A new method is developed and used to determine compressional- and shear-wave models for the crust in the northernmost portion of the Mississippi embayment. A nonlinear least-squares process fits contiguous straight lines to data obtained using a seismic network, and yields apparent velocities and points of intersection of the lines.
The compressional-wave model consists of a 5.6-km-thick layer with a velocity of 6.19 km/sec overlying an 18.1 -km-thick layer with a velocity of 6.38 km/sec. These are underlain by material having a velocity of 7.17 km/sec. Since the shear-wave data are characterized by a large amount of scatter, only a two-layer model was obtained. It consists of a 21.5-km-thick layer having a velocity of 3.62 km/sec above material with a velocity of 4.10 km/sec. These models are overlain by variable thicknesses of sediments of the Mississippi embayment.
The travel times of compressional waves at short distances indicate that the great preponderance of earthquake foci lie at shallow depths. Although the travel times are relatively insensitive to small changes in depth for these shallow earthquakes, few data points imply depths greater than about 10 km, and most are compatible with shallower depths.