Wide-angle Reflection Profiling with Quarry Blasts in the Eastern Tennessee Seismic Zone

Abstract

This study evaluates the feasibility of using relatively small, delay-fired quarry blasts as energy sources for wide-angle reflection profiling of the middle to lower crust beneath the southern half of the eastern Tennessee seismic zone. The test recordings targeted structure on both sides of the New York–Alabama lineament, a prominent northeast-trending zone of steep magnetic gradients. The lineament marks a near-vertical boundary between zones of high and low seismic activity and also correlates with a linear boundary between blocks of high- and low-velocity crust imaged by joint hypocenter-velocity inversion of regional P and S arrival time data. Timed blasts at four crushed-stone quarries were recorded with a portable array of 19 PRS-4 recorders and three-component, 4.5-Hz geophones with stations at roughly 200 m intervals. Source-receiver offsets ranged from 16 to 67 km. Blasts were timed with a PRS-4 deployed at the quarry sites. The blasts ranged from 75 to 542 ms in duration and from 259 to 942 lbs. explosive per delay. The effects of ripple firing are clearly evident in trace autocorrelations and as coherent peaks and nodes in the amplitude spectra for each shot; the latter show signal energy at frequencies up to 45 Hz. Source wavelets estimated by minimum entropy deconvolution have durations that are consistent with the reported durations of the blasts. Unprocessed field records recorded over distances between 16 and 29 km show strong reflections from the basal decollement (depth 4.5 km) and deeper interfaces within the basement. Shot gathers and slant stacks for blasts recorded at offsets between 62 and 67 km show coherent energy with apparent slownesses and travel times consistent with precritical P and S reflections from the lower crust and Moho. Although sparsely sampled, preliminary migrated sections suggest the presence of several highly reflective structures, including a concentration of reflectors at a depth of about 25 km, near the maximum reported depth for earthquakes in the region, a zone of south-dipping reflectors at depths between 27 and 40 km, northwest of the New York–Alabama lineament, and a clustering of reflectors at depths between 40 and 50 km, possibly marking the crustmantle transition. They also show north-dipping reflectors that correlate with the top of a prominent wedge of reflective crust imaged in nearby vertical-incidence profiles. The results demonstrate that signals from relatively small delayfired blasts can be quite useful for imaging the crust.