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Dispersion of Rg waves in southeastern Maine: Evidence for lateral anisotropy in the shallow crust

WESTON OBSERVATORY DEPARTMENT OF GEOLOGY AND GEOPHYSICS BOSTON COLLEGE, WESTON, MASSACHUSETTS 02193
DEPARTMENT OF EARTH, ATMOSPHERIC AND PLANETARY SCIENCES MASSACHUSETTS INSTITUTE OF TECHNOLOGY, CAMBRIDGE, MASSACHUSETTS 02139

Abstract

Group velocities of short-period Rayleigh waves (Rg) are sensitive to variations in the velocity structure of the upper few kilometers of the crust. In this study, we analyzed dispersive properties of Rg waves recorded from blasts detonated as part of the Maine seismic refraction experiment conducted in 1984 by the U.S. Geological Survey. The period range of the Rg waves in this study is 0.4 to 1.6 sec; group velocities in this period range are sensitive to the velocity structure at depths ranging from very near the surface to about 2 km. Group velocities were found to vary significantly from one path to the next. A simple explanation of the observed pattern of group velocities is that higher velocities were observed for paths parallel to the structural grain of the Appalachians than for paths transverse to the grain. For example, 1-sec Rg waves have group velocities of about 3.0 to 3.2 km/sec for paths parallel to the grain and about 2.6 to 2.7 km/sec for paths transverse to the grain. Thus, our results suggest that the shallow crust beneath the Appalachians of southeastern Maine is laterally anisotropic.

Rg group velocities from this study were inverted to obtain shear wave velocity models of the upper 1.5 to 2.0 km of the crust underlying southeastern Maine. To compare our results with refraction and laboratory results, V_p was estimated from V_s using a V_p/V_s ratio of 1.78. Inverting the average group velocities for all paths in this study resulted in a V_p model with velocities ranging from 4.9 km/sec in the upper 0.3 km to 6.2 km/sec at 2 km depth. The range in observed Rg group velocities from one path to the next is evidence of a variation in shallow crustal structure depending on geographical location and/or orientation of the paths. In the upper 0.3 km, V_p appears to range from about 4.8 to 5.3 km/sec, and at 2 km depth V_p appears to range from about 5.6 to 6.6 km/sec.

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