths of 40 km to 150 km. We use a grid-search approach to find the best 1-D model, starting with the previously developed S25 regional model. The results support the existence of an 8° discontinuity in P arrivals caused by a negative velocity gradient in the lithosphere between depths of 40 km to 120 km followed by a small (∼1%) jump and then a positive gradient down to 165 km. The P velocity then decreases by 2% from 165 km to 200 km, and we define this zone as the regional lithosphere-asthenosphere boundary (LAB). Beneath northern profiles, waves reflected from the 410 discontinuity (410) are delayed by up to 1 s relative to those turning just below the 410, which we explain by an anomaly just above the discontinuity with P velocity reduced by ∼3%. The 660 discontinuity (660) appears to be composed of two smaller velocity steps with a separation of 16 km. The inferred low-velocity anomaly above 410 may indicate high water concentrations in the transition zone, and the complexity of the 660 may be related to Farallon slab segments that have yet to sink into the deep mantle.
Additional Information© 2012 American Geophysical Union. 28 July 2011; Revised 11 January 2012; Accepted 11 January 2012; Published 18 February 2012. We would like to thank Thorsten Becker, the associated editor, and two anonymous reviewers. Their suggestions and comments made significant improvements to the manuscript. Seismic data in this study was obtained from IRIS data management center. This work was supported by National Science Foundation through grant EAR-0639507 and the Caltech Tectonic Observatory. Contribution number 10071 of the Seismological Laboratory, California Institute of Technology.
Published - Chu2012p17534Geochem_Geophy_Geosy.pdf