Fundamental mode Rayleigh waves travelling along the direction of the essentially linear array of long-period seismographs at TRI, BLZ, BOL, GSO, OLB have been studied for two earthquakes. Phase velocities have been obtained in the period range 25-250 sec. The phase velocities for this region show a remarkable similarity at long periods to those obtained for paths in other tectonically active zones, such as the East African Rift and the great basin of the western United States. An inversion to the phase velocity data for this region has shown that any high velocity material overlying a low-velocity channel is either very thin or absent. Low velocity material with S-wave velocities in the range 4.2-4.4 km/sec may start from just below the Moho and extend to great depths. Attempts to resolve the structure into segments corresponding to the Po Valley, the Apennines and the northernmost Tyrrhenian Sea cannot be made because of the small distance scale of the array. However, it is likely that the structure of the Tyrrhenian Sea between GSO and OLB is more nearly continental than oceanic. © 1979.
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