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Southern Methodist University Department of Geological Sciences, Dallas, Texas 75275-0395
Abstract
Nuclear explosions from the Nevada Test Site and earthquakes in southern and central California are recorded with offsets of 800 to 2000 km at the Lajitas seismic station in Texas. These broadband, far-regional data are from a single high-dynamic range station with three-component instruments at an exceptionally quiet site and bridge the distance range between regional refraction studies of the crust and uppermost mantle and teleseismic investigations with long-period data. Seismograms from 15 events were used to constrain the upper-mantle structure to a depth of about 250 km by matching the complete waveforms, including body and surface waves. A simple, one-dimensional model is proposed that is consistent with the broadband body and surface waveforms as well as arrival times. While the P-wave velocity structure is well constrained by a model similar to that proposed by Olsen et al. (1980), the shear-wave velocity structure is less controlled since the only identifiable shear arrivals are the Lg phase, trapped in the crustal wave guide. There are no identifiable Sn or mantle S waves below a distance of 1900 km. In contrast, mantle P waves, turning in the depth range below 120 to 150 km are the P-wave arrivals that dominate beyond 1200 km, although Pn is observed in cases of good signal-to-noise ratios to distances of 1400 to 1500 km as an emergent phase preceding these mantle arrivals. Due to the emergent Pn wave packet observed at all distances, the mantle-lid P-wave velocities are believed to exhibit a negative gradient zone, as proposed by Olsen et al. (1980) in contrast to the model developed by Burdick and Helmberger (1978). The medium parameters for S waves can only be constrained by the observed dominant Lg energy. By introducing a strong velocity contrast across the crust-mantle boundary, including a high-velocity S lid, the Lg energy can be efficiently trapped in the crust. This effect is further enhanced by including a laminated structure for the lower crust. Surface waves are most sensitive to the velocities in the uppermost crust, which consists of low-velocity gradient layers typical for sediments starting at 4 and 2 km/sec for P and S waves, respectively, for the Basin and Range province. Low Q values (20 to 50) in the shallow crustal layers are further required to match these waves.
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