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The Upper Mantle Seismic Structure of Northeastern North America

J. T. Hertzog and J. E. Ebel
Boston College Department of Geology and Geophysics

Previous seismic studies have shown laterally varying P-wave seismic structures in the uppermost mantle throughout northeastern North America. Using the seismic refraction technique new data are analyzed to delineate with improved spatial resolution the upper mantle P-velocity structure throughout the Avalon, northern Appalachians, and Grenville terranes of the northeastern US and southeastern Canada. A total of twenty-five earthquakes have been analyzed, utilizing over one-hundred seismic stations throughout northeastern North America. The uppermost mantle P-wave velocity structure is best resolved horizontally from the seismic data in southeastern Canada Grenville terrane and the Avalon terrane in southeastern parts of New England. The P-wave velocity of the upper mantle Moho through the Avalon terrane in southeastern New England is found to be uniformly 8.1 km/s in this study, consistent with the results of previous studies (8.1 km/s). Pn velocities throughout the Grenville terrane show an average velocity of 8.15 km/s to 8.30 km/s compared to previous found upper mantle velocities of 8.12 km/s and 8.32 km/s. The northern Appalachian terrane P-wave velocity structure is not as well constrained laterally or horizontally in comparison to the Avalon and Grenville terranes due to lack of seismic stations in this area; however, a P-wave velocity of 8.15 km/s is measured in this study. The analysis of the new earthquake data has yielded few P-wave velocities from deeper below the Moho in the upper mantle. Although the depth to Moho is not well constrained due to a lack of seismic stations, a P-wave velocity increase with depth is recognized in the Grenville terrane, with a velocity as fast as 8.50 km/s at epicentral distances over 350 km away from the event. This implies that there is an observable increase with depth in the upper mantle seismic structure immediately below the the Moho within the Grenville terrane, whereas in the Avalon terrane there is only one distinguishable velocity recognized (8.1 km/s) for epicentral distances up to 900 km found using the seismic refraction method.

AAPG Search and Discover Article #90087 © 2008 AAPG/SEG Student Expo, Houston, Texas