On October 20, 1982, we collected seismic refraction data on a 250 km line segment between Butte, Montana and Challis, Idaho. These data are from two open pit mine blasts; one at Anaconda Minerals Companys' southeast Berkely pit and the other at Cyprus Mines Corporations' Thompson creek mine. Eight of our ten recorders wrote clear records of both blasts with measured travel times accurate to +/−0.2 seconds. We supplemented these data with data from permanent stations in the Butte area. The Pg (crustal) velocity is 5.8 km/s to the southwest and 6.0 km/s to the northeast. The Pn (mantle) velocity is 7.9 km/s to the northeast and 8.1 km/s to the southwest; total travel‐times are equal. The S‐wave velocity in the crust is about 3.5 km/s. The symmetry of the travel‐time results, with their intersection within one kilometer of the midpoint between the pit blasts, strongly suggests the refraction line overlies a horizontal Moho or parallels the strike line of a dipping Moho surface. These data indicate the crust in southwestern Montana and east‐central Idaho is approximately 33 km thick. Contrasting tectonic processes, compression and thickening during the Laramide orogeny with extension and thinning since the Paleocene, resulted in a crust with slightly less than normal
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