THE BRIGHT MOHO REFLECTION IN THE 1986 NEVADA PASSCAL SEISMIC EXPERIMENT

Detailed study of the reflected phases from the Moho, identified in th e seismic reflection recordings of the 1986 Nevada PASSCAL lithospheri c seismic experiment, place important constraints on the fine structur e and geological nature of the Mohorovicic discontinuity. Two differen t spectral ratio techniques are used to estimate the reflectivity of t he Moho. We evaluate the ratio of the reflected PmP phase to the incid ent 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 refle ctivity within the range 0.28-0.39 was obtained. The analysis of the s pectral characteristics suggests a layered structure for the Moho with an average thickness between 85 and 250 m. These results require a la yered Moho structure with high impedance contrasts. Amplitude modeling using a reflectivity algorithm demonstrates that constructive interfe rence from a 1.0-1.5-km-thick layered structure produces amplitudes an d waveforms comparable to the data. To achieve such high reflectivitie s, 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. Geol ogically, the low velocities are consistent with velocities characteri stics of basaltic magmas (partial melts). The high velocities are char acteristic of mantle material. The partial melts may represent an expr ession of present-day underplating.