A seismic-refraction survey of crustal structure in central Arizona

The U.S. Geological Survey conducted a seismic-refraction study of the earth's crust and upper mantle near the Tonto Forest Seismological Observatory (TFO), located 10miles south of the Mogollon Rim near Payson in central Arizona. Two recording lines 400 km long intersect in the approximate form of a cross at TFO; one line trends southeast and the other northeast. The sedimentary layer at most places southwest of the rim is less than 1 km thick, but north of the rim it is 2 to 3 km thick. The velocity in this uppermost layer ranges from 2.6 to 4.7 km/sec, with the higher limit measured near or north of the rim. Arrivals refracted in the upper crust (P_g) can be attributed to two layers for all the shot points south of the rim. The velocity in the upper layer is about 5.9 km/sec with thickness ranging from 2 to 8 km; beneath the upper layer the velocity is about 6.1 km/sec. The upper layer seems to be absent northeast of the rim, where two shot points generated P_g arrivals that show only a velocity of 6.2 km/sec. A Poisson ratio of 0.22 for the upper crustal layers was measured from shear and compressional arrivals. The lower crust could not be identified from the first and later refraction arrivals; however, minimum depths to the intermediate layer were determined. An average crustal velocity of 6.2 km/sec was measured from wide-angle reflection alignments. A thin intermediate layer would explain the seismic measurements.

A delay-time method was used to map the configuration of the M-discontinuity. The depth below sea level is about 36 km along the northwest-trending line. The northeast-trending line shows a shallow depth of 21 km near Gila Bend, increasing depth to about 34 km under TFO, and a flat M-discontinuity at 40 km depth under the Mogollon Mesa northeast to Sunrise Springs. There is evidence of an abrupt depth change of about 4 km on the M-discontinuity in the vicinity of TFO. The velocity in the upper mantle is 7.85 km/sec. The relation of topographic elevation to crustal thickness suggests an approach to isostatic equilibrium, which is deduced from a near-zero regional free-air gravity anomaly. However, lateral density change in the upper mantle is required to make the crustal-refraction model fit the observed gravity-anomaly values, provided that velocity and density are linearly related.