THE BRIGHT MOHO REFLECTION IN THE 1986 NEVADA PASSCAL SEISMIC EXPERIMENT

Detailed study of the reflected phases from the Moho, identified in the seismic reflection recordings of the 1986 Nevada PASSCAL lithospheric seismic experiment, place important constraints on the fine structure and geological nature of the Mohorovicic discontinuity. Two different spectral ratio techniques are used to estimate the reflectivity of the Moho. We evaluate the ratio of the reflected PmP phase to the incident source amplitude. We also calculated the spectral ratio of the PmP phase with a reference reflection. Reflectivity estimates, calculated using both spectral ratio techniques, are very consistent. High reflectivity within the range 0.28-0.39 was obtained. The analysis of the spectral characteristics suggests a layered structure for the Moho with an average thickness between 85 and 250 m. These results require a layered Moho structure with high impedance contrasts. Amplitude modeling using a reflectivity algorithm demonstrates that constructive interference from a 1.0-1.5-km-thick layered structure produces amplitudes and waveforms comparable to the data. To achieve such high reflectivities, 20-25% of the Moho consists of layers with velocities below 6.5 km/s. The remaining layers have velocities between 7.7 and 8.1 km/s. Geologically, the low velocities are consistent with velocities characteristics of basaltic magmas (partial melts). The high velocities are characteristic of mantle material. The partial melts may represent an expression of present-day underplating.